NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2score-in-chemistry.weebly.com/uploads/4/8/7/1/... · NJC Preliminary Examination 9647/01/14 [Turn Over NATIONAL JUNIOR

1

NJC Preliminary Examination 9647/01/14 [Turn Over

NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION

Higher 2

CANDIDATE

NAME

SUBJECT

CLASS

REGISTRATION

NUMBER

CHEMISTRY Paper 1 Multiple Choice

Additional Materials: Multiple Choice Answer Sheet

Data Booklet

9647/01

Thu 18 Sep 2014

1 hour

READ THESE INSTRUCTIONS FIRST

Write in soft pencil.

Do not use staples, paper clips, highlighters, glue or correction fluid.

Write your name, subject class and registration number on the Answer Sheet in the spaces provided unless this has been done for you.

There are 40 questions on this paper. Answer all questions. For each question there are four possible answers A, B, C and D.

Choose the one you consider correct and record your choice in soft pencil on the separate Answer Sheet.

Read the instructions on the Answer Sheet very carefully.

Each correct answer will score one mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet.

This paper consists of 18 printed pages.

2

NJC Preliminary Examination 9647/01/14

Section A

For each question there are four possible answers, A, B, C, and D. Choose the one you consider to be correct.

1 Tin(IV) iodide can be prepared by refluxing 0.04 moles of tin with 0.03 moles of iodine

dissolved in 50 cm3 of tetratchloromethane (boiling point: 77 oC).

Sn + 2I2 SnI4

Orange crystals of the product are obtained by filtering the hot reaction mixture and then cooling the filtrate. Which of the following would indicate that the reaction is complete?

A The boiling point of the reaction mixture is 77 oC.

B No tin remains in the reaction flask.

C Crystallisation occurs in the boiling solvent.

D No more purple vapour is observed in the reaction vessel.

2 When an excess of KI was added to 5.0 cm3 of a 0.400 mol dm3 solution of AO43,

8.00 cm3 of 0.500 mol dm-3 of Na2S2O3 is required to react completely with the iodine liberated. What is the final oxidation state of A?

A 3 B +1 C +2 D +3

3 Which equation corresponds to the third ionisation energy of titanium (Ti)?

A Ti(g) Ti3+(g) + 3e

B Ti2+(s) Ti3+(g) + e

C Ti2+(g) Ti3+(g) + e

D Ti3+(g) + e Ti2+(g)

4 A 1.0 dm3 sample of gas is heated from 27 °C to 54 °C under constant pressure. What will be the new volume of the gas at 54 °C?

A 0.5 dm3 B 0.9 dm3 C 1.1 dm3 D 2.0 dm3

3


5 The following graphs show the ideal gas relationship for 2 different ideal gases. Gas P has twice the amount of particles as gas Q. Which graph is correct?

A

B

C

D

T

p

V

0

Q

P

T

pV

0

Q

P

T

V

p

0

P

Q

p

pV

T

0

Q

P

4


6 Three carboncarbon bonds are labelled in the diagram.

xy

z

Which bond is made up of an sp2sp3 overlap?

A y only

B z only

C x and y only

D x, y and z

7 Which of the following species has a bond angle of 120 around the central atom?

A NCl3

B N3

C BBr3

D H3O+

8 Enthalpy changes for the following reactions can be determined experimentally:

N2(g) + 3H2(g) 2NH3(g) Hrxn = 93 kJ mol1

H2(g) + ½O2(g) H2O(l) Hrxn = 242 kJ mol1

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(l) Hrxn = 906 kJ mol1

What is the enthalpy change of formation of NO(g)?

A + 90 kJ mol1

B +180 kJ mol1

C +360 kJ mol1

D +1270 kJ mol1

9 61.8 kJ of energy is required to vaporise 320 g of liquid bromine at its boiling point of 59 C.

What is the entropy change of vaporisation for bromine at 59 C?

A +0.093 kJ mol1 K1

B +0.186 kJ mol1 K1

C +0.523 kJ mol1 K1

D +1.050 kJ mol1 K1

5


10 The following reaction is used industrially to produce a combustible gas from coal.

H2O(g) + C(s) H2(g) + CO(g) A mixture of powdered coal and steam at a pressure of 1 atm was allowed to reach equilibrium at a constant temperature of 500 K. At equilibrium, the total pressure had increased to 1.9 atm. What is the numerical value of the equilibrium constant, Kp, at 500 K?

A 1.9

B 3.24

C 8.1

D 81

11 The titration curve below shows the reaction between a solution of 0.10 mol dm3 weak acid,

H2C2O4, and 0.20 mol dm3 aqueous sodium hydroxide.

Which statement is correct?

A Methyl orange cannot be used as an indicator in a titration to verify the concentration of H2C2O4.

B The volume of H2C2O4 used in this titration is 40 cm3.

C There are equal amounts of HC2O4 and C2O4

2 present at point A.

D The resultant solution formed has more H+ than OH when 20 cm3 of NaOH has been added.

Volume of NaOH/ cm3

pH

0 20

A

10

6


12 Which statement is not a correct description of a strong monoprotic acid?

A It can have a higher pH than a weak acid.

B It has a relatively high electrical conductivity in dilute solution.

C It requires the same volume of alkali for complete neutralisation compared to a weak monoprotic acid of the same concentration.

D It forms an acidic buffer when 0.1 mole of sodium hydroxide is added to 0.2 mole of a strong monoprotic acid.

13 Hydrogen peroxide reacts with acidified iodide ions liberating iodine according to the equation below:

H2O2(aq) + 2H+(aq) + 2I(aq) I2(aq) + 2H2O(l)

The kinetics of this reaction were investigated and it was found to have the following rate equation:

rate = k[H2O2][I]

Two series of experiments were conducted giving rise to Graph A and Graph B. The concentrations of reagents used for both experiments were as follows:

[H2O2] = 0.1 mol dm–3, [I–] = 0.1 mol dm–3, [H+] = 0.01 mol dm–3

Graph A Graph B rate of reaction y

x time

Which combination shows the correct labelling of the x-axis for Graph A and y-axis for Graph B?

x-axis for Graph A y-axis for Graph B

A [I–][H+] / mol2 dm–6 [H2O2] / mol dm–3

B [H2O2][I–] / mol2 dm–6 [H+] / mol dm–3

C [H2O2] / mol dm–3 [H+] / mol dm–3

D [I–] / mol dm–3 [H2O2] / mol dm–3

7


14 Use of the Data Booklet is relevant to this question. When a solution of sulfuric acid is electrolysed, gas J and gas K are produced at the cathode and the anode respectively in the molar ratio 2:1. In another experiment, the same quantity of electricity is used to electrolyse a saturated sodium chloride solution and a gas L is evolved at the anode. What is the molar ratio of J : K : L?

A 2:1:1 B 2:1:2 C 4:2:1 D 4:2:3

15 The diagram below is an experimental set-up by a student to purify a piece of impure

copper.


A The student should place the impure copper as electrode Z.

B The blue Cu(NO3)2 solution fades during the process of purification of copper.

C The change in mass for each electrode is different.

D The mass of copper deposited is 1.43 g when a current of 0.2 A flows for 3 hours in the above setup.

16 Aluminium is the third most abundant element in the Earth’s crust. Which statement is not true of its compounds?

A A solution of aluminium chloride turns blue litmus red.

B Aluminium chloride can be prepared by passing chlorine gas over heated aluminium metal.

C Aluminium fluoride has a much higher melting point than aluminium bromide.

D The pH of aluminium oxide in water is 7 because it can react with both aqueous acids and alkalis.

Cu(NO3)2 (aq) at

1 mol dm3

Y Z

8


17 The graph below shows the variation in the enthalpy change of vaporisation of eight consecutive elements in Period 2 and 3 of the Periodic Table.


A Oxide of Q is basic.

B Oxide of V, when dissolved in water, produces an acidic solution.

C Chloride of V, when dissolved in water, produces a neutral solution.

D Chloride of W when dissolved in water, produces an alkaline solution.

18 Barium sulfate occurs naturally as barite, which is a solid ore. Magnesium sulfate, however, occurs mainly in the solution. Which statement best explains the above observation?

A Barium ions are less readily hydrated than magnesium ions.

B Barium sulfate has a stronger crystalline lattice.

C Magnesium sulfate can react with water while barium sulfate cannot.

D Barium sulfate is not as polar as magnesium sulfate, hence it is much less soluble in a polar solvent such as water.

9


19 In which reaction is chlorine gas formed?

A NaCl is heated strongly with I2.

B NaCl is heated with concentrated H2SO4.

C NaCl is heated with Na2S2O3.

D NaCl is heated with concentrated H2SO4 and MnO2.

20 X(aq) undergoes the following reaction.

What is the identity of X?

A AgNO3 B Ag2SO4 C Pb(NO3)2 D PbSO4

21 How many stereoisomers does this organic molecule have?

A 24 B 25 C 26 D 27

pale yellow precipitate

X(aq) KI (aq) white precipitate

BaCl2 (aq)

Excess NH3 (aq)

Excess NH3 (aq)

precipitate does not dissolve

colourless solution

CH3

CO2HH3C

HO

CH3

10


22 Limonene, a main odour constituent of citrus fruits, is added to cleaning products such as hand cleansers to give a lemon-orange fragrance.

Which statement about limonene is correct?

A It reacts with hot acidified concentrated KMnO4 to give 2 organic compounds.

B The major product formed from the reaction of limonene with HBr(g) has chiral carbon(s).

C It reacts with H2 in the presence of nickel to form an alkane with formula C9H18.

D It reacts with Br2 in organic solvent to form a product with 4 chiral carbons.

23 The reaction conditions for four different transformations are given. Which transformation has the correct conditions?

A

KMnO4, NaOH(aq)

heat

C C

H H

C

H

OH

C

OH

H

B

KMnO4, H2SO4(aq)

heatC

H

OH

C

OH

H

C

O

OH2

C

K2Cr2O7, H2SO4(aq)

heat2 CH3CH2COOHCH3CH2CH=CHCH2CH3

D

(CH3)2C=CHCHOKMnO4, NaOH(aq)

heat (CH3)2CO + 2 CO2

11


24 Malic acid, HO2CCH(OH)CH2CO2H, is found in apples. Which statement about malic acid is not true?

A It can form esters with both ethanoyl chloride and ethanoic acid.

B 1 mole of malic acid can react with 3 moles of phosphorus(V) chloride.

C 1 mole of malic acid can react with 3 moles of hydrogen bromide gas.

