2015 O Level Pure Chemistry 5073/02 Answers

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2015 OLEVEL CHEMISTRY 5073/02

SECTION ASECTION ASECTION ASECTION A

1. The table shows some common oxidation states for some elements in their compounds.

elementelementelementelement common oxidation statescommon oxidation statescommon oxidation statescommon oxidation states metal / nonmetal / nonmetal / nonmetal / non----metalmetalmetalmetal

A 2 nonnonnonnon----metalmetalmetalmetal

B +2; +3; +4; +6; +7 metalmetalmetalmetal

C +1 non-metal

D +3 metalmetalmetalmetal

E 1 nonnonnonnon----metalmetalmetalmetal

(a) Complete the table by indicating whether each element listed is a metal or non-metal. [1]

[No marks is awarded for any wrong indication][No marks is awarded for any wrong indication][No marks is awarded for any wrong indication][No marks is awarded for any wrong indication]

(b) Answer the following questions using the letter of the elements. [3]

(i) Which element is likely to be hydrogen? CCCC

(ii) Which element is likely to be in Group VI? EEEE

(iii) Which element is likely to form coloured compounds? BBBB

(c) No elements from Group 0 are listed in the table. Use the information in the table to explain

why. [2]

Elements in Group 0 have full valence electron shellfull valence electron shellfull valence electron shellfull valence electron shell and do not have to gain or losedo not have to gain or losedo not have to gain or losedo not have to gain or lose electronselectronselectronselectrons to obtain stable noble gas configuration, hence will not form compoundswill not form compoundswill not form compoundswill not form compounds. [To use information in table is to state no compo[To use information in table is to state no compo[To use information in table is to state no compo[To use information in table is to state no compound is formed (for 1mark)]und is formed (for 1mark)]und is formed (for 1mark)]und is formed (for 1mark)]

2. Metals can be extracted from their oxides by reduction reactions. The table given below gives the

minimum temperature needed for reduction of some metal oxides by carbon.

metal oxidemetal oxidemetal oxidemetal oxide min. temperature needed (min. temperature needed (min. temperature needed (min. temperature needed ())))

calcium oxide 2100

zinc oxide 900

copper oxide 100

magnesium oxide 1600

lead oxide 400

(a) (i) How does the temperature needed relate to the reactivity of the metal? [1]

The more reactive the metalmore reactive the metalmore reactive the metalmore reactive the metal, the higher the minimum temperature higher the minimum temperature higher the minimum temperature higher the minimum temperature needed for reduction.




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(ii) Predict the minimum temperature needed for reduction of iron oxide by carbon.

Explain your answer. [2]

606060600000. Iron is less reactive than zincless reactive than zincless reactive than zincless reactive than zinc, but more reactive than leadmore reactive than leadmore reactive than leadmore reactive than lead as it is between zinc and lead in the reactivity series, thus the temperature is between the 2 metals. [Any value between 400 and 900[Any value between 400 and 900[Any value between 400 and 900[Any value between 400 and 900]]]]

(b) Metal oxides also react with metals.

Use the list below for the following questions.

copper silver oxide

zinc sodium oxide

magnesium calcium oxide

iron potassium oxide

Which metal and metal oxide pair will likely react to give the most vigorous reaction? [1]

MagnesiumMagnesiumMagnesiumMagnesium and silver oxisilver oxisilver oxisilver oxidededede. [Both substances must be correct for 1mark][Both substances must be correct for 1mark][Both substances must be correct for 1mark][Both substances must be correct for 1mark]

(c) Zinc metal reacts with steam. Gives the names of the two products. [2]

Zinc oxideZinc oxideZinc oxideZinc oxide and hydrogen gashydrogen gashydrogen gashydrogen gas. [No marks awarded for chemical formulae][No marks awarded for chemical formulae][No marks awarded for chemical formulae][No marks awarded for chemical formulae]

3. (a) The table shows some information of certain organic compounds.

