106
- d -block - , A f-- Sc Ti V Cr Mn Fe Co Ni Cu Zn 21 22 23 24 25 26 27 28 29 30 r-- y Zr Nb Mo Tc Ru Rh Pd Ag Cd 39 40 41 42 43 44 45 46 47 48 r-- '5B La to Hf Ta W Re as Ir P1 Au Hg 57 71 72 73 74 75 76 77 78 19 BO r-- f-- Ac 90 Ku to 104

Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

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Page 1: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

-

d-block -, A

f--Sc Ti V Cr Mn Fe Co Ni Cu Zn21 22 23 24 25 26 27 28 29 30

r--y Zr Nb M o Tc Ru Rh Pd Ag Cd39 40 4 1 42 43 44 45 46 47 48

r-- '5BLat o

Hf Ta W Re as Ir P1 Au Hg57 71 72 73 74 75 76 77 78 19 BO

r-- f--Ac 90 Ku

~to

104~ ~

Page 2: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

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Page 3: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

UNIT

~

Transition Elements .

liIllllllllll1m~N27899

Page 4: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

r\JAT'ONd~L

~ Inner London Education Authority 1984

First published 1984by John Murray (Publishers) Ltd50 Albemarle StreetLondon W1X 4BD

All rights reserved.Unauthorised duplicationcontravenes applicable laws

Printed and bound in Great Britain byMartin's of BerwicK

British Library Cataloguing in Publication DataIndependent Learning Project for Advanced

ChemistryTransition Elements. - (ILPAC; Unit 15)1. ScienceI. Title II. Series500 0161.2ISBN 0 7195 4053 4

/I

Page 5: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

CONTENTS

PREFACEAcknowledgementsKey to activity symbols and hazard symbols

vvi

vii

INTRODUCTIONPre-knowledgePre-test

23

LEVEL ONEVARIABLE OXIDATION STATEElectron configurations of the elementsFormation of ionsThe pattern of oxidation statesWhy scandium and zinc are not typical transition metalsREDOX CHEMISTRY OF VANADIUM AND MANGANESEVanadiumExperiment 1 - illustrating the oxidation states of vanadiumThe oxidation states of manganeseExperiment 2 - illustrating the oxidation states of manganeseCHARACTERISTICS OF TRANSITION ELEMENTSColoured compoundsFormation of complexesCatalysis involving transition elementsParamagnetismSimilar physical and chemical propertiesGENERAL PHYSICAL AND CHEMICAL PROPERTIESSimilarity in physical propertiesTransition elements as structural materialsSimilarity in chemical propertiesTHE EXTRACTION OF IRON IN THE BLAST FURNACESteel productionLEVEL ONE CHECKLISTLEVEL ONE TEST

5

556899

1010131417171 92020212121232526283031

LEVEL TWOFORMATION AND STABILITY OF COMPLEX IONSStability constantsAn application of complex ion chemistryCompetition for cationsPolydentate ligandsRelative stabilities of complex ionsExperiment 3 - relative stabilities of some complex ionsUse of edt a in titrationsSHAPES OF COMPLEX IONS AND ISOMERISMDifferences between organic and inorganic isomerismOptical isomerismSTOICHIOMETRY OF COMPLEX IONSExperiment 4 - determining the formula of a complex ionACIDITY OF COMPLEX IONSThe effect of pH on redox potentialsTHE EFFECT ON REDOX CHEMISTRY OF CHANGING LIGANDSSOME REDOX CHEMISTRY OF COPPERExperiment 5 - some redox chemistry of copperTRANSITION ELEMENTS IN CATALYSIS

3535363737383940424243454647505253535557

iii

Page 6: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

Experiment 6 - investigating the use of cobalt(II) ions asExperiment 7 - catalysing the reaction between iodide ions

peroxodisulphate ionsHeterogeneous catalysisTransition elements in biological systemsLEVEL TWO CHECKLIST

END-Of-UNIT PRACTICAL TESTEND-Of-UNIT TEST

a catalyst 60and

63666667

6869

7373

797980

81

APPENDIX ONEELLINGHAM DIAGRAMS AND THE EXTRACTION Of METALS

APPENDIX TWOLigand field theoryParamagnetism

ANSWERS TO EXERCISES

iv

Page 7: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

PREFACE

This volume is one of twenty Units produced by ILPAC. the Independent LearningProject for Advanced Chemistry. written for students preparing for the AdvancedLevel examinations of the G.C.E. The Project has been sponsored by the InnerLondon Education Authority and the materials have been extensively tested inLondon schools and colleges. In its present revised form. however. it isintended for a wider audi~ncej the syllabuses of all the major ExaminationBoards have been taken into account and questions set by these boards have beenincluded.

Although ILPAC was initially conceived as a way of overcoming some of thedifficulties presented by uneconomically small sixth forms. it has frequentlybeen adopted because its approach to learning has certain advantages over moretraditional teaching methods. Students assume a greater responsibility fortheir own learning and can work, to some extent, at their own pace. whileteachers can devote more time to guiding individual students and to managingresources.

By providing personal guidance. and detailed solutions to the many exercises.supported by the optional use of video-cassettes. the Project allows studentsto study A-level chemistry with less teacher-contact time than a conventionalcourse demands. The extent to which this is possible must be determinedlocally; potentially hazardous practical work must. of course, be supervised.Nevertheless. flexibility in time-tabling makes ILPAC an attractive propo-sition in situations where classes are small or suitably-qualified teachersare scarce.

In addition. ILPAC can provide at least a partial solution to other problems.Students with only limited access to laboratories, for example, those studyingat evening classes, can concentrate upon ILPAC practical work in the laboratory.in the confidence that related theory can be systematically studied elsewhere.Teachers of A-level chemistry who are inexperienced. or whose main disciplineis another science. will find ILPAC very supportive. The materials can be usedeffectively where upper and lower sixth form classes are timetabled together.ILPAC can provide 'remedial' material for students in higher education.Schools operating sixth form consortia can benefit from the cohesion that ILPACcan provide in a fragmented situation. The project can be adapted for use inparts of the world where there is a severe shortage of qualified chemistryteachers. And so on.

A more detailed introduction to ILPAC. with specific advice both to studentsand to teachers. is included in the first volume only. Details of the ProjectTeam and Trial Schools appear inside the back cover.

LONDON 1983

v

Page 8: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

ACKNOWLEDGEMENTSThanks are due to the following examination boards for permission toreproduce questions from past A-level papers:

Joint Matriculation Board;Teacher-marked Exercise, p62(3)[1980)

Oxford Delegacy of Local Examinations;Teacher-marked Exercise, p77(1981)

Southern Universities Joint Board;End-of-Unit Test 13(1982)

The Associated Examining Board;Exercise 10(1980)Teacher-marked Exercise p78(1980)Level One Te st, 11 (1980)

University of London Entrance and School Examinations Council;Exercises 27(1981), 51(N1978)Teacher-marked Exercises p25(1&77), p62(1)(1974), p62(2)[N1982)Level One Test 1-5(1983), 6(1977), 7(1978), 8(1977)' 9(1981)

10(1979), 12(1983)End-of-Unit Test 1-4(1978), 5[N1975), 6(1979), 7(N1978), 8(N1980)

9(1980), 10(1981), 11(1977), 12(1980), 14(1976)15(1976), 16a(1981), 16b(1982)

Where answers to these questions are included, they are provided by ILPACand not by the examination boards.Questions from papers of other examining boards appear in other Units.

Photographs are reproduced by permission as follows:Fig. 6: Space shuttle - PopperfotoFig. 7A: Brass - J. Allan Cash PhotolibraryFig. 7B, 7C, 70, 7F - Tony LanghamFig. 7E: Railway lines - PopperfotoFig. 9: B~ast furnace - PopperfotoFig. 10: Steel production - PopperfotoFig. 11: Atomic models - Tony LanghamPhotographs of students - Tony Langham

vi

Page 9: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

SYMBOLS USED IN ILPAC UNITS

& Reading

~Exercise

\\\;t Test

oo--[2]

Revealing exercises

~ Discussion

~ Computer programme

'A' Level question ~ Experiment

I[ ]1 Video programme00'A' Level part question

[Q8 Film loop

'A' Level questionSpecial paper

Model-making

Worked example

Teacher-marked exercise

INTERNATIONAL HAZARD SYMBOLS

[Xl Harmful

[i] Flammable

~l:V i,l, Corrosive--

[~ Toxic

• Explosive

~J Oxidising

Radioactive

VII

Page 10: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements
Page 11: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

INTRODUCTIONThe term 'transition elements' can include all the elements in the centralpart of the Periodic Table - between the block of typical metals in Groups Iand II and the block of elements, mostly non-metals, in Groups III to O.This position helps to explain the use of the word 'transition', although theterm is not altogether satisfactory. You have probably already used the termad-block' instead in your study of the Periodic Table as a whole, althoughthis too is not entirely satisfactory.

s-block~

I II

p-block

III IV V VI VII 0r-- -

- ....-

Sc Ti V Cr Mn Fe Co Ni Cu Zn21 22 23 24 25 26 27 28 29 30

Y Zr Nb Mo Tc Ru Rh Pd Ag Cd39 40 41 42 43 44 45 46 47 48

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Hg57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Ku89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104

f-block - inner transition elements

d-block - transition elements

Fig.1. The Periodic Table

We confine a detailed study in this Unit to the elements 21.SC to 30Zn,although the discussion is sometimes limited to the elements 22Ti to 29CUfor reasons which we explain. Many of the general statements apply also tothe other rows. The whole block is characterised by similarity of properties.not only in vertical groups as elsewhere in the Periodic Table, but in hori-zontal rows as well.

In Level One of this Unit we explore the general properties which are charac-teristic of transition elements, especially variable oxidation states. Weuse standard electrode potentials, E&, to determine the feasibility of reaction.as described in Unit P6 (Equilibrium III: Redox Reactions). At the end ofLevel One we include a Section on the extraction of iron from its ores in theblast furnace. If you need to study Ellingham diagrams in this connectionyou should look at Appendix One at this point.

We extend your study of the general characteristics of transition elements inLevel Two, where we consider more fully the formation and stability of complexions and the mechanism of homogeneous catalysis. In Appendix Two you can,if you wish, take your study of complex ions a little beyond A-level inorder to consider a more satisfactory explanation of variations in colourand paramagnetism.

1

Page 12: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

There is a"video-programme entitled 'Transition Metals' made to accompanythis Unit .. It is not essential but, if it is available, you should try tosee it at the appropriate times. You might watch it right through before youstart the Unit, and later use sections to help you with individual topics.

Transition metals

There are seven experiments in this Unit, two in Level One and five in LevelTwo. Your teacher may advise you not to do all of them. There is also anend-of-Unit practical test.

PRE-KNOWLEDGEBefore you start work on this Unit, you should be able to:

(1) identify the d-block in an outline Periodic Table;(2) write electron configurations for elements up to 22SC both in s, p, d

notation and in lelectrons in boxesl form;(3) deduce oxidation numbers of elements in compounds, given their formulae;(4) use standard electrode potentials to predict the feasibility of redox

reactions;(5) recognise examples of different types of isomerism;(6) write an expression for the equilibrium constant for a given reaction.

PRE-TESTTo find out whether you are ready to start Level One, try the follo- ~wing test. You should not spend more than 40 minutes on this test. •Hand your answers to your teacher for marking. ~~

2

Page 13: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

PRE-TEST \\\;l81 PI d notation for the element1. Write electron configurations in

13Al and the ion 802-. (2)

2. Write electron configurations in 'electrons in boxes' form for theelement N and the ion K+. (2)

3. Use the simple rules to calculate the oxidation number of phosphorusin these compounds and ions:

( 4 )

4. Use the following electrode potentials to write balanced equations.with appropriate ~ values. for(a) a reaction which probably goes to completion.(b) a reaction for which the equilibrium mixture contains a

significant proportion of both reactants and products.Zn2+(aq) + 2e- ? Zn(s)Cr3+(aq) + 3e- ? Cr(s)Ag+(aq) + e- ~ Ag(s)

Etr = -0.76 V

Etr = - 0 • 74 V

ge- = +0.80 V ( 4 )

( 1 )Why was the word 'probably' used above?

5. Write an expression for the equilibrium constant for the reaction:Cu2+(aq) + 4NH3(aq) ? Cu(NH3)42+(aq)

What is the unit for this constant?( 2)

( 1 )

6. Select from the following structures. all of which aretrich1orobut-1-enes:(a) two geometric isomers.(b) two optical isomers.(c) two structural isomers.Name the isomers in each pair. showing how prefixes are used todistinguish between the two. (In (b) you are not expected toknow which is which~)(i) C1 C1

"" /C=C CH3/ -, /H C

/""Cl H

(iii) H Cl"" /C==C CH3/ -, /

Cl C/ -,

Cl H

(ii) C1 H'" /

C=C CH3/ <, /Cl C

/ '"Cl H

(iv) H Cl"" /

C= C CH3/ -, /

C1 C/'"

H C1 ( 9 )

(Total 25 marks)

3

Page 14: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements
Page 15: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

LEVEL ONEThe metals you have studied so far, those in Groups I and II, each have asingle oxidation state. In the transition elements you study metals whicheach have several oxidation states, and this is the key to much of theircharacteristic chemistry.

VARIABLE OXIDATION STATEObjectives. When you have finished this section you should be able to:

(1) write electron configurations for the atoms of the first transitionseries from scandium to zinc;

(2) write electron configurations for some of the more common ions oftransition elements;

(3) explain why transition elements have variable oxidation states;(4) explain the general Qattern of oxidation states in the transition

elements;(5) explain why scandium and zinc are often excluded from the study of

transition elements.

Electron configurations of the elementsFirst, read the introductory section of the chapter on transitionelements in your textbook(s). For the time being, focus yourattention on electron conf~gurations and oxidation states. Othercharacteristics, such as catalysis, complex ions, colour and para-magnetism will be mentioned, but you will study these more fullylater in the Unit.

Exercise 1 ( a ) Write down the electron configurations, in s, p, d ~notation, for the elements 21SC to 30Zn. Show allthe sub-shells for scandium but thereafter represent ~~the electron arrangement which is common to all bythe symbol of a noble gas.

(b) Write down the electron configurations again, butthis time use the 'electrons-in-boxes' notation forthe 3d and 4s orbitals.

(An swers on page 81)

You have noted that atoms of chromium and copper have only one 4selectron, whersas all th~ others have two. For the next exercise,find out from your textbook(s) why this is so. You may find ituseful to revise from Unit S2 (Atomic Structure] an explanation of'steps' between nitrogen and oxygen and between phosphorus and sulphurin the plots of ionization energy against atomic number.

5

Page 16: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

Exercise 2 (a) Suggest reasons why atoms of chromium and copper ~have only one 48 electron while the other elementsin the full transition series have two 48 electrons. ~~

(b) What would you expect to be the electron configura-tions of molybdenum and silver atoms?

(Answers on page 81 )

Having considered the electronic structure of atoms, we now consider the lossof electrons to form ions.

Formation of ionsThe first point to grasp is that when transition elements form ions it isalways the s-electrons which are lost first. You have learned that in thehypothetical process of 'building up' atoms by adding electrons and protonsone by one the 48 orbitals are filled before the 3d - you might thereforeexpect the 3d electrons to be lost first.

Fig.2. The order in which orbitals are filled.

3d electrons would be lost first if protons were lost at the same time but,of course, in the formation of ions, only electrons are lost, leaving theatom with an excess positive charge. This excess charge has the effect ofpulling all the electrons a little closer and, in the process, the 3dorbitals occupy a lower energy level than the 4s.

This is one consequence of the fact that the 3d and 4s energy levels arefairly close togsther for the transition elements, as shown in Fig. 3.At scandium, the effect is t~at the energy levels are 'inverted' and theenergy of the 3d orbitals is less than that of the 4s.

6

Page 17: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

~1'0UrJ)

B(5c>OJQ)ca>

--<,

""- -....Sd-5p

5s

<,<,

<,<, 4f--- 4d-4p

4s

---- "-'---- ~-"-3d-3p

3s

--2p2s

-----1s

10 30 40 10060 9050 70 8020atomic number

Fig.3. Variation of energy levels with atomic number.

You should now be able to see that a number of different electron configura-tions could be reasonably stable for ions of transition elements, instead ofthe more usual single stable configuration for other metals.

Draw together what you have learned about stable electron configurations, inthis Unit and earlier, in order to do the next exercise.

Exercise 3 Consider the loss of electrons one at a time from typical ~transition elements. What resulting electron configura- ..tions do you thinK might be reasonably stable? ~~(Answer on page 81 )

7

Page 18: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

Most transition elements do indeed form a number of different ions, andtheir configurations often conform to the criteria for stability we listedin our answer to Exercise 3. However, other configurations are stable, asyou see in the next exercise.

Exercise 4 Write lelectrons-in-boxesl configurations for the

~

following ions, all of which exist although some are morestable than others.(a) Sc3+ (e ) V3+ ( L) Fe2 + (m) Ni2+(b) Ti2 + (f) Cr+ (j) Fe3+ ( n ) Cu+(c) Ti3 + (g) Cr3+ (k) C02+ (0 ) Cu2+(d) V2 + (h) Mn2+ (I) C03 + ( p l Zn2 +(Answers on page 81

The stability of a particular ion depends to some extent on the anion withwhich it combines. For instance, fluoride, and to a lesser extent chloride,which are small and not easily polarized, stabilize high oxidation states.On the other hand, iodide, and to a lesser extent bromide, stabilize loweroxidation states.