D 1 mole of malic acid can react with 2 moles of sodium hydroxide.

25 Smoke from a bonfire contains a compound that causes irritation to the eyes. This compound readily decolourises aqueous bromine and produces a brick red precipitate with

alkaline copper(II) tartrate. It also produces an insoluble product when reacted with acidified

potassium dichromate(VI). What is a possible structure of the compound?

A CH3CH2CH2OH

B CH2=CHCHO

C

CH=CH2

CH2CHO

D

CHO

OH

12


26 One industrial preparation of ethanoic acid is the direct carbonylation of methanol, using a rhodium catalyst.

CH3OH + CO CH3COOH

Which compound could be expected to produce by this method?

A

OH

C

CH2OH

CH2CO2HH

B

OH

C CO2H

CH2OH

H

C

OH

C

CO2H

CH2CO2HH

D HC CH2CO2H

CH2CO2H

CH2OH

27 Butanone, CH3CH2COCH3 , may be used as the starting point for synthesising compound Z, CH3CH=C(CO2H)CH3. Which is the shortest possible process for producing Z?

A H2, Pt KCN, ethanol, heat dil. H2SO4, reflux

B KCN, ethanol, heat excess, c. H2SO4, heat dil. H2SO4, reflux

C HCN, trace NaCN Al2O3, heat dil. H2SO4, reflux

D NaCN, H2SO4 PCl5 NaOH, ethanol, heat dil. H2SO4, reflux

Rhodium

catalyst

CO2H

C CH2CO2H

CH2CO2H

H

13


28 Which statement about Paracetamol is correct?

HO

N

H

C O

CH3

Paracetamol

A It reacts with hot, aqueous sodium hydroxide to give sodium ethanoate.

B Yellow precipitate is observed upon addition of aqueous alkaline iodine.

C White fume is observed on addition of SOCl2.

D It decolourises hot acidified KMnO4.

29 The reaction below is carried out to synthesise an acid anhydride.

CH3 CH3

COCl

CO2H

CO2H

C

C

OO

O

Which type of reaction is not involved in the above synthesis route?

A Condensation

B Elimination

C Hydrolysis

D Oxidation

30 Which pair shows the correct trend for pKb value?

A NH3 > NH2Cl

B C6H5CH2NH2 > CH3CH2NH2

C CH3NH2 > C6H5NH2

D CH3CONH2 > CH3CH(OH)NH2

14


Section B For each of the questions in this section, one or more of the three numbered statements 1 to 3 may be correct. Decide whether each of the statements is or is not correct (you may find it helpful to put a tick against the statements that you consider to be correct). The responses A to D should be selected on the basis of

A B C D

1, 2 and 3 are

correct

1 and 2 only are correct


1 only is

correct

No other combination of statements is used as a correct response.

31 Which reactions/processes will result in a positive entropy change?

1 Addition of 1 mole of Cl2 (g) at 298K to 1 mole of N2 (g) at 298K.

2 Conversion of diamond to graphite.

3 Combustion of ethane at room temperature and pressure.

32 The following shows the equilibrium reaction in the Contact process to form SO3.

2SO2(s) + O2(g) 2SO3(g) ∆H < 0

Which of the following correctly describes how the system will respond to the proposed change?

change value of Kc partial pressure of SO3(g)

at new equilibrium forward rate constant, kf

1 increase temperature decrease increase increase

2 remove SO3 no change decrease no change

3 add catalyst no change no change increase

15


The responses A to D should be selected on the basis of

A B C D

1, 2 and 3 are

correct



1 only is

correct


33 The position of equilibrium lies to the right in each of these reactions.

Reaction 1: N2H4 + HBr N2H5+ + Br–

Reaction 2: N2H5+ + NH3 NH4

+ + N2H4

Based on this information, which statements are correct?

1 Br– is the conjugate base of HBr.

2 N2H5+ is the acid in Reaction 2.

3 The order of acid strength is HBr > N2H5

+ > NH4+.

34 This question is about sparingly soluble salts.

AgCl AgI HgI2

Ksp 1.0 1010 8.3 1017 1.1 1028

From the above information, which statements are true?

1 For a solution containing 1 mol dm3 of NaCl and NaI, the concentration of iodide ions

when AgCl just starts to precipitate is 8.3 107 mol dm3.

2 Solubility of HgI2 is 3.02 1010 mol dm3.

3 Solubility of HgI2 increases when dilute nitric acid is added to it.

16



A B C D

1, 2 and 3 are

correct



1 only is

correct


35 The carbonates of Group II decompose according to the following equation.

MCO3(s) MO(s) + CO2(g)

For this reaction, H increases on descending the group.

carbonate Mg Ca Sr Ba

H/ kJ mol1 +101 +178 +235 +269

Which properties can be used to explain this trend?

1 The ionic radius of the metal ion.

2 The melting point of Group II metal carbonate.

3 Group II oxide is a stronger ionic lattice than Group II carbonate.

36 Use of the Data Booklet is relevant to this question.

Which statements are correct?

1 CoF3 reacts with H2O to give O2 gas.

2 Effervescence is observed when Fe3+(aq) is mixed with Na2CO3.

3 Na2CrO4 can be oxidised to Na2Cr2O7.

17



A B C D

1, 2 and 3 are

correct



1 only is

correct


37 A catalytic converter is part of the exhaust system of many modern cars.

Which reactions occur in a catalytic converter?

1 2CO + 2NO 2CO2 + N2

2 CO2 + NO CO + NO2

3 2SO2 + 2NO 2SO3 + N2

38 Compound T is reacted with excess NaOH(aq) under reflux while compound V is reacted with NaBH4 in methanol.

Which pairs of T and V give the same product?

T V

1 C6H5CH(Cl)CH3 C6H5COCH3

2 BrCH2CH2CH2OH CHOCH2CO2H

3 CH3CH2CH2Cl (CH3)2CO

18



A B C D

1, 2 and 3 are

correct



1 only is

correct


39 Use of the Data Booklet is relevant to this question.

Quinone (cyclohexadiene1,4dione) can be formed by oxidising quinol (benzene1,4diol).

OO + 2 H+ + 2e

HO OH

quinone quinol Which statements are correct?

1 C bonded to O in quinone has an oxidation state of +2 while C bonded to O in quinol has an oxidation state of +1.

2 Eocell for the reaction between quinol and acidified KMnO4 is positive.

3 LiAlH4 can be used to reduce quinone to quinol.

40 In order to form a new bond between 2 reactant molecules containing hydroxyl groups, a

dehydrating agent such as concentrated sulfuric acid can be used. What are the possible products formed when ethanol reacts with ethanoic acid under reflux in the presence of concentrated sulfuric acid?

1 C O CH2CH3CH3

O

2 CH3CH2 O CH2CH3

3 C O CCH3

O O

CH3

Eo = +0.70 V

– END OF PAPER –

Paper 1 : MCQ Answer Key

1 D 11 C 21 B 31 B

2 D 12 D 22 D 32 C

3 C 13 B 23 B 33 A

4 C 14 B 24 C 34 B

5 C 15 C 25 C 35 D

6 A 16 D 26 A 36 B

7 C 17 C 27 C 37 D

8 A 18 A 28 A 38 D

9 A 19 D 29 B 39 B

10 C 20 A 30 D 40 A

NJC Preliminary Examination 9647/02/14 [Turn over

NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2

CANDIDATE NAME

SUBJECT CLASS

REGISTRATION NUMBER

CHEMISTRY

Paper 2 Structured Answer on the Question Paper. Additional Materials: Data Booklet

9647/02

Wednesday 3 Sept 2014

2 hours

READ THE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use paper clips, highlighters, glue or correction fluid. Answers all questions. The number of marks is given in brackets [ ] at the end of each question or part question.

For Examiner’s Use

1 /12

2 /16

3 /16

4 /17

5 /11

Total

/72

This document consists of 15 printed pages and 1 blank page.


2 For Examiner’s

Use

BLANK PAGE

3


For Examiner’s

Use Answer all the questions.

1 Planning (P) A student was provided with a spirit burner. He was asked to determine the enthalpy

change of combustion under laboratory conditions, Hc, for ethanol using the following setup.

Knowing that there could be significant heat loss arising from the above experimental setup, he decided to calibrate the calorimeter by burning 0.60 g of methanol (CH3OH) to determine the calorimeter’s heat capacity, Ccalorimeter, which accounts for both the water and the copper can. Heat capacity is defined as the number of Joules of heat needed to raise the temperature of the calorimeter by one Kelvin or one degree Celsius. The temperature of the calorimeter rose from 25.0°C to 33.8°C.

The same calorimeter was then used to measure the enthalpy of combustion of ethanol.

(a) Given the enthalpy change of combustion of methanol is − 715 kJ mol–1, use the information above to calculate the heat capacity of the calorimeter, Ccalorimeter, stating its units.

[2]

calorimeter (copper can containing

50 cm3 of water)

draught shield

wick

spirit burner containing selected alcohol

stirrer

lid

thermometer


4 For Examiner’s

Use

(b) Given the enthalpy change of combustion of ethanol is approximately −1370 kJ mol–1, calculate the minimum mass of ethanol required to give the same temperature change as that in the calibration.

[2]

(c) Write a plan to determine the enthalpy change of combustion, Hc, of ethanol that

the student will carry out.

You may assume that you are provided with:

a thermometer with divisions of 0.2 °C division;

the apparatus normally found in a school or college laboratory.

Your plan should include details of

the procedure to determine the enthalpy change of combustion of ethanol;

the readings recorded using appropriate table(s), including units;

precautions taken to ensure reliability of the experiment

an outline of how the results would be used to determine the enthalpy change of combustion of ethanol based on the plan that you have written using arbitrary values.

………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… …………………………………………………………………………..…………………….. ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… …………………………………………………………………………………………………

5


For Examiner’s

Use

………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ……………………………………………………………………………………………….[7]

(d) Identify one potential safety hazard in this experiment and state how you would minimise this risk. ....................................................................................................................................... ....................................................................................................................................... ...................................................................................................................................[1]

[Total: 12]


6 For Examiner’s

Use

2 The simplest chemical reactions are those that occur in the gas phase in a single step,

such as the transfer of a chlorine atom from ClNO2 to NO.

ClNO2(g) + NO(g) NO2(g) + ClNO(g)

(a) (i) An equimolar mixture of ClNO2(g) and NO(g), at a total initial pressure of

3 atm, was allowed to react in a closed vessel at 1000 K. When equilibrium

was attained at the 5th minute, the partial pressure of ClNO2 was found to be

0.57 atm. Calculate the value for the equilibrium constant, Kp , of this system.

