Chemistry Broad Questions

8/2/2019 Chemistry Broad Questions

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CHEMISTRYBROADQUESTION

ANSWERS

NAME: INTEKHAB HOSSAIN

CLASS: X


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January 2004 P2

8. a)Metals are extracted from their oresby reduction processes. These involveelectrolysis or reaction with carbon orcarbon monoxide.i) Explain what is meant by reduction interms of the loss or gain of electrons byan ion. Choose a suitable example andwrite an equation to illustrate youranswer.

Ans: Reduction is the gain of electronsExample: Fe 3+ + e - Fe2+

When Fe 3+ gain 1 electron it formsFe2+

ii) Explain why metals high in thereactivity series are usually extracted by

electrolysis.

Ans: Ions of reactive metals do not accept electrons easily.


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Their compounds are difficult to reduce

Electrolysis is a powerful form of

reduction which is betterthan coke reduction. Hence theyare usually extracted by

electrolysis.

iii) Magnesium is extracted frommagnesium chloride by electrolysis. Stateone essential condition required for thisprocess and explain why is it necessary.Write equations for the reactions thatoccur at the cathode and at the anode.

Ans: The essential condition required for the electrolysis of magnesiumchloride is MgCl2 must be molten.

In the solid state the ions are fixed in their position so they cannotconduct electricity. So to make theions mobile it must be molten.

Reaction at cathode: Mg 2+ + 2e -

MgReaction at anode: 2Cl - - 2e -

Cl2

iv) Tin is a metal that is just below ironin the reactivity series. Suggest howtin is likely to be extracted from its


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oxide, SnO2. Write an equation forthe reaction that occurs.

Ans: Tin can be extracted from SnO 2 by

reduction.The reduction can be carried out by

heating with coke/ CO2/ CO/ thermiteprocess ( aluminium)

SnO 2 + 2C Sn + 2Coor

SnO2 + C Sn + CO2

OrSnO2 + 2CO Sn + 2CO2

b) i) After extraction, copper requiresfurther refining. Describe how this iscarried out on an industrial scale. Writeequations for the reactions that occur but

a diagram is not required.

Ans: Further refining of copper is done by electrolysis.

Copper(ii) sulphate/ copper(ii) nitrate can be used as electrolyte.

Impure lump of copper is used as anode.

Pure copper strip is used as cathode.

Copper is transferred from anode to cathode.


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The impurities fall to the bottom of the cell.

Anode: Cu + Cu2+ + 2e

Cathode: Cu

2+

+ 2e-

Cu+

ii) Give two uses of copper.

Ans: Electrical wiring ( good conductor of electricity)

Water pipes ( good conductor of heat)

c) A metal such as sodium can beobtained from sodium chloride byusing electrolysis, but a non metalsuch as phosphorus cannot be

obtained from its chloride cannot beobtained from its chloride by thismethod. Explain why this is so.

Ans: Sodium chloride is an ionic compound

The ions (in the molten or in

aqueous state) can carry the currentPCl 3 or PCl5 is a covalent molecule,means consist of molecules.

There are no charged particles to conduct electricity.


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9. a) Explain each of the followingstatements.

i) Graphite is a soft material whereasdiamond is very hard, but they bothhave high melting points.

Ans: Graphite is having layer structures.

Weak forces (Van Der Walls force) exist between the layers.

Diamond is tetrahedral in structure.Both diamond and graphite are

macromolecular.Strong bonds exist between the

atoms of both diamond and graphite.Hence lot of energy is needed to

break the bonds.

ii) The molar enthalpy (heat) ofvaporization , H, for, the covalentcompound hydrogen chloride is 16 kJmol-1 whereas H for the ionic

compound lithium chloride is 151mol-1.

Ans: Hydrogen chloride consist of molecules


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Weak forces of attraction exist between them.

In lithium chloride oppositely

charged ions are arrangedHence strong forces of attraction exist between them.

May 1997 P1

7. Describe how you would obtain apure sample of:

a) Sodium chloride from a mixture ofsodium chloride and sand.

Ans: The mixture is taken in a beaker and water is added to dissolve the

mixture.NaCl becomes soluble and the sand is filtered off, it will beremaining in the filter paper asresidue.

The filtrate is then evaporated in an evaporating basin and placed onsteam bath until the point ofcrystallization.

It is then allowed to cool.The crystals are then transferred

to filter paper and left to dry.


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b) Barium sulphate from a mixture ofbarium sulphate and sodiumsulphate.

Ans: The mixture is taken into a beaker and water is added

Na 2CO3 becomes soluble and afterfiltering Ba2SO4 being insoluble arecollected as residue.

c) Nitrogen from a mixture of nitrogenand carbon dioxide.

Ans: The mixture is passed through a wash bottle containing KOH.

CO 2 being acidic will react with thealkali to form potassium carbonate.2KOH + CO2 K2CO3 + H2O

Nitrogen may be collected by the

displacement of water or by gassyringe.

d) Copper from a mixture of copper andzinc.

Ans: The mixture is taken in a beaker

and dilute HCl/ H2SO4 is added to itin excess, to dissolve zinccompletely.


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Zinc will react with dilute HCl to form ZnCl and hydrogen. Copper willbe unreacted.

The content is poured in the filter

paper which will be washed by hotwater several times.

e) Petrol from a mixture of petrol andwater.

Ans: The mixture is taken in a separating funnel.

Shake the mixture and leave it to stand for some time.

Petrol and water will be separated as two layers, petrol at the top.

By opening the tap, the water is removed.

Petrol will be left in the funnel.

May 2000 P1

7. a) Write a brief account of theformation of ethanol byfermentation. Your account shoulddescribe the reaction conditions and


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include a word equation. Details ofthe apparatus used are notrequired.

Ans: Glucose is fermented to ethanol byits catalytic decomposition by thehelp of enzymes secreted by yeastat room temperature in absence ofair.

ProcedureGlucose is dissolved in a little hot water and the solution is poured in alarge bottle containing cold water tohave a temperature around 300C.Brewers yeast is mixed with water to form a paste and then added tothe glucose solution.