The pattern of oxidation statesIons with charges greater than 3+ are rarely found. but higher oxidationstates than 3 or 4 (in which the electrons may be regarded as Ipartiallylost' to more electronegative atoms in covalent bonding) are very common incompounds of the transition elements. Remember that, in this context,oxidation number is merely a formalism and does not relate to the actualcharge on the atom in the compound.

Higher oxidation states are most often seen in oxides and oxo-anions, someof which are the subject of the next exercise.

Exercise 5 Work out the oxidation states of the transition elements

~

in the following ions and compounds:1 2 -1-6 -~ )'V --z

(a) Mn04 - (e) K2Cr04 ( L) VOC12tl; -~ ~s(b) Mn02 (f) Cr203 (j ) V205

L. 6 - '[; I ~ - t

(c ) K2Mn04 (g) KCr03CI (k) NH4V03(, S

(d ) Cr2072- (h) V02 +

(Answers on page 81 )

The following table summarises the known oxidation states of elements of thefirst transition series - the most common states are ringed. Use it toanswer the next exercise.

8

Page 19: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

Table 1

Sc Ti V Cr Mn Fe Co Ni Cu Zn

0® 6 6

® 5 5 5 5

0 4 4 4 4 4 4@ 3 3 @ 3 @ 3 3 3

2 2 2 @ @ 0 0 @ 01 1 1 1 1 1 1 1

Exercise 6 What patterns can you see in the common oxidation states: ~(a) from scandium to manganese,(b) from manganese to zinc? ~~(Answers on page 81

Table 1 shows that scandium and zinc have only one known oxidation state.This sometimes leads to their exclusion from a list of transition metals.

Why scandium and zinc are not typical transition metalsAll zinc compounds have a filled 3d sub-shell while all scandium compoundshave an empty 3d sub-shell. The typical properties of compounds of transi-tion elements are associated, as you will see, with partly-filled d orbitals.For this reason, scandium and zinc may be excluded from a study of transitionelements. We will mention another reason for their exclusion when we dealwith physical properties.

In the next section you study experimentally some of the oxidation states oftwo typical transition elements, vanadium and manganese.

REDOX CHEMISTRY OF VANADIUM AND MANGANESEObjectives. When you have finished this section you should be able to:

(6) describe the preparatipn of a variety of oxidation states for vanadiumand manganese;

(7) describe some examples of disproportionation and reverse disproportion-ation involving vanadium and manganese.

. 9

Page 20: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

VanadiumThe oxidation states of vanadiumwhich you can see in the section'Transition Metals' dealing with

are best illustrated by an experimentof the ILPAC video-programme I[ IIvariable oxidation states. .'

00

tmHowever, you should, if possible, do the experiment yourself.

Illustrating the oxidationstates of vanadium

EXPERIMENT 1

AimThe purpose of this experiment is to illus-trate the presence of several differentoxidation states for vanadium, and to showhow it is possible to change from oneoxidation state to another.

IntroductionStarting with a solution containing vanadium(V) in acid conditions, you usepowdered zinc as a reducing agent. Colour changes. indicate the formation ofother oxidation states. After this first series of reactions you performsome .changes between oxidation states by using a variety of oxidizing andreducing agents.

Requirementssafety spectacles and glovesconical flask, 100 cm~spatulaammonium polytrioxovanadate( V), NH4V03 (ammonium metavanadate)measuring cylinder, 25 cm3sulphuric acid, dilute, 1 M H2S04 ~)(~sulphuric .acid, concentrated, H2S04 - - - - - - - - - - - - - l~ .:,

5 test-tubes and holder ~--__••~test-tube rackzinc dust, ZnBunsen burner, tripod, gauze and bench matfilter funnel and paperpotassium manganate(VII) solution, 0.02 M KMn04 (permanganate)sodium sulphite, Na2S03potassium iodide solution, 0.05 M KIsodium thiosulphate solution, 0.1 M Na2S203

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Hazard warningC9ncentrated sulphuric acid is corrosive and reactsviolently with water. Therefore you MUSTWEAR SAFETY SPECTACLES AND GLOVES.WHEN DILUTING, ADD ACID TO WATER, NOT WATER TO ACID.MOP UP SMAL~ SPILLAGES WITH EXCESS WATER.

ProcedureFirst complete a copy of Table 2 to assist you in identifying the differentoxidation states of vanadium.Table 2

Ion (hydrated) *V02+ V02+ V3 + V2+

Colour yellow blue green violet

Oxidation state

Name

(Answer on page 82)

*The V03- ion in the ammonium salt is converted to V02+ by acid:V03-(aq) + 2H+(aq) ? V02+(aq) + H20(I)

1. Place about 0.25 g (one spatula measure) of ammoniumtrioxovanadate(V) in a conical flask and add about 25 cm3 ofdilute sulphuric acid. Carefully add about 5 cm3 of concentratedsulphuric acid and swirl the flask until you obtain a clearyellow solution.

2. Pour about 2 cm3 of this vanadium[V) solution into each of two test-tubes ready for later tests.

3. To the conical flask add 1 - 2 g (one spatula measure) of zinc dust, alittle at a time. Swirl the flask at intervals and record any observedcolour changes in a copy of Results Table 1.

4. When the solution has become violet (you may need to heat the flask forthis final change), filter about 2 cm3 into each of three test-tubes.

5. To one of the three tubes add, a little at a time, an excess of acidifiedpotassium manganate(VII) solution, shaking after each addition, until nofurther change is observed.

6. Keep the other test-tubes of solutions for later tests. Answer the firsttwo questions (page 12) and complete Results Table 1 as far as you canbefore doing further tests.

7. To one of the tubes containing vanadium(V) add a little sodium sulphiteand shake. Filter if cloudy. Now boil carefully (at a fume cupboard)to remove excess sUlphur dioxide and add about the same volume of thevanadium(II) solution. Record your observations.

S. To the second of the tubes containing vanadium(V), add about 2 cm3potassium iodide solution and mix. Then add about 2 cm3 of sodiumthiosulphate solution. Record your observations.

9. Keep the last tube of vanadium(II) solution for a final experiment youmay wish to do after answering question 7.

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Results Table 1

Test Observations Summary of reaction

Ammonium vanadate + acid White solid dissolved +5 +5V03- -+ V02 +

to a yellow solution.

Vanadium(V) + zinc

Vanadium(II) + manganate( VII)

Vanadium(V) + sulphiteAdd vanadium(II)

Vanadium(V) + iodide+ thiosulphate

Vanadium(II) + - - - - -

(Specimen results on page 82 )

Questions1. How do you explain the first appearance of a green colour in the

solution?2. What are the subsequent changes in colour and why do these changes occur?

You will need the following electrode potentials in order to answer some ofthe remaining questions:

Zn2+(aq) + 2e- ~ Zn(s) £B- -0.76 VV3+(aq) + e- ~ V2+(aq) EB- -0.26 VV02+(aq) + 2H+(aq) + e ~ V3+(aq) + H2O(1) EB- +0.34 VV02+(aq) + 2H +(aq) + e ~ V02+(aq) + H2O(1) EB- +1 .00 VS042-(aq) + 4H+(aq) + 2e- ~ H2S03(aq) + H2O(1) £B- +0.17 VI2(aq) + 2e- ~ 2I-(aq) EB- +0.54 V

3. What did you observe when you added iodide ions to vanadium(VJ? Whatcaused .this colour?

4. Why did you add sodium thiosulphate?5. Why does reduction with iodide not give the same result as reduction

with zinc?6. What did you observe when you added sulphite ions to acidified

vanadium(V) solution? Does this result correspond with a predictionmade using the EB- values? (Hint: sulphite ions and acid react togive what?)

7. How would you set about finding a suitable oxidizing agent for theoxidation of vanadium(II) to vanadium(III) and no further? Does oneappear in the table above?

(Answers on page 82 )

In the next section we consider the various oxidation states of manganese.

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The oxidation states of manganeseThe worked example and exercises which follow help you to predict ways ofmaking the less common oxidation states of manganese from the readilyavailable Mn(VII), Mn(IV) and Mn(II). You may then test your predictionsin Experiment 2.

Worked example. It has been proposed that Mn(VI) compounds might beformed by reaction between Mn(VII) and Mn(IV).Predict the feasibility of such reactions in aqueoussolution of varying pH.

Solution.The first requirement is to find electrode potentials for appropriate half-cells. These are quoted for 'standard' acid conditions, namely[H+(aq)] ~ 1 mol dm-3 or pH = 0:

(VI -+ IV):E-G- = 0.56 VEB- = 2.26 V

A cell reaction converting Mn(VII) and Mn(IV) to Mn(VI) is obtained byreversing the second half-equation and adding to the first:2Mn04-(aq) + Mn02(s) + 2H20(I) ~ 3Mn042-(aq) + 4H+(aq);

bEB- = -2.26 V + 0.56 V = -1.70 VSince bE-& is large and negative, the reaction will not proceed from left toright to any measurable extent. Indeed, any Mn(VI) obtained by other meanswould be expected to disproportionate completely in acid conditions. (Thesame conclusion may be reached by applying the 'anticlockwise rule' to thetwo half-equations.)We also see that reducing [H+(aq)] would shift equilibrium to the right andmake the desired reaction more likely. We should therefore consideralkaline conditions.Electrode potentials for 'standard' alkaline conditions, namely[OH-(aq)] = 1 mol dm-3 or pH = 14, are as follows:(VII -+ VI): 2Mn04-(aq) + 2e- ~ 2Mn042-(aq); EB- 0.56 V(VI -+ IV): Mn042-(aq) + 2H20(I) + 2e- ~ Mn02(S) + 40H-(aq); E& - 0.60 VA cell reaction converting Mn(VII) and Mn(IV) to Mn(VI) is obtaiRed byreversing the second half-equation and adding to the first:2Mn04-(aq) + Mn02(s) + 20H-(aq) ~ 3Mn042-(aq) + 2H20(I); bEB- = -0.04 VHere we see that 6EB- is negative again, but this time very small, so that wemight expect the reaction to produce an equilibrium mixture containingsignificant proportions of both reactants and products. Furthermore, weshould be able to shift the equilibrium to the right by increasing[OH-(aq)]. thus producing more Mn(VI) in the equilibrium mixture.

You can test this prediction in Experiment 2, together with predictions youmake in the next three exercises.

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Exercise 7 Predict the feasibility of making Mn(III) from Mn(II) and ~Mn(IV) under conditions of varying pH. Use theseelectrode potentials: ~~Mn02(s) + 4H+(aq) + e ? Mn3+(aq) + 2H20(1); EtT = 0.95 VMn3 + (aq) + e

Mn (OH) 3 ( s ) + e

? Mn2 + (aq) ;? Mn(OH)2(S) + OH-(aq);

Mn02(s) + 2H20(1) + e(Answer on page 83)

E€r = 1.51 VEtT = - O. 10 V

E-& = 0.20 V

Exercise 8 Predict the feasibility of making Mn(III) from Mn(II) and~Mn(VII) in acid conditions. The relevant half-cellpotentials are: ~~Mn3 + (aq) + e ? Mn2+ leq l j E€r = 1.51 VMn04-(aq) + BH+(aq) + 5e- ~ Mn2+(aq) + 4H20Cl); E€r = 1.50 V(Answer on page 83)

cell potentials are: ~EtT 2.0 V

Exercise 9 Predict the feasibility of making Mn(V) from Mn(IV) andMn(VI) in conditions of varying pH. The relevant half-

Mn042-(aq) + 2H+(aq) + o ? MnD3 -(aq) + H2°(1);Mn03 -(eq) + 2H+(aq) + e ? Mn02(s) + H2O(1);1'1nO4 2-(aq) + H2°(1) + e ? 1'1n03-(aq) + 20H-( aq) ;Mn03-(aq) + H2°(1) + e ? Mn02(S) + 20H-( aq) ;(Answer on page 83 )

EtT = 2.:§, V

E-& = 0.34 V

E€r = 0.84 V

Now you should be able to do Experiment 2. Be sure to have youranswers to the last three exercises available for comparispn.

The oxidation states of manganeseEXPERIMENT 2

Aim and IntroductionThe purpose of this experiment is to makesamples of some of the less commonoxidation states of manganese, usingmethods predicted in the previousexercises. There are three parts, withquestions following each.

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Requirementssafety spectacles and protective gloves6 test-tubes and rackpotassium manganate(VII) (permanganate) solution, 0.01 M KMn04sulphuric acid, dilute, 1 M H2S04sodium hydroxide solution, 2 M NaOH - - - - - - - - - - - - - - -manganese(IV) oxide (manganese dioxide), Mn02spatulastirring rodfilter funnel and 3 papersmanganese(II) sulphate-4-water, MnS04·4H20sulphuric acid, concentrated. H2S04 - - - - - - - - - - - - -

Hazard warningConcentrated sulphuric acid is corrosive and reactsviolently with water. Therefore you MUST:WEAR SAFETY SPECTACLES AND GLOVES.WHEN DILUTING, ADD ACID TO WATER, NOT WATER TO ACID.MOP UP SMALL SPILLAGES WITH EXCESS WATER.

Procedure. Part ARefer to the Worked Example on page 13 where we predicted that it should bepossible to make Mn[VI) from Mn[VII) and Mn[IV).1. Put about 5 cm3 of potassium manganate(VII) solution in each of three

test-tubes.2. To one of the three tubes add about 3 cm3 dilute sulphuric acid and to

another add about 3 cm3 sodium hydroxide solution.3. To each of the three tubes add a little solid manganese[IV) oxide and

stir for about a minute.4. Filter enough of each mixture into a clean tube to see the colour of

the filtrate clearly. Use a fresh filter paper for each mixture.S. One of the tubes should now have in it a clear green solution of Mn[VI).

Add to this a little dilute sulphuric acid.

Questions. Part A.1. Explain why only one of the three mixtures reacted to give green Mn(VI).2. What happened when acid was added to Mn[VI)? Explain.(Answers on page 83

Procedure. Part BRefer to your answer to Exercise 7 in which you predicted that it should bepossible to make Mn(III) from Mn(II) and Mn(VII).6. Dissolve about 0.5 g manganese(II) sulphate in about 2 cm3 of dilute

sulphuric acid in a test-tube.7. Carefully add about 10 drops concentrated sulphuric acid and

cool the tube under a running tap.B. Add a few drops of potassium manganate[VII) solution to obtain a deep

red solution of Mn(III).9. Dilute the red solutio~ with about five times its volume of water, wait

a few moments, and note any colour change.

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Questions. Part B.3. Explain what happened when the Mn(III) solution was diluted.(Answer on page 83)

Procedure. Part CRefer to your answer to Exercise BJ in which you predicted that it shouldalso be possible to make Mn(III) from Mn(II) and Mn(IV).10. In each of two test-tubesJ dissolve a little manganese(II) sulphate in

water and add an equal volume of sodium hydroxide solution to obtain aprecipitate of manganese(II) hydroxide.

11. To one of the two tubesJ add a little manganese(IV) oxide and stir.12. Let both tubes stand for a few minutesJ and note any changes.

Questions. Part C.4. Can you see any sign of Mn(III) in the tubes?5. What is different about the conditions of this experiment (part C)

compared with the last (part B) which makes its success less likely?6. What happens in the test-tUbe which had no manganese(IV) added? Suggest

an explanation. (Hint: is the change greater in the upper part of themixture?)

(Answers on page 83)

The method suggested in Exercise 9 for making Mn(V) was shown not tobe worth tryingJ but your teacher may demonstrate another method.

Similar interconversions from one oxidation state to another are possiblefor other transition elementsJ but we do not discuss these in detail.HoweverJ in the next exercise you apply some of what you have learned to thetransition metal chromium.

Exercise 10 Write down the electronic configuration (sJ PJ delectrons) off L) an atom of chr-orn.i um, (ii) a chr-omi urnl Ll l ion.

(b) State the oxidation state (number) of chromium in

(a)

(c) Classify the following changes as oxidationJ reductionand/or acid-baseJ giving a reason in each case.

(d) By reference to standard electrode potentialsJ suggestthree reagents which would effect the change in (c)(ii).*Hint: what must be included to balance the equation?

(Answers on page 83)

Variable oxidation state is just one of the characteristics of transitionelements. In the next section we look at a number of othersJ but all of themare related in some way to electron configuration and oxidation state.

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CHARACTERISTICS OF TRANSITION ELEMENTSYou have probably studied transition elements briefly in your pre-A-levelcourse and will be aware of some of their general characteristics. A morecomplete list is as follows:1. Variable oxidation state.2. Compounds are almost always coloured.3. Complex ions are formed.4. Catalysts almost always contain transition elements.5. Paramagnetism is observed.6. Physical properties, which make the elements useful as metals, are

broadly similar.

Objectives. When you have finished this section you should be able to:

(8) list five general characteristics of transition elements in additionto variable oxidation state;

(9) relate these characteristics to electron arrangement;(10) explain the terms complex ion, ligand and co-ordination number;(11) name complex ions systematically given formulae;(12) write formulae of complex ions given names;(13) give examples of the use of transition elements or their compounds as

catalysts.

Having discussed variable oxidation state in some detail, we now look brieflyat each of the other general characteristics in turn in order to link themtogether. A more detailed study of some of these characteristics will appearin Level Two and Appendix Two.

Coloured compoundsYou will have observed some of the striking colours of transition elementcompounds in Experiments 1 and 2. What may not yet be apoarent is thatvirtually all compounds of transition elements are coloured. Also, fewinorganic compounds of other elements are coloured, as you can confirm foryourself in the next exercise.