(ii) At the 10th minute, more ClNO2 gas was pumped into the vessel at 1000 K,

increasing the partial pressure of ClNO2 to 1 atm.

Suggest how the position of the equilibrium would change.

…………………………………………………………………………………………...........

…………………………………………………………………………………………...........

…………………………………………………………………………………………...........

(iii) Hence illustrate clearly, in the pressuretime graph below, the changes in the

partial pressures of ClNO2 and NO2 when

(I) the above gaseous system first reached equilibrium at the 5th minute,

(II) more ClNO2 gas was added into the vessel at the 10th minute and a new

equilibrium was attained at the 15th minute.

Pressure

Time 0

7


For Examiner’s

Use (iv)

Suggest whether the addition of an inert gas into the vessel would affect the position of the equilibrium.

…………………………………………………………………………………………...........

…………………………………………………………………………………………...........

[7] (b) ClNO2 can behave as an ideal gas under certain experimental conditions.

(i) State the two assumptions of kinetic theory of ideal gas.

…………………………………………………………………………………………...........

…………………………………………………………………………………………...........

…………………………………………………………………………………………........... (ii) Predict whether ClNO2 behaves ideally under high pressure.

…………………………………………………………………………………………...........

…………………………………………………………………………………………...........

…………………………………………………………………………………………........... [4] (c) (i)

Draw the dot-and-cross diagrams of NO and ClNO. Suggest the shape and

bond angle of ClNO.

(ii) Hence, suggest why the formation of ClNO from NO is favoured.

…………………………………………………………………………………………...........

…………………………………………………………………………………………...........

[5]

[Total:16] 3 The four most abundant salts in sea water are given in the table below.


8 For Examiner’s

Use

salt kg m3

Sodium chloride 27.5

Magnesium chloride 6.75

Magnesium sulfate 5.63

Calcium sulfate 1.80

The following flow scheme shows the series of reactions that involve seawater.

(a) Seawater contains chloride and sulfate ions. To verify the presence of chloride and sulfate ions instead of bromide and sulfite ions in sea water, the following chemicals can be used:

HNO3(aq), AgNO3(aq), BaCl2(aq), NH3(aq)

(i) Explain the importance of adding HNO3(aq) before other chemicals during this

verification process.

…………………………………………………………………………………………............

…………………………………………………………………………………………............

…………………………………………………………………………………………............

(ii) Suggest a sequence of chemical tests that can be used to verify the presence

of chloride and sulfate ions in sea water after acidification.

…………………………………………………………………………………………............

…………………………………………………………………………………………............

…………………………………………………………………………………………............

…………………………………………………………………………………………............

…………………………………………………………………………………………............

…………………………………………………………………………………………............

…………………………………………………………………………………………............ [3]

Seawater Step 1 controlled addition of

CO32(aq); filter

filtrate Step 2 addition of

OH(aq)

M(OH)2

Step 3 heat

MO Step 4

MCl2 (aq) Step 5

M(s)

9


For Examiner’s

Use

(b) (i) Calculate the concentration of magnesium and calcium ions in 1 dm3 of seawater.

(ii) Write an expression for the solubility product of magnesium carbonate.

(iii) Determine the range of concentrations of CO32(aq) required to cause selective

precipitation of only one cation in seawater given the following data.

compound solubility product

Magnesium carbonate 1.0 × 105

Calcium carbonate 5.0 × 109

(iv) Hence deduce the identity of M in the above flow scheme.

…………………………………………………………………………………………... [6]


10 For Examiner’s

Use

(c) (i) Write an equation with state symbols for Step 3 using M to represent your cation.

…………..……………………………………………………………………………....

(ii) Suggest and explain why magnesium hydroxide has a lower thermal stability

than calcium hydroxide.

…………………………………………………………………………………………...

…………………………………………………………………………………………...

…………..…………………………………………………………………………….....

…………………………………………………………………………………………...

…………..…………………………………………………………………………….....

…………..…………………………………………………………………………….....

[3]

(d) (i) If the mass of MO isolated in Step 3 is 0.05 g, calculate the volume of

0.1 mol dm3 of HCl required to convert all MO to MCl2.

(ii) Suggest how Step 5 can be carried out.

…………………………………………………………………………………………...

…………………………………………………………………………………………...

…………..…………………………………………………………………………….....

…………………………………………………………………………………………...

[4]

[Total: 16]

11


For Examiner’s

Use

4 Methanol is the simplest alcohol, and is a volatile, colourless and flammable liquid. Methanol is mainly used as an antifreeze, solvent or fuel.

(a) Define the standard enthalpy change of combustion of methanol in words.

…………………………………………………………………………………………............

…………………………………………………………………………………………............ [1] (b) (i) Using appropriate data from the Data Booklet, calculate the ∆Hr of the following

reaction.

2CH3OH(l) + 3O2(g) 2CO2 (g) + 4H2O(l) ∆Hr

(ii) Hence determine the standard enthalpy change of combustion of methanol.

[3] (c) The theoretical standard enthalpy change of combustion of methanol is

−715 kJ mol–1. Give two reasons to explain the discrepancy between this value and your answer in (b)(ii).

…………………………………………………………………………………………............

…………………………………………………………………………………………............

…………..…………………………………………………………………………….............

…………………………………………………………………………………………...........

[2]


12 For Examiner’s

Use

(d) Other than direct burning of methanol to generate energy, methanol can also be used in a fuel cell. Methanol fuel cells are more practical than hydrogen fuel cells. Carbon dioxide and water are the products of the methanol fuel cell.

(i) For each electrode X and Y, name the type of electrode and state its polarity.

Electrode X Type of electrode: ………………………………………... Polarity: ………………………………………...................

Electrode Y Type of electrode: ………………………………………... Polarity: ………………………………………...................

(ii) On the diagram above, indicate the direction of electron flow in the wire with an arrow.

(iii) Write the half-equation of the reaction taking place at each electrode. X: ………………………………………..................…………………………………

Y: ………………………………………..................…………………………………

(iv) Explain why methanol fuel cells are more practical than hydrogen fuel cells.

…………………………………………………………………………………………............

…………………………………………………………………………………………........... (v) Use appropriate data from the Data Booklet to explain why an acidic electrolyte

is often preferred to an alkaline or neutral electrolyte.

…………………………………………………………………………………………............

…………………………………………………………………………………………............

[8]

13


For Examiner’s

Use

(e) In a laboratory, methanol can also be oxidised to other organic compounds using suitable chemicals.

Give the appropriate reagents and conditions required for the following conversion.

I: Methanol Methanal Reagents and conditions: …………………………………………………………………..

II: Methanol Methanoic acid

Reagents and conditions: …………………………………………………………………...

III: Methanol Carbon dioxide Reagents and conditions: …………………………………………………………………...

[3]

[Total:17]


14 For Examiner’s

Use

5 Lactic acid is a carboxylic acid with the formula CH3CH(OH)CO2H. (a) State the type of stereoisomerism exhibited by lactic acid. Draw appropriate

structures to illustrate your answer.

[2] (b) In solution, lactic acid can lose a proton from the carboxyl group, producing the

lactate ion CH3CH(OH)CO2−. Compared to ethanoic acid, lactic acid is more acidic

as it deprotonates ten times more easily than ethanoic acid.

Other than the electron withdrawing effect of –OH group, suggest another factor which explains the additional stability of the lactate ion. Illustrate your answer with a diagram.

[2]

15


For Examiner’s

Use

(c) A student tries to synthesise lactic acid from chloroethane using a series of reactions

as shown.

(i) Fill in the intermediate organic compounds A and B in the following flow scheme.

CH3CH2Cl

Step 1 Step 2

HC

CH3

O

Step 3

HCN, trace NaCN

Step 4

H

C

CH3

OH

COOH

compound A

compound B

(ii) State the reagents and conditions for Step 1. Reagents and conditions: …………………………………………………………………...

(iii) State the type of reaction for Step 4 and write a balanced equation for this step.

Type of reaction: …………………………………………………………………………….. Balanced equation: …………………………………………………………………………..

(iv) Lactic acid formed in the above flow scheme is a racemic mixture while lactic acid formed via bacteria fermentation contains only one optical isomer. Explain.

…………………………………………………………………………………………............

…………………………………………………………………………………………............

…………..……………………………………………………………………………..............

[7]

[Total: 11]

CH3CH2Cl

NJC Preliminary Examination H2 Chemistry Paper 2 Solutions

1 Planning (P) A student was provided with a spirit burner. He was asked to determine the enthalpy

change of combustion under laboratory conditions, Hc, for ethanol using the following setup.

Knowing that there could be significant heat loss arising from the above experimental setup, he decided to calibrate the calorimeter by burning 0.60 g of methanol (CH3OH) to determine the calorimeter’s heat capacity, Ccalorimeter, which accounts for both the water and the copper can. Heat capacity is defined as the number of Joules of heat needed to raise the temperature of the calorimeter by one Kelvin or one degree Celsius. The temperature of the calorimeter rose from 25.0°C to 33.8°C.

The same calorimeter was then used to measure the enthalpy of combustion of ethanol.

(a) Given the enthalpy change of combustion of methanol is − 715 kJ mol–1, use the information above to calculate the heat capacity of the calorimeter, Ccalorimeter, stating its units.

[2] Heat evolved from methanol = (

) × 715000

= 13400 J

Heat gained by calorimeter = CT = C × 8.8 Heat evolved from methanol = Heat gained by calorimeter 13400 = (8.8)C C = 1520 J K–1

calorimeter (copper can containing

50 cm3 of water)

draught shield

wick

spirit burner containing selected alcohol

stirrer

lid

thermometer

(b) Given the enthalpy change of combustion of ethanol is approximately −1370 kJ mol–1, calculate the minimum mass of ethanol required to give the same temperature change as that in the calibration.

[2] For the combustion of ethanol to give the same temperature

change as that in the calibration, it means: Heat evolved from ethanol = Heat gained by calorimeter

ethanol × (1370 × 103) = 13400

ethanol =

( )

= 9.871 × 10–3 mol Minimum mass of ethanol = (9.871 × 10–3) × 46.0 = 0.450 g

(c) Write a plan to determine the enthalpy change of combustion, Hc, of ethanol that

the student will carry out.

You may assume that you are provided with:

a thermometer with divisions of 0.2 °C division;

the apparatus normally found in a school or college laboratory.