Ammonium phosphate and potassium nitrate are also added toprovide food for the yeast cells.Yeast also feeds on sugar.Yeast supplies an enzyme called zymase, which catalyses thedecomposition of glucose to

ethanol.C 6H12O6 C2H5OH + 2CO2The mixture is then filtered to remove solid impurities and thesolution is concentrated by


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fractional distillation (temperatureis maintained from 70oC to 80oC)

c) Ethanol may be oxidized tocompound X. Ethanol reacts withcompound X to form an ester,compound Y.

i) Give the name and structural formulaof compound X.

Ans: Ethanoic Acid.

H OH C C O HH

ii) Give the reaction conditions requiredto make compound Y.

Ans: Concentrated sulphuric acidTemperature maintained below

boiling point.

iii) Draw the structural formula ofcompound Y.


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Ans: CH3COOH2CH3

H O H HH C C O C CH

H H H

d) Butanol is another member of thesame homologous series as ethanol.

It has the molecular formula C4H10O.i) Draw two possible structures for

butanol.

Ans:2-methylpropan-

1-ol

CH2CH3 CH CH2 OH

2-methylpropan-2-ol

CH3CH3 C CH3


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ii) State two ways in which the physicalproperties of butanol will differ fromthose of ethanol.

Ans: Butanol is having high boiling point as more bonds are present.

Butanol will be less volatile than ethanol.

e) Ethanol has a relative molecularmass of 46 and boils at 78oC. CarbonDioxide has a similar relativemolecular mass of 44 but boils at-78oC. Suggest reasons why theboiling points of these twosubstances are so different.

Ans: Carbon dioxide is a gas because

of weak intermolecular force ofattraction.Ethanol is a liquid due to more

intermolecular force of attraction,due to the presence of Hydrogenbonds.To overcome these attractive forces

the boiling must be higher inethanol.Molecular mass is no matter for the existence of its state.

Jan2007 P2


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6. a) Describe how you would preparecrystals of each salt using the

method indicated. Write an equationfor each reaction.i) Sodium nitrate, NaNO3, by reacting

an acid with an alkali.

Ans: Transfer 25 cm 3Sodiumhydroxide into a conical flask withthe help of pipette filler. Add a fewdrops of phenolphthalein indicator,and titrated with nitric acid from theburette.,till the end point.

By knowing the exact volume ofacid and base from the titration,NaOH and HNO3should be mixedwithout using the indicator

NaOH + HNO 3 NaNO3 + H2OTake the solution into an evaporating dish.Heat it onto a water bath to reduce the volume (till the point ofcrystallization )Leave it to cool until crystals are

formed.The crystals are washed with little cold distilled

water and leave it to dry. Pure crystalsof NaNO3 are formed.


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ii) Anhydrous iron(III) chloride, FeCl3, bydirect reaction of the elements.

Ans: A piece ofIron coil is taken and heat it strongly. Dry chlorine gas ispassed over the heated iron coil.

Iron(III) chloride sublimes and collected at the chamber, (It is adeliquescent substance.)2Fe + 3Cl 2 2FeCl3

iii) Copper(II) sulphate, CuSO4.5H2O, byreacting an acid with a metal oxide.

Ans: Some dilute sulphuric acid is taken in a beaker and warmed.

Add copper(II) oxide little by little until in excess.

The excess copper(II) oxide is removed by filtration.

The filtrate is taken in an evaporating dish and is placed on awater bath.

Heat it till the point of crystallization.Leave it in a cool place to form the crystals.


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The crystals are poured on a piece of filter paper and left it to dry.CuO + H 2SO4 CuSO4 + H2O

b) The mineral kieserite has the formulaMgSO4.H2O

i) Calculate the percentage by mass ofmagnesium in kieserite.

Ans: Mr of MgSO4.H2O =24+32+24+2+16

= 138% by mass of Magnesium = 24/138

100

ii) Describe one test to show thatkieserite contains sulphate ions.

Ans: Kieserite is dissolved in water.Add barium chloride solution

followed by hydrochloric acidA white precipitate of barium

sulphate is formed shows thepresence of sulphate ions.

Ba + SO4--BaSO4

7. a) Describe the reactions of sodiumand potassium with water, giving


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three similarities and onedifference. Write an equation forone of the reactions.

Explain, in terms of their electronicconfigurations, why the elementssodium and potassium have similarchemical properties. State the trendin reactivity on descending Group 1and suggest a reason for it.

Ans: Both floats on waterMelts to a silvery ballEventually dissolvesForms alkaline solution which can be tested by litmus

Potassium reacts violently and hydrogen produced ignites

immediately but sodium does not.2K + 2H 2O 2KOH + H2Both have 1 electron in their outer shell. Ex- Na 2,8,1Reactivity increases down the groupAtoms get bigger as we go down the group, so there is less attraction for

the outer electronDue to less attraction the electron in the outermost shell is lost quickly.


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Ans: 2Cl - - 2e - Cl2chlorine bleaches the litmus

ii) Write an equation for the reactionoccurring at the cathode and explainwhy the litmus blue. What otherobservation would you see at thecathode?

Ans: 2H + + 2e - H2H + discharged as H2 gas leaving theOH ion in waterThe OH ion turns the litmus blueBubbles of H 2 gas will be seen

iii) Name an important compound thatis produced industrially by theelectrolysis of brine and give one

large scale use for it.

Ans: Sodium HydroxideIt is used in the manufacture of

soap.

d) Describe a simple experiment toshow whether a white powder is asodium compound or a potassiumcompound.


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Ans: By performing a flame testSodium will produce a persistent

golden yellow flame

Potassium will produce lilac flame

8. Crude oil is a source of manycompounds that are of greatimportance in the chemical industry.Study the scheme below and answerthe questions that follow.

ProcessA

EtheneC

2H

4

Poly(ethene)

Process B

ReactionC

Oxidation

ReactionD

Crude NapthaC H -

Ethanol

C2H5 Compound E


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a) Give the name of Process A.Describe how the process of separating crude oil into fractions,

including naphtha, is carried out.