Fill in the blanks for missing names, formula and processes. [5]

namenamenamename structural formulastructural formulastructural formulastructural formula process(es) for making process(es) for making process(es) for making process(es) for making

compoundcompoundcompoundcompound

etheneetheneetheneethene

Fractional distillation of crude oil crude oil crude oil crude oil

followed by catalytic crackingcrackingcrackingcracking

poly(ethene)poly(ethene)poly(ethene)poly(ethene)

Addition polAddition polAddition polAddition polymerisationymerisationymerisationymerisation

of ethene.

ethanol

Two processes.

Process 1: Process 1: Process 1: Process 1: Catalytic addition addition addition addition of

steamsteamsteamsteam to ethene

ethanoic acid

Oxidation Oxidation Oxidation Oxidation of ethanol.

[0.5mark each][0.5mark each][0.5mark each][0.5mark each]






(b) Dilute ethanoic acid and hydrochloric acid react with metal oxides.

(i) How are the reactions between metal oxide and each of the two acids similar? [1]

Both reactions produce a a a a saltsaltsaltsalt and waterwaterwaterwater as the only products.

(ii) The rate of the reaction of dilute ethanoic acid with metal oxides is slower than that of

dilute hydrochloric acid of the same concentration and temperature. Why is this so? [2]

Ethanoic acid is a weak acidweak acidweak acidweak acid with partial ionisation in water to form partial ionisation in water to form partial ionisation in water to form partial ionisation in water to form GH ionsionsionsions, but hydrochloric acid is a strong acid strong acid strong acid strong acid with complete ionisation in water to form complete ionisation in water to form complete ionisation in water to form complete ionisation in water to form GH ions. Hence, ethanoic acid contains lesser lesser lesser lesser GH ionsionsionsions, resulting in a slower reaction.

4. Aluminium is used to make drink cans. The metal used can be obtained either from recycling or by

extraction from bauxite by electrolysis.

processprocessprocessprocess recyclingrecyclingrecyclingrecycling electrolysiselectrolysiselectrolysiselectrolysis

separation

Physical sorting.

Steel and aluminium cans are

separated using a magnet.

Aluminium cans are shredded into

small pieces.

Physical extraction and breaking up of

ore.

Concentrated sodium hydroxide is

used to dissolve aluminium oxide to

separate it from insoluble metal oxide

impurities.

main process Heating to 700.

Dissolving in ionic solvent at 900.

Electrolysis to form molten

aluminium.

Carbon dioxide is formed in the

process.

finishing Cooling and shaping of molten

aluminium.

Cooling and shaping of molten

aluminium.

(a) Use the information above to estimate the melting point of aluminium. [1]

700700700700 [Actual melting point = 660[Actual melting point = 660[Actual melting point = 660[Actual melting point = 660]]]]

(b) The extraction of aluminium uses 95% more energy than the recycling process. Use the

information to explain why. [2]

The amount of energyenergyenergyenergy required to dissolvedissolvedissolvedissolve aluminium in ionic solvent is almost 30% (900 vs 700) more thanmore thanmore thanmore than that needed to heatheatheatheat aluminium cans. And, a huge amount ofhuge amount ofhuge amount ofhuge amount of energy energy energy energy is required to ensure bauxite stays molten ensure bauxite stays molten ensure bauxite stays molten ensure bauxite stays molten for extraction to occur. [Mention of 30% is the enhancement for the centr[Mention of 30% is the enhancement for the centr[Mention of 30% is the enhancement for the centr[Mention of 30% is the enhancement for the centres students]es students]es students]es students]






(c) The extraction of aluminium creates waste that can harm the environment. Two such waste

are concentrated sodium hydroxide and carbon dioxide. Why would each of these wastes

cause harm when released into the environment? [2]

NaOH is a strong alkalistrong alkalistrong alkalistrong alkali and corrosive. corrosive. corrosive. corrosive. When leaked into the sealeaked into the sealeaked into the sealeaked into the sea, it will raise pH of waterraise pH of waterraise pH of waterraise pH of water greatly above 7greatly above 7greatly above 7greatly above 7, killing marine lifes killing marine lifes killing marine lifes killing marine lifes that dont survive in alkaline conditions. COK is a greenhouse gasgreenhouse gasgreenhouse gasgreenhouse gas that raise global temperature, melting polar raise global temperature, melting polar raise global temperature, melting polar raise global temperature, melting polar caps, raising seacaps, raising seacaps, raising seacaps, raising sea----level andlevel andlevel andlevel and causes floodingsfloodingsfloodingsfloodings.