Your plan should include details of

the procedure to determine the enthalpy change of combustion of ethanol;

the readings recorded using appropriate table(s), including units;

precautions taken to ensure reliability of the experiment

an outline of how the results would be used to determine the enthalpy change of combustion of ethanol based on the plan that you have written using arbitrary values.

[7] Procedure

1. Use the same calibration apparatus setup for the experiment.

2. Rinse the spirit burner with ethanol and allow the wick to dry off/ use a new wick with same length.

3. Weigh the spirit burner containing 1.0 g of ethanol using an electronic balance.

4. Measure the initial temperature of the water using a 0.2oC division thermometer.

5. Light the wick of the spirit burner. 6. Monitor the temperature of water using the thermometer with constant stirring

using the stirrer. 7. Extinguish the flame with a cap when the rise of temperature of the water

reaches about 8.8oC.

8. Measure the final temperature of the water.

3

9. Weigh the spirit burner with the remaining ethanol again.

10. Repeat the experiment until the % difference of T/m < 5%. Tabulation of results

Initial temperature of water / oC T1

Final temperature of water / oC T2

T / oC T2 – T1 = T

Initial mass of spirit burner with ethanol / g x

Final mass of spirit burner with remaining ethanol / g y

Mass of ethanol burnt, m / g x – y = z

T/m

Calculation

Heat gained by water and calorimeter = C × T = (1520)T J = (1.52)T kJ

Amount of ethanol =

mol

Hc (ethanol) = – (1.52)T/ (

) kJ mol–1

(d) Identify one potential safety hazard in this experiment and state how you would

minimise this risk. [1]

Methanol is toxic. Wear glove. Methanol is toxic and volatile. Conduct the experiment in a fume hood. Methanol and ethanol are highly flammable. Alcohols are placed away from flame when not in used in experiment.

[Total: 12]

2 The simplest chemical reactions are those that occur in the gas phase in a single step,

such as the transfer of a chlorine atom from ClNO2 to NO.


(a) (i) An equimolar mixture of ClNO2(g) and NO(g), at total initial pressure of 3 atm,

is allowed to react in a closed vessel at 1000 K. When equilibrium is attained at the 5th minute, the partial pressure of ClNO2 is found to be 0.57 atm. Calculate the value for the equilibrium constant, Kp , of this system.


Initial/ atm 1.5 1.5 0 0

Change/ atm 0.93 0.93 +0.93 +0.93

Eqm/ atm 0.57 0.57 0.93 0.93

( ) ( )

( ) ( )

( )

( )

Kp = 2.67 (no units)

(ii) At the 10th minute, more ClNO2 gas was pumped into the vessel at 1000 K,

increasing the partial pressure of ClNO2 to 1 atm. Suggest how the position of the equilibrium would change.

By LCP, the position of the equilibrium will shift forward to consume some of the extra ClNO2 in the reaction to partially decrease its partial pressure.

(iii) Hence illustrate clearly, in a pressuretime graph below, the changes in the

partial pressures of ClNO2 and NO2 when (I) the above gaseous system first reaches equilibrium at the 5th minute, (II) more ClNO2 gas was added into the vessel at the 10th minute and a

new equilibrium is attained at the 15th minute.

Time

Pressure

0

ClNO

1.5

0.57

1.0

5 10 15

0.93

NO2

5

(iv) Suggest whether the addition of an inert gas into the vessel would affect the

position of the equilibrium.

The position of the equilibrium is not affected by the addition of the inert gas as the partial pressure of all gaseous reactants and products remained unchanged

[7] (b) ClNO2 can behave as an ideal gas under certain experimental conditions.

(i) State the two assumptions of kinetic theory of ideal gas.

There are no/ negligible intermolecular forces of attraction or repulsion

between the gaseous particles. A gas is composed of tiny gaseous particles that have a negligible volume compared to the volume of the gas container.

(ii) Predict whether ClNO2 behaves ideally under high pressure.

Under high pressure, the gaseous particles are very close together in a

small volume.

They experience significant intermolecular forces of repulsions/ attractions between them. OR

The volume of the gas particles is no longer negligible compared to the volume of the container.

So it is not behaving ideally under high pressure. [4] (c) (i) Draw the dot-and-cross diagrams of NO and ClNO. Hence, suggest the shape

and bond angle of ClNO.

Shape : bent Bond angle : Stating a value that is within the range –

110 x < 120

(ii) Hence, suggest why the formation of ClNO from NO is favoured.

NO contains a single unpaired electron. Formation of N-Cl bond allows N to

achieve noble gas/ octet configuration drives the formation of ClNO. [5] [Total: 16]

N O × × ×

×× × Cl O N

××

××

××

× ×

××

××

3 The four most abundant salts in sea water are given in the table below.

salt kg m3

Sodium chloride 27.5

Magnesium chloride 6.75

Magnesium sulfate 5.63

Calcium sulfate 1.80

The following flow scheme shows the series of reactions that involve seawater.

(a) Seawater contains chloride and sulfate ions. To verify the presence of chloride and sulfate ions instead of bromide and sulfite ions in sea water, the following chemicals can be used:

HNO3(aq), AgNO3(aq), BaCl2(aq), NH3(aq)

(i) Explain the importance of adding HNO3(aq) before other chemicals during this

verification process.

It is to react the sulfite ion away. This is so that when BaCl2 is added, the white precipitate is due to BaSO4 instead of BaSO3.

(ii) Suggest a sequence of chemical tests that can be used to verify the presence

of chloride and sulfate ions in sea water after acidification.

Step 1: divide acidified sea water into two portions Step 2: add BaCl2 to one portion, precipitation verifies presence sulfate ion. Step 3: add AgNO3 to the second portion. Step 4: Add NH3(aq) to mixture obtained in step 3 until in excess. If white precipitate

obtained is soluble in excess NH3(aq), presence of chloride ions are verified.

Flowchart is also acceptable. [3]

(b) (i) Calculate the concentration of magnesium and calcium ions in 1 dm3 of

seawater. [MgCl2] = 67.5 kg in 1 m3

= 67500g in 1000 dm3

= 67.5 g in 1 dm3 [CaSO4] = 1.8 g in 1 dm3

[MgSO4] = 5.625 g in 1 dm3

Seawater Step 1 controlled addition of

CO32(aq); filter

filtrate Step 2 addition of

OH(aq)

M(OH)2

Step 3 heat

MO Step 4

MCl2 (aq) Step 5

M(s)

7

)0.641.321.40(

8.1][ 2

Ca = 0.0132 mol dm3

)0.713.24(

75.6][

2

2

MgClMg = 0.07029 mol dm3

)0.641.323.24(

63.5][

4

2

MgSOMg = 0.046761 mol dm3

[Mg2+]total = 0.07029 + 0.046761 = 0.117 mol dm3 (3sf)

Answers in kg dm3 or g dm3 are also accepted.

(ii) Write an expression for the solubility product of magnesium carbonate.

Ksp = [Mg2+][CO32]

(iii) Determine the range of concentrations of CO3

2(aq) required to cause selective precipitation of only one cation in seawater given the following data.

compound solubility product

Magnesium carbonate 1.0 × 105

Calcium carbonate 5.0 × 109

CaCO3 will precipitate first.

In order for CaCO3 to ppt, ionic product [Ca2+][CO32]> Ksp.

min [CO32] =

013216.0

100.5 9=3.79 × 107 mol dm3

To prevent precipitation of MgCO3, ionic product [Mg2+][CO32] < Ksp

max [CO32] =

01175.0

100.1 5=8.51 × 104 mol dm3

(iv) Hence deduce the identity of M in the above flow scheme. Mg [6] (c) (i) Write an equation with state symbols for Step 3 using M to represent your

cation. Mg(OH)2 (s) MO (s) + H2O (g) (ii) Suggest and explain why magnesium hydroxide has a lower thermal stability

than calcium hydroxide.

Mg(II) ion has higher charge over size ratio than Ca(II) ion, hence it polarises (distorts) the electron cloud of hydroxide ion to greater extent, covalent bond in hydroxide ion is weakened to greater extent therefore lower energy needed to cause decomposition. Thus it has lower thermal stability.

[3]

(d) (i) If the mass of MO isolated in Step 3 is 0.05 g, calculate the volume of

0.1 mol dm3 of HCl required to convert all MO to MCl2.

molMO

310241.1)0.163.24(

05.0

MO + 2HCl MCl2 + H2O

molHCl

33 1048.210241.12

Vol of HCl = 3

3

8.2410001.0

1048.2cm

(ii) Suggest how Step 5 can be carried out.

Evaporate MgCl2 solution to dryness, to obtain MgCl2 salt. Carry out electrolysis of MgCl2 at molten state.

[4]

[Total: 16]

4 Methanol is the simplest alcohol, and is a volatile, colourless and flammable liquid.

Methanol is mainly used as an antifreeze, solvent or fuel.

(a) Define the standard enthalpy change of combustion of methanol in words.

Standard enthalpy change of combustion of methanol is the heat evolved when 1

mole of methanol is burnt in excess oxygen under standard conditions (298K, 1 atm).

[1] (b) (i) Using appropriate data from the Data Booklet, calculate the ∆Hr of the following

reaction.

2CH3OH(l) + 3O2(g) 2CO2 (g) + 4H2O(l) ∆Hr

∆Hr = 2[3BE(C-H) + BE(C-O) + BE(O-H)] + 3BE(O=O) – 4BE(C=O) – 8BE(O–H) = 2 [3(410) + 360 + 460)] + 3(496) – 4(740) – 8(460) = –1052 kJ mol–1

(ii) Hence determine the standard enthalpy change of combustion of methanol.

∆Hc = ½ × (–1052) = –526 kJ mol–1

[3]

9

(c) The theoretical standard enthalpy change of combustion of methanol is −715 kJ mol–1. Give two reasons to explain the discrepancy between this value and your answer in (b)(ii).

The bond energy values from the Data Booklet are average values of a particular bond.

Methanol and water are in liquid states. Bond energy values are used for gaseous molecules.

[2] (d) Other than direct burning of methanol to generate energy, methanol can also be

used in a fuel cell. Methanol fuel cells are more practical than hydrogen fuel cells. Carbon dioxide and water are the products of the methanol fuel cell.

(i) For each electrode X and Y, name the type of electrode and state its polarity.

Electrode X Type of electrode: anode Polarity: negative

Electrode Y Type of electrode: cathode Polarity: positive

(ii) On the diagram above, indicate the direction of electron flow in the wire with an arrow.