Ans: By fractional distillation crude oil can be fractionated to producenaphtha.The crude oil should be vaporized at

450oC- 500oCThe different compounds will be

separated according to their points.Low boiling point compounds are

collected at the top of the columnHigh boiling point compounds are collected at the bottom of thecolumn.

b) Give the name of Process B, in whichthe naphtha fraction is converted intomany different compounds, includingethene. Describe how this is done andexplain what happens to the originalmolecules present in the naphtha.

State the importance of this processin the petrochemical industry.


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Write an equation using C6H14 to showthe formation of ethene and one othercompound.

Ans: The name of Process B is cracking.

It is carried out at high temperature (700oC)

Long chain hydrocarbons break downA small alkane is formedAlkenes like ethene and propene are also formedUnsaturated molecules are used for manufacture of plastics, bypolymerization.

It also produces petrol

C 6H14 C2H4 + C4H10or

C6H14 2C2H4 + C2H6

c) Describe how ethene is convertedinto ethanol in Reaction C. Write anequation for the reaction, using

displayed formula.

Give one advantage and onedisadvantage that this method of


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producing ethanol has over thefermentation method.

Ans: Ethene is converted to ethanol

by reacting with steam, which iscalled hydrolysis.

Ethene and steam should be heated at 300oCA pressure of 70 atm is maintainedPhosphoric acid is used as catalystC 2H4 + H2O C2H6O

Advantage: 1. It is a continuous process

2. It produces pure product3. The reaction is passed

Disadvantage: 1. It is non renewable starting material

d) Name the type of polymerizationtaking place in Reaction D. Draw therepeating unit of the polymer.

Ans: Reaction D is addedpolymerization.

H H H HC=C C CH H H H n


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e) i) Compound E has the followingpercentage composition by mass: C=38.7% H= 9.7% O= 51.6%

i) Use these figures to calculatethe empirical formula of E.

Ans:C H O

Molarmass

12 1 16

Mole 3.225 9.7 3.225Molarratio

1 3 1

Emperical Formula: CH3O

ii) Given that 0.05 mol of E has amass of 3.1 gm, calculate therelative molecular mass of E

hence determine its molecularformula.

Ans: 0.05= 3.1/xX= 3.1/0.05

Mr= 62Molecula formula= Emperical

formula/nn= molecular mass/ empirical

formula mass= 62/31= 2


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C2H6O2

9. Use your knowledge of structure andbonding to explain each of thefollowing observations.a) Diamond is one of the hardestsubstances known whereas graphiteis soft and is used in pencils.

Ans: Diamond: In diamond each carbon atom has a tetrahedralstructure

All bonds are strong covalent bondsGraphite: It has layer structure

There is weak force of attraction between the layers.

The layers slide over each other

b) Sodium metal is food conductor of

electricity when solid and whenmolten. Sodium chloride does notconduct electricity when solid andwhen molten.


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Ans: Metals have delocalized electronsThe electrons flow throughout the metal which can conduct electricity.

Sodium chloride is an ionic

compound, in solid state the ionsare fixed in their position.In molten sodium chloride the ions are mobile, which can carryelectricity.

c) At room temperature, MgO is asolid with a high melting pointwhereas H2O is a liquid that vaporizes easily.

Ans: Mgo is an ionic compoundThe oppositely charged ions Mg 2+

and O2-

There is a strong electrostatic force of attraction between the oppositelycharged ions.H 2O is a covalent compoundIt consist of moleculesThere is weak intermolecular forcesMore heat is needed to separate the

ions than molecules Thus MgO has high melting point

than H2O


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d) Argon and nitrogen are bothunreactive at room temperature.Nitrogen will combine with other

elements at high temperature.

Ans: Argon has 8 electrons in its outermost shell as its electronconfiguration is 2,8,8As the outermost shell is full, hence it cannot gain, lose or share anyelectrons.Nitrogen has triple bond in N 2

moleculeTo break up the bond requires high energy

e) There is more than one compoundwith the molecular formula C2H4Cl2

Ans: 1,1- dichloroethane1,2- dichloroethane

January 2003 P2

7. a) Name the liquid raw material that

is a mixture of hydrocarbons.Describe how the mixture isseparated.


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Ans: The liquid raw material is crude oilThe mixture is separated by

fractional distillationCrude oil is heated at 500 oC until ofit vaporizesThe vapour passes up through the tower which is made up ofperforated shelvesAs the perforated shelves have bubble caps, when the bubble goesup it cools down.At different heights different fractions are collectedLower boiling point are collected at the top and higher boilingcompounds at the bottom.

b) Methane and chlorine react to formseveral possible products. Underwhat conditions does this reactiontake place? Suggest how the organicproducts of this reaction could beseparated and give the formulae oftwo of them.

Ans: Methane and chlorine should be placed in diffused sunlight


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Different products formed are chloromethane (CH3Cl), CH2Cl, CHCl,CCl4

The different fractions are

separated by fractional distillationas they have different boilingpoints.

c) Ethene also reacts with chlorine butin a different type of reaction. Inwhat way is this reaction differentfrom the reaction of methane andchlorine? Draw the structuralformula of the organic product ofthis reaction and name it.

Ans: When ethene reacts with chlorine, the reaction is addition

reaction, chlorination.The reaction takes place at room temperature and atmosphericpressure.H H HH

C C + Cl----Cl Cl----

C----C-----Cl H HH H

Methane reacts with chlorine only in the presence of diffused


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sunlight and the reaction issubstitution reaction.

d) The following table contains average

bond dissociation energies that youwill need to answer both parts ofthis question.

Bond Averagedissociation energy/

kJ mol-1

C---H 435C O 805C O 1077

H----O 464O O 498

i) Methane burns in a good supply ofair according to the followingequation.

CH4(g) + 2O2(g) CO2(g) +2H2O(g)By considering the bonds brokenand bonds formed, calculate theenthalpy change for this reaction.