(d) Recycling uses less energy and produces less waste than extraction. Give one other reason

why recycling metals is important. [1]

Recycling reduces the needreduces the needreduces the needreduces the need for more extraction of metals for more extraction of metals for more extraction of metals for more extraction of metals such as aluminium which are finite resourcesfinite resourcesfinite resourcesfinite resources and will deplete over time.and will deplete over time.and will deplete over time.and will deplete over time.

(e) Some aluminium is used to make duralumin. It is commonly used to make aircraft bodies.

Duralumin is a mixture of aluminium and other elements such as copper, manganese and

magnesium.

(i) What is the name given to mixtures such as duralumin? [1]

Alloy

(ii) Suggest why duralumin is more useful as aircraft bodies than aluminium. [1]

It is stronger and harder than aluminiumstronger and harder than aluminiumstronger and harder than aluminiumstronger and harder than aluminium as layers of atoms in duralumin are less likely to slide.

5. Dilute sodium chloride solution forms hydrogen and oxygen gases during electrolysis.

(a) Write ionic equations for the reactions at the cathode and anode. [2]

Cathode: 2MH(NO) + 2PQ MK(S) Anode: 4TMQ(NO) TK(S) + 2MKT(U) + 4PQ






(b) The gases are collected and measured. Theoretically, the ratio of the volume of hydrogen to

oxygen collected should be 2:1.

Oxygen gas is more soluble than hydrogen in water. This changes the ratio of gases that are

collected.

(i) Explain the theoretical volume of the two gases. [2]

For 2moles of electrons form 1mole of MK, hence for the same circuit,2moles of electrons(anode) form 0.5moles of TK, resulting in twice the volume of MK than TK.

(ii) Explain how and why the solubility of oxygen affects the ratio of hydrogen to oxygen

that is collected. [2]

When some TK dissolves, the volume collected decdecdecdecreasesreasesreasesreases and is and is and is and is less than half less than half less than half less than half thatthatthatthat of of of of MK. As such, the the the the actual yield is loweractual yield is loweractual yield is loweractual yield is lower, resulting in a greater ratio difference a greater ratio difference a greater ratio difference a greater ratio difference between the 2 gasesbetween the 2 gasesbetween the 2 gasesbetween the 2 gases.

(iii) The difference from the expected ratio is greater when the electrolysis starts but less

noticeable after the electrolysis has been running for some time. Suggest why. [1]

The volume of oxygen gas produced exceeds the volume of space in container atexceeds the volume of space in container atexceeds the volume of space in container atexceeds the volume of space in container at anode. anode. anode. anode. Hence, over time, the difference difference difference difference remains consistent. remains consistent. remains consistent. remains consistent.

(c) What happens to the concentration of sodium chloride during electrolysis? Explain. [1]

The concentration concentration concentration concentration increasesincreasesincreasesincreases. The amount of water in electrolyte decreases amount of water in electrolyte decreases amount of water in electrolyte decreases amount of water in electrolyte decreases as MH and TMQ ions are preferentially discharged, removing water while amount of sodium chloride remains unchanged.

(d) The same apparatus can be used to electrolyse concentrated aqueous sodium chloride. Give

one similarity and one difference between the products of the two electrolysis. [2]

(e) Platinum metal electrodes are used. Why is platinum a suitable material? [1]

Platinum is inertinertinertinert and as anodeanodeanodeanode, does not oxidised and breaks downdoes not oxidised and breaks downdoes not oxidised and breaks downdoes not oxidised and breaks down, allowing the Respective anions to be preferentially discharged instead.

Both processes

produced hydrogen gas at the cathodes.