Arrow is drawn from anode to cathode

(iii) Write the half-equation of the reaction taking place at each electrode. X: CH3OH + H2O CO2 + 6H+ + 6e

Y: O2 + 4H+ + 4e 2H2O (iv) Explain why methanol fuel cells are more practical than hydrogen fuel cells.

Methanol is in liquid state and it is more compact to be transported/

Methanol gives out more energy when burnt compared to hydrogen as it released more electron per mole or methanol.

(v) Use appropriate data from the Data Booklet to explain why an acidic electrolyte

is often preferred to an alkaline or neutral electrolyte.

In neutral or alkaline medium, O2 + 2H2O + 4e 4OH– Eº = +0.40 V In acidic medium, O2 + 4H+ + 4e 2H2O Eº = + 1.23 V

Since the Eº value in acidic medium is more positive, O2 is more easily reduced in acidic medium/ reduction is more feasible.

[8] (e) In a laboratory, methanol can also be oxidised to other organic compounds using

suitable chemicals.

Give the appropriate reagents and conditions required for the following conversion.

I: Methanol Methanal Reagents and conditions: K2Cr2O7, dil. H2SO4, distill

II: Methanol Methanoic acid

Reagents and conditions: K2Cr2O7, dil. H2SO4, reflux

III: Methanol Carbon dioxide Reagents and conditions: KMnO4, dil. H2SO4, reflux or Burn in excess oxygen

[3]

[Total:17]

5 Lactic acid is a carboxylic acid with the formula CH3CH(OH)CO2H. (a) State the type of stereoisomerism exhibited by lactic acid. Draw appropriate

structures to illustrate your answer. Optical isomerism

H

CH3

HO2COH

H

CH3

CO2HOH

[2]

11

(b) In solution, lactic acid can lose a proton from the carboxyl group, producing the

lactate ion CH3CH(OH)CO2−. Compared to ethanoic acid, lactic acid is more acidic

as it deprotonates ten times more easily than ethanoic acid.

Other than the electron withdrawing effect of –OH group, suggest another factor which explains the additional stability of the lactate ion. Illustrate your answer with a diagram.

Intramolecular hydrogen bond is possible in lactate ion between the carboxylate ion and the protonic H in –OH.

H

CH3

O

HO

-

O

H bonding

..

..

..

. .

[2] (c) A student tries to synthesise lactic acid from chloroethane using a series of reactions

as shown.

(i) Fill in the intermediate organic compounds A and B in the following flow scheme.

CH3CH2Cl

Step 1 Step 2

HC

CH3

O

Step 3

HCN, trace NaCN

Step 4

H

C

CH3

OH

CO2H

compound A

compound B

CH3CH2OH

H

C

CH3

OHCN

(ii) State the reagents and conditions for Step 1. Reagents and conditions: NaOH(aq), heat

(iii) State the type of reaction for Step 4 and write a balanced equation for this step.

Type of reaction: Hydrolysis

Balanced equation:

H

C

CH3

OHCN

+ 2H2O H+

+H

C

CH3

OHCO2H

+ NH4

+

(iv) Lactic acid formed in the above flow scheme is a racemic mixture while lactic

acid formed via bacteria fermentation contains only one optical isomer. Explain.

Carbonyl carbon is trigonal planar and the CN– nucleophile can attack the carbonyl carbon from top and bottom of the plane with equal propability, forming equal amount of each optical isomers, resulting in a racemic mixture. In bacteria fermentation, the attack can only take place from one side since enzyme has specific reaction site, thus only 1 optical isomer is formed.

[7]

[Total: 11]

1

[Turn Over

NATIONAL JUNIOR COLLEGE PRELIMINARY EXAMINATIONS Higher 2

CANDIDATE NAME

SUBJECT CLASS

REGISTRATION NUMBER

CHEMISTRY Paper 3 Free response

9647/03 Wed 17 Sep 2014

2 hours

READ THESE INSTRUCTIONS FIRST Answer any four questions. Start your answer to each question on a fresh piece of paper. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely behind the cover page.

This paper consists of 11 printed pages and 1 cover page.

2

[Turn Over

1 Iodine monochloride, ICl, is an interhalogen compound that is formed when chlorine gas is

passed through iodine crystals. Generally, interhalogen compounds have chemical properties similar to that of halogens.

(a) (i) Predict the physical state of ICl at room temperature.

(ii) By considering the partial charges in ICl, explain how water reacts with ICl.

(iii) Hence construct an equation for the reaction of ICl with water.

[3] (b) Methylbenzene reacts with ICl in the presence of FeCl3 to form compound A.

(i) Draw the structure of A.

(ii) Describe the mechanism of this reaction, clearly indicating the role of FeCl3.

[4] (c) FeCl3 and FeF3 are both formed from a metal and a non-metal. However, in molten

state, only FeF3 is able to conduct electricity and FeCl3 does not.

(i) Suggest the structures of FeCl3 and FeF3 and explain why iron forms two different types of compounds when reacted with the different halogens – fluorine and chlorine.

(ii) Using the information given below as well as relevant data from the Data Booklet, construct an energy level diagram for the formation of FeF3 from its elements to determine its lattice energy.

enthalpy change of atomisation of Fe +340 kJ mol–1

first electron affinity of F –328 kJ mol–1

standard enthalpy change of formation of FeF3 –989 kJ mol–1

[7]

Suggest an equation for the

reaction of ICl with water

3

[Turn Over

(d) Iron is a transition metal, a d-block element that varies from s-block metals in terms of physical properties. The table below gives data about some physical properties of the elements – iron and calcium.

property calcium iron

relative atomic mass 40.1 55.8

atomic radius/ nm 0.197 0.126

density/ g cm–3 1.54 7.86

electrical conductivity Good Good

(i)

Write down the full electronic configurations of calcium and iron.

(ii) Explain why the atomic radius of iron is smaller than that of calcium.

(iii) Suggest why iron has a higher density than calcium.

(iv) Explain which of the 2 metals would be expected to have a higher electrical conductivity.

[6]

[Total:20]

4

[Turn Over

2 (a) In the qualitative analysis of organic compounds, the presence of a carbonyl

functional group can be confirmed by using 2,4dinitrophenylhydrazine.

When 2 cm3 of 2,4dinitrophenylhydrazine solution is added to separate test tubes containing 2 cm3 propanone and 2 cm3 propanal respectively, a bright orange precipitate is observed in both test tubes.

Write a balanced equation for the reaction of 2,4dinitrophenylhydrazine with propanal. Hence, identify the type of reaction.

[2] (b) Fehling's solution, an alkaline copper(II) tartrate complex, can be used to distinguish

between propanal and propanone. A positive test would show the formation of a brick red solid, Cu2O.

(i) Identify which of the two organic compounds above would give a positive test with the Fehling’s solution. Give the structure of the organic product for this reaction.

The brick red solid, Cu2O, is isolated through filtration. It is soluble in excess ammonia to give a colourless solution B. When left exposed to air, the colourless solution B turns into a deep blue solution, containing the complex ion [Cu(NH3)4]

2+.

(ii) Explain why the solution containing the complex ion [Cu(NH3)4]2+ is deep blue

in colour.

(iii) Identify the oxidation state of copper in the copper containing species in the colourless solution B, and write its electronic configuration. Hence explain why solution B is colourless.

[6] (c) [Cu(NH3)4]

2+ can also be prepared by adding excess aqueous NH3 to aqueous copper(II) sulfate. Three reactions involving aqueous copper(II) sulfate are illustrated below.

I (aq)

Excess

conc HCl

NH3 (aq) Excess

NH3 (aq)

Pale blue ppt

C

Yellow solution D

[Cu(NH3)4]2+

CuSO4 (aq)

White ppt E in brown solution



5

[Turn Over

(c) (i) Explain fully, with the aid of equations, how the pale blue precipitate C is formed when NH3 (aq) is added to CuSO4 (aq).

(ii) Write an equation for the formation of the yellow solution D. Hence, explain what will be observed when water is added to solution D.

When hexane is added to the brown solution containing the white precipitate E and shaken, a purple hexane layer is formed above a colourless aqueous layer containing the white precipitate E.

(iii) By referring to the types of intermolecular interactions, explain why the aqueous solution turns from brown to colourless upon mixing of hexane.

(iv) Construct an ionic equation for the formation of the white precipitate E when

I–(aq) is added to CuSO4 (aq).

[6] (d) Ethane−1,2−diamine, NH2CH2CH2NH2 (abbreviated as en when it acts as a ligand),

can undergo ligand exchange reaction with [Cu(NH3)4]2+ to form [Cu(en)2]

2+.

The standard entropy change for this reaction has a value of 88 J mol1 K1.

(i) Write an equation for this reaction and hence suggest the sign of the entropy change.

(ii) Draw the structure of the complex ion [Cu(en)2]2+ and identify its shape.

[4] (e) Ethane–1,2–diamine, NH2CH2CH2NH2, can be synthesised by reacting

1,2dichloroethane with ammonia under suitable conditions. ClCH2CH2Cl NH2CH2CH2NH2

(i) State the type of reaction occurring in the above synthesis.

(ii) A student attempted to carry out the above experiment. One of the side

products obtained has the molecular formula C4H10N2. Suggest the structure of this side product.

[2]

[Total:20]

NH3

6

[Turn Over

3 Dental erosion is the irreversible loss of tooth structure due to chemical dissolution by acids. Our dental enamel is composed primarily of hydroxyapatite, Ca10(PO4)6(OH)2.

Ca10(PO4)6(OH)2 (s) + aq 10Ca2+ (aq) + 6PO43 (aq) + 2OH (aq)

When a tooth is placed in distilled water of pH 7, a small amount of it will slowly dissolve. In contrast, our saliva fluid contains calcium, phosphate and hydroxide ions that prevent the dissolution of our dental enamel. Acidic foods and drinks, such as Coca Cola which contains phosphoric acid, may damage our teeth when it is frequently consumed. The solubility of our enamel increases about 10 – fold for each unit decrease in pH.

(a) (i) Phosphoric(V) acid, H3PO4, is a triprotic acid. The magnitude of the second

acid dissociation, Ka2, is much smaller than the first acid dissociation Ka1. Suggest an explanation for this observation using relevant equations.

(ii) Identify two reasons for the increased solubility of enamel at low pH under

constant temperature. Explain your answer.