Ans: (4354) + (2498) (2805)

+ (4464)2736 3466Enthalpy change= 2736 3466

= -730


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ii) In a poor supply of air the poisonousgas carbon monoxide

(C O) is formed.

CH4(g) + 3/2 O2(g) CO(g) +2H2O(g)

In terms of enthalpy changes, explainwhy the production of

carbon monoxide by a gas heater

reduces the efficiency. ( Youare

not required to calculate the enthalpychange for this reaction).

Ans: When CO is formed less heat energy is produced hence reduces

the efficiency of the gas heater.

8. Explain the difference between eachof the following. Give suitableexamples to support your answer.


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a) Addition polymerisation andcondensation polymerization

Ans: Additionpolymerization

Condensationpolymerization

The monomers are unsaturatedcompounds

The double bond provides the linkagebetween themonomers

Ex- Nylon

The monomers contain differentgroups

Loss of small molecules providesthe bond between themonomers (H2O orHCl)

Ex- Polythene

b) The bonding in a covalent compoundand in an ionic compound

Ans:Covalent Bonding Ionic Bonding


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atoms are bound by sharing electrons

A metal ion and a

non metal ion make acompoundEx- NaCl 2

the atoms are bound by attractionof opposite ions

Two non metal ions

make a compoundEx- Cl 2

c) An acidic oxide and a base oxide.

Ans:Acidic oxide Basic oxide

They form from non metals

When they dissolve in water they formacids

They dissolve in alkali to form salt

Ex- Sulphur dioxide + Water

Sulphurousacid

They form from metals

When they dissolve in water they formalkalis

They dissolve in acid to form salt

Ex- Na 2O + H2ONaOH

d) A saturated hydrocarbon and anunsaturated hydrocarbon.

Ans:Saturated

HydrocarbonUnsaturatedHydrocarbon


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Contains CC (single) bond

Reacts with

halogen in diffusedsunlight, thereaction is asubstitutionreaction.

Ex- H HC---CH H

Contains C C or C C(double or triple)

bondReacts with halogen in duffusedsunlight, thereaction is aaddition reaction.

Ex- H HC CH H

f)Isotopes and allotropes.

Ans:Isotope Allotrope

These are atoms of

same element havingsame atomic butdifferent massnumber.

They have different mass number as thenumber of neutrons

are differentEx- Cl 35, Cl37

They are the

different form of thesame element

They exist in the same state

Ex- Diamond and

graphite are differentallotropes of carbon


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January 2005 P2

7. Explain how you would use theexperimental results given below toidentify each of the unknownsubstances A, B, C, D, E and F. Writebalanced equations for the reactionsthat occur.

a) A is a white solid. When dilutehydrochloric acid is added to A, acolourless gas is formed which turnspotassium dichromate (VI) solutionfrom orange to green. A platinum

wire, when dipped in the solid andthen put in a Bunsen flame, producesa lilac colour.

Ans: The gas produced is sulphur dioxide.

Hence A is a sulphide ion (SO 2-3)

The lilac colour of the flame indicates the presence of potassiumionK 2SO3 + 2HCl KCl + SO2 + H2O


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b) B is a blue solution. When a piece ofmagnesium ribbon is put in thesolution, a brown deposit forms on

the magnesium and the blue colourdisappears. After filtering, addition ofdilute nitric acid followed by aqueoussilver nitrate produces a whiteprecipitate.

Ans: The blue solution indicates the presence of copper(ii) ion

The brown deposit forms and the blue colour disappears indicatingthe reaction is a displacementreaction

Mg(s) + Cu +(aq) Mg+(aq) + Cu(s)Addition of dilute nitric acid

followed by silver nitrate giveswhite precipitate of silver chlorideshows the presence of Cl-.Ag + Cl - AgClHence compound B is copper(ii) chloride.

c) C is a gaseous hydrocarbon whichdecolourises bromine water. When 1mole of C burns in excess oxygen, 3moles of carbon dioxide and 3 molesof water are formed.


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Ans: The bromine water decolourises, confirms the hydrocarbon is

unsaturated (alkene)3 moles of CO 2 is formed shows thepresence of 3 carbon atoms in thehydrocarbon3 moles of H 2O shows the presenceof 6 hydrogen atomsHence c is propene, C 3H6C 3H6 + 9/2 O2 3CO2 + 3H2O

d) D is a hard grey metal. Whenplaced in water, D sinks and acolorless gas is evolved whichexplodes if mixed with air and ignited.The cloudy suspension formed turnsred litmus blue.

Ans: D is calcium.When placed in water the gas evolved is hydrogen, which explodesif mixed with air and ignited2H + 2O 2 2H2OCa + H 2O Ca(OH)2 + H2

The cloudy suspension is Ca(OH)2which is alkaline and turnsred litmus blue.


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e) E is a colouless liquid with a molecularformula C2H6O. When it reacts withphosphorus pentachloride, PCl5,steamy fumes and a compound Fcontaining carbon, hydrogen andchlorine are formed.

Ans: E is ethanol, C 2H6OThe steamy fumes are HCl, signifies the presence of OH groupF is chloroethane.C 2H5OH + 3PCl5 C2H5Cl + POCl3 + HCl

8. a) i) Describe, giving essential

practice details, how you wouldperform a titration to find theconcentration, in mol dm3, of asolution of sulphuric acid. You areprovided with aqueous sodiumhydroxide of concentration 0.50 moldm3 and phenolphthalein indicator.

Ans: NaOH solution is taken in a biurette, and the initial reading isnoted.


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25 cm 3 of H2SO4 acid is transferredto a conical flask by the pipette byusing pipette filler

A few drops of phenolphthalein are

added in the conical flask, no colourchange is observedNaOH solution is added from the biurette, a little at a time andswirled the flask to complete thereactionNear the end point, NaOH should be added dropwiseAt the end point, the colourless solution will turn pink.The final biurette reading is noted.The difference of the two readings is the volume of the alkali needed tomeutralise the acid

The titration should be repeated several times for more accuracy.

ii) The equation for the reactionbetween sodium hydroxide andsulphuric acid isH2SO4 + 2NaOH Na2SO4 + 2H2OIn a titration, 25.0 cm3 of

H2SO4 required 26.5 cm3

of 0.50mol dm3 NaOH forneutralization. Calculate theconcentration of H2SO4 in moldm3.