Dilute

electrolyte Concentrated

electrolyte

Similarity Difference

Oxygen gas produced at anode.

Chlorine gas produced at anode.






6. Farmers often buy soil improvers that contain substances that neutralise acidity. The table below

shows some common substances found in soil improvers.

substancesubstancesubstancesubstance chemical chemical chemical chemical

compositcompositcompositcompositionionionion

effectiveness in effectiveness in effectiveness in effectiveness in

neutralisingneutralisingneutralisingneutralising other pointsother pointsother pointsother points

limestone CaCOV fair Insoluble in water. Needs to be

ground to a very fine powder.

quicklime CaO Very high

Made by heating limestone to a

high temperature.

Reacts exothermically with

water to make an alkaline

solution.

slaked lime Ca(OH)K Very high

Made by adding water to

quicklime.

Slaked lime is an alkali.

blast furnace slag

Mixture of

CaCOV and

CaSiOV with other

impurities

fair

Insoluble in water. Impurities

include silicon oxides and other

non-metal compounds.

Composition of mixture varies.

(a) Use the information in the table to suggest why limestone is less effective at neutralizing

acidity than quicklime and slaked lime. [2]

Limestone is insolubleinsolubleinsolubleinsoluble and hence does not dissociate does not dissociate does not dissociate does not dissociate in water to form hydroxide ionsin water to form hydroxide ionsin water to form hydroxide ionsin water to form hydroxide ions that can effectively neutralise the hydrogen ions in acids. Therefore, it is limited in limited in limited in limited in contact with acids as solidscontact with acids as solidscontact with acids as solidscontact with acids as solids unlike the mobile ions in the other two to allow faster reaction.

(b) (i) Suggest an advantage of making improvers from slag rather than the other substances. [1]

Slag contains other non-metal compounds that may include componentsinclude componentsinclude componentsinclude components such as nitrogennitrogennitrogennitrogen or phosphorusor phosphorusor phosphorusor phosphorus, which are essential for good plant growth.essential for good plant growth.essential for good plant growth.essential for good plant growth. [Accept idea of slag as waste t[Accept idea of slag as waste t[Accept idea of slag as waste t[Accept idea of slag as waste that have no demands in markethat have no demands in markethat have no demands in markethat have no demands in market and increase costs from and increase costs from and increase costs from and increase costs from

disposal]disposal]disposal]disposal]

(ii) Slag may contain impurities of Group IV and V oxides, making it less effective at

neutralizing acidity. Explain why this is so. [2]

These oxides are nonnonnonnon----metal oxides metal oxides metal oxides metal oxides that are mmmmostly acidic oxides. ostly acidic oxides. ostly acidic oxides. ostly acidic oxides. Therefore Increasing acidityIncreasing acidityIncreasing acidityIncreasing acidity and require more improvers to be usedmore improvers to be usedmore improvers to be usedmore improvers to be used.






(c) The calcium content of the substances is important because it adds to the mineral content of

the soil. Show by calculation that quicklime has a higher percentage by mass of calcium than

both of limestone and slaked lime. [3]

Percentage by mass of calcium in quicklime

= XY

XYHZ[ 100%

= 71.4% (3sf)

Percentage by mass of calcium in limestone

= XY

XYHZKHZ[V 100%

= 40%

Percentage by mass of calcium in slaked lime

= XY

XYHK(Z[HZ) 100%

= 54.1% (3sf)

Therefore, quicklime has a higher percentage by mass of calcium than limestone and slaked

lime.

(d) Quicklime is made by strongly heating limestone. The reaction produces carbon dioxide.