[5] (b) One other important function of our saliva is its buffering ability. There are three

possible buffer systems in saliva:

the protein buffer,

the carbonic acid/ hydrogencarbonate buffer,

the phosphate buffer. The carbonic acid/ hydrogencarbonate buffer maintains the oral cavity at about pH 6.3 to reduce the risk of dental erosion.

(i) Write an equation to explain how the carbonic acid/ hydrogencarbonate buffer

prevents dental erosion when small amounts of acid are produced from the digestion of sugars.

(ii) A solution of 0.5 mol dm3 carbonic acid, H2CO3, has a pH of 3.34.

(I) Calculate the acid dissociation constant, Ka, of carbonic acid.

(II) Hence, determine the ratio of [HCO3]/[H2CO3] present in the mouth to

maintain the pH of the oral cavity at 6.3.

(iii) A student decides to create a phosphate buffer to study its effectiveness in maintaining the pH of the oral cavity in the mouth.

Given that Ka of KH2PO4(aq) is 6.23 × 108 mol dm3, what volume of

0.1 mol dm3 NaOH(aq) must be added to 100 cm3 of

0.1 mol dm3 KH2PO4 (aq) to prepare a buffer solution of pH 6.3?

[7]



7

[Turn Over

(c) Phosphoric(III) acid, H3PO3, on heating at 200 C converts to phosphoric(V) acid, H3PO4, and phosphine, PH3.

(i) Explain, with reference to structure and bonding, the relative boiling points of PCl3, PH3 and H3PO3.

(ii) Describe, with the aid of an equation, how H3PO3 can be prepared from PCl3.

(iii) When reacted with sulfur at 180 C, PCl3 produces a compound F with Mr = 169.6. F reacts with water readily to form an acidic solution and a colourless gas with a rotten egg smell.

(I) Suggest a formula for F, and describe its shape.

(II) Hence, by considering the partial charges in F, write an overall

equation for the reaction of F with water.

[8]

[Total:20]

8

[Turn Over

4 4-ethylphenol, G, and its chlorinated products have useful antiseptic properties.

CH3C

H

H

OH

A

Depending on the conditions of the reaction, G can react with chlorine in two different ways, giving the two isomers J and K. In these reactions chlorine reacts in a similar manner to bromine.

IIA

I

Cl2(aq)C8H8Cl2O C8H8Cl2O

B C

(a) (i) Suggest a structural formula for J.

(ii) Explain whether you would expect J to be more or less acidic than phenol.

(iii) Give the reagent and conditions for the formation of K.

[4] (b) Under suitable conditions, 4-ethylphenol can be converted to compound L.

C

O

Cl C

H

H L

(i) J, K and L are halogen–containing compounds which can react with a nucleophile under suitable conditions. Explain why the reactivity of J, K and L towards nucleophilic substitution is as follows:

L > K > J

(ii) Hence, describe suitable chemical tests to distinguish J, K and L.

[5]





G

K J

G

9

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(c) Compound L can easily form other acid derivatives when reacted with suitable reagents such as phenol and methylamine.

(i) Phenol reacts with L to form an ester. Suggest and explain the condition required to carry out such a conversion.

(ii) Suggest the structural formula of the product formed when methylamine is

reacted with

C

O

C

H

H

HO

instead of L.

[2] (d) (i) Compound M, CxHyO2, has a relative molecular mass of 152. Given that 3.04 g

of M gives 4.32 dm3 of CO2 and 2.16 g of H2O at room temperature and pressure, determine the values of x and y.

(ii) The following are a series of reactions carried out to elucidate the structure of M.

1. M reacts with acidified potassium dichromate(VI) to form N, C9H8O3.

2. M does not decolorise aqueous bromine.

3. When M reacts with acidified potassium manganate(VII), it forms

benzene–1,2– dicarboxylic acid.

4. Both M and N react with alkaline iodine to give a

yellow precipitate.

5. The other organic product obtained from M in reaction 4 is acidified to

form compound P, C8H8O3.

6. Compound Q, C8H6O2, is formed when P is reacted with concentrated

H2SO4.

Deduce, with reasoning, the structures of M, N, P and Q.

[9] [Total:20]

10

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5 Enzymes are large protein molecules that adopt highly specific three-dimensional structures.

(a) (i) Briefly describe how an enzyme maintains its three-dimensional shape.

The diagram below shows the structure, at pH 7, of a fragment from polyphenoloxidase enzyme which contains three amino acid residues, R, S, and T.

HNCHCONHCHCONHCHCO

CH2 CH2SH OH

A B C

(ii) Draw a diagram to show how the side chain on amino acid S could be involved

in maintaining the tertiary structure of polyphenoloxidase. State the type of reaction that occurs.

(iii) Cut fruits such as apples turn brown on exposure to air due to a chemical reaction caused by polyphenoloxidase. To prevent the browning of cut apple, lemon juice is commonly added to the fruits. Explain how lemon juice helps to prevent the browning of cut apple.

(iv) Draw the structures of the amino acids formed when polyphenoloxidase is heated under reflux with 3 mol dm–3 potassium hydroxide, and state the type of reaction occurring.

[10] (b) Enzymes, trysin and chymotrypsin, are secreted by the pancreas to aid the digestion

of proteins in the small intestine. When trypsin was added to polypeptide U, the following fragments were obtained.

phe gly-lys-phe ser-lys-ala-phe

When chymotrypsin was added to polypeptide U, the following fragments were obtained.

ala-phe-gly-lys phe-ser-lys phe

Using the above results, deduce the shortest possible amino acid sequence of polypeptide U. Show your reasoning.

[2]



R S T

11

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(c) Histidine is an essential amino acid in human. It can be catalysed by an enzyme to produce histamine that is responsible for allergic body reactions. The following shows the structure of histidine.

CO2H

NH2N

HN

histidine

(i) By considering the number of electron domains about the nitrogen atoms, state the type of hybridisation of the nitrogen atoms, aN and bN, in histidine.

(ii) In histidine, aN is less basic than bN and *N is found to be inert to chemical reaction. There are three pKa values associated with histidine: 1.8, 6.1, 9.2. Make use of these pKa values to suggest the major species present in solutions of histidine with the following pH values.

pH 1

pH 3

pH 7

[5]

(d) The hydrolysis of sucrose can be catalysed by the enzyme sucrase, or by dilute acids. The initial reaction of the hydrolysis can be represented by:

C12H22O11 + H2O C6H12O6 + C6H12O6 sucrose glucose fructose The following results were obtained using hydrochloric acid as the catalyst.

experiment [sucrose] / mol dm−3

[HCl] / mol dm−3

initial rate / mol dm−3s−1

1 0.05 0.05 0.100

2 0.06 0.06 0.144

3 0.08 0.06 0.192

Use these data to deduce the order of reaction with respect to HCl and sucrose. Hence, write the rate equation for the reaction.

[3]

[Total:20]

b a

*





1

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NATIONAL JUNIOR COLLEGE PRELIMINARY EXAMINATIONS Higher 2

CANDIDATE NAME

SUBJECT CLASS

REGISTRATION NUMBER

CHEMISTRY Paper 3 Free response

9647/03 Wed 17 Sep 2014

2 hours

READ THESE INSTRUCTIONS FIRST Answer any four questions. Start your answer to each question on a fresh piece of paper. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely behind the cover page.

This paper consists of 11 printed pages and 1 cover page.

2

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1 Iodine monochloride, ICl, is an interhalogen compound that is formed when chlorine gas is

passed through iodine crystals. Generally, interhalogen compounds have chemical properties similar to that of halogens.

(a) (i) Predict the physical state of ICl at room temperature.

Liquid / solid. Mr of ICl : 162.5 Mr of Br2 : 159.8 Strength of td-id expected to be similar to that of Br2, hence liquid state. Td-id stronger than Cl2 but weaker than I2.

(ii) By considering the partial charges in ICl, explain how water reacts with ICl.

The lone pair of electrons on the oxygen of H2O attacks the + I, breaking the

I-Cl bond, forming HOI. The H+ then combines with the Cl¯.

(iii) Hence construct an equation for the reaction of ICl with water.

ICl + H2O HOI + HCl [3] (b) Methylbenzene reacts with ICl in the presence of FeCl3 to form compound A.

(i) Draw the structure of A.

2-iodo methylbenzene

(ii) Describe the mechanism of this reaction, clearly indicating the role of FeCl3. Electrophilic substitution reaction

FeCl3 + ICl [FeCl4]¯ + I+

H+ + [FeCl4]¯ FeCl3 + HCl

[4]

3

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(c) FeCl3 and FeF3 are both formed from a metal and a non-metal. However, in molten state, only FeF3 is able to conduct electricity and FeCl3 does not.

(i) Suggest the structures of FeCl3 and FeF3 and explain why iron forms two different types of compounds when reacted with the different halogens – fluorine and chlorine. FeCl3 : Simple covalent molecules FeF3 : Ionic lattice structure Fe3+ has a high charge density and is hence able to polarise the larger electron cloud of the chlorine atoms resulting in more extensive sharing of electrons hence forming a covalent bond. Fluoride ions due to their small size have low polarisability.

(ii) Using the information given below as well as relevant data from the Data Booklet, construct an energy level diagram for the formation of FeF3 from its elements to determine its lattice energy.

enthalpy change of atomisation of Fe +340 kJ mol–1

first electron affinity of F –328 kJ mol–1

standard enthalpy change of formation of FeF3 –989 kJ mol–1

+340 + 762 + 1560 + 2960 + 3/2 x 158 + 3 x -328 + LE = -989 LE = -5860 kJmol-1

[7]


Fe(s) + 3/2F2(g) +577

+5282 Fe(g) + 3F(g)

Fe3+ (g) + 3e + 3F(g)

Fe3+ (g) + 3F¯(g) -984

LE

0

Enthaply/ kJmol-1

-989

FeF3(s)

4

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(d) Iron is a transition metal, a d-block element that varies from s-block metals in terms of physical properties. The table below gives data about some physical properties of the elements – iron and calcium.

Property calcium Iron

relative atomic mass 40.1 55.8

atomic radius/ nm 0.197 0.126

density/ g cm–3 1.54 7.86

electrical conductivity Good Good

(i)

Write down the full electronic configurations of calcium and iron. Ca : 1s22s22p63s23p64s2 Fe : 1s22s22p63s23p63d64s2

(ii) Explain why the atomic radius of iron is smaller than that of calcium. Iron has more protons and hence has a higher nuclear charge than calcium. The additional d electrons in iron are poor shielding electrons and hence the shielding effect is negligible. Iron hence has a higher effective nuclear charge. The valence electrons in iron are held more tightly resulting in a smaller atomic radius.