(Answer on page 83 )

Exercise 11. Name one inorganic compound in common use which iscoloured but does not contain a transition element.

Before we look for an explanation of the colour of these compounds, we willremind you of the reason for any coloured appearance.

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In general, a substance appears coloured because it absorbs some of thelight which falls on it. The light which is then reflected or transmittedto the observer's eye is not a complete spectrum of \he wavelengths whichmake up white light, and appears to have a colour complementary to that ofthe absorbed light. For e~ample, copper sulphate solution appears bluebecause it absorbs red light, as shown in Fig. 4.

Fig.4.

All substances absorb some wavelengths of the electromagnetic spectrum in avariety of ways - this enables us to identify substances by infra-red andultraviolet spectroscopy_ However, the absorption of visible wavelengthsalways involves promotion of electrons from one energy level to anotherfairly close level, as shown in Fig. 5.

>OJa;cQ)

-S:J- _] energy lost in

] - small steps-] - absorbed as

] - heat by solventJ;_3i-

lower energy level 1-£1 ~----------~ - - - - - - - - - - - - - - - - - - - - - - - - -"------

higher energy level- - - - - - - - - - - - - .,.....-------------,

(excited state)

energy gainedfrom radiationof frequency v£2-£1=hv

(ground state)

Fig.5.

Although the 3d and 48 levels are generally very close in compounds oftransition metals, this is only a part of the reason for their colour, asyou will see if you tackle Appendix Two. At this stage, it is sufficientfor you to note that because of their particular electron configurations,compounds of transition elements have energy levels sufficiently close forthe~ to absorb visible light.

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Read about coloured compounds of transition elements before doing thenext exercise. Your textbooK may include an introductory account ofthe splitting of 3d energy levels, which you should find interestingalthough it is not liKely to be examined.

oW

Exercise 12 Why do you think that the following compounds arecolourless - ScC13, ZnS04, CuCl?(Answer on page 83)

A more detailed study of the electron transitions involved in the absorptionof visible light appears in Appendix Two. ASK your teacher whether youshould study it. Now we consider another of the general characteristics oftransition elements.

Formation of complexesRead the appropriate section of your textbooK to find out whatcomplex ions are, and how they are formed by transition elements.MaKe sure that you learn some examples of simple ligands, bothcharged and uncharged, and some examples of complex ions of differentco-ordination number so that you can attempt the following exercises.You should also find out about the systematic naming of complex ions.

oW

In your reading you may also find a treatment of the shapes of complex ions,their relative stabilities, and isomerism. We will deal with these aspectsin Level Two.

Exercise 13 In the following complexes identify the centralthe ligand(s) and the co-ordination number.

ion,

(a ) Cu (NH3 )42+

(b) CrCl(H20)52+(Answers on page 84

(c) CO(OH)2(H20)4(d) (NH4)3VF6

Note that many texts enclose the formula of a complex ion in square braCKets,e.g. [Cu(NH3)4J2+. We do not follow this convention in this Unit to avoidconfusion with the square braCKets used for concentrations.

Exercise 14 Given the co-ordination numbers shown, write down the ~formulae and names of the complex ions/formed between:

~\\\(a) Cu2 + and H2O, (C.N. 4)(b) Fe3+ and CN-, (C.N. 6)(c) Ni2+ and Cl-, (C.N. 4)(d ) Ag+ and NH,3. (C.N. 2)(Answers on page 84

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Exercise 15 Identify four types of bonding in crystals of coppersUlphate-5-water, CuS04'5HzO.(Answer on page 84)

Now we take a brief look at the fourth and fifth items in our list ofcharacteristics of transition elements.

Catalysis involving transition elementsYou have learned in Unit P5 that a catalyst speeds up a chemical reactionby providing an alternative route with lower activation energy. Clearly, thecatalyst must take part in the sequence of reactions, but since the eventualproducts are the same, the catalyst must be regenerated.

You study the mechanism of homogeneous catalysis in Level Two of this Unit,and you will find that this general characteri5tic of transition element5 i5a150 related to variable oxidation state. To emphasise the importance oftran5ition element5 in catalysis, try the next exercise.

Exercise 16 Write equations for at least six reactions in which a ~transition element (or compound) is used as a catalyst.At least four of these should be used in industry. Name ~~the catalyst. (Hint: don t t forget organic chemistry!)(Answer on page 84 )

ParamagnetismAny substance which i5 attracted into a magnetic field is said to be para-magnetic, while if it is repelled it is diamagnetic. Transition elements andtheir ions are generally paramagnetic, whereas most others are diamagnetic.

Paramagnetism in transition elements is associated with unpaired electronsfound in their partially-filled d orbitals. We will discuss this further inAppendix Two; ask your teacher whether you need to study it.

Note that ferromagnetism (which we usually refer to simply as 'magnetism')can be regarded as a very powerful paramagnetism and is confined to theelements iron, cobalt and nickel.

The last general characteristic of transition elements in our list is thebroad similarity in physical and chemical properties. We look at thisbriefly here before going on to a more detailed study in subsequent sections.

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Similar physical and chemical propertiesIt should be no surprise to find similar chemical properties, because chemicalreactions always involve the outer electrons, and the atoms in each transitionseries have the same outermost electron shell (48, 58, or 68), nearly alwayscontaining two electroITs.

Physical properties also depend on the outer electrons (especially metallicconductivity), and also on the way electron arrangement determines atomicradius. Similar atomic radius generally gives rise to similar physicalproperties.

GENERAL PHYSICAL AND CHEMICAL PROPERTIESObjectives. When you have finished this section you should be able to:

(14) relate physical properties of transition elements to electronarrangement and atomic radius;

(15) show how physical properties determine the practical uses oftransition elements;

(16) give some examples of the chemical reactions of transition elementswith air, water, acids and non-metals.

Similarity in physical propertiesEarlier in the Unit (page 9), we stated that scandium and zinc are often notregarded as transition elements because their compounds do not have partiallyfilled d orbitals.

Another reason for their exclusion is concerned with tne physical propertiesof the elements, and you explore this in the next exercise.

Exercise 17 LooK up the densities and melting-points of the elements~from scandium to zinc. Suggest a reason for excludingscandium and zinc from the transition metals. ~\\\(Answer on page 84 )

We confine our attention now to the first transition series from Ti to Cu,in which we examine trends in physical properties.

Use Table 3 to help you answer the following exercises.

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Table 3

Property Ti V Cr Mn Fe Co Ni Cu

Metallic radius 0.145 0.132 0.137 0.137 0.124 0.125 0.125 0.128r /nmm

Ionic radius (M2 + ) 0.090 0.088 0.080 0.088 0.076 0.074 0.072 0.069r./nm

l

1st ionization 660 650 650 720 760 760 740 750energy/kJ mol-l

Electronegativity 1.5 1.6 1.6 1.5 1.8 1.8 1.8 1.9(Pauling)

Exercise 18 Describe the Beneral trends observed in Table 3 andexplain them in terms of electron arrangement.(Answer on page 84)

It is instructive to compare these trends with the trends in another hori-zontal row of elements in the Periodic Table, say those in Period 1 from Lito Ne. However, if you were asked to construct a table similar to Table 3for Period 1, you would encounter difficulties with ionic radii, as you cansee by reference to your data book.

Exercise 19 Why is it difficult to compare ionic radii of transition~elements with those of Period 1 elements?(Answer on page 84) ~\\

Nevertheless, a broad comparison can be made between transition elements andelements in a short period, and some firm conclusions drawn. Study your databook to help you do this in the next exercise.

Exercise 20 (a) How do the trends in radii, ionization energy, and ~electronegativity for the elements in a shortperiod resemble the trends for transition elements? ~~

(b) How do these trends differ?(c) Explain the differences noted in (b) in terms of

electron arrangement.(Answers on page 84 )

Just as ionization energy and electronegativity are related to atomicradius and electron arrangement, so are many other physical proper-ties. You have touched on this relationship in Units 11 (s-Block Elements),12 (The Halogens), 13 (The Periodic Table), S2 (Atomic Structure) and S4(Bonding and Structure).

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Exercise 21 For each of the following properties, briefly describe ~and explain how transition elements are broadly similarto one another and different from the s-blocK metals: ~~(a) density, (b) melting-point, (c) boiling-point.(Answers on page 85 )

The physical properties of transitionelements maKe them idea~ly suited for~ ~use as structural materlals. We C2222 - --------------- - ====sconsider some of these uses in thenext section.

Transition elements as structural materialsTransition metals are often used as structural materials because they combinestrength and durability with other desirable features such as electricalconductivity, ductility and pleasing appearance.

Read the section in your textbooK dealing with the uses of transition 0metals, looKing particularly for the reasons why titanium is used CDextensively in rocKet technology. Also lOOK for reasons whytransition metals are used in alloys.

Fig.6. Space-shuttle taunch from Cape Canaveral.

The most important example of a useful transition metal is, of course, iron.Its great advantage of being relatively cheap to produce outweighs itsgreat disadvantage of suffering far worse corrosion than other more expen-sive transition metals. We consider the extraction of iron and the manu-facture of steel in a later section, but it is relevant here to mention theincorporation of small amounts of other transition elements into alloysteels in order to modify their properties.

ASK your teacher whether you should study Ellingham diagrams in Appendix One(page 73) before going on to the following sections.

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As well as the alloy steels, a number of other important alloys containtransition elements. Some of these are illustrated in Fig. 7, which is thesubject of the next exercise.

A B

c

E F

Fig.7. Some uses of alloys containing transition metals.

Exercise 22 Name two transition elements used in each of theapplications illustrated in Fig. 7.(Answers on page 85 )

The extensive use of transition elements in alloys is related to theiratomic radii. You are asked about this in the next exercise.

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Exercise 23 Why does the pattern of atomic radii of transitionelements make them suitable for use in alloys?(Answer on pa8e 85

(a) atomic radius,(b) maximum oxidation state,

(c) boiling-point,

To consolidate and organise what you have learned so far about thetransition elements, attempt the following Teacher-marked Exercise.You may need to do some additional reading.

Teacher-marked Describe and explain how the following propertiesExercise change along the first series of transition elements:

Cd) enthalpy of fusion.As far as possible, relate your answers to the electronicstructure of the elements and their bonding.

Having dealt with the physical properties of transition elements, we considertheir chemical reactions.

Similarity in chemical propertiesInstead of asking you to work through a long list of the reactions ofindividual elements, we simply note the following general statements. Thereare exceptions, of course, but the few you need to know are mentioned in then8xt exercise.

Transition elements generally:1. react with non-oxidizing acids (e.g. dilute H2S04, and Hel of any

concentration) to give salts (commonly with the metal in the +2oxidation state) and hydrogen;

2. react with oxidizing acids (e.g. concentrated H2S04, and HN03 of anyconcentration) to give salts (sometimes with the metal in a higheroxidation state) and a gaseous reduction product of the acid (S02 or amixture of oxides of nitrogen);

3. react with oxygen, the halogens and sulphur, to give a variety of oxides,halides and sulphides (refer to the oxidation states in Table 1 on page9 for the most likely products);

4. are not attacked by water, but react at high temperatures with steam togive an oxide (lower oxidation state) and hydrogen;

5. have similar reactivities in the above reactions.

In the next exercise you apply these general statements to a few 0specific examples, and also recall some exceptions which you should ~already know about from your pre-A-level work. You should, of course ~refer to your textbook where necessary.

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Exercise 24 (a) Name a transition element which does not react with~non-oxidizing acids.Name a transition element wh i ch does not react with ~\\oxygen.

( b)

(c) Write equations for reactions between:( .i ) manganese and dilute sulphuric acid,

[i.i ) nic Kel and concentrated sulphuric acid,(iii) chromium and iodine,(iv) chromium and fluorine,(v) vanadium and oxygen,

(vi) titanium and sulphur.[Answers on page 85)

In the next section we consider the extraction of iron from iron ore and themanufacture of steel. We do not deal with the extraction of other transitionelement s.

THE EXTRACTION OF IRON IN THE BLAST FURNACEYou have probably learned something of this process in your pre-A-Ievelstudies, in which case you may heed only to revise your notes and add oneor two further points. However, some,syllabuses require you to discuss theprocess with reference to Ellingham diagrams relating free energy changes,6G~, to temperature.

ASK your teacher whether you should study Ellingham diagrams in Appendix One(page 73 ) before going on to the following sections.

Fig.B. The base of a blast furnace in operation.

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Objectives. When you have finished this section you should be able to:

(17) describe the reactions occurring in the operation of a blast furnacefor the extraction of iron;

(1B) state the chief impurities in cast iron and the effect of these on itsproperties.

Read about the blast-furnace in your textbook(s). Look for a set of <=)simple equations for the relevant chemical reactions and the approxi- ~mate temperatures of the regions of the furnace where these reactions ~occur. This will enable you to do the following exercises.

Exercise 25 Label a larger copy of Fig. 9 to show the following:(a) the approximate dimensions,(b) the hopper feed and its composition,(c) the composition of the waste gases,(d) the tuyeres,(e) the tapping-holes for molten iron and slag,(fJ the parts of the furnace where

the temperatures are approxi-mately 250°C, 700 °C, 1150 °cand 1500 °C.

(gJ equations for the main chemicalreactions opposite the appro-priate parts of the furnace.(Assume that the iron ore ismainly Fe203• J

(Answers on page 85

Fig.9. Extraction of iron in a blast furnace.

Exercise 26 Most of the ircrn produced by blast furnaces is made into Jsteel, but some is used as 'cast iron' and a very small ~proportion is purified (formerly known as 'wrought iron').(a) What are the chief impurities in cast iron?(b) How do these impurities affect the properties of iron?(c) state two uses of cast iron.(Answers on page 86 )

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Exercise 27 ( a ) (i ) Give the names and formulae of the iron com-pounds contained in two important iron ores.Give the names of two impurities, commonlypresent in iron ores, which have to be removedin making iron and turning it into steel.

(b) (i) One of the sUbstances fed into the blast furnace isthere to remove an impurity or impurities. Give thename of this substance.

( Ld )

(ii) Write the equation(s) for a reaction between thissubstance and the impurity.

(iii) In what state does the impurity actually leave thefurnace?

(c) Give the names of two substances which act as reducingagents in the blast furnace. In each case, give an equa-tion showing its reducing action.

(d) Conditions are so arranged that the gases leaving theblast furnace will burn.(L) 'What is the flammable gas?

(ii) Give one use for the emergent gases.(e) Draw a labelled diagram of the apparatus you would use to

prepare some anhydrous iron(III) chloride from iron.(Answers on page 86 )

You have seen that cast iron is of limited use largely because of its highcarbon content, which makes it brittle. Now we look briefly at the ways inwhich cast iron is made into steel of different types.

Steel productionThere is a wide variety of different steels, all of which consist mostly ofiron. However, their properties differ quite considerably due to thepresence of small, carefully controlled proportions of elements such ascarbon and other transition metals. Three steps may be involved in themaking of a particular steel, the first being necessary in all cases.1. Complete or partial removal of the impurities in cast iron.2. Controlled addition of alloying elements.3. Heat treatment.

Objective. When you have finished this section you should be able to:

(19) outline the production of steel from cast iron.

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Read about steel production in a modern textbook. We suggest you <=)focus your attention on the L.D. or Kaldo converters (based on the ~folder Bessemer converter) rather than the open-hearth process, which ~is becoming much less important. Unless you are studying a metal-lurgy option (in which case your teacher will advise you) we suggestyou need not learn details of percentage compositions or micro-structures of different types of steel.

Fig.10. Casting steel.

The next exercise is about the main features of steel production.

Exercise 28 (a) What is the function of the oxygen which is blownthrough the impure iron obtained from tile blastfurnace?

(b) Why is oxygen preferred to air, which was formerlyused in the older Bessemer converter?

(c) What is the composition of the lining of a converter andwhat is its function?

(d) What does the floating slag in a converter consist of,and under what circumstances is it used as a fertilizer(known to farmers as 'basic slag')?

(e) Outline what is meant by the term 'tempering of steel'.(Answers on page 86 )

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LEVEL ONE CHECKLISTYou have now reached the end of Level One of this Unit. The following is asummary of the objectives in Level One. Read carefully through them andcheck that you have adequate notes. At this stage you should be able to:

(1) write electron configurations for the atoms of the first transitionseries from scandium to zinc;

(2) write electron configurations for some of the more common ions oftransition elements;

(3) & (4) explain the general pattern of variable oxidation states in thetransition elements;

(5) explain why scandium and zinc are often excluded from the study oftransition elements;

(6) describe the preparation of a variety of oxidation states for vanadiumand manganese;

(7) describe some examples of disproportionation and reverse dispropor-tionation involving vanadium and manganese;

(8) list five general characteristics of transition elements in additionto variable oxidation state;

(9) relate these characteristics to electron arrangement;(10) explain the terms complex ion, ligand and co-ordination number;(11) & (12) name complex ions systematically given formulae and write

formulae from names;(13) give examples of the use of transition elements or their compounds as

catalysts;(14) relate physical properties of transition elements to electron arrange-

ment and atomic radius;(15) show how physical properties determine the practical uses of

transition elements;(16) give some examples of the chemical reactions of transition elements

with air, water, acids and non-metals;(17) descfibe the reactions occurring in the operation of a blast furnace

for the extraction of ~ron;(18) state the chief impurities in cast iron;(19) outline the ~oduction of steel from cast iron.

LEVEL ONE TEST~tryTo find out how well you have learned the material in Level One,

the test which follows. Read the notes below before starting.