Assuming that limestone is pure calcium carbonate, calculate the volume of carbon dioxide

that is produced. [3]

No. of moles of limestone = K^ZY_`

XYHZKHZ[V

= 2.50 10^mol

Equation : CaCOV CaO + COK

So,

CaCOV COK

ratio 1 : 1

moles 2.50 10^ 2.50 10^abU

Volume of carbon dioxide = 2.50 10^ 24daV

= 6.06.06.06.0 efghij






SECTION SECTION SECTION SECTION BBBB

7. Table 1Table 1Table 1Table 1 shows the melting and boiling points of alkanes.

alkanealkanealkanealkane formulaformulaformulaformula melting pointmelting pointmelting pointmelting point

(((()))) boiling pointboiling pointboiling pointboiling point

(((())))

ethane CKH[ 183 89

propane CVHl 188 42

butane CXHZY 138 1

pentane C^HZK 130 36

hexane C[HZX 95 69

octane ClHZl 57 126

Table 1Table 1Table 1Table 1

GraphGraphGraphGraph 1111 below shows the graph of melting/boiling points plotted against number of carbon atoms

in alkane.




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The flashpoint of a compound is the minimum temperature at which that compound gives off

enough vapour to burn in air. It is important when considering the amount of hydrocarbons

blended to make fuels. Compounds with lower flashpoints evaporate and burn more easily at lower

temperatures and burn more smoothly in car engines.

In a petrol refinery, isomerization is used to convert straight chain alkanes into branched alkanes.

The flashpoints of some straight chain and branched alkanes are provided in Table 2Table 2Table 2Table 2 and Table 3Table 3Table 3Table 3

respectively.

alkanealkanealkanealkane formuformuformuformulalalala flashpointflashpointflashpointflashpoint

(((())))

propane CVHl 104

butane CXHZY 71

pentane C^HZK 49

hexane C[HZX 23

heptane CmHZ[ 4

octane ClHZl 13


number of carbon number of carbon number of carbon number of carbon

atomsatomsatomsatoms formulaformulaformulaformula

flashpointflashpointflashpointflashpoint

(((())))

brancbrancbrancbranchhhhed ed ed ed

alkane 1alkane 1alkane 1alkane 1 5

57


alkane alkane alkane alkane 2222 5

65


alkane alkane alkane alkane 3333 8

4


(a) What trends are shown by the data in Table 1Table 1Table 1Table 1 and Graph 1Graph 1Graph 1Graph 1? [1]

Generally, as the number of carbon atoms increases, the melting and boiling points increases.the number of carbon atoms increases, the melting and boiling points increases.the number of carbon atoms increases, the melting and boiling points increases.the number of carbon atoms increases, the melting and boiling points increases. Both the melting and boiling points do not incrdo not incrdo not incrdo not increase in value linearly. ease in value linearly. ease in value linearly. ease in value linearly. [Note the plural in trends][Note the plural in trends][Note the plural in trends][Note the plural in trends]

(b) One of the alkanes appears to have a melting point that is different from the expected value.

Name the alkane and explain your choice. [1]

Propane. The expected value of the melting point of propane is 160160160160 and the listed value is much lower to give a clear trend in melting points.much lower to give a clear trend in melting points.much lower to give a clear trend in melting points.much lower to give a clear trend in melting points.

(c) Predict the melting and boiling of heptane, CmHZ[. [1]

Melting point : 79 and boiling point : 90 [Acceptable : in range of [Acceptable : in range of [Acceptable : in range of [Acceptable : in range of 70s and 90s respec70s and 90s respec70s and 90s respec70s and 90s respectively]tively]tively]tively]

[No marks for just 2 values given without stating melting and boiling point][No marks for just 2 values given without stating melting and boiling point][No marks for just 2 values given without stating melting and boiling point][No marks for just 2 values given without stating melting and boiling point]






(d) The boiling points and flashpoints of straight chain alkanes have similar trends. Describe

these trends. [1]

Both boiling points and flashpoints incincincincrease as the number of carbon atoms per moleculerease as the number of carbon atoms per moleculerease as the number of carbon atoms per moleculerease as the number of carbon atoms per molecule increaseincreaseincreaseincreases.s.s.s.