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Ans: 25 SA/26.5 0.5= 1/2SA = 0.265 mol/dm3

iii) Sodium sulphate forms crystals offormula Na2SO4.xH2O. Describebriefly how a sample of the pure,dry crystals can be obtained ( usethe information given in (ii) above )

Ans: Mix 26.5 cm 3 of NaOH and 25 cm3

of H2SO4No indicator should be addedTransfer the solution in an evaporating dish and heat it on awater bathEvaporation should be continued till the point of crystallization

Leave the evaporating dish in a cool place to form the crystalsTransfer the crystals on a filter paper and wash it with cold waterThe crystals are transferred on a porous plate to get dry


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b) 6.44 g of Na2SO4.xH2O was heated toremove the water of crystallizationand a loss in mass of 3.60 g was

recorded. Use these figures tocalculate the number of moles ofwater of crystallization, x, in 1 moleof Na2SO4.xH2O.

Ans: Mass of anhydrous salt= 6.44- 3.6

= 2.84R.F.M of Na2SO4= 142Na2SO4 : xH2O

2.84 : 3.6142 : 18xx= 10.1

c) In an experiment, a solution

containing 0.05 mole HNO3 wasneutralized by aqueous sodiumhydroxide and 2.8 kJ of heat energywas evolved.

i) Calculate the enthalpy change when 1mole of HNO3 is neutralized. Writean ionic equation for the

neutralization reaction and use it toexplain why the reaction isexothermic.


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Ans: 0.05 mole of Nitric acid gives out2.8 kJ

1 mole of Nitric acid gives out 2.8/0.05

kJ = 56kJH+ + OH - H2OAs the bonds are formed energy is

released. So it is exothermic.

ii) Draw an energy profile for thereaction and include the value forH.

Ans:Energy

Reaction path

May 2005 P2


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6. When hydrochloric acid is added tosodium hydroxide solution, thefollowing neutralization takes place.

NaOH + HCl NaCl + H2Oa) State and explain how the progress of

this reaction can be followed byadding a few drops of eitherphenolphthalein or methyl orange tosodium hydroxide solution.

Ans: 25 cm 3 of NaOH is transferred ina conical flask wit the help of apipette by using pipette fillerAdd few drops of phenolphthalein indicator to the solutionThe colourless solution will turn pinkThe HCl is taken in a biurette and initial biurette reading is taken

The acid is added little by little until there is sharp end point

b)An experiment was performed to findthe volume of hydrochloric acid ofconcentration 0.050 mol dm-3 neededto neutralize 25 cm3 of sodium

hydroxide solution of unknownconcentration.The experiment was carried out threetimes.


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Experiment 1

Experiment 2

Experiment 3

Final

volume ofhydrochloric acid/

cm3

48.25 44.30 46.05

Initialvolume ofhydrochloric acid/

cm3

18.30 14.25 16.05

Volume ofhydrochlo

ric acidused/ cm3

29.95 V 1 V2

i) Name four pieces of apparatus

needed to carry out thisexperiment accurately.

Ans: PipetteBiuretteConical flaskStand


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c) i) Calculate the volumes, V1 and V2, of

hydrochloric acid used in experiments2 and 3. Use these values to calculatethe average volume of hydrochloricacid used in the three experiments.

Ans: C= n/v0.05= n/301000n= 1.5 10-3

ii) Calculate the number of moles ofhydrochloric acid used toneutralize 25 cm3 of the aqueoussodium hydroxide.

Ans: NaOH : HCl

1 : 1 1.5 10-3 : x

x= 1.5 10-3

iii) Calculate the concentration ofsodium hydroxide solution.

Ans: VASA/VBSB = a/b

30 0.05/ 25 SB = 1/1SB = 0.06 mol/ dm3


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d) Describe how a pure, dry sampleof sodium chloride crystals could beobtained from this experiment.

Ans: Repeat the experiment by using 25 cm3 of NaOH and 30 cm3 of HCl

No indicator is usedTake the solution in an

evaporating basin and place it in asteam bath

Heat the solution till the point of crystallization

Allow the solution to cool and crystallize

Transfer the crystals on a filter paper using a filter funnel

Wash the crystals with a little, cold distilled water

Leave it to dry in the air

7. b) Aluminium potassium sulphate is adouble salt made by a crystallizing asolution containing aluminiumsulphate and potassium sulphate insuitable proportions. It has the

formula Al2 (SO4)3.K2SO4.xH2O.

Ans: Dissolve the salt in waterConnect the solution in a circuit

with a bulb


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The bulb will glow, shows a current is passing Potassium ion

If a flame test is carried out with

the help of a nichrome wireLilac colour flame is produced Sulphate ionPrepare the solutionHCl is addedBarium chloride solution is addedA white precipitate of Barium

sulphate is formed Ba2+ + SO42- BaSO4

c) The following reaction occurs at200oC.

Al2(SO4)3.K2SO4.xH2OAl2(SO4)3.K2SO4 + xH2O

Hydrated compoundAnhydrous compound

When 9.48 g of the hydratedcompound was heated at 200oC,5.16 g of the anhydrous compoundwas formed.

i) Calculate the relative formula

mass of anhydrous aluminiumpotassium sulphate,Al2(SO4)3.K2SO4.

Ans: R.F.M of Al 2(SO4)3.K2SO4


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= 27 2 ( 32 + 16 4) 3 39 2 32 + 16 4

= 516

ii) Calculate the value of x in theformula AL2(SO4)3.K2SO4.xH2O.

Ans: Mass of H2O = 9.48- 5.16= 4.32

Al2(SO4)3.K2SO4 : xH2O5.16 4.32516 18x

92.88x = 2229.12x = 24

January 2004 P2

1. When 1.40 g of a metal, M, wasadded to an excess of copper(ii)sulphate solution, brown copper wasdeposited and the blue colour of thesolution faded until it was almostcolourless.