1. You should spend about 1 hour on this test.2. Hand your answers to your teacher for marking.

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LEVEL ONE TESTQuestions 1-5 concern the preparation of a complex iron compound offormula BaFe04•

A solution of iron(III) nitrate in water is placed in a dropping funnel andadded at the rate of1 drop every 15 seconds to a boiling solution of sodiumhydroxide in 2 M sodium chlorate(I) (sodium hypochlorite). The solution.which is purple in colour. is boiled for a minute or two and filtered intoan aqueous solution of barium nitrate. The red precipitate of BaFe04 isfiltered off and dried.2Fe(N03)3(aq) + 3NaOCl(aq) + 10NaOH(aq) ~ 2Na2Fe04(aq) + 6NaN03(aq)

Na2Fe04(aq) + Ba(N03)2(aq) ~ BaFe04(s) + 2NaN03(aq)

1. The purple colour is most likely to be caused byo N03-

2. The most likely reason for using sodium chlorate(I) would be toA act as an oxidizing agentB remove nitrate ions from solutionC neutralize the sodium hydroxideo act as a reducing agentE buffer the solution

3. If the iron(III) nitrate was added rapidly to a cold solution ofalkaline sodium chlorate(I), the most likely result would be thatA the mixture would effervesce vigorouslyB xa precipitate of sodium nitrate would formC /the iron(III) ions would be converted into iron(II) ionso Xiron(III) hydroxide would precipitateE chlorine would be liberated

4. The most suitable name for BaFe04 would beA barium iron(II) oxide o barium ferrate(IV)B barium iron(III) oxide E barium ferrate(VI)C barium ferrate(III)

s. In the first reaction, the sodium ions from the alkali arBA'-oxidized o disproportionated

E reducedB unchangedC preCipitated from solution

31

( 1 )

( 1 )

( 1 )

( 1 )

( 1 J

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In Questions 6 to 8 inclusive one, or more than one, of the suggestedanswers may be correct. You should answer as follows:

A if only 1, 2 and 3 are correctB if only 1 and 3 are correctC if only 2 and 4 are correct0 if only 4 is correctE if some other response, or combination, is correct

6. Values for the standard electrode potentials (E~) of three systems0are given below: ~System 'E~/VV3+(aq) + e ~ V2+(aq) +0.20

+0.77-0.04

If equal volumes of aqueous 1.0 M iron(III) and aqueous 1.0 M vanadium(II)are mixed1 Fe3+(aq) is reduced to Fe2+(aq)2 Fe3+(aq) is reduced to Fe(s)3 V2+(aq) is oxidized to V3+(aq)4 there is no reaction ( 1 )

7. The electronic configurationcould represent

3d 4sAr I tf I tit I tit I D

1 Mn(II) 2 Fe(III) 3 Co(III) 4 Ni(II) ( 1 )

1

2

C1207 \(

Fe (CN)64-

3

4

8. In which of the following is there an element with the sameoxidation number as that of chromium in K2Cr207?

( 1 )

For Questions 9 and 10 choose an answer from A to E as follows:

A Both statements true: second explains first.B Both statements true: second does not explain first.C First true: second false.0 First false: second true.E Both false.

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First statement Second statement

9. In the ion [Ni(H20)6J2+water is a bidentateligand.

The oxygen atom in the water moleculehas two lone pairs of electrons.

10. The Eradation in proper-ties along a series oftransition elements ismore marked than thatalong a period of non-transition elements.

Inner quantum levels are being filledwith electrons along a series oftransition elements.

11. (a) State five properties which are typical of the transitionelements and illustrate each with a specific example. (10)

(b) For each of the oxidation numbers +2, +3, +4, +6 and +7,write the formula of one compound or ion in which thetransition element either chromium or manganese has thatoxidation number.

( 1 )

( 1 )

( 5 )

(c) For each of the five examples chosen in (b), name a reagentwhich will change the oxidation number and write the redoxequation for the reaction. (10)

12. Cobalt, copper, iron and manganese ar8 d-block elements.(a) What is meant by the term 'd-block element'?(b) Write the electronic configurations of Cu, Fe2+ and Mn2+.

(Use the symbol Ar followed by 'arrows-in-boxes' for the 3dand 4s orbitals.)

( 2)

(3 )

(c) Explain in terms of their electronic configurations whyFe2+ ions are readily oxidized to Fe3+ ions, but Mn2+ ionsare not readily oxidized to Mn3+ ions. (2)

(d) (i) Give the formula of a compound or ion containingmanganese in an oxidation state of +7.

(ii) How do you account for the existence of the +7oxidation state for manganese? ( 3)

(e) Cobalt forms a complex compound of formula [Co(NH3J4C12J+ Cl-(i) What is the oxidation state of cobalt in this compound?

(ii) Give the name of the complex ion contained in this compound.(iii) How many moles of silver chloride would be immediately

precipitated from one mole of this compound in aqueoussolution by the addition of an excess of silver nitrate? (3)

Continued on next page.

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13. Ca) Describe qualitatively and explain the trend in atomic radiusfrom titanium to copper.

Cb) What bearing does this trend have on the use of the transitionelements in alloys?

(c) Give two reasons why the metals potassium and copper have suchwidely different densitites and boiling-points. (4)

14. Write equations for the reactions between:(a) vanadium and fluorine,(b) cobalt and dilute hydrochloric acid,(c) manganese and iodine,(d) copper and concentrated sulphuric acid. ( 8 )

(Total 60 marks)

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LEVEL TWO

In Level Two we consider in more detail some of the characteristics oftransition metals which you studied briefly in Level One. We begin withcomplex ions.

Formation and stability of complex ionsMost transition metal chemistry takes place in aqueous solution; wefrequently refer to aqueous ions. However, when we write, say, Cu2+(aq)we refer not to simple Cu2+ ions surrounded by more or less mobile watermolecules, but to the definite identifiable species Cu(H20)42+. This is anexample of a complex ion; the four molecules of water are ligands attachedby dative covalent bonds, as shown below. This ion is surrounded by loosely-attached water molecules and is, therefore, best written as Cu(H20)42+(aq).

H H 2+""", /

H 0 H-. T /Oi----+Cu~O

/ J. -.H

°H

H/~H(Note that in some complex ions, the ligands are attached by ion-dipolebonds rather than dative covalent bonds, but we think the distinction is notimportant at A-level.)

In the formation of other complex ions, the water molecules are replaced byother ligands in equilibrium reactions such as:

Cu(H20J42+(aqJ + 4Cl-(aqJ ~ CuC14

2-(aqJ + 4H20(lJThis equation is often simplified to:

Cu2+(aq) + 4Cl-Caq) ~ CuC142-Caq)in just the same way as we often use H+Caq) to represent H30+Caq).

The equilibrium constants for reactions such as this are called stabilityconstants, and you can apply to them what you learned about equilibriumconstants in Unit P2 CEquilibrium I: Principles).

In the sections which follow, you learn how to use stability constants topredict ligand replacement reactions for various types of complex ions andhow to compare stabilities experimentally. Also, we describe a practicaluse of complex ions in titrimetric analysis and, in order to make anotherlink with work you have already done, we compare complex ion formation withacid-base reactions.

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Stability constantsThe term 'stability constantl is simply a special name given to an equili-brium constant applied to the formation of a complex ion, e.g.

Cu2+(aq) + 4Cl-(aq) ~ CuC142-(aq)K = [CuC142-(aq)]st [Cu2+(aq)] [Cl (aq)]4

Objectives. When you have finished this section, you should be able to:

(20) use the equilibrium law to write expressions for stability constantsof complex ions;

(21) use stability constants in simple calculations.

Read the section in your textbook which deals with the formation ofcomplexes, looking for references to equilibrium and stabilityconstants, so that you can attempt the exercises which follow. Sometextbooks do not refer to stability constants by name. If you cannotfind an account, all you need is to recall the equilibrium law andapply it to the appropriate equations.

oill

( a )

( b)

What is the name of the ion CuC142-?the ;)

~

Exercise 29The formation of the CuC142- ion, summarised inequation above, takes place in four stages, thewater molecules being replaced one by one. For eachstage, write an equation and an expression for thestability constant.

(c) Show that the product of the four intermediate stabilityconstants is equal to the overall stability constantobtained directly from the overall equation.

(d) What is the unit of the overall stability constant?(Answers on page 86 )

Note that overall stability constants are the ones most often used, sincethe ligand is usually in excess.

Exercise 30 The stability constant of the dicyanoargentate(I) ion1.0 x 1021 mol-2 dm6• Explain what is meant by thisstatement, including an equation in your answer.(Answer on page 87)

is ;)

~

Exercise 31 What is the concentration of silver ions in a solutionwhere the concentrations of cyanide ions and dicyano-argentate(I) ions are 0.50 mol dm-3 and 0.10 mol dm-3

respectively?(Answer on page 87)

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An application of complex ion chemistryWhen articles are plated with silver or gold, it is necessary to have a verylow concentration of metal ions in the electrolyte in order to achieve evenplating which adheres well, but there must also be a substantial 'reserve' ofmetal ions so that an adequate amount of metal can be deposited on thecathode. These requirements are met by complexing the metal ions with addedcyanide ions. As plating proceeds, the silver ions removed from solutionare replaced by disturbing the equilibrium:

AgCH20)2+Caq) + 2CN-Caq) ? AgCCN)2-Caq) + 2H20Caq)Silver ions are also replenished by the dissolving of a silver anode, butthese are not immediately available at the cathode. The complex ion acts asa 'carrier' of silver between anode and cathode.

The replacement (or displacement) of ligands in the formation of a complexion can also be regarded as a competition for the cation between the twoligands. In this way, the formation of a complex ion is similar to an acid-base reaction in which two bases compete for a proton, as we now show.

Competition for cationsWhen more than one ligand is available in excess for bonding to a transitionmetal ion, one product will predominate, depending on the relative stabilityconstants.

There is a competition between the ligands for the cation somewhat similarto the competition for a proton between two bases. In the simplest examplesof competition, one of the two competing species is water. For example,

H+~CH3C02H(aq) + H20(I)

H+~

? CH3C02-(aq) + H30+(aq)acid base base acid

Cu2+

~Cu(H20)42+(aq) + 4CI-(aq)complex ligand

Cu2+

~? 4H20(I) + CuC14-(aq)

ligand complex

In the first reaction above, CH3C02- is a stronger base (has a greateraffinity for H+) than H20 so that the equilibrium lies toward the left. Theequilibrium constant, here called the dissociation constant, is very small.

In the second reaction, the Cl- ion has a greater affinity for Cu2+ than hasH20. The equilibrium constant, here called a stability constant, is large.

We now move to the study of some other, less simple, ligands.

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Polydentate ligandsThese are ligands with more than one site for dative covalent bonding.

Objectives. When you have finished this section you should be able to:

(22) explain the terms polydentate and chelate;(23) explain why polydentate ligands generally give very stable complex ions.

Read again the section in your textbook on complex ions, lookingparticularly for the meanings of the terms 'polydentate' and'chelate'. This will help you to do the following exercises.

Exercise 32 Name the ligand H2NCH2CH2NH2 and state how many bondsyou would expect it to make with a central cation.Explain your choice.(Answer on page 87 )

oill

A ver¥ useful hexadentate ligand is the ion formed from bis(bis(carboxymethyl)-aminoJethane.

You may be pleased to know that this very clumsy (but very informative) nameis rarely used. The ion is known as edta, which is the abbreviation for thetraditional name ethylene-diamine-tetra-acetate and its formula is shownbelow. The parent acid can be written as H4edta, although you may findreference to EDTA! (This is just one of the hazards of studying chemistryat a time when the use of systematic and recommended nomenclature is notunive:r:sal!)

/CH2C02HCH2 - N "<,

ICH2C02H

/CH2C02HCH2 - N "<,

CH2C02HH4edta

The carboxylic acid group is often encountered in ligands but it is usuallythe ion rather than the acid which is involved in bonding and only one of thetwo oxygen atoms forms bonds. You explore this in the next exercise, whichuses edta as an example.

To do t rus exercise, you should first construct a model of the edtaligand from the structural formula given, using a 'ball and spoke'model kit. You need one sphere with six evenly s~aced holes torepresent the central cation. 1\

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Exercise 33 (a) Which atoms in an edta ion might be able to form ~dative bonds? Does your answer explain why edta isdescribed as being hexadentate? .~~Suggest reasons( b)

(i) why it is the ion rather than the acid whichis involved in bonding~

(ii) why only one of the two oxygen atoms in each carboxylgroup forms bonds.

(c) Suggest a reason why polydentate ligands generally givemore stable complexes than monodentate ligands.

(Answers on page 87 )

Now that you have encountered a variety of ligands~ you study how stabilityconstants vary with the nature of the ligand.

Relative stabilities of complex ionsYou investigate the relative stabilities of complexes formed from variousligands in a series of short experiments~ but to help you predict what willhappen you should first do the next exercise.

Cu(NH3J42+: KstCuC142-: Kst

1.58 X 1013 mol-4 dm12

3.98 x 105 mol-4 dm12

Exercise 34 The stability constants of two complex ions are:

(a) Write the equations to which these constants refer.(b) Use the equations and stability constants to predict what

would happen if ammonia solution were added drop by dropto a solution of CuC142- ions.

(Answers on page 88 )

Now you should be ready for the next experiment~ which illustratesthe relative stabilities of some complex ions. If your laboratorytime is limited~ you should maKe the predictions in Results Table 3before you reach the laboratory.

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EXPERIMENT 3Relative stabilities of some complex ions

AimThe purpose of this experiment is to testpredictions of ligand replacementreactions made using stability constants.

IntroductionIn a series of test-tube reactions you examine a number of ligand replacementreactions. One type, in which charged and uncharged ligands are representedas lig- and LIG respectively, can be represented:

CuCLIGJ42+ + 4 lig- ~ CuCligJ42- + 4 LIGYou use stability constants to predict the outcome before mixing suitablepairs of solutions.

Requirements6 test-tubestest -tube rac kcopperCIIJ sulphate solution, 0.20 M CuS04wash-bottle of distilled watersodium chloride solution, saturated NaClsodium ethanedioate solution, 0.20 M (C02Na)2 (sodium oxalate]1,2-diaminoethane solution, 0.10 M (CH2NH2)2

edta soluti6n (sodium salt), 0.10 M C1oH1408N2Na2

Procedure1. Complete the four prediction columns (headed 'P') in a copy of Results

Table 3, using the stability constants given. Use a '/' to indicate'replacement' and a 'x' to indicate 'no reaction',

2. Place about 1 cm3 of copper sulphate solution in each of five test-tubes.3. To the first tube add about 5 cm3 distilled water. This tube is for

colour comparison with the others.4. To the second tube add about 5 cm3 sodium chloride solution a little at

at time, noting any colour changes.5. To the third tube add about 5 cm3 sodium ethanedioate in the same way.6. Similarly, to the fourth and fifth tubes add diaminoethane and edta

solutions respectively.7. The colours in the five tubes are predominantly due to the five complex

ions shown in Results Table 3, Write down the colours in your table,and fill in the first results column (headed 'R'), Again, use a '/'or a 'x' to show whether or not replacement of ligands has occurred.

Note that, in addition to the abbreviation edta, we use 'ox' torepresent the ethanedioate (oxalate) ligand and 'en' to represent the1,2-diaminoethane ligand (~thylenediami~e)

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Results Table 3

Ligand H2O Cl- C204 2- NH2C2H4NH2 C~OH~408N2 4-= OX = en = edta

Complex ion Cu(H2o)4 2 + CuC14 2- CU[OX)2 2- Cu(en)2 2+ Cu[edta)2-

Colour

Stabilityconstant

Test

Add ox

4.0 X 105mo14 dm-~2

2.1 x 1o~omo12 dm-6

6.3 X 1o~8mo1 dm-3

P

Predictions [P) and results (R). I = replacement, x = none

RP R P R RP P R

Add en

Add edta

(Specimen results on page 87 )

8. Divide the solution containing CuC142- ions into four parts. To each ofthese add an excess of one of the other four ligands in turn - use theoriginal ligand solutions and not the complex ion solutions you havemade. Complete the second results column according to whether or not youthink replacement has occurred.

9. Repeat step 8 for the remaining complex ion solutions and complete theremaining results columns.

Questions1. Could you have made any tentative predictions without knowing stability

constants?2. Estimate the approximate stability constant for Cu(en)22+.3. One of the ligand exchange reactions appeared to be readily reversible.

Which one was this?4. Calcul~te the ratio [Cu(H20)42+(aq)]/[CuC142-(aq)] for the following

values of [Cl-(aq)]:(a) 5.0 mol dm-3 (in saturated NaCl)(b) 0.050 mol dm-3 (in dilute NaCl).How do these ratios help to explain the reversible nature of the ligandexchange?

(Answers 0 n page 87 )

The ion edta forms very stable complexes with many other metal ions as wellas copper. We now consider an application of this fact.

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Use of edta in titrationsObjective. When you have finished this section you should be able to:

(24) explain the use of complexes in titrimetric analysis.

Edta forms such stable complexes with many metal ions that the reactions canbe used in titrations to measure their concentrations. Another complex, lessstable but more highly coloured, is used as an indicator. This.is simply anapplication of the exchange of ligands you have just studied, as you will seein the next exercise.