(e) How is the flashpoint of a straight chain alkane affected by isomerisation? Use the

information in the table to explain. [4]

When isomerise, the flashpoint dedededecreasescreasescreasescreases as compared to a straightchain alkane of the sameof the sameof the sameof the same number of carbon atomsnumber of carbon atomsnumber of carbon atomsnumber of carbon atoms. By comparing pentane and branched alkane 1, the flashpoint decreased from 49to 57. Also, the more the more the more the more the number of branchings, the lower the number of branchings, the lower the number of branchings, the lower the number of branchings, the lower the flashpointflashpointflashpointflashpoint of the same number of carbon asame number of carbon asame number of carbon asame number of carbon atoms. toms. toms. toms. By comparing branched alkane 1 and 2, when the no. of branchings increased from 1 to 2, the flashpoint dropped from 57 to 65.

[2[2[2[2markmarkmarkmarkssss for compfor compfor compfor comparing straightchain to isomer; 2aring straightchain to isomer; 2aring straightchain to isomer; 2aring straightchain to isomer; 2markmarkmarkmarkssss for comparing no. of branchings per for comparing no. of branchings per for comparing no. of branchings per for comparing no. of branchings per

isomerisomerisomerisomer. Respective ex. Respective ex. Respective ex. Respective examples must be quoted from the tableamples must be quoted from the tableamples must be quoted from the tableamples must be quoted from the table]]]]

(f) Petrol contains straight chain octane. Other compounds are added to improve the

performance of petrol. In cold countries, winter blend petrol is sold. This petrol contains

increased amounts of compounds that include pentane and branched alkane 2.

(i) Explain how and why this mixture works better than pure octane alone. [2]

Pure octane has a higher flashpointhigher flashpointhigher flashpointhigher flashpoint and requires more energy for fuel to burnmore energy for fuel to burnmore energy for fuel to burnmore energy for fuel to burn. When mixed, petrol will have a lowered meltinglowered meltinglowered meltinglowered melting pointpointpointpoint where its flashpoint can go below below below below op, , , , making the fuel more efficient in burning.

(ii) Engine tanks in cars are not air tight. Winter blend petrol is not sold in very hot

weather conditions because it leads to an increased loss of petrol from the tank.

Explain how and why petrol is lost. [2]

Winter blend petrol has boiling pointboiling pointboiling pointboiling point lower than room temperature lower than room temperature lower than room temperature lower than room temperature during hot weather conditions. Hence, it vapourises very readilyvapourises very readilyvapourises very readilyvapourises very readily and escapes uncombusted inescapes uncombusted inescapes uncombusted inescapes uncombusted in engines in the form of uuuunburnt hydrocarbonsnburnt hydrocarbonsnburnt hydrocarbonsnburnt hydrocarbons, resulting in mass loss.






8. Car engines are adjusted to work at a particular air:fuel ratio. The amount of air that is mixed with

the fuel affects the temperature of the engine, the amount of pollutant gases that form and how

efficiently the catalytic converter works. Two major pollutants are carbon monoxide and nitrogen

monoxide.

(a) A lean burn engine runs with a higher ratio of air to fuel than a normal car engine. This

means that the mixture contains a higher amount of air compared to fuel. One effect of this is

a lower running temperature of the engine.

How will a lean burn engine affect the amount of carbon monoxide and nitrogen monoxide

production compared to a normal car engine? Explain. [3]

Production of CO decrdecrdecrdecreases in lean burn engineeases in lean burn engineeases in lean burn engineeases in lean burn engine. With higher air:fuel ratio, the presence of more oxygenmore oxygenmore oxygenmore oxygen allows for greatergreatergreatergreater chance of complete combustionchance of complete combustionchance of complete combustionchance of complete combustion. Thus, reducing formation ofreducing formation ofreducing formation ofreducing formation of CO.CO.CO.CO. At a lower running temperature of engine, the amount of NO produced decreases.the amount of NO produced decreases.the amount of NO produced decreases.the amount of NO produced decreases. Nitrogen gas is inertinertinertinert and only combusts under high temperatureunder high temperatureunder high temperatureunder high temperature. As such, lesser tendencylesser tendencylesser tendencylesser tendency of formation of NOformation of NOformation of NOformation of NO in lean burn engine.

(b) A catalytic converter removes pollutants by redox reactions.