(iii) Suggest why iron has a higher density than calcium. Iron has a larger atomic mass, but a smaller atomic radius that results in a smaller volume than calcium. Since density is mass/volume, iron has a higher density.

(iv) Explain which of the 2 metals would be expected to have a higher electrical conductivity. Both the 3d and 4s electrons in iron can be delocalised into the sea of electrons as they are of similar energy. Hence iron is expected to have a higher electrical conductivity due to the larger number of charge carriers.

[6]

[Total:20]

5

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2 (a) In the qualitative analysis of organic compounds, the presence of a carbonyl

functional group can be confirmed by using 2,4dinitrophenylhydrazine.

When 2 cm3 of 2,4dinitrophenylhydrazine solution is added to separate test tubes containing 2 cm3 propanone and 2 cm3 propanal respectively, a bright orange precipitate is observed in both test tubes.

Write a balanced equation for the reaction of 2,4dinitrophenylhydrazine with propanal. Hence, identify the type of reaction.

[2]

C O

H

CH3CH2

+ NN

H

H

H

N

NO2

O2

NN

H

C

N

NO2

H

CH3C

O2

H2

+ H2O

Type of reaction: Condensation reaction

(b) Fehling's solution, an alkaline copper(II) tartrate complex, can be used to distinguish

between propanal and propanone. A positive test would show the formation of a brick red solid, Cu2O.

(i) Identify which of the two organic compounds above would give a positive test with the Fehling’s solution. Give the structure of the organic product for this reaction. Propanal will react with Fehling’s solution.

Organic product is C O-

CH3CH2

O

The brick red solid, Cu2O, is isolated through filtration. It is soluble in excess

ammonia to give a colourless solution B. When left exposed to air, the colourless solution B turns into a deep blue solution, containing the complex ion [Cu(NH3)4]

2+.

(ii) Explain why the solution containing the complex ion [Cu(NH3)4]2+ is deep blue

in colour. In the presence of NH3 ligands, the d orbitals of Cu2+ split into two different

energy levels with an energy gap, E. he electron from the lower energy d orbital can be promoted to the higher energy d orbital by absorbing

wavelength of light with energy corresponding to the energy gap, E. Wavelength of light not absorbed will be reflected and seen as the complementary colour, deep blue.

6

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(iii) Identify the oxidation state of copper in the copper containing species in the colourless solution B, and write its electronic configuration. Hence explain why solution B is colourless.

Oxidation state of Cu in B = +1 Electronic configuration: [Ar] 3d10 In Cu+ complex, the d orbitals are fully filled. Hence, it is not possible to have

dd transition and all the wavelengths of visible light spectrum are reflected. The solution appears colourless.

(c) [Cu(NH3)4]

2+ can also be prepared by adding excess aqueous NH3 to aqueous copper(II) sulfate. Three reactions involving aqueous copper(II) sulfate are illustrated below.

(c) (i) Explain fully, with the aid of equations, how the pale blue precipitate C is formed when NH3 (aq) is added to CuSO4 (aq).

NH3 + H2O NH4

+ + OH

Cu2+ + 2 OH Cu(OH)2

(ii) Write an equation for the formation of the yellow solution D. Hence, explain

what will be observed when water is added to solution D.

[Cu(H2O)6]2+ + 4Cl [CuCl4]

2 + 6H2O

Blue Yellow By Le Chatelier’s Principle, when water is added to solution D, the above equilibrium will shift to the left, and the yellow solution will turn to blue.

I (aq)

Excess

conc HCl

NH3 (aq) Excess

NH3 (aq)

Pale blue ppt

C

Yellow solution D

[Cu(NH3)4]2+

CuSO4 (aq)

White ppt E in brown solution

7

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When hexane is added to the brown solution containing the white precipitate E and shaken, a purple hexane layer is formed above a colourless aqueous layer containing the white precipitate E.

(iii) By referring to the types of intermolecular interactions, explain why the aqueous solution turns from brown to colourless upon mixing of hexane.

Both I2 and hexane are non-polar and has intermolecular forces of temporary

dipoleinduced dipole attraction (td-id) between molecules of I2 and between

molecules of hexane. The energy released from forming the solvent-solute interaction (td-id) is sufficient to overcome the solvent-solvent and solute-solute interaction. I2 is more soluble in hexane than in H2O, and thus the hexane layer turns purple, while the aqueous layer turn colourless.

(iv) Construct an ionic equation for the formation of the white precipitate E when

I–(aq) is added to CuSO4 (aq).

2Cu2+ + 4 I 2CuI + I2

[6] (d) Ethane−1,2−diamine, NH2CH2CH2NH2 (abbreviated as en when it acts as a ligand),

can undergo ligand exchange reaction with [Cu(NH3)4]2+ to form [Cu(en)2]

2+.

The standard entropy change for this reaction has a value of 88 J mol1 K1.

(i) Write an equation for this reaction and hence suggest the sign of the entropy change. [Cu(NH3)4]

2+ + 2 en [Cu(en)2]2+ + 4 NH3

There are a greater number of particles in the products of the reaction. Hence the system becomes more disordered after the reaction, thus entropy increases. Entropy change is a positive value.

(ii) Draw the structure of the complex ion [Cu(en)2]2+ and identify its shape.

Shape is square planar or tetrahedral (Coordination number = 4)

[4]

8

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(e) Ethane–1,2–diamine, NH2CH2CH2NH2, can be synthesised by reacting

1,2dichloroethane with ammonia under suitable conditions. ClCH2CH2Cl NH2CH2CH2NH2

(i) State the type of reaction occurring in the above synthesis.

Nucleophilic substitution

(ii) A student attempted to carry out the above experiment. One of the side products obtained has the molecular formula C4H10N2. Suggest the structure of this side product.

Further substitution reaction between NH2CH2CH2NH2 and ClCH2CH2Cl

[2]

[Total:20]

NH3

9

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3 Dental erosion is the irreversible loss of tooth structure due to chemical dissolution by acids. Our dental enamel is composed primarily of hydroxyapatite, Ca10(PO4)6(OH)2.

Ca10(PO4)6(OH)2 (s) + aq 10Ca2+ (aq) + 6PO43 (aq) + 2OH (aq)

When a tooth is placed in distilled water of pH 7, a small amount of it will slowly dissolve. In contrast, our saliva fluid contains calcium, phosphate and hydroxide ions that prevent the dissolution of our dental enamel. Acidic foods and drinks, such as Coca Cola which contains phosphoric acid, may damage our teeth when it is frequently consumed. The solubility of our enamel increases about 10 – fold for each unit decrease in pH.

(a) (i) Phosphoric(V) acid, H3PO4, is a triprotic acid. The magnitude of the second

acid dissociation, Ka2, is much smaller than the first acid dissociation Ka1. Suggest an explanation for this observation using relevant equations.

H3PO4 + H2O H3O+ + H2PO4

1aK

H2PO4 + H2O H3O

+ + HPO42

2aK

Protons are more easily lost from neutral H3PO4 molecules than from H2PO4

anions. OR

The formation of the more stable conjugate base H2PO4, compared to HPO4

2, favoured the first acid dissociation.

(ii) Identify two reasons for the increased solubility of enamel at low pH under

constant temperature. Explain your answer.

Ca10(PO4)6(OH) (s) + aq 10Ca2+(aq) + 6PO43(aq) + 2OH(aq) --- eqm (1)

Ksp = [Ca2+]10[PO43]6[OH]2

At low pH (high [H+]),

H+ reacts with OH to form H2O.

H+ reacts with base PO43 to form HPO4

2.

The reduction of [OH] and [PO43] results in IP < Ksp significantly.

OR

The reduction of [OH] and [PO43] favours the forward direction of equilibrium

(1). Thus the enamel dissolves more at low pH.

[5]

10

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(b) One other important function of our saliva is its buffering ability. There are three possible buffer systems in saliva:

the protein buffer,

the carbonic acid/ hydrogencarbonate buffer,

the phosphate buffer. The carbonic acid/ hydrogencarbonate buffer maintains the oral cavity at about pH 6.3 to reduce the risk of dental erosion.

(i) Write an equation to explain how the carbonic acid/ hydrogencarbonate buffer

prevents dental erosion when small amounts of acid are produced from the digestion of sugars.

HCO3– + H+ H2CO3 ( CO2 + H2O)

(ii) A solution of 0.5 mol dm3 carbonic acid, H2CO3, has a pH of 3.34.

(I) Calculate the acid dissociation constant, Ka, of carbonic acid.

(II) Hence, determine the ratio of [HCO3]/[H2CO3] present in the mouth to

maintain the pH of the oral cavity at 6.3.

pH = 3.34 [H+] = 103.34 = 4.571 × 104 mol dm3

]CO[H

]][H[HCOK

32

3a

3.34-

-3.34-3.34

a100.5

))(10(10K

= 4.18 × 107 mol dm3

(II)

]CO[H

][HCOlgpKpH

32

3a

6.3 = 6.379 + ]CO[H

][HCOlg

32

3

]CO[H

][HCO

32

3

= 0.835 or

11

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(iii) A student decides to create a phosphate buffer to study its effectiveness in maintaining the pH of the oral cavity in the mouth.

Given that Ka of KH2PO4(aq) is 6.23 × 108 mol dm3, what volume of

0.1 mol dm3 NaOH(aq) must be added to 100 cm3 of

0.1 mol dm3 KH2PO4 (aq) to prepare a buffer solution of pH 6.3?

H2PO4 + OH HPO4

2 + H2O

pKa of KH2PO4(aq) = 7.206

Let the amount of NaOH to be added be x cm3.

Amount of HPO42 = Amount of NaOH = x mol

Amount of H2PO4 = (0.01– x)

]PO[H

][HPOlgpKpH

42

4a

2

6.3 = 7.206 + ]PO[H

][HPOlg

42

4

2

]PO[H

][HPO

42

4

2

= 0.124

In the buffer, final total volume is the same, thus:

42

4

POH of Amount

HPO of Amount2

= 0.124

1240010

.)x.(

x

x = 0.001106 mol

Volume = 01106010

0011060.

).(

. dm3 = 11.10cm3

[7]

12

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(c) Phosphoric(III) acid, H3PO3, on heating at 200 C converts to phosphoric(V) acid, H3PO4, and phosphine, PH3.

(i) Explain, with reference to structure and bonding, the relative boiling points of PCl3, PH3 and H3PO3. All three compounds are made up of simple covalent molecules.