The copper was filtered, washed anddried; its mass was found to be 1.59g. Sodium hydroxide solution was


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added to the filtrate and a greenprecipitate was formed.a) Given that 1 mole of M displaces

1 mole of copper, use theinformation given above to calculatethe relative atomic mass, Ar, of themetal M, and identify this metal.

Ans: moles of copper = 1.59/63.5= 0.025

Ar(M) = 1.40/0.025= 56

Therefore it is iron.

b) Wite an equation for the reactionbetween the metal and copper(ii)sulphate solution.

Ans: Fe(s) + Cu+ Cu(s) + Fe+

c) Explain why the blue colour fadedduring the reaction.

Ans: The blue colour is caused by

copper ionAs copper ion is displaced by copper metal by the displacement ofcopper(ii) hence the solution fades


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d) Describe a test that could havebeen used to show that sulphateions were present in the filtrate.

Ans: Add aqueous HCl followed by Barium chloride

A white precipitate of Barium(ii) sulphate will form

e) i) Identify, by name of formula, thegreen precipitate.

Ans: Copper(ii) hydroxide

ii) Write an equation for the reactionthat produces the green precipitate.

Ans: FeSO 4 + 2NaOH Fe(OH)2 +

Na2SO4Fe 2+ + OH - Fe(OH)2

2. The hydrocarbons methane, CH4,propane, C3H8, and pentane, C5H12are all members of the alkanehomologous series.

a) i) Explain what is meant by the

term hydrocarbon.

Ans: These are the compoundscontaining carbon and hydrogen only.


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ii) State two characteristics of anhomologous series.

Ans: Same general formula

Differ from the next compound by CH2

iii) Write the molecular formulae ofthe two alkanes that followpentane in the series.

Ans: Hexane: C6H14, Heptane: C7H16

b) i) Explain what is meant by theterm structural isomerism.

Ans: These are the compounds havingsame molecular but different

structural formula.

c) When pentane is heated to a veryhigh temperature, it may undergothe reaction represented by theequation:

C5H12 C2H6 + C3H6

i) What is the name given to thistype of reaction?

Ans: Cracking


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ii) Pentane could decompose to give

two different products. Completethe following equation by giving thestructural formula of one possiblehydrocarbon that could be formed inaddition to methane.

Ans: C5H12 CH4 +C4H8

3. The electron configurations of fourelements represented by the lettersW, X, Y and Z are:W 2,7 X 2,8,3 Y 2, 8,5

Z 2,8,8,2Use the letters given to answer thefollowing questions.

a) i) Which two elements aremetals?

Ans: X, Z

ii) State the group to which eachmetal belongs.

Ans: Gr 1 Y, Gr 3- X

b) i) Which element would form anion with a charge of 3 -?


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e) i) Element Y forms a compoundwith hydrogen. What type ofbonding is found in this

compound?

Ans: Covalent

4. Nickel, Ni, is a transition metal inthe same row of the Periodic Tableas copper.a) Describe, giving essentialpractical details, how you couldprepare crystals of nickel(ii)sulphate, NiSO4.6H2O, starting fromnickel(ii) oxide and dilute sulphuricacid.

Ans: Take some dilute H 2SO4 and

heat it gentlyAdd nickel oxide little by little

until in excessExcess nickel (ii) oxide id

filtered offThe solution of nickel(ii)

sulphate is taken in an

evaporating dish and heated ona steel bath till the point ofcrystallization

Transfer the crystals on a filter paper and leave it to dry.


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b) The overall equation for thepreparation is:

NiO(s) + H2SO4(aq) + 5H2O(l)NiSO4.6H2O(s)Use the equation to calculate themaximum mass, in grams, of nickel(ii) sulphate crystals that could beobtained from 3.75 g of nickel (ii)oxide.

Ans: 75 2633.75 x x = 13.15

d) State two characteristic propertiesof transition metals that makedifferent from Group 2 metals.

Ans: They form variable valancies. Ex- Fe2+, Fe3+

They all form coloured compounds. Ex- Copper(ii)sulphate is blue

5. Describe simple experiments ortests that could be performed in thelaboratory to support each of thefollowing statements. Diagrams andequations not required.


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a) Sodium chloride is an electrolytewhereas sugar is a non electrolyte.

Ans: NaClDissolve in water when an

electric circuit is applied to abulb, the bulb will light up,shows it is an electrolyte. SugarSame experiment is carried out

but bulb will not light up showsit is not an electrolyte.

b) Sodium sulphate and sodiumsulphite behave differently when

added to dilute hydrochloricacid.

Ans: Na 2SO4: when dilute HCl isadded, no reaction is observed

Na 2SO4: when dilute HCl isadded sulphur dioxide gas is

evolved which turns potassiumdichromate orange to green


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c) Magnesium carbonate is lessstable to heat than sodiumcarbonate.

Ans: Mg 2CO3 produces CO2 inheating which turns limewatermilky

Na 2CO3 is thermally stable

January 2008 P2

8. a) A group in the Periodic Tablesometimes described as a familyof elements. Discuss how thisdescription applies to Group 1 (Lito K) and Group 7 ( Cl to I) byconsidering:

the number of electrons in the outer shellthe loss or gain of electrons to

form a stable electronicconfiguration


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b) In the Periodic Table, metals in thesame period become less reactivefrom left to right.

Support this statement by describingthree contrasting observationsmade during the reactions ofpotassium and calcium with water.Write equations for the reactionsthat occur.

Ans:Potassium Calcium

Moves on the surface of the waterand melts to a silveryball and disappears, aclear solution isformed.

2K + H2O 2KOH +H2O

reacts violentlyH 2produced ignites

immediately with alilac flame

Calcium sinksforms a cloudy

suspension due tothe formation ofCa(OH)2 whichsparingly soluble.

Ca + H 2O Ca(OH)2+ H2

reacts steadilyH 2 produced does

not ignite

c) Explain why the elements in Group 0are inert. Give one use for argonthat depends on this inertness.