Write an overall equation for the two reactions above. [1]

2CO + 2NO 2COK + NK

(c) The amount of air in pollutants that enter the converter affects the reactions. The graph

shows the percentage of pollutants successfully removed.

(i) Describe and explain how increased amounts of air affect the removal of carbon

monoxide and nitrogen monoxide. [3]

Initially, as amount of air increases, % of CO removed increases% of CO removed increases% of CO removed increases% of CO removed increases with maximummaximummaximummaximum amt.amt.amt.amt. of NO removed. of NO removed. of NO removed. of NO removed. When maximum amount of CO is removedmaximum amount of CO is removedmaximum amount of CO is removedmaximum amount of CO is removed, increasincreasincreasincreasing amt. of air ing amt. of air ing amt. of air ing amt. of air will cause the % of NO removed to decrease% of NO removed to decrease% of NO removed to decrease% of NO removed to decrease with no effects on COno effects on COno effects on COno effects on CO. At lower amount of air, NO can be reduced to reduced to reduced to reduced to qr more effectively more effectively more effectively more effectively without without without without combusting in aircombusting in aircombusting in aircombusting in air. However, increasing amt. of air allows for oxidation of CO to increasing amt. of air allows for oxidation of CO to increasing amt. of air allows for oxidation of CO to increasing amt. of air allows for oxidation of CO to str. Eventually, in the lack of COlack of COlack of COlack of CO but more air, more air, more air, more air, qr formed oxidises back into NO, reducing % of removal of NO.

CO + [O] COK

from oxidising agent

2NO NK + 2[O]

to reducing agent






(ii) In the converter, apart from reacting with each other, carbon monoxide and nitrogen

monoxide react with other substances as well. How does the graph show that? [1]

Both the pollutants do not have zero percent removalzero percent removalzero percent removalzero percent removal, which indicates that there is presence of both gases in converter reacting with other substances.

EITHER

9. The diagram shows the energy changes when lithium chloride and sodium chloride are dissolved

in water.

(a) Describe the differences in the energy changes and temperature changes that happen as each

compound dissolves. [4]

When lithium chloride dissolves, the reaction is exothermicexothermicexothermicexothermic and the surrounding temp.the surrounding temp.the surrounding temp.the surrounding temp. increasesincreasesincreasesincreases.... As the energy level of product is less than that of reactant, more more more more heat is released heat is released heat is released heat is released iiiinto nto nto nto surroundingsurroundingsurroundingsurrounding. However, when sodium chloride dissolves, the reaction is endothermic endothermic endothermic endothermic and the surrounding temperature decreasessurrounding temperature decreasessurrounding temperature decreasessurrounding temperature decreases. The energy level of product is greater than that of reactant, hence more more more more heat is abheat is abheat is abheat is absorbed from the surrounding.sorbed from the surrounding.sorbed from the surrounding.sorbed from the surrounding.

(b) A student measured the temperature change when 4.0g of potassium chloride was dissolved

in excess water.

temperature at start (temperature at start (temperature at start (temperature at start ()))) 20

lowest temperature recorded lowest temperature recorded lowest temperature recorded lowest temperature recorded after dissolving (after dissolving (after dissolving (after dissolving ())))

12

calculated energy change (J)calculated energy change (J)calculated energy change (J)calculated energy change (J) +720

(i) Explain why the calculated energy change includes a + sign. [1]

The reaction is endothermicendothermicendothermicendothermic, hence the enthalpy change is positiveenthalpy change is positiveenthalpy change is positiveenthalpy change is positive.

(ii) Use the results to calculate the enthalpy change when one mole of potassium chloride

dissolves in excess water. Give your answer in kJ/mol to 3 significant figures. [3] No. of moles of KCU

= X.Y

VwHV^.^

= 0.05369mol

H per mole of KCU = (HmKY)Y.Y^V[w

ZYYY

= = = = 0.03870.03870.03870.0387 kJ/mol (3s.f.)kJ/mol (3s.f.)kJ/mol (3s.f.)kJ/mol (3s.f.)