The stronger H-bonding between H3PO3 molecules requires largest amount of energy to overcome them compared to the weak temporary dipole – induced dipole interactions in PCl3 and PH3. Thus H3PO3 has the highest boiling point.

PCl3 molecules, with greater ease of distortion of their larger electron cloud size, will have stronger temporary dipole – induced dipole interactions between each other compare to PH3 molecules. So PCl3 has a higher boiling point than PH3.

(ii) Describe, with the aid of an equation, how H3PO3 can be prepared from PCl3.

PCl3 + 3H2O 3HCl + H3PO3 PCl3 undergo hydrolysis to form acidic H3PO3.

(iii) When reacted with sulfur at 180 C, PCl3 produces a compound F with Mr = 169.6. F reacts with water readily to form an acidic solution and a colourless gas with a rotten egg smell.

(I) Suggest a formula for F, and describe its shape.

PSCl3

Shape: Tetrahedral

(II) Hence, by considering the partial charges in F, write an overall equation for the reaction of F with water. PSCl3 + 4H2O → H3PO4 + H2S + 3HCl

[8]

[Total:20]

13

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4 4-ethylphenol, G, and its chlorinated products have useful antiseptic properties.

CH3C

H

H

OH

A

Depending on the conditions of the reaction, G can react with chlorine in two different ways, giving the two isomers J and K. In these reactions chlorine reacts in a similar manner to bromine.

IIA

I

Cl2(aq)C8H8Cl2O C8H8Cl2O

B C

(a) (i) Suggest a structural formula for J.

2,6-dichloro-4-ethylphenol

CH3C

H

H

OH

Cl

Cl

(ii) Explain whether you would expect J to be more or less acidic than phenol. J is a stronger acid. Both conjugate base from J and phenol are resonance stabilised but J has two electronegative Cl (effect outweigh one electron releasing alkyl group) which helps to further disperse the charge of the conjugate base from J, making it more stable than phenoxide. Therefore J is a stronger acid.

(iii) Give the reagent and conditions for the formation of K.

Cl2(g), UV light or high temp

[4] (b) Under suitable conditions, 4-ethylphenol can be converted to compound L.

C

O

Cl C

H

H L

G

K J

G

14

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(i) J, K and L are halogen–containing compounds which can react with a nucleophile under suitable conditions. Explain why the reactivity of J, K and L towards nucleophilic substitution is as follows:

L > K > J When compounds J, K and L react with nucleophiles, C-Cl bond is substituted. L is most susceptible to nucleophilic attack because the carboxyl carbon is the most partial positive due to additional highly electronegative O attached. On the other hand, carbon in carboxyl carbon of J is the least partial positive as the continuous overlap of p orbital of Cl with pi orbital of benzene partially offset the partial positive charge of this carbon. This makes it most resistant towards nucleophilic reaction. OR C-Cl bond of J is the strongest as the continuous overlap of p orbital of Cl with pi orbital of benzene results in double bond character in C-Cl bond.This makes it most resistant towards nucleophilic reaction as it is more difficult to overcome the stronger C-Cl bond.

(ii) Hence, describe suitable chemical tests to distinguish J, K and L.

Upon addition of AgNO3(aq) to three separate samples of J, K and L, L gives white ppt while the J and K do not. When J and K are separately (i) heated with aq NaOH (ii) cool, excess HNO3 (iii) add AgNO3(aq) White ppt observed with C only.

OR

J K L

AgNO3 (aq) No precipitate No precipitate White precipitate

(i) heated with aq

NaOH (ii) cool,

excess HNO3 (iii) add

AgNO3(aq)

No precipitate White precipitate

[5]

15

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(c) Compound L can easily form other acid derivatives when reacted with suitable reagents such as phenol and methylamine.

(i) Phenol reacts with L to form an ester. Suggest and explain the condition required to carry out such a conversion. NaOH reacts with phenol to form phenoxide. This is to increase the electron density at oxygen making it a stronger nucleophile.

(ii) Suggest the structural formula of the product formed when methylamine is

reacted with

C

O

C

H

H

HO

instead of L.

CH3NH3+ C6H5CH2COO

(d) (i) Compound M, CxHyO2, has a relative molecular mass of 152. Given that 3.04 g

of M gives 4.32 dm3 of CO2 and 2.16 g of H2O at room temperature and pressure, determine the values of x and y.

molCO 18.024

32.42

molOH 12.00.18

16.22

molH 24.0

Simplest ratio of C:H = 3: 4

Mr of (C3H4)nO2 = 152

n = 3 x and y are 9 and 12 respectively.

(ii) The following are a series of reactions carried out to elucidate the structure of

M.

1. M reacts with acidified potassium dichromate(VI) to form N, C9H8O3.

2. M does not decolorise aqueous bromine.

3. When M reacts with acidified potassium manganate(VII), it forms

benzene–1,2– dicarboxylic acid.

4. Both M and N react with alkaline iodine to give a

yellow precipitate.

5. The other organic product obtained from M in reaction 4 is acidified to

form compound P, C8H8O3.

6. Compound Q, C8H6O2, is formed when P is reacted with concentrated

H2SO4.

Deduce, with reasoning, the structures of M, N, P and Q.

16

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E: C9H12O2 vs C9H20

Degree of unsaturation is 4: benzene likely to be present.

Reaction Explanation Deduction

1 oxidation

E:C9H12O2 --- F: C9H8O3

-4H + 1O

F likely to have a ketone and COOH

E has 1o and 2

o alcohol at side chain

Or E has alcohol groups that can be oxidised

2 Absence of activated benzene ring, absence of

C=C

3 Side chain oxidation Benzene has two substituent at 1,2 position to

each other.

4 F has methyl ketone while E has methyl

2oalcohol (-CH(OH)CH3)

Therefore there must be a -CH(OH)CH3 and -

CH2OH substituent at 1,2 position of benzene.

5 G

6 G to H involves elimination of

H2O

Intramolecular reaction between COOH and

alcohol.

C

OH

H

CH3

CH2OH

E:

F: C

O

CH3

COOH

G:

COOH

CH2OH

H: O

O

[9] [Total:20]

17

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5 Enzymes are large protein molecules that adopt highly specific three-dimensional

structures.

(a) (i) Briefly describe how an enzyme maintains its three-dimensional shape. The three-dimensional shape of enzyme is maintained by the R group interactions between the amino acids along the polypeptide chain, namely ionic bond, hydrogen bond, disulfide bridge and van der Waals interaction.

The diagram below shows the structure, at pH 7, of a fragment from polyphenoloxidase enzyme which contains three amino acid residues, R, S, and T.

HNCHCONHCHCONHCHCO

CH2 CH2SH OH

A B C

(ii) Draw a diagram to show how the side chain on amino acid S could be involved

in maintaining the tertiary structure of polyphenoloxidase. State the type of reaction that occurs. Type of reaction: Oxidation

(iii) Cut fruits such as apples turn brown on exposure to air due to a chemical reaction caused by polyphenoloxidase. To prevent the browning of cut apple, lemon juice is commonly added to the fruits. Explain how lemon juice helps to prevent the browning of cut apple. The low pH of lemon juice denatures polyphenoloxidase enzyme as H+ protonates COO– which disrupts the ionic bonds or H+ protonates NH2 which disrupts H bonds that stabilise the tertiary structure by of the enzyme causing it to lose its catalytic ability and thus preventing the browning of the cut apple.

R S T

CH2–S–S–CH2

Disulfide bridge

18

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(iv) Draw the structures of the amino acids formed when polyphenoloxidase is heated under reflux with 3 mol dm–3 potassium hydroxide, and state the type of reaction occurring. Type of reaction: Alkaline hydrolysis

[10] (b) Enzymes, trysin and chymotrypsin, are secreted by the pancreas to aid the digestion

of proteins in the small intestine. When trypsin was added to polypeptide U, the following fragments were obtained.

phe gly-lys-phe ser-lys-ala-phe

When chymotrypsin was added to polypeptide U, the following fragments were obtained.

ala-phe-gly-lys phe-ser-lys phe

Using the above results, deduce the shortest possible amino acid sequence of polypeptide U. Show your reasoning. Reasoning phe phe-ser-lys ser-lys-ala-phe ala-phe-gly-lys gly-lys-phe phe phe-ser-lys-ala-phe-gly-lys-phe

[2]

CH2

C

H

COOK+H2N

CH2

C

H

H2N COOK+

SH

C

H

H2N COOK+

OH

19

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(c) Histidine is an essential amino acid in human. It can be catalysed by an enzyme to produce histamine that is responsible for allergic body reactions. The following shows the structure of histidine.

CO2H

NH2N

HN

histidine

(i) By considering the number of electron domains about the nitrogen atoms, state the type of hybridisation of the nitrogen atoms, aN and bN, in histidine. aN: sp2 hydridisation bN: sp3 hydridisation

(ii) In histidine, aN is less basic than bN and *N is found to be inert to chemical reaction. There are three pKa values associated with histidine: 1.8, 6.1, 9.2. Make use of these pKa values to suggest the major species present in solutions of histidine with the following pH values.

pH 1

pH 3

pH 7

pH = 1 pH = 3

pH = 7

[5]

b a

*

CO2H

NH3+

N

HN

H+

CO2

NH3+

N

HN

H+

CO2

NH3+

N

HN

20

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(d) The hydrolysis of sucrose can be catalysed by the enzyme sucrase, or by dilute acids. The initial reaction of the hydrolysis can be represented by:

C12H22O11 + H2O C6H12O6 + C6H12O6 sucrose glucose fructose The following results were obtained using hydrochloric acid as the catalyst.

experiment [sucrose] / mol dm−3

[HCl] / mol dm−3

initial rate / mol dm−3s−1

1 0.05 0.05 0.100

2 0.06 0.06 0.144

3 0.08 0.06 0.192

Use these data to deduce the order of reaction with respect to HCl and sucrose. Hence, write the rate equation for the reaction. Using Expt 2 & 3, [sucrose]Expt 3 = 4/3 [sucrose]Expt 2, rateExpt 3 = 4/3 rateExpt 2 1st order of reaction w.r.t sucrose Using Expt 1 & 2,

[ ] [ ]

[ ] [ ]

(

) (

)

n = 1 1st order of reaction w.r.t HCl rate = k[sucrose][HCl]

[3]

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NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2score-in-chemistry.weebly.com/uploads/4/8/7/1/... · NJC Preliminary Examination 9647/01/14 [Turn Over NATIONAL JUNIOR