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Ans: All group 0 elements are inert dueto the complete duplet or octet inthe outermost shell.

Uses- Argon is used in bulb light andin metallurgical process.

January 2005 P2

6. Chlorine, bromine and iodine aremembers of group of elementscalled the halogens.

a) i) Describe how a dry sample ofchlorine gas, free from hydrogen

chloride, can be prepared andcollected in the laboratory startingfrom hydrochloric acid and asuitable oxidizing agent. Give theconditions needed and theapparatus required. Write anequation for the reaction.

Ans: The reagents are: Concentrated HCl and MnO2.

MnO 2 is taken in a flask andconcentrated HCl is poured


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through a thistle funnel. Shake themixture and heat it.

The reaction produces

effervescence and the gas evolvedis passed through a wash bottlecontaining distilled water toremove hydrogen chloride gas.

Then the gas is dried through the wash bottle containingconcentrated H2SO4.

The gas is chlorine, as it is denser than air it is collected bydownward delivery or upwarddisplacement.

4HCl + MNO 2 Cl2 + 2H2O +MnCl2

ii) Describe a simple test for chlorine

gas.

Ans: It turns damp litmus paper firstred then bleach

iii) State two ways in which thephysical properties of the

halogens change as the group isdescended.

Ans: The colour darkensThe density increases


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b) Describe what is seen when chlorineis bubbled through a solution ofpotassium iodide. Write an equation

for the reaction and explain in termsof electron transfer why thisreaction involves both oxidation andreduction. Identify the oxidizingagent in this reaction.

Ans: The colour solution turns dark brown

Cl 2 + 2I - 2 Cl-+ I2Chlorine gains electrons whereas

iodide loses electronsHence it is a redox reactionChlorine is the oxidizing agent in

this reaction

c) When dry chlorine gas is passedover heated iron fillings, a vigorousreaction occurs and a black solid isformed.

i) Name the black solid and write anequation for its formation.

Ans: The black solid is iron(iii) chloride2Fe + 3Cl 2 2FeCl3


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The brown precipitate is due to the presence of Fe(OH)3

ii) Copper(ii) sulphate

Ans: A blue precipitate of copper (ii) hydroxide will form when littleammonia is added

When excess ammonium hydroxide solution is added theprecipitate and a deep bluesolution is formed

The deep blue solution is formed due to the formation of [ Cu(NH3)4.2H2O]+2

c) The equation for the reactionbetween ammonia and sulphuric

acid is:2NH3(aq) + H2SO4(aq)(NH4)2SO4(aq)

i) Calculate the volume of ammoniagas at room temperature andpressure that would have to bedissolved in water to give a

solution containing 6.80 g ofammonia.

Ans: 2NH 3 + H2SO4 (NH4)2SO4


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January 2008 P2

9. a) i) What is allotropy?

Ans: Existing of same element in twoor more different forms in thesame physical state.

ii) State two ways in which thestructures of diamond andgraphite are similar.

Ans: Both of them are macromolecules of carbon atoms

Both of them are covalently bonded

iii) State on way in which thestructures of diamond andgraphite are different.

Ans: In diamond each carbon atom is

tetrahedralGraphite has hexagonal layer structure


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iv) Explain why diamond is very hardwhereas graphite is soft.

Ans: In diamond atoms are bonded

with four other carbon atoms as aresult atoms are closely packedwithin the crystal lattice, that iswhy diamond is hard

In graphite carbon atoms are arranged in hexagonal layerstructure and between the layersthere is weak forces so it is softand can slide over.

b) When burnt in air, carbon can formtwo gaseous products. Identify theproducts and state the conditionsunder which each forms. Write an

equation formation of each gas.

Ans: The gases are CO 2 and COCO 2 forms when carbon is burned

in excess airC + O 2 CO2CO is formed when carbon is

burned in limited airC + O 2 COCO when inhaled combines with

haemoglobin and formcarboxyhaemoglobin, which is


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toxic and produces less heatenergy.

ii) Give a test that would distinguishbetween the gases.

Ans: When CO 2 is passed throughlime water it turns milky but Codoes not.

When a lighted taper is introduced to a gas jar of CO2, itextinguishes, but in CO it burnswith a blue flame

iii) One of the gases can act as

reducing agent at hightemperatures. Describe what youwould see if this gas is passedover heated copper (ii) oxide.Write an equation for the reaction.

Ans: The gas is CO

CO is a reducing agent, if it is passed over heated copper (ii)oxide, black copper (ii) oxide isturned to reddish brown copper.

CO + CuO Cu + CO 2


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May 2002 P2

6. Elements of the same group of thePeriodic Table show similaritiesboth in atomic structure and inproperties. This question is aboutfirst three elements in Group 1 ofthe Periodic Table shown on theback cover of this booklet.

a) Write the first three elements ofGroup 1 in order of theirreactivity, starting with the mostreactive, and give the electronicconfiguration of each. Whatsimilarity in electronic structurewould you expect all of the otherand show that the solution is

alkaline.

Ans: The first three elements of Group 1 in order of their reactivityare: Potassium, Sodium, Lithium.

Lithium 2,1

Sodium 2,8,1Potassium 2,8,8,1All Group 1 elements have

single electron in their outermostshell


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b) Give a suitable observation for thereaction of each of the first three

elements of Group 1 with waterthat shows the order of reactivity.Write an equation including statesymbols, for the reaction of anyone of these elements with water.Describe how you would identifythe gas produced and show thatthe solution formed is alkaline.

Ans: If a small piece of potassium is dropped into a trough of water: itmelts to a silvery ball and reactsviolently, gives out hydrogenwhich ignites spontaneously.

2K + 2H 2O 2KOH + H2

When a piece of sodium is placed on water, it moves on the surfacerapidly, melts to a silvery ball andfinally disappears.