Exercise 35 25.0 cm3 of an orange solution of Fe3+ ions of unknown ~

concentration was titrated with 0.10 M edta, using aLi.ttle 2-hydroxybenzoic acid as an indicator. When ~\\20.0 cm3 of edta solution had been added, the violetcolour of the 2-hydroxybenzoate complex disappeared,leaving a clear yellow solution.(a) Explain the colour changes.(b) Assuming the complex ion formed to be Fe(edta)-,

write an equation for its formation.ec) Calculate the concentration of Fe3+ ions in the

original solution.(Answers on page 88 )

Having considered stability constants and the relative stabilities of variouscomplex ions, we now move on to consider their shapes.

SHAPES OF COMPLEX IONS AND ISOMERISMYou have learned that co-ordination numbers 2, 4 and 6 are common in complexions. These co-ordination numbers are associated with different three-dimensional shapes, and consideration of these shapes in this section leadsyou to study how the various types of isomerism you have met in organicchemistry can also arise in inorganic chemistry.

Objectives. When you have finished this section you should be able to:

(25) describe shapes commonly adopted by complex ions;(26) explain how structural, geometric and optical isomerism can arise in

complexes, giving examples.

Read the section on shapes of complex ions in your textbook(s) anduse it to do the next exercise.

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Exercise 36 What four shapes are commonly adopted by complex ionsof coordination numbers 2, 4 and 6? Give an exampleof each.(Answer on page 88)

Most of your work on isomerism concerns organic compounds, and you havelearned about the main types of isomerism in Unit 03 (More Functional Groups).Both structural isomerism and stereoisomerism occur in inorganic chemistryas well, particularly in complexes, but there are fewer subdivisions. Themain differences between organic and inorganic isomerism are as follows.

Differences between organic and inorganic isomerism1. Inorganic structural isomerism is simpler; there are no subdivisions

(chain-branch, positional, functional group, metamerism).2. Inorganic optical isomerism is not concerned with asymmetric carbon

atoms. Occasionally, there may be some other tetrahedral chiral centrebut, more often, optical isomerism arises from the different ways ofattaching bidentate ligands in octahedral complexes.

3. Inorganic geometrical isomers do not usually arise from dif·~erentarrangements around a double bond. However, the prefixes cis- andtrans- are used, but with reference to some other structural feature.

Read the section on isomerism in complexes in your textbook(s).Look for examples of structural isomerism, optical isomerism andcis-trans-isomerism (or geometrical isomerism). You should then beable to do the following exercises.

You may be able to manage without making models of complexes for Asome of the exercises, but you will certainly need them sooner orlater. You can use the same 'ball and spoke' type that is so usefulin organic chemistry, but a simpler type, using coloured plastictubing (like drinking straws), is particularly suited to complex ionisomerism (see Fig. 11). Whichever type of model you use, you mustbe able to represent a central atom forming six evenly-spaced bonds (inoctahedral directions).

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Exercise 37 There are two isomers with the formula Cr(H20)sCIBr.(a) Identify the two different complex ions in these

compounds.(b) What shape do these ions have?(c) What type of isomerism i$ this?(An swers on page 88 )

When hydrogen chloride is passed into cold chromium(III) ~nitrate solution, violet crystals A, CrC13"6H20, areobtained. These lose no water when stood over concen- ~~trated sulphuric acid in a desiccator, and 1 mol of Agives 3 mol of AgCl immediately, on addition of silvernitrate to the solution. Measurements of the electrical conduc-tivity suggest that each 'molecule' of A gives four ions insolution.

Exercise 36

On cooling a hot solution of chromium(III) chloride, greencrystals B, CrC13"6H20, are formed. B loses water over concen-trated sulphuric acid to give CrC13"4H20.On addition of silver nitrate, 1 mol of B gives 1 mol of AgClat once. Another 2 mol of AgCl are slowly formed, and thecolour of the solution changes at the same time. Conductivitymeasurements suggest that each 'molecule' of B gives two ions"By treatment of a solution of B successively with sulphuric andhydrochloric acids, the green crystals C, also with the formulaCrC13"6H20, can be made. 1 mol of Closes 1 mol of water overconcentrated sulphuric acid, gives 2 mol of AgCl at once andanother 1 mol more slowly, and has a conductivity correspondingto the formation of three ions in solution.(a) Suggest structures for compounds A, Band C" Give reasons.(b) Suggest a structure for yet another compound with the

formula CrC13"6H20. What would be its behaviour withsilver nitrate and with concentrated sulphuric acid? Whatkind of electrical conductivity would it have?

(An SW8rs on page 89)

Exercise 39 The compound Co(NH3)3(N02)2Cl is soluble in water, andhas two possible structures:

N02

H3 N/'-'.:-t z=: C1/ ./ Co I

H3N ~--1:::"'/NH3

NO.:A

N02 +

H3N/~-~/70H2I Co I C1-

H3N &::::-::-~I~I NH3

N02B

(a) How would you show that B, and not A, is the correctstructure?

(b) How would you find out if there is an equilibrium betweenB and A in the solution?

(Answers on page 89 )

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Exercise 40 How many isomeric ionsare there with theformula Co(NH3)4C12+?Sketch them using theoutline shown.

(Answer on page 89 )

You should now be clear about structural and geometric isomerism in transi-tion element compounds. We move on to discuss optical isomerism in moredetail.

Optical isomerismExamples of optical isomerism you have met in organic chemistry have alwaysbeen concerned with an 'asymmetric carbon atom' (or more than one), i.e. acarbon atom attached by four tetrahedrally-directed bonds to four differentatoms or groups.

The parallel situation, where four different ligands are attached tetra-hedrally to a transition metal ion, is rarely encountered -. However,asymmetry frequently arises in octahedral complexes containing bidentateligands, and we now consider some examples.

Although the geometry is not quite so simple as in tetrahedral asymmetry,the prinCiple of left-handedness and right-handedness (or mirror-imageisomerism) still applies, as we show in the next exercise. In order to dot~e exercise, you need a molecular model kit with at least two atoms whichcan make six bonds each in octahedral directions and, if possible, a smallmirror.

First, make a model of the chromium(III) complex ion shown in Fig. 12. AThis complex has four monodentate ligands (NH3) and one bidentateligand (H2NCH2CH2NH2). As you saw in Experiment 3, we can representthis ligand by the symbol 'en', both in the diagram and in theformula, Cr(NH3)4en3+. If you have the appropriate type of model kit,you can represent the bidentate ligand by one piece of tube bent back onitself.

NH3

H3N/~-~~':7 NH2-CH/ Cr / I 2

H3N~t~ NH,--CH2

NH3

NH3

H Nr:::::.- I__ :;:::.;=:3/~i~/~

I Cr I enH3N~-t~NH3

Fig.12.

Note that, for simplicity, we have not included the ove~all charge in thediagrams.

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Now use your model, and adaptations of it, to do the following exercise.

Exercise 41 Is the ion Cr(NH3)4en3+ optically active? Ifnecessary, make another model and use a mirror.Make models of the geometric isomers ofCr(NH3)2(en)23+ by exchanging two NH3 ligands for'one lenl ligand. Sketch them in a similar way toAre they optically active? How many isomers ofCr(NH3J2(enJ2

3+ are there in all?(c) Make models of the isomers of Cr(en)33+ and sketch

them. State the type of isomerism.

( a )

( b)

(Answers on page 89)

Fig. 12.

In order to help you consolidate the work you have done, look at the OJsection on complex ions in the ILPAC video-programme IThe Transition ~Metalsl, if it is available.

00

Now we consider how the formulae of complex ions can be determined.

STOICHIOMETRY OF COMPLEX IONSIt should be clear to you by now that large numbers of different complexescan be formed from anyone transition metal ion, and you may well wonder howtheir structures have been determined and how they can be distinguished fromone another.

We do not pursue this matter in great detail at A-level but we caneasily make a start by considering how the formula of a complex ioncan be determined. This is the subject of the next experiment, inwhich you use a colorimeter. We give full details of the operationof this instrument, although you may already have used one in UnitP5 iChemical Kinetics).

Objective. When you have finished this section you should be able to:

(27) describe how the formula of a complex ion can be determined bycolorimetry.

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EXPERIMENT 4Determining the formula of a complex ion

AimThe purpose of this experiment is to usecolorimetry to find the formula of thecomplex ion formed from copper(II) ionsand ammonia. The same method can be usedfor many other complex ions.

IntroductionUsing different proportions of copper(II) ion and ammonia you preparemixtures and examine them in a colorimeter. The mixture which shows themost intense colour (and, therefore, the greatest absorbance) indicates theproportions of Cu2+ and NH3 in the complex ion.

Requirementssafety spectacles3 beakers, 100 cm33 burettes, 50 cm3 or 3 graduated pipettes, 10 cm3

3 small funnels for filling burettesB test-tubes, to fit colorimeterB corks or bungs to fit test-tubestest-tube rackammonium sulphate solution, 2.0 M (NH4)2S04copperCII) sulphate solution, 0.10 M CuS04ammonia solution, 0.10 M NH3colorimeter, with filters

Procedure1. Prepare a different mixture in each of B test-tubes by running in from

burettes or graduated pipettes the volumes of the thr~e solutions shownin Results Table 4. Cork each tube and shake to mix thoroughly.

Results Table 4

Tube number 1 2 3 4 5 6 7 6

Volume of (NH4)2S04/cm3 15 5 5 5 5 5 5 5

Volume of CuSO 4/cm3 0.0 1.0 1.5 2.0 2.5 3.0 4.0 5.0

Volume of NH3/cm3 0.0 9.0 6.5 6.0 7.5 7.0 6.0 5.0

Colorimeter reading 0.0

(Specimen results on page 90)

2. Choose a suitable filter for this experiment. If you are not quite surewhich filter to use, look at the notes at the end of this proceduresection.

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3. With the filter in position, put tube 1 into the colorimeter, cover itto exclude stray light, and turn the adjusting knob so that the metershows zero absorbance (or, on some colorimeters, 100% transmission).Ideally, this should set the intensity of the light reaching the tubesfor the whole experiment but, because simple colorimeters tend to givea light intensity which is not constant, you should repeat this stepimmediately before each reading you take.

4. Still with tube 1 in the colorimeter, check for constancy of meterreading with the tube rotated and moved to slightly different positions.Mark the tube so that you can replace it in such a way as to get thesame reading each time.If necessary, change the tube until you are satisfied. (With some of thesimplest colorimeters, it may not be possible for you to be too fussy!)

5. Immediately after setting zero with tube 1, replace it with tube 2 andtake a reading of absorbance (or % transmission). To minimise errorsdue to irregular tube diameter and position, rotate the tube and take anaverage reading. (If you are lucky enough to be using matched tubes, putthe mark in the same position each time.)Also, check the zer~ again after the reading and repeat if necessary.Record the average reading in a copy of Results Table 4.

6. Repeat steps 3 and 4 before taking further colorimeter readings for eachof the remaining coloured solutions in turn.

7. Plot a graph of meter reading against volume of reactants as shown inFig. 13. If your meter shows absorbance, the graph will show a peak;if it shows transmittance, the graph will show a trough.

OJc:0ro~...Q)a;E.~oou

o 2 3 4 5 6 7 8 9 10

volume of 0.1 M CuS04

10 9 8 7 6 5 4 3 2 o

Frg.13.volume of 0.1 M NH3

B. From the graph, determine the volumes of CuS04 and NH3 corresponding tothe peak (or~trough if you are working with transmittance). Thesevolumes may not necessarily be those used in any of the tubes, but shouldgive a simple whole number ratio.

9. Use the simple whole number ratio obtained from the graph to write theformula for the complex ion.

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~hoosing a filterIdeally, the filter lets t~rough onlylight of the particular wavelengthwhich is absorbed by the complex ion.So for a blue complex ion, whichabsorbs yellow light, you need ayellow filter - yellow is thecomplementary colour to blue. SeeFig. 14.If you are not clear which filter hasthe complementary colour of the solu-tion you are using, then follow thedetailed procedure below.1. Put anyone of the available filters into the slot in the colorimeter,

and insert tube 1, covering it to exclude stray light. Turn the adjustingknob so that the meter shows zero absorbance (or, on some colorimeters,100% transmission). Mark the rim of the tube so that you can replace it

Fig.14.

in the same position.2. Replace tube 1 with tube 5 and take a reading of absorbance (or of

% transmission). Mark this tube too so that you can replace it in thesame position.

3. Repeat steps 1 and 2 above for all the filters in turn.4. Choose the filter which gives the greatest absorbance (or least trans-

mission) and go back to step 3 of the procedure section.

Questions1. What is the function of the ammonium SUlphate in this experiment? If

you do not know the answer, try adding ammonia solution to copper sulphatesolution without any ammonium sulphate present. If you are not in thelaboratory, or if you still need a clue, look up your notes on the commonion effect - Unit P2 (Equilibrium I: Principles].

2. Why does the absorbance decrease after the peak while the amount of addedcopper ions increases?

3. Why does thE! absorbance increase up to the peak while the amount of addedammonia decreases?

4. How would your results be affected by using the wrong filter?5. How would your results be affected by using no filter at all?(Answers on page 90)

ExtensionThe method described above can be used with very little modification(if any) to find' the formulae for many other complex ions. You maywish to try, for example, using a solution of diaminoethane (ethylene-diamine) in place of the ammonia in Experiment 4. If you do this, theammonium sulphate is not necessary and should be replaced with water.Other complexes you might try are those made from:

nickel(II) ions and edtairon(III] ions and thiocyanate ions

Note that the volumes given in Results T~ble 4 may need adjusting fordifferent complexes. Ask your teacher for advice.

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Now we move on to consider another aspect of the chemistry of complex ions.

ACIDITY OF COMPLEX IONSSolutions of some complex ions, particularly the aqua-complexes, are foundto be definitely acidic. The extent of this acidity varies with the natureand charge of the central ion.

Objectives. When you have finished this section you should be able to:

(28) explain why solutions of some complex ions are acidic;(29) recognise that pH often affects redox potentials.

You study this acidity in a revealing exercise, in which you applywhat you have already learned about acids and about polarization ofions. This exercise concerns the ion FeCH20)63+ but the principlesapply also to other ions.

1. A solution of iron(III) chloride has a pH of 4.0. What ions must bepresent, and in what concentration, to give the solution this pH?

A1. Hydrogen ions must be present, in concentration 1.0 x 10-4 mol dm-3•

02. What is the source of these hydrogen ions?

A2. The hydrogen ions must come from the water.

03. Since pure water has a pH of 7, the low pH of the solution must be dueto interaction of water molecules with Fe3+ ions and/or [1- ions.What should lead you to expect more interaction with Fe3+ ions thanwith [1- ions?

A3. The greater charge on Fe3+ ion attracts polar water molecules morestrongly than [1- ion. As you have seen, this leads to fairly strongbonding in the complex ion FeCH20)63+.

04. What is the 'polarizing power' of a cation, and what factors tend toincrea se it?

A4. In Unit S4 (Bonding and Structure), you learned that the polarizingpower of a cation is its ability to distort the charge cloud of aneighbouring anion, leading to partial covalent bonding. Polarizingpower increases with increasing charge and with decreasing size.

nccy:J. Adjacent molecules can also be polarized by cations.What might be the effect of polarization on the bonds in the Fe(H20J63+

complex?

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A5. The further distortion of the charge cloud in a water molecule, alreadypolar due to the differing electronegativities of oxygen and hydrogen,leads to a strengthening of the Fe - ° bonds by shifting electrondensity from the O-H bonds. The O-H bonds thus become weaker, andthe release of protons becomes more likely.

QG. Write an equation for the transfer of one proton from the hexaaqua-iron(III) ion to a water molecule.

AG.

This change is shown in diagram form below.H20

H20 ,~l:.;:-; OH2/ Fe I

H20 ~t~OH2

H20

H20

H2 0 ,~~ -:.;::-'7 OH/ Fe I

H20 ~r~OH2

H20

3+ 2+

Q7. Why is the transfer of a second proton less likely to occur than thetransfer of the first?

A7. The charge on Fe is effectively reduced from 3 to 2. The remaining H20ligands are thus less polarized and release protons less readily.However, a small amount of Fe(H20)4(OH)2+ ion is formed.

Q8. In gBneral terms, which transition metal ions would you expect to havethe most strongly acidic aqua-complexes?

A8. The most strongly acidic aqua-complexes would be those formed from ionsof small size and high charge. Elements in the first transition serieshave smaller ions of the same charge than the second and third transi-tion series and, within a series, ions of the same charge becomeslightly smaller from left to right across the Periodic Table.

You can apply what you have learned to the following exercises.

Exercise 42 The compound VC14 dissolves in water with a violent ~reaction. Explain why the resulting solution isstrongly acidic and contains oxovanadium(IV) ions, V02+. ~~Assume a coordination number of G.(Answer on page 90)

Exercise 43 (a) Show how the manganate(VII) ion, Mn04-, could beregarded as derived from interactton between themanganese(VII) ion, Mn7+, and water molecules.

(b) The manganate(VII) ion could also be regarded as a complexion formed from Mn7+ and 02- ions. Suggest reasons whyit is not usually regarded as a complex ion.

(Answers on page 91 )

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The hydrolysis of aqua-complexes has some bearing on the variation of redoxpotentials with pH~ as you see in the next section.

The effect of pH on redox potentialsYou learned in Unit P6 (Equilibrium 3: Redox Reactions) how the redoxpotential of a half-cell varies from the standard value when the concentra-tion of any species appearing in the half-equation varies from the standard1.0 mol dm-3• Use this knowledge to do the next exercise.

Exercise 44 (a) Write a half-equation for the reduction ofmanganate(VII) to manganese(II) in acid solution.