M1M1M1M1

M1M1M1M1 M1M1M1M1






(iii) Describe what happens to the arrangement and movement of the particles in

potassium chloride during dissolving. [2]

The ions ions ions ions are closelycloselycloselyclosely----packed together in an orderly arrangement, vibrating about packed together in an orderly arrangement, vibrating about packed together in an orderly arrangement, vibrating about packed together in an orderly arrangement, vibrating about their fixed positionfixed positionfixed positionfixed position as solids. When added to water, the ions become widely separatedwidely separatedwidely separatedwidely separated amongstamongstamongstamongst the water moleculesthe water moleculesthe water moleculesthe water molecules, vibrating randomly and sliding over one ano, vibrating randomly and sliding over one ano, vibrating randomly and sliding over one ano, vibrating randomly and sliding over one anotherthertherther.... [Note that excess water means a dilute solution][Note that excess water means a dilute solution][Note that excess water means a dilute solution][Note that excess water means a dilute solution]

OR

9. Fluorine is in Group VII.

(a) A jet of fluorine gas is aimed at a filter paper soaked with potassium bromide solution. The

solution on the paper quickly turns brown.

(i) Explain why this happens. Include an ionic equation to support your answer. [3]

Fluorine is more reactive than is more reactive than is more reactive than is more reactive than brominebrominebrominebromine, hence it displaces bromine from potassiumdisplaces bromine from potassiumdisplaces bromine from potassiumdisplaces bromine from potassium bromidebromidebromidebromide to form the brown bromine solution.bromine solution.bromine solution.bromine solution. Ionic equation : FK(S) + 2ByQ(NO) 2FQ(NO) + BrK(NO)

(ii) The experiment is repeated with chlorine and iodine gases replacing fluorine in

different setups with potassium bromide solution. State and explain what you would

expect to see in each setup. [3]

Chlorine is more is more is more is more reactive than reactive than reactive than reactive than brominebrominebrominebromine, hence it displaces bromine from potassiumdisplaces bromine from potassiumdisplaces bromine from potassiumdisplaces bromine from potassium bromidebromidebromidebromide. . . . When added, the colourlesscolourlesscolourlesscolourless solution turns brownbrownbrownbrown. But, iodine is less reactive than bromineiodine is less reactive than bromineiodine is less reactive than bromineiodine is less reactive than bromine, hence it doesn not displace bromine from itsdoesn not displace bromine from itsdoesn not displace bromine from itsdoesn not displace bromine from its solutionsolutionsolutionsolution. . . . When added, iodine gas dissolves in solution dissolves in solution dissolves in solution dissolves in solution and turns it yellowishyellowishyellowishyellowish----brownbrownbrownbrown. [Avoid brown for iodine[Avoid brown for iodine[Avoid brown for iodine[Avoid brown for iodine and confuse with the bromine colour stated]and confuse with the bromine colour stated]and confuse with the bromine colour stated]and confuse with the bromine colour stated]

(b) Fluorine also reacts with iron. When a jet of fluorine is aimed at some iron wool, the wool

glows and appears to burn. An ash of iron(III) fluoride is left behind.

(i) Write an equation, with state symbols, for this reaction. [2]

3FK(S) + 2Fe(z) 2FeFV(z)

(ii) Explain, using ideas of oxidation state and electron transfer, why fluorine is

considered an oxidising agent in the reaction. [2]

Fluorine caused iron to be caused iron to be caused iron to be caused iron to be oxidisedoxidisedoxidisedoxidised as the oxidation state of iron oxidation state of iron oxidation state of iron oxidation state of iron inininincreasedcreasedcreasedcreased from 0 in 0 in 0 in 0 in FeFeFeFe to +3 in +3 in +3 in +3 in FeFj , , , , and is itself reduced to FeFj. Also, each molecule of fluorine gains each molecule of fluorine gains each molecule of fluorine gains each molecule of fluorine gains an electron an electron an electron an electron from iron, causing iron to be reduced to FeFj. . . . Therefore, fluorine acts as an oxidising agent.

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2015 O Level Pure Chemistry 5073/02 Answers