2Na + 2H 2O 2NaOH + H2When a small piece of lithium is

placed on water, it moves slowly,

does not melt, does not catch fire,the reaction keeps on steadily.2Li + 2H 2O 2LiOH + H2


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If a burning splint is introduced into a gas jar it burns with asqueaky pop

A litmus paper will turn blue

showing the presence of alkali.

d) Each of the following compoundsbehaves differently when heated.For each compound, state what youwould see happen. Where a reactionoccurs, give the name or theformula of each of the products.

i) Sodium Carbonate

Ans: No reaction takes place

ii) Copper (ii) carbonate

Ans: It turns from green to blackCuCO 3 CuO + CO2

iii) Sodium nitrate

Ans: It melts to liquid

2NaNO 3 2NaNO2 + O2

iv) Copper (ii) nitrate

Ans: Blue solid turns black


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Brown gas is given off2CuNO 3 2CuO + NO2 + O2

January 2007 P2

8. Describe the reactions of sodiumand potassium with water, givingthree similarities and onedifference. Write an equation forone of the reactions.Explain in terms of their electronic

configuration, why the elementssodium and potassium have similarchemical properties. State the trendin reactivity on descending Group 1and suggest reason for it.

Ans: Similarities

Both moves on the surface of the waterMelts to a silvery ballEventually dissolves


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Forms alkaline solution, which can be tested with a

litmus paper

DifferencesPotassium reacts violently and Hydrogen produced ignitesimmediately but sodium does not

2K + 2H 2O 2KOH + H2Explanation: Both have one

electron in the outer shell. Ex- Na2,8,1 K 2,8,8,1

Reactivity increases down the groupAtoms get bigger as we go down

the group, so there is lessattraction for the outer electron.

Due to less attraction the electron is lost more easily.

b) Rubidium, Rb, is the fourthmember of Group 1. Name the

products and write an equation forany reaction that occurs when thefollowing compounds are heated.

i) Rubidium carbonate


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Ans: No reaction takes place

ii) Rubidium nitrate

Ans: Rubidium nitrite and oxygen are formed

2RbNO 3 2RbNO2 + O2

c) Aqueous sodium chloride (brine)containing litmus indicator iselectrolysed in apparatus thatprevents the products formed atthe electrodes from mixing. Theindicator turns blue near thecathode and colourless near theanode.

i) Write an equation for the

reaction occurring at the anodeand explain why the litmus turnscolourless.

Ans: 2Cl 2e Cl 2Chlorine bleaches litmus

ii) Write an equation for the reactionoccurring at the cathode andexplain why the litmus turnsblue. What other observationwould you see at the cathode.


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Ans: 2H + 2e H 2H + discharged leaving OH

OH

turns litmus blueBubbles of gas can also be seen

iii) Name an important compoundthat is produced industrially bythe electrolysis of brine and giveone large scale use of it.

Ans: Sodium hydroxide is produced

It is used for making soap

iv) Describe a simple experiment toshow whether a white powder isa sodium compound or a

potassium compound.

Ans: Take the white compound in a nichrome wire and place it in thenon luminous part of the flame.

If yellow flame is given than the compound is sodium

compound.If lilac colored flame is given then the compound is potassiumcompound.


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May 2004 P2

1. Use the in formation in the table

to answer the questions thatfollow.

Element Atomicnumber

Relativeatomic mass

Calcium 20 40Flourine 9 19

Neon 10 20Oxygen 8 16Sodium 11 23

a) Give the name or symbol of theatom that:

i) has the electron configuration2,8,8,2; Calciumii) forms a cation by the loss of oneelectron; Sodiumiii) forms an anion by gaining twoelectrons; Oxygen

b) State the electron configurationof neon and explain why neon ischemically inactive.


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Ans: Neon has electronconfiguration 2,8. It shows that it iscomplete and stable. That is why it

does not take part in any chemicalreaction.

c) Some of the elements in thetable can combine to formcompounds.

i) Give the formula of a compoundformed from two of the elements inthe table.

Ans: Na2O

ii) State, with a reason, whether thebonding in this compound is ionic or

covalent.

Ans: Ionic because the bond is madeup of metal and non metal. Transferof electrons are from metal to nonmetal.

iii) State whether the compound issolid, liquid or gas.

Ans: Solid


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d) i) Draw labeled diagram showingthe structure of a typical metal.

Ans:

ii) Explain why magnesium conductselectricity but sulphur does not.

Ans: Magnesium is a metal whichhas two electrons in its outermostshell that are loosely held, so theelectrons dissociates from the metaland form an electron cloud. Thesefree electrons carry the charge.Sulphur does not have any freeelectrons and do not conductelectricity.

8. Describe, in detail, a simplelaboratory experiment to show thateach of the following statements iscorrect. State the observations you

- - - -

- - - -

- - --

+

+

+

+

+

+

+ + +

+ ++

+ + +


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would expect to make and giveequations where appropriate.

a) Copper (ii) sulphate crystals give

off water when heated and thereaction is readily reversible.

Ans: Heat copper (ii) sulphate in a test tube

Liquid condenses on cooler part of tube

Test liquid with cobalt chloride paper

Colour changes from blue to redCopper (ii) sulphate blue to

whiteColour change reverses when

water added CuSO4.5H2O CuSO4 + 5H2O

b) Solid ammonium nitratedissolves in water endothermicallyand the ions move freely in theresulting solution.

Ans: Take water in a beaker

Add thermometer Add solid ammonium nitrate and stir

Temperature falls


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Place electrodes in resulting solution

Add a circuit including bulb

Bulb lights up showing current

are flowing.

c) Ammonia is very soluble in waterand forms an alkaline solution.

Ans: A gas jar full of ammonia is taken

Invert it over trough of waterWater rises up the gas jarUniversal indicator is then added

to the solutionColour changes from green to

blueNH 3 + H2O NH4OH

d) Chlorine can displace a lower

element in Group 7 from an aqueoussolution of one of its compounds.

Ans: We have to take solution of potassiumbromide


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Add chlorine water to itThe colour of the solution

changes to orange

This happens due to the

presence of bromine2Br -+ Cl2 2Cl- + Br(l)2

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Chemistry Broad Questions