(b) The standard redox potential for the half-cell inwhich this reaction occurs is 1.51 V. How wouldyou expect the potential to change if the pH were adjustedto 1.0 without altering the other concentrations? (Aqualitative answer only is expected~ but you may wish totry to calculate a new value using the Nernst equation.)

(Answers on page 91 )

You may be surprised to learn that pH can also affect redox potentials insome cells where hydrogen ions do not appear in the half-equation. Thisarises when hydrolysis of one or more of the component ions occurs toproduce an acidic solution~ as we showed in the previous section.

fot instance~ the half-cell reaction normally written as:

does not represent all the changes which occur in the half-cell~ becauseboth fe3+(aq) and f~(aq) are hydrolysed to some extent. This means thatother redox equilibria~ such as

fe(H20)sOH2+(aq) + e- ? fe(H20)sOH+(aq)also contribute to the electrode potential. The extent of their contribu-tion varies with the pH of the solution~ because changing [H+(aq)] altersthe concentrations of the hydrolysed ions by shifting the position ofequilibrium in acid-base reactions such as:

Fe(H20)63+(aq) ? Fe(H20)sOH2+(aq) + H+(aq)

Therefore~ the electrode potential for fe3+/Fe2+ does vary a little with pH~and the same is true for other systems where hydrolysis occurs. for thisreason~ electrode potentials for the transition metals are listed at zeropH. Under these conditions~ the extent of hydrolysis is negligible.

Note that the stabilization by acid of Fe2+ solutions to oxidation by air isa kinetic effect and is not due to the dependence of electrode potential onpH.

You have seen how pH can affect electrode potentials and the relativBstabilities of different oxidation states. Now we show how changing ligandscan also affect the relative stabilities.

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THE EFFECT ON REDOX CHEMISTRY OF CHANGING LIGANDSWe usually discuss the redox chemistry of the transition elements byreference to electrode potentials for the hydrated ions, i.e. complex ionswith water molecules as ligands. When other ligands replace water molecules,the redox chemistry is modified and the relative stabilities of the differentoxidation states may be changed.

Objective. When you have finished this section you should be able to:

(30) recognise that redox potentials for changes in oxidation state oftransition elements vary according to the ligands present.

You should achieve this objective by doing the next exercise.

Exercise 45 Here are some redox potentials:

~Fe(CN)63-(aq), Fe(CN)64-(aq) I Pt; Etr = 0.36 V

12(aq), 21-(aq) I Pt; Etr = 0.54 V

Fe3+(aq), Fe2+(aqJ I Pt; Etr = 0.77 V

(a) What would you expect to observe when aqueous iodine isadded to:(t l FeSO 4 (aq ),

(b) What would you expect to observe when aqueous potassiumiodide is added to:

(c) Summarise the information above in one sentence referringto the relative stabilities of Fe(1I) and Fe(1I1).

(Answers on page 91 )

The next section deals with the relative stabilities of copper(1) andcopper(11) compounds. Again, the nature of the ligands is important.

SOME REDOX CHEMISTRY OF COPPERAs well as providing another example of the way ligands affect redoxchemistry, this section also contains another application of solubilityproducts and another example of disproportionation. You have met these twotopics before - in Unit P2 (Equilibrium 1: Principles) and Unit 12 (TrleHalogens) respectively.

Objectives. When you have finished this section you should be able to:

(31) explain why copper(1) ions cannot exist in solution (except in minuteconcentration);

(32) describe and explain how some insoluble copper(1) compounds can be made.

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Read about copper(l) compounds in your textbook. Look for methods of c=2preparation and reasons why the only common ones are insoluble. ~

The first exercise in this section requires you to make predictions about thecopper(l) oxidation state by using E~ values.

Exercise 46 H3p03(aq) + 2H+(aqLH3p02(aq) + H2o(1) I Pt;Cu2+(aq), Cu t Laq ) I Pt;Cu+ (aq) I Cu(sl :

E~ -0.50V~E~ 0.15

~\\\E~ = 0.52 V

(a) Use the E~ values above to explain why an attempt to reducecopper(II) to copper(I) in aqueous solution usingdioxophosphoric(IJ acid would probably not be successful.

(b) Copper(l) sulphate is reasonably stable as a dry solid butnot in solution. What would you expect to happen if youdissolved it in water. What type of change is this?

(Answers on page 91 )

The next exercise concerns the relative stabilities of copper(I) andcopper(II) in the presence of cyanide ions.

Exercise 47 In an excess of cyanide ions a copper(l) complex exists, ~CU(CN)43-, but there is no corresponding copper(II)cDmplex. The relevant redox potentials are: ~~CU(CN)43-(aq) + e ~ Cu(s) + 4CN-(aq)j E~ -1.09 VCu2+(aq) + 4CN-(aq) + e ~ CU(CN)43-Caq); E~ 1.77 V,(a) What effect would the presence of cyanide ions have on the

attempted reduction referred to in Exercise 46?(b) Would copper metal be an effective reducing agent in these

circumstances? Write an equation.(Answers on page 91 )

You may well have used a reaction similar to the one referred to in Exercise47(bJ to prepare copper(I) chloride. The first step is to make a copper(I)complex, CuC12-, by heating metallic copper and a copper(II) compound withan excess of chloride ions:

Cu(s) + Cu2+(aq) + 4Cl-(aq) ~ 2CuC12-(aqJThis complex is in equilibrium with a minute concentration of Cu+(aq),according to the equation:

CuC12-(aqJ ~ Cu+(aqJ + 2Cl-(aq); K = 5.0 X 10-6 mo12 dm-6cWhen the solution is diluted, the equilibrium shifts to the right* andsufficient copper(I) ions are produced to cause precipitation of copper(I)chloride.

Cu+(aq) + Cl-(aqJ ~ CuCl(s); K = 3.1 X 106 mol-2 drn"c*It is .easier to see why the equilibrium shifts to the right by applyingLe Chatelier's principle to the equilibrium written in the form:

Cu(H20)2C12-(aq) + 2H20(I) ~ Cu(H20)4+(aq) + 2CI-(aq)

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In the next experiment you apply the principles we have'just outlined t~to the practical preparation of copper(I) compounds. LJJL\

Some redox chemistry of copperEXPERIMENT 5

Aim and IntroductionThe purpose of this experiment is toprepare two copper(I) compounds byreduction of copper(IIJ. Differentconditions are used in Parts A, Band C.Requirementssafety spectaclesboiling-tube and holderspatulacopper turningscopper(IIJ chloride-6-water, CuCI2'6H20sodium chloride, NaCIwash-bottle of distilled waterBunsen burner and bench matb se k sr-, 250 crrr'4 test-tubestest-tube racksodium sulphite-7-water, Na2S03'7H20copper(II) sulphate solution, 0.10 M CuS04potassium iodide solution, 0.10 M KIsodium thiosulphate-5-water, Na2S203'5H20

Procedure - Part A1. Put 1 spatula measure of copper turnings, two of copper(II) chloride and

two of sodium chloride in a boiling-tube and add about 10 cm3 of distilledwater.

2. Heat the tube till the contents just boil, and maintain this temperature,swirling the tube from time to time, for several minutes until thesolution appears not to be darKening in colour any further.

3. Allow the tube to cool a little, and pour the solution into about 50 cm3

of distilled water in a beaKer.4. Put the beaker on one side while you do Part B of the experiment and then

re-examine it.

Procedure - Part B1. Dissolve one spatula measure of copper(II) chloride in water in a tast-

tube about one-third full.

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Procedure - Part 8 (continued)2. Dissolve one spatula measure of sodium sulphite in water in another test-

tube about one-third full.3. Mix the two solutions and allow to stand for a few minutes.4. If necessary, pour off the solution to examine the white solid.

Questions - Parts A and B1. What is the white solid produced in each case, and why has it no colour?2. What is the reducing agent in each case?3. Why was heat required in Part A and not in Part B?4. What is the function of the excess of chloride ions in Part A?5. What causes the dark brown colour in Part A?6. Why does the solution in contact with the white solid in Part A slowly

turn blue on standing?(Answers on page 92

Procedure - Part C1. Mix a little copper(II) sUlphate solution in a test-tube with about

twice the volume of potassium iodide solution and allow to settle.2. Explain the change in appearance after step 2.

Questions - Part C1. What do you observe in step 1?2. Explain the change in appearance after step 2.3. Why is the solution no longer blue after step 2?4. What, therefore, is the creamy white solid?(Answers on page 92)

Summarise the whole experiment in a larger copy of Results Table 5.

Results Table 5

Method Observations Equat ion (s)

A. Preparation of

B. Preparation of

C. Preparation of

(Specimen results on page 92 )

The reaction you performed in Part C of the experiment can be represented bythe equation:

2Cu2+(aq) + 4I-Caq) -* 2CuICs) + I2(aq)In the next exercise you consider this reaction as consisting of two half-reactions (redox) followed by precipitation.

Cu2+(aq) + e ~ Cu+(aq); g& 0.15 V~2(aq) + 2e- ~ 2I-(aq); E~ 0.54 V

~ CuI(s); Ko 11K s

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Exercise 46 ( a) Do the electrode potentials above lead you to expect~iodide ions to reduce copper(II) to copper(I)?Explain. ~~What is the maximum concentration of Cu+(aq) therecan be in a suspension of copper(I) iodide where[I-(aqJ] = 0.10 mol dm-3?

(b)

(c) The electrode potentials quoted above refer to standardcon dition s; in part icular, [Cu+(aq)J = 1.0 mo1 dm-3 •

What is the effect on the electrode potential of reducing[Cu+(aq)] to the value calculated in (b)? (Either give aqualitative an swe. or, if you are familiar with the Nernstequation, calcula da new VEJue ,)

(d) In the light of your answer to (c), suggest a reason whythe addition of iodide ions to copper(II) sulphate solutionprecipitates copper(I) iodide while the correspondingreaction with bromide ions does not occur.

2Br2(aq) + 2e- ? 2Br-(aq); E~ = 1.09 V(Assume CuBr has similar solubility to CuI.)

(Answers on page 92 )

We now move on to an important application of the redox chemistry of thetransition metals - their use as catalysts, which we touched on in Level 1.

TRANSITION ELEMENTS IN CATALYSISIn Unit P5 (Chemical Kinetics), you learned the distinction between homo-geneous and heterogeneous catalysis. In this section we deal mainly withthe mechanism of homogeneous catalysis; examples of heterogeneous catalysisappear in Unit 16 (Selected p-Block Elements).

Objectives. When you have finished this section you should be able to:

(33) explain a general mechanism for homogeneous catalysis involvingtransition metal ions, and give some examples;

(34) use standard redox potentials to predict whether particular ions mightfunction as catalysts for a given reaction;

(34) describe very briefly the importance of transition metals in biologicalsystems.

If it is available, watch the section on catalysis in the ILPAC I[ 11video-programme 'Transition Metals' either now as an introduction,or at the end of this section as a summary. 00

Read the section in your textbook(s) dealing with homogeneous ~catalysis paying particular attention to the importance of transition ~Ielements. '-J-I

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You should now be able to do the following Revealing Exercise, whichdeals with the catalytic oxidation of 2,3-dihydroxybutanedioate(tartrate) ions by aqueous hydrogen peroxide. The exercise will helpyou to revise some aspects of redox chemistry you have studied earlier.

01. Write the formula for 2,3-dihydroxybutanedioate ion, both in structuraland condensed forms.

A1.

[

0 'HO H OJ2-,,~ I I //'C-C-C-C/

/ I I "0/;/ H OH ~'oor

CH(OH)C02-

ICH(OH)C02-or

02. Deduce the half-equation for its oxidation to carbon dioxide andmethanoate ion. In order to balance atoms and charges you need toinclude hydrogen ions and water.

03. Write a half-equation for the reduction of hydrogen peroxide to water.

04. The standard electrode potentials for half-cells in which thesereactions occur are as follows:2HC02-Caq) + 2C02Cg) + 6H+Caq) + 6e- ~ C4H4062-Caq) + 2H20Cl);

E-6- 0.20 VEB- 1.77 V

Do you think that hydrogen peroxide will oxidize 2,3-dihydroxybutane-dioate ion?

A4. The reaction is energetically favourable. ~ is 1.57 V, which islarge enough to suggest that the reaction would go to completion.

05. Write the overall equation for the reaction by combining the two half-equations.

OB. Although it is energetically favourable, the reaction is very slow,even when heated, unless a catalyst is present. How, in general terms,does a catalyst speed up a reaction?

AB. A catalyst provides an alternative reaction pathway with loweractivation energy.

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Since this particular reaction involves a transfer of electrons, acatalyst could act in,two possible ways.Either: catalyst + H202 ~ intermediate

intermediate + C4H6062- ~ products + catalystcatalyst + C4H6062- ~ intermediate

intermediate + H202 ~ products + catalyst

Q7. What type of reaction would be involved in both of these possibilities?

A7. Each would be a redox reaction.

QB. What feature of transition elements makes their ions likely candidatesfor catalysts in redox reactions?

AB. Transition elements have more than one oxidation state. Electronscould, therefore, be transferred between reactants and products bymeans of the catalyst changing between two oxidation states.

The electron transfer can be represented generally as:e

~Reactant A + Catalyst (Low ox.) ~ Product A + Catalyst [High ox.)

8

~Reactant B + Catalyst (High ox.) ~ Product B + Catalyst (Low ox.)Overall: Reactant A + Reactant B ~ Product A + Product B

QB. Cobalt(II) has been suggested as a suitable catalyst, with intermediateformation of cobaltCIIIJ. Assuming for the moment that simple ionsC02+[aq) and Co3+[aq) are involved, write equations for a possiblemechanism. (Combine half-equations as before.)

AB. H202[aq) + 2H+[aq) + 2e- ~ 2H2O(1)2C02+(aq) ~ 2C03+[aq) + 2e-

H202[aq) + 2C02+[aq) + 2H+[aq) ~ 2C03+(aq) + 2H20(1) ... (1)C4H4062-(aq) + 2H20(1) ~ 2C02 + 2HC02-(aq) + 6H+(aq) + 6e-6C03+[aq) + 6e- ~ 6C02+(aq)C4H4062-[aq) + 2H2O(1) + 6C03+[aq) ~ 2C02[g) + 2HC02-(aq)

+ 6H+(aq) + 6C02+[aq) ... (2)The combination of equations (1) and (2) gives the same overallreaction as before.

Q10. For this mechanism to be feasible, the standard electrode potential forthe C02+/C03+ half-cell must lie within a certain range of values.What range is this?

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A10. E~ should lie between 0.2 V and 1.77 V so that Co(III) can oxidize2,3-dihydroxybutanedioate ion and Co(II) can reduce hydrogen peroxide.

011. E~ for the half-cell C03+(aqJ,Coz+(aqJ I Pt is 1.84 V, which is outsidethe required range. What two factors might, nevertheless, make Co(IIJable to act as a catalyst for this reaction?

A11. (a) Using concentrations different from the standard concentrationsimplied by E~ values might make the reaction feasible. Inparticular, Coz+ in high concentration might reduce Hz02 in lowconcentration. However, concentration changes do not alter DEvalues very much.

(b) The cobalt ions can form complexes with the 2,3-dihydroxybutane-dioate ions. Electrode potentials for half-cells involvingcomplexes are often substantially different from those involvingsimple ions.

Unfortunately, ~ for the half-cell containing cobalt complexes with2,3-dihydroxybutanedioate is not quoted in the literature. Even if it were,and the reaction were shown to be energetically favourable, we could stillnot be certain that Co(II) would be a catalyst because we have no means ofpredicting kinetic feasibility.

Investigating the use ofcobalt(II) ions as a catalyst

Clearly, you must do an experiment.

EXPERIMENT 6

AimThe purpose of this experiment is to testwhether cobaltCII) ions will catalyse thereaction discussed in the last exercise (theoxidation of 2,3-dihydroxybutanedioate ionsby hydrogen peroxide) and to look forevidence of intermediate compound formation.

IntroductionIn this short experiment, you simply mix the hot reactants and add thepot~ntial catalyst. If you see a coloured intermediate, you can attempt tostabilize it by cooling the mixture quickly.

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Requirementssafety spectaclesbeaker, 250 cm3distilled waterspatulapotassium sodium 2,3-dihydroxybutanedioate, CO,K(CHOH),C02Na'4H20

(also known as potassium sodium tartrate or Rochelle salt)Bunsen burner and bench mattripod and gauzethermometer, 0 - 100°Cmeasuring cylinder, 25 cm3

~hydrogen peroxide solution, 20-volume H~O~ -- - -- - -- - - ----. ~r. r. ~_2 test-tubes and rackcobalt(II) chloride, CoC12'6H20stirring rodteat pipette

Procedure1. Dissolve about 1 g of potassium sodium 2,3-dihydroxybutanedioate in about

50 cm3 water in a beaker.2. Heat the solution to about 70°C.3. Add about 20 cm3 hydrogen peroxide solution, heat again to about 70°C

and then remove from heat. At this stage there should be little or nosign of reaction.

4. Dissolve a few small crystals (about 0.25 g) of cobalt(II) chloride in alittle water (about 5 cm3) in a test-tube and add this to the hotsolution. There will be an induction period before a reaction starts,so be patient! The length of the induction period decreases withincreasing concentrations of reactants" and catalyst and with increasingtemperature. These can be varied if necessary.

5. As soon as the solution appears to be dark green, quickly transfer about5-10 cm3 by teat pipette to a test-tube and cool it under the tap. Ifthe green colour has disappeared before you have cooled the tube, repeatthe experiment in such a way that the reaction proceeds more slowly.

Questions1. The gas given off when the reaction starts has two components. What are

these?2. What is the dark green solution and why does the colour return to pink?3. Why does cooling the test-tube preserve the green colour for a while?(Answers on page 92 )

Another experiment (optional) on catalysis follows, but you have now tIDcovered enough ground to be able to do one or more of the followingTeacher-marked Exercises, for which you have to make a broad review ~of all the characteristics of transition elements. Ask your teacher Jlwhich question(s) to attempt.

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Teacher-markedExercise

The elements scandium to zinc are generallylisted in a separate 'block' of the PeriodicTable.

Review the major characteristics commonto this block of elements which are notgenerally shown by elements in other blocks of thePeriodic Table. Explain how the chemical properties ofthese elements or their compounds can be understood interms of electronic structure.

( a )

(b) Illustrate these characteristics by reference to ONEmember of the series scandium to zinc, and discusscritically the extent to which this member exhibitsall the characteristics outlined in your answer to (a).

Teacher-markedExercise

Discuss, with appropriately chosen examples to illus-trate your answers, some of the typical properties ofthe transition elements and their compounds.Outline briefly THREE important applications ofcompounds of the transition elements.

Teacher-marked (a) Show, by stating essential reagents and condi-Exercise tions only, how the following conversions may

be brought about.(i) Mn2+(aq) ~ Mn02(s)

(i i ) Mn0 2 ( s ) ~ MnO 4 2 - ( aq )(iii) Mn04

2-(aq) ~ Mn04-(aq)lLv l Mn2+(aqJ ~ Mn04-(aq)(v) Cr3+(aq) ~ Cr04

2-(aq)(vi) Cr042-(aq) ~ Cr2072-(aq)

(b) Sketch the structures of the ions Cr042- and Cr2072-.(c) Both potassium manganate(VII) and potassium dichromate(VI)

may be used as oxidizing agents in volumetric analysis.What are the advantages and disadvantages of each?

(d) A solution containing 0.124 g of arsenic(III) oxide(AsZ03J required 25.0 cm3 of acidified 0.02 Mpotassium manganate(VII) for complete oxidation. Whatwas the oxidation state of the arsenic after oxidation?

For the next experiment you should make predictions about possiblecatalysts in the same way as we did in the Revealing Exercise.Then you will test your predictions. To make the best use of yourlaboratory time, you should make the predictions beforehand.

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EXPERIMENT 7Catalysing the reaction between iodide ionsand peroxodisulphate ions

AimThe purpose of this experiment is to find andtest suitable catalysts, from a range ofreadily available transition metal ions, forthe reaction:

IntroductionYou can make predictions about tho suitability of possible catalysts byassuming that the mechanism of catalysis consists of two stages, either ofwhich can be the first:

(i) higher oxidation state of catalyst oxidizes 1-,

(ii) lower oxidation state of catalyst reduces S20e2-.You then perform a number of experiments, each with fixed starting concen-trations of 1-, S20e2- and possible catalyst. Thiosulphate ions and starchare also added to enable you to measure the rate of reaction, and thus testyour predictions.As the reaction proceeds, any iodine formed immediately reacts with thethiosulphate:

I2(aq) + 2S2032-(aqJ ~ S4062-[aq) + 2I-[aq)When all the thiosulphate has reacted, the free iodine gives a deep bluecolour with the starch. If t is the time taken for the blue colour toappear, then 1/t is a measure of the initial rate of reaction - see Unit P5(Chemical Kinetics).

Requirementssafety spectacles4 test-tubes and rackpotassium iodide solution, 0.2 M KIsulphuric acid, dilute, 1 M H2S04potassium manganate[VII) solution, 0.1 M KMn04potassium chromate(VI) solution, 0.1 M K2Cr04iron(III) chloride solution, 0.1 M FeCl3potassium peroxodisulphate solution 0.2 M K2S20emanganese(IIJ sulphate solution, 0.1 M MnS04

chromium(III) chloride solution, 0.1 M CrCl3iron[II) sulphate solution, 0.1 M FeS044 burettes and stands }4 beakers, 100 cm3 [shared with other students)4 small funnelsstarch solution, 0.2%sodium thiosulphate solution, 0.01 M Na2S203conical flask, 150 cm3

boiling-tubestopclock

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Proc8dur81. In a copy of R8sults Tabl8 6 writ8 th8 appropriat8 818ctrod8

pot8ntials from your data booK.2. US8 th8 818ctrode pot8ntials to pr8dict th8 feasibility of 8ach St8P in

th8 propos8d m8chanism. You should wait till th8s8 steps have beent8st8d b8for8 pr8dicting th8 oV8rall f8asibility.

R8sults Tabl8 6

Transition 818m8nt Chromium Mangan8se Iron

Oxidation stat8s uS8d Cr(VI) Cr(III) Mn(VII) Mn(II) F8(III) F8(II)

El8ctrod8 pot8ntial/V(high8r ox. state ~ low8r)

El8ctrod8 pot8ntial/V(12 + 28- ~ 21-)

El8ctrod8 potent iaI/V(S20S2- + 28- ~ 2S04

2-)

008S high8r Pr8dictionox. stat8oxidiz8 I-? Practically

D08s low8r Predictionstat8 r8duc8S20S 2-? Practically

Do you exp8ct catalysis?

Tim8 for blu8 colourto app8arls

(Sp8cim8n r8sults on pag8 92 J

3. In a t8st-tub8, mix a f8w drops of potassium iodid8 solution with a fewdrops of dilut8 sulphuric acid. Add a f8w drops of on8 of th8 solutionscontaining a transition 8lem8nt in a higher oxidation state. LooK for8vid8nc8 of r8action and fill in your tabl8 accordingly.

4. In a t8st-tub8, mix a f8w drops of potassium peroxodisulphat8 solutionwith a f8w drops of a solution containing th8 same transition 818ment ina low8r oxidation stat8. LooK for evidenc8 of reaction and fill in yourtabl8 accordingly.

5. R8p8at steps 3 and 4 for the other two transition eleme~ts. From yourresults pr8dict which of th8 giv8n solutions ar8 liK8ly to b8 catalysts.To cheCK wh8ther catalysis actually occurs, proc88d as follows. elf youdo not hav8 time to ch8cK 8v8ry pr8diction, shar8 your r8sults withoth8r stud8nts.)

6. Fill 4 bur8tt8s with on8 each of the following solutions:potassium iodide,starch,

potassium p8roxodisulphate,sodium thiosulphate.

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7. From the burettes run into a conical flask these volumes:10 crrr' of potassium iodide sol ution,10 cm3 of sodium thiosulphate solution,5 crrr' of starch solution

Run 20 cm3 of potassium peroxodisulphate solution into a boiling-tube.B.

9. Quickly pour the contents of the boiling-tube into the flask and startthe stopclock. Swirl the flask twice to mix the contents and then standit on the bench while you look for the appearance of a blue colour.

10. As soon as the solution turns blue, stop the clock and record the timetaken.

11. Repeat steps 7 to 10, but in addition to the first three reagents, add5 drops of a solution containing a transition metal ion.

12. If you have time to repeat the procedure before trying another possiblecatalyst then, of course, your results should be more reliable.

Questions1. For one of the three transition elements you used, all of the predictions

were confirmed by experiment. Which one was this? Describe briefly apossible mechanism (without equations) for catalysis involving thiselement.

2. One of the three transition elements did not catalyse the reaction.Which one was this? Why could it not function as a catalyst?

3. One of the three transition elements worked well as a catalyst, eventhough its lower oxidation state did not appear to react with peroxo-disulphate. Which one was this? Suggest a possible explanation. (Hint:look at Experiment 2.)

4. Two students performed this experiment on different days using the sameequipment and solutions. They found that their results did not agreevery well. Suggest an explanation.

(Answers on page 93)

The experiments you have just done, together with the preceding exercise,illustrate the type of mechanism which is believed to operate in many examplesof homogeneous catalysis. We now consider, very briefly, heterogeneouscatalysis.

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Heterogeneous catalysisTransition elements and their compounds are widely used in industry as solidcatalysts in gas phase reactions - you have already met a number of examples.Where the catalyst is a compound, e.g. vanadium(V) oxide in the manufactureof sulphuric acid, the mechanism probably involves electron transfer betweentwo oxidation states in a manner similar to the homogeneous catalysisalready described. This may also be the case when the catalyst is a metalinitially but is converted to a compound by one of the reactants.

However, the mechanism of heterogeneous catalysis often involves a processknown as chemisorption. Reacting gases are fairly strongly but reversiblybonded to the surface atoms of a transition metal, using the d orbitals.In the process some of the bonds within the gas molecules become stretched oreven broken. In addition, the concentration of gases near the surface isincreased. Both these factors increase the rate of reaction.

We do not discuss in detail any mechanisms for heterogeneous cata-lysis but you can find accounts in your textbooks if you wish to readabout them. Ask your teacher if your syllabus includes this topic.

oGJ

Catalysis is also important in biological systems, and this is mentionedbriefly in the final section of this Unit.

Transition elements in biological systemsAlthough this aspect of transition element chemistry is rarely examined atA-level, your study would be incomplete without considering the greatimportance of transition elements in biological systems.

Ask your teacher for some references for reading on this topic if you 0cannot find much in your textbooks. Some items you may wish to ~Ipursue are as follows: ~

Anaemia and the role of iron in haemoglobin.Cobalt in vitamin B12.Transition elements as coenzymes (i.e. substances which enable

enzymes to function as catalysts).Diseases associated with deficiencies of copper, zinc and manganese.Trace elements in soil.

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LEVEL TWO CHECKLISTYou have now reached the end of this Unit. In addition to what was listedat the end of Level One, you should now be able to:

(20) & (21) use the equilibrium law to write expressions for stabilityconstants of complex ions and to use them in simple calculations;

(22) explain the terms polydentate and chelate;(23) explain why polydentate ligands generally give very stable complex

ions;(24) explain the use of complexes in titrimetric analysis;(25) & (26) describe the shapes commonly adopted by complex ions and discuss

examples of structural, geometric and optical isomerism;(27) describe how the formula of a complex ion can be determined by colori-

metry;(28) explain why solutions of some complex ions are acidic;(29) recognise that pH often affects redox potentials;(30) recognise that redox potentials for changes in oxidation state of

transition elements vary according to the ligands present;(31) explain why coppereI) ions cannot exist in solution (except in minute

concentration);(32) describe and explain how some insoluble copper(r) compounds can be

made;(33) explain a general mechanism for homogeneous catalysis involving ions

of transition elements, and give some examples;(34) use standard redox potentials to predict whether particular ions might

function as catalysts for a given reaction;(35) describe very briefly the importance of transition metals in biological

systems.

Check that you have adequate notes before going on to the End-of-Unit Test.

There is also an End-of-Unit Practical Test, which appears overleaf. Askyour teacher whether you should do this test as well.

END-OF-UNIT TESTTo find out how well you have learned the material in this Unit, try ~?the test which follows. Read the notes below before starting.1. You should spend about 1~ hours on this test. ~~2. Hand your answers to your teacher for marking.

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END-Of-UNIT PRACTICAL TESTThis test is intended primarily for studentspreparing for a practical examination.Ideally, it should be done under examinationconditions. We do not include a requirementslist here because we assume that all thematerials you need will be provided for you.

Ask your teacher whether you may refer totextbooks and/or notes during the test.

You are provided with two saltsC and D. Carry out the followingtests and record your observationsand inferences in (a larger copy of)the table provided. Then answer thequestions which follow the table.

Results Table 7Test Inferences

1. Warm three-quarters of your sample of C with4-5 cm3 of aqueous sodium hydroxide.

Observations

2. Add approximately 10 cm3 of dilute sulphuricacid to the remainder of your sample of C ina boiling-tube and warm·the mixture in orderto dissolve the solid. Use portions of thesolution for the following tests:(a) To 1-2 cm3 of the solution of C add an

equal volume of aqueous potassiumiodide. Then add. dropwise. aqueoussodium thiosulphate until there is nofurther change.

(b) To 2 cm3 of the solution add a littlecopper powder and warm.

(c) To 3-4 cm3 of the solution add a littlezinc powder and allow the mixture tostand. It is suggssted that you makeobservations for about 5 minutes andthen allow the mixture to stand for afurther 30 minutes. making observationsfrom time to time. During this time youshould proceed with other tests.

3. Carry out a flame test on substance D.

4. Dissolve some D in the minimum quantity ofdistillsd water and use portions of thesolution for the following tests:(a) Add a few drops of this solution to a

mixture of 1-2 cm3 of aqueous potassiumiodide with an equal volume of dilutesulphuric acid. Then add aqueous sodiumthiosulphate dropwise.

(b) To 1 cm3 of the solution of D add aque-ous sodium hydroxide. Then add dilutesulphuric acid until in excess.

(c) To about 2 cm3 of the solution of D addan equal vol.ume of dilute sulphuric acid.Then add hydrogen peroxide solution drop-wise until there is no further change.

(d) Transfer the solution from (c) to aboiling-tube and add about 10 cm' ofaqueous sodium hydroxide and 1 cm3 ofhydrogen peroxide solution. Heat theresulting solution.

The anion in 0 contains oxygen and a metal. Give the oxidation statesof the metal which are indicated in the reactions in 4(a), 4(b) and 4(d).

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END-OF-UNIT TEST ~\;lQuestions 1-4 concern the following metals from the first d-block series inthe Periodic Table:A Scandium (atomic number 21)B Titanium (atomic number 22)C Manganese (atomic number 25)o Iron (atomic number 26)E Copper (atomic number 29)

Select, from A to E, the metal which

1. has the greatest number of unpaired electrons in its atom C_ ( 1 )

2. forms a colourless ion of oxidation state +4 6 ( 1 )

3. forms the smallest cation of oxidation state +2 ~ 1) ( 1 )

4. displays the highest oxidation number C ( 1)

5. Titanium has the electronic structure 1s22s22p63s23p63d24s2Which of the following suggested compounds of titanium isUNLIKELY to exist?

( 1J

6. A compound X has the formula CrCH20)6C13 and a relative formulamass of 266. 20.0 cm3 of a solution containing 13.30 g dm-3 ofX required 10.0 cm3 of 0.1 M AgN03 for complete precipitation ofthe chloride ions.Which of the following is the best structural formula for X?

A Cr(H20)63+3Cl- 0 CrC13(H20)3"3H20B CrCl(H20)s2+2Cl-"H20

(£) CrC12(H20)4+Cl-"2H20 ( 1)

In Questions 7 to 10 inclusive one, or more than one, of the suggestedanswers may be correct. You should answer as follows:A if only 1, 2 and 3 are correctB if only 1 and 3 are correctC if only 2 and 4 are correct0 if only 4 is correctE if some other response, or combination, is correct

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••

7. In which of the following reactions is the named element oxidized?1. Thallium in 2Tl+(aq) + Zn Is ) ~ 2Tl(s) + Znz+(aq) ),.-2. Silver in AgzS(s) + 4CN-(aq) - 2[Ag(CN)z]-(aq) + SZ-(aq) /3. CoP per in 2Cu2+(aq ) + 4I-(aq ) ~ 2CuI (s ) + IZ (aq l -x

4. Gold in 4Au(s) + 5CN-(aq) + 2HzO(1) + 02(g) ~ 4[Au(CN)z]-(aq) //( 1 )

5. In which of the following ions does the metal have an oxidationnumber of +3?1• V02 + »: 2. AIOz - /3. (1)

9. Which of the following compounds contain manganese in its highestoxidation state? I-~1. MnZ074'l~ 2. MnC204 ( 1 )

10. Chromium forms the complex ion [Cr(NH3)6]3+. Which of the follo-wing statements would you expect to be true?

~. The correct name is the hexammine-chromium(IIIJ ion.2. The ·complex ion is colourless./

'/3. The complex ion is octahedral.X4. A solution of the ions gives an immediate precipitate of

chromium(III) hydroxide on addition of cold aqueous sodiumhydroxide. ( 1)

For Questions 11 and 12, choose an answer from A to E as follows:A Both statements true: second explains first.8 80th statements true: second does not explain first.C First true: second false.o First false: second true.E Both false.

First statement Second statementThe oxidation numbers of the11. In Co(NH3)6C13' the oxidation

number of cobalt is +6.'<

constituents of a compound mustadd algebraically to zero. (1 )

12. Chromium (atomic number 24)has a maximum oxidationnumber of VI.

Chromium has six electrons in itshighest energy level.

c( 1 )

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(b)

Explain the meaning of 'complex ion', as applied to transi-·tion metals, incorporating the terms 'ligand' and 'centralion' in your answer. (3)Complete the equations:

M(H20)42+(aq) + 4NH3(aq) -+

M(H20)42+(aq) + 4CI-(aq) -+

13. (a)

( 2 )

(c) Suggest two pos~ible shapes for the two different complexions formed in reaction (b) (2)

(d) Name the TYPE of bonding common to both these complex ionsin (c). ( 1 )

(e) Say why ammonia molecules and chloride ions are able to formthese complexes. (2)

(f) Explain, using an example, why the electronic configurationsof transition metal ions are particularly appropriate for theformation of complex ions. (2)

(g) Write the formula of:(i) the hexacyanoferrate(II) ion;

(ii) the hexacyanoferrate(III) ion.(h) Explain why chlorine can interconvert the complexes in (g)

and write an equation for the reaction.(i) Suggest the observations that could be recorded if sulphur(IV)

oxide (sulphur dioxide) gas is bubbled into a hot, acidifiedsolution containing Cr2072- ions and say what type of reactionis involved. (3)

( 2 )

( 3)

14. (a) In aqueous solution, the colours of certain vanadium ionsare as follows:V02+ [i.e. V(V)), yellow; V4+, blue; V3+, greenWhen sulphur dioxide is passed through an acidic solutionammonium vanadate (NH4V03), the solution turns green. Howwould you find out experimentally whether the green colourwas due to V3+, or to a mixture of vanadium in oxidationstates IV and V?

oof

( 4 )

(b) The reaction between peroxodisulphate ions, S2082-, andiodide ions is catalysed by some transition metal ions.Describe how you would compare the catalytic effect of Cr3+and Fe3+ ions on this reaction. (7)

(c) In fact, Fe3+is found to be a catalyst, but not Cr3+.Suggest a possible mechanism, in view of the following stan-dard electrode potentials:

Fe 3 +(aq l, Fe2+(aq )S2082-(aq), 2S042-(aq)I2(aq), 2I-(aq)

+0.77 V+2.01 V+0.54 V

What prediction can be made about the standard electrodepotential of Cr3+(aq), Cr2+(aq) ions in view of the obser-vation that Cr3+ ions do not catalyse the reaction? (6)

71

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15. Consider the following standard electrode potentials, all of 0which refer to aqueous solutions.Cr3 +, Cr2 + I Pt E-9- = -0.41 V

(Cr2072- + 14H+), (2Cr3 + + 7H20 J I Pt Et7 = +1.33 V

Ce4+, Ce3 + I Pt p;B- = +1.70 V

(a) Write down the equations for the half-cell reactions corres-ponding to these three standard electrode potentials. (3)

(b) Find the e.m.f. of the following cells, all concentrationsbeing unity.

I

Pt Cr2 +, Cr3 + : Ce4+, Ce3 + I PtPt ( 3)

( c) In view of the e.m.f)s so calculated, what would be expectedto happen when a solution containing an equal number ofmoles of Ce3+ and Ce4+ ions was added to an acidified solu-tion of Cr3+ ions? Write the equation for the reaction. ( 3)

16. (a) (i) How would you obtain an aqueous solution of thecomplex ion [Cr(NH3)6J3+?

(ii) What is the oxidation state of chromium in this ion?(iii) Give the stereo-structure of the ion.(iv) Give the name of the ion, [Cr(NH3)4CI2]+' and draw the

stereo-structures of two isomers of it. (10)(b) (i) Write an expression for the stability constant of the

tetramminecopper(II) ion, [Cu(NH3J4J2+.(ii) When aqueous sodium hydroxide is added to a solution of

this complex in water, a small quantity of copper(II)hydroxide is formed, but when hydrogen sulphide gas ispassed through such a solution almost all the copper isprecipitated as copper(II) SUlphide. Suggest an expla-nation for this difference in behaviour. (7)

(Total 75 marks)

72

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APPENDIX ONE

ELLINGHAM DIAGRAMS AND THE EXTRACTION OF METALSThis section is primarily for students following syllabuses which includethe interpretation of Ellingham diagrams. If your teacher recommends thatyou study this topic, you should already have some knowledge of standardfree energy change ~G~, and its relationship with enthalpy of reaction,entropy and temperature:

~G-E7 = M~ - TD,S~

Before you begin, you may need to revise the work you did at the end ofUnit S3 (Chemical Energetics). Alternatively, your teacher may have decidedthat you study that section now for the first time.

Objectives. When you have finished this section you should be able to:

(36) state the information conveyed by a single line on an Ellinghamdiagram;

(37) construct a simple Ellingham diagram from given thermodynamic data;(38) interpret Ellingham diagrams so as to predict the feasibility of the

reduction of metal oxides by carbon and carbon monoxide.

Read about Ellingham diagrams in your textbook(s) and their use indeciding the best method of extraction of metals. You should thenbe able to do the following exercises.

Exercise 49 ( a ) What information is represented by a single lineon an Ellingham diagram?For many chemical reactions M-E7 and D,S-E7 vary solittle with temperature that they may be taken aseffectively constant. This enables us to calculatevalues of ~G-E7 over a range of temperatures.

( b)

Mtr( 298 K) D,Str( 298 K)Reaction /kJ mol-~ /J mol-~ K-~

2C(graphite) + °2(g) -+ 2CO -221 +1802Fe(s) + °2(g) -+ 2FeO(s) -533 -151

Calculate ~G-E7 for each reaction at the following tempera-tures:

(i) 298 K(ii) 1000 K

(iii) 1500 K(iv ) 1693 K

(v) 1808 K(vi) 2000 K

(c) Using one pair of axes, plot an Ellingham diagram for thetwo reactions. Join adjacent points by straight lines,i.e. assume that intermediate values fallon the lines.

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Exercise 49 (continued).(d) Why do you think 'kinks' appear in the Fe ~ FeD line?

(Hint: look up the melting-points of Fe and FeD.)(e) Why does the C ~ CO line have a negative slope and the

Fe ~ FeD line have a positive slope?(Answers on page 93 )

In the Revealing Exercise which follows you explore the use ofEllingham diagrams to give information about the best method ofextraction of metals.

Metallic oxides may be reduced by heating with carbon provided that a tempe-rature at which 6G& for the reaction is negative can be achieved. We shallrepresent this reduction by the general equation:

2MO(s) + 2C(s) ~ 2M(s) + 2CO(g) - - - - - - - - (A)

Equation (A) can be thought of as a combination of two reactions:2C(s) + 02(g) ~ 2CD(g)2M(s) + 02(g) ~ 2MO(s)

- -(8)- -(C)

Q1. How would you combine equations (8) and (C) to obtain equation (A)?

A1. If equation (C) is reversed (see equation (D) below) and added toequation (8). equation (A) is obtained.

2MD (s) ~ 2M (s) + O2 (g ) - - - - - - - (D)2C(s) + D2(g) ~ 2CD(g)2C(s) + 2MO(s) + 02(g) ~ 2M(s) + 2CO(g) + 02(g)

Q2. If 6G& for reaction (8) is -x kJ mol-1• and 6G~ for reaction (C)is -y kJ mol-~. what is the value of 6G~ for reaction (A)?

A2. 6G& for reaction (A) 6G~ for reaction (B) -6G&for reaction (C).-x - (-y) kJ mol-1 = (-x + y) kJ mol-1

Study the Ellingham diagram below.°C.

Note that the temperatures are given ino

'"

r-- /'/-r--r-- V-- r---r---~v

,// r---I- r--VV

/V

f..-- ----...--...--f--

2Mg + O2 -> 2MgO

-;-0 -500E-,~¢'(!)

~

-1000

2C + 02~ 2CO

1000 1500 2000Fig.15. temperaturel'C

74

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03. What are the values of ~G~ for the reactions:2C(s) + o2(g) ~ 2Co(g)2Mg( s) + O2 (s) ~ 2Mgo( s)

at 1630 o[ (the point of intersection)?

A3. 2C(s) + o2(g) ~ 2Co(g);2Mg(s ) + O2 (g ) ~ 2Mg0(s ) ;

-570 kJ mol-1

-570 kJ mol-1

04. What is the value of ~G~ at 1630 °c for the following reaction?2Mgo(s) + 2C(s) ~ 2Mg(s) + 2Co(g)

Show your working.

A4. ~G~ = -570 kJ mol-1 - (-570 kJ mol-1) = 0 (See A2.)

05. Use the Ellingham diagram to obtain values of ~G-E? for the same reactionat 1000 °c and 2000 °C.

A5. At 1000 °C, ~G~

At 2000 °C, ~G~

-460 kJ mol-1 -(-940 kJ mol-1)

-650 kJ mol-1 -(-340 kJ mol-1)

+480 kJ mol-1

-310 kJ mo1-1

06. Is the reduction of Mgo by C feasible at 1000 °c and 2000 °C? Explain.

A6. The reduction of Mgo by C at 1000 °c is not feasible because ~Ge- islarge and positive but at 2000 °c it is feasible because ~G-E? is largeand negative.

07. What is the significance of the point of intersection of the two lines?

A7. The point of intersection gives the mlnlmum temperature at which thereaction is feasible since that is the temperature at which ~G~ = o.

Strictly, the reaction proceeds to an equilibrium mixture if ~G-E? liesbetween ± 30 kJ mol-1 but we can ignore this here since it representsonly a very small temper31:ure range - see Unit S3 (Chemical Energetics).

Now attempt the next exercise, which is about the reduction of iron oxide.It helps to explain the operation of the blast furnace, which we describein the section beginning on page 26 of this Unit, and uses a diagram similarto the one you drew in Exercise 49.

75

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Exercise 50 Iron ore contains either iron(III) oxide. Fe203' oriron(II) diiron(III) oxide. Fe304. These are readilyreduced to iron(II) oxide. FeD. by carbon or carbonmonoxide. The Key step in the process is the finalreduction of iron(II) oxide. FeD.Study the Ellingham diagram in Fig. 16 and answer the questionswhich follow.

-200

I

"0E...., -400.:o/!¢'(!)

<l

-600

0

2Fe+02- 2FeO

2C + O2- 2CO

500 700 1000 1500

ternperaturer'C

Fig.16. Ellingham diagram for the reduction of iron oxide.

Over what temperature ranges might iron(II) oxide be reduced by(a) carbon. (b) carbon monoxide?Explain your answers.(Answers on page 93)

To consolidate your understanding of Ellingham diagrams. try the followingexercise. in which you consider the feasibility of reducing metal oxides byvarious means. You will need to study Fig. 17.

°1C(s) + 02(g) - CO2(g)

~ -400 t--\"":?-""~---:::::~=--""'"E....,~¢'(!)<l

II

Sn(s) + 02(g) - Sn02(s) 2Ni(s) + 02(g) - 2NiO(s)

III

L--------2H2(9) + 02(g) - 2H20(g)

II

IIIII

III

2C(s) + 02(g) - 2CO(g)

500 1000 1500

Fig.17.temperaturerC

76

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Exercise 51 From the Ellingham diagram above, it can be deduced that(a) carbon can reduce chromium(III) oxide at 1000 °c(b) nickel(II) oxide can be reduced by hydrogen at 1500 °c(c) below 300°C nickel could reduce tin(IV) oxide(d) both hydrogen and carbon could reduce tin(IV) oxide at

500°C(Answer on page 93 )

Finally in this Appendix, we suggestfollowing Teacher-marked Exercises.tional reading to help you plan yourwriting.

that you try one or both of theYou may need to do some addi-answer(s) before you start

Teacher-marked Fig. 1B shows the variation of the standard freeExercise energy change with temperature for the reaction of

certain elements with oxygen according to the equa-tions shown.

2Zn + O2 ~ 2ZnO

0

-200I

"0E -400J~<D-

-600<::><l

-800

-1000

-1200

500 1000 1500

Fig.18.temperature/K

2Mg + O2 --> 2MgO

2C + 02~ 2CO

(a) The standard enthalpy changes for all these reactionshardly vary with temperature. Explain qualitatively whythe standard free energy change for the reaction2C + O2 ~ 2CO decreases with a rise in temperature,whereas for metals, the standard free energy change forthe reaction 2M + O2 ~ 2Mo increases with a rise intemperature.

(b) Explain carefully how and why the information presented inFig. 1Benables the feasibility to be assessed of thereduction of zinc oxide to zinc by (i) carbon at BOO K,(ii) magnesium at 1000 K and (iii) carbon at 1500 K.

(c) Use Fig. 1B to comment on the thermal stability Qfmagnesium oxide, with respect to decomposition intomagnesium and oxygen, at 1500 K.

(d) Why is it that several metals are prepared by electrolyticmethods?

77

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Teacher-marked For the reaction:Exercise the free energy changes at various temperatures are

given in the following table:

Temperature/K 300 600 1200 2000~G/kJ mol-l. -856 -801 -276 -330

and for the reaction, 2C(s) + o2(g) ~ 2Co(g)the free energy changes at various temperatures are givenin the following table:

Temperature/K~G/kJ mol-~

300-276

600-330

1200-440

2000-582

(a) For the two sets of data, plot ~G/kJ mol-~ (on the Yaxis)against temperature/K (on the X axis), using the same axes.

(b) What information do the graphs provide about the feasibilityof producing titanium by the reduction of titanium(IV)oxide by carbon?

(c) Suggest the possible difficulties which prevent such amethod being used for the commercial production oftitanium.

(d) Outline how titanium is produced from its chloride.(e) Give a use of titanium and state a characteristic property

of the element upon which its use depends.

78

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APPENDIX TWOIn this Appendix we pick up two of the topics mentioned earlier in the Unitand take them a little further. This is because some of the accountsusually given at A-level (and expected in examinations) are rather inadequate,as you may already have realised.

Objectives. When you have finished the section you should be able to:

(39) give a simple account of the colour of transition metal compounds interms of the ligand field theory;

(40) explain the variation in paramagnetism between different complex ions.

We begin with an elementary description of the ligand field theory toaccount for the colours of transition metal compounds.

Ligand field theoryIn Level One we briefly explained the existence of highly-coloured compoundsby stating that transition elements have electronic energy levels sufficientlyclose for the absorption of visible light by the promotion of electrons fromone level to another. We did not say which levels are involved because thesituation is not as simple as it might appear.

Although the 3d and 4s levels are very much closer in transition metal ionsthan are, say, the 2p and 3s levels in a sodium ion, the gap is usually stilltoo large for visible light to cause electron transitions.

The ligand field theory explains how the presence of ligands cause the3d-orbitals, normally at the same energy level, to split into two levels witha gap suitable for the absorption of visible light. The reason is that thespatial arrangement of the d-orbitals (five of them, remember) is not the sameas that of the ligands. Orbitals closer to ~he ligands are raised to aslightly higher energy level than those further away.

For more details, looK up ligand field theory or crystal field theoryin an advanced text. Some A-level texts may deal with the subject inan elementary way, without using these titles, in sections dealingwith the colours of complex ions.

The distribution of electrons in the doubly-degenerate d-orbitals not onlyexplains the colours of complex ions, but is also significant in othertopics such as paramagnetism, as you see in the next section.

79

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ParamagnetismWe noted in Level One that paramagnetism in transition element chemistryis associated with unpaired electrons. The way electrons are paired insimple ions is adequate to explain the variation in paramagnetism (themeasured quantity is known as paramagnetic susceptibility) of hydrated ionsalso~ as shown in Table 7.Table 7

Unpaired ParamagnetismIon 3d electrons electrons (relative)

Sc3+ I I I I I 0 0

Ti3 + t I I I I I 1 1

V3+ + It I I I I 2 2

Cr3 + t It It I I I 3 3

Mn2 + It It It It It I 5 5

Fe3+ It It It It It I 5 5

Fe2+ I t+ It It It It I 4 4

C02+ I t+ I 1'+ + It It I 3 3

Ni2+ I 1'+ I t+ 1'+ I t It I 2 2

Cu2+ I t+ 11'+ t+ I t+ It I 1 1

Zn2+ I t+lt+ t+lt+lt+1 0 0

The numbers of unpaired electrons shown in Table 7 do not, however, explainthe relative paramagnetism of all complexes. For instance, while Fe(H20)62+

is strongly paramagnetic due to the same four unpaired electrons as in Fe2+,

the apparently similar complex ion Fe(CN)64- is not paramagnetic at all!

Ligand field theory explains this too. The splitting of the d energy levelsvaries in extent from ligand to ligand. If the energy gap is great enough,electrons may pair up in the lower sub-level, leaving the upper sub-levelempty.

For further discussion of paramagnetism~ read the appropriatesection of your textbook or reference book.

Also~ look again at the section on magnetism in the ILPAC video-programme 'The Transition Metals', if it is available.

80

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Page 94: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

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Page 102: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

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Page 103: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements
Page 104: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements
Page 105: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements

CONTRIBUTORS

Project Director

Colin Robertson , Inspector for Science , ILEA

Writing team

The materials were written and revised by practising teachers seconded to theProject for limited periods :

Lambros AtteshlisLesley BulmanMike FoleyAnn FriendLawrence HalsteadTerence Kelly

Production Team

Tony LanghamVanda ChanJohn SangwinPeter FaldonDawn DevereuxConstance Godfrey :Stella Jefferies

Frank McManusLeona rd RoselaarFran RoweAlec ThompsonSteve Wa xman

i/c production and cover designGraphicsAVA TechnicianAVA TechnicianOffice and typingTypingTyping and layout

Videotapes

Brian Babb, Producer, Educational Television Centre , ILEA

Reader

John Stephens , Department of Natural Sciences, South London College

Evaluator

John Gilbert , Institute for Educational Technology , University of Surrey

ILPAC trial schools

The following schools and colleges took part in the trials of the IndependentLearning Project for Advanced Chemistry . The Inner London Education Authoritywishes to thank the teachers in these schools and their students for their help .

Abbey Wood SchoolAcland Burghley SchoolBacon's C.E . SchoolBrooke House SchoolDunraven SchoolElliott SchoolEltham Hill SchoolEnsham SchoolForest Hill SchoolHighbury Grove SchoolHull College of Further EducationHydeburn School.

John Roan SchoolLadbroke SchoolLondon Nautical SchoolMorpeth SchoolNorth Westminster Community SchoolQuintin Kynaston SchoolSt . Mark's C.E . SchoolSydenham SchoolThomas Calton SchoolWalsingham SchoolWoodberry Down SchoolWoolverstone Hall

Page 106: Science;ILPAC Unit I5: Transition elements...Transition Elements. - (ILPAC; Unit 15) 1. Science I. Title II. Series 500 0161.2 ISBN 0 7195 4053 4 /I CONTENTS PREFACE Acknowledgements