2012 olevel pure chemistry paper 2 questions and answers

METHODOLOGYMETHODOLOGYMETHODOLOGYMETHODOLOGY

OF STUDIES OF STUDIES OF STUDIES OF STUDIES Block 513 Bishan St 13 #01-514 (2nd Floor) Singapore 570513 | ROC No. : 53176143J

Office Tel: 63540340635403406354034063540340

METHODOLOGY OF SCIENCEMETHODOLOGY OF SCIENCEMETHODOLOGY OF SCIENCEMETHODOLOGY OF SCIENCE

REALISE YOUR MOSTREALISE YOUR MOSTREALISE YOUR MOSTREALISE YOUR MOST

CHEMISTRY

OOOO----LEVELLEVELLEVELLEVEL 2012201220122012 QUESTIONS AND ANSWERSQUESTIONS AND ANSWERSQUESTIONS AND ANSWERSQUESTIONS AND ANSWERS SECTION A (50 marks)SECTION A (50 marks)SECTION A (50 marks)SECTION A (50 marks) 1. The diagran below shows reactions involving dilute sulfuric acid. Six unknown substances A to FA to FA to FA to F

are shown. (a) Using the letters AAAA to FFFF, (i) which substance is Universal Indicator? [1]

EEEE

(ii) which substance is carbon dioxide? [1]

BBBB (iii) which substance is a metal? [1]

CCCC (iv) which substance could be sodium hydrogencarbonate? [1]

AAAA

[5678 96: ;<<7=: ;98>7?8 @9 8A;BB <;=8 @7. ′D′] (b) FFFF is an oxide of metal. Suggest a possible name of this substance. [1]

Copper(II) oxide Copper(II) oxide Copper(II) oxide Copper(II) oxide [F6 A;?G8 @H (II)@8 96: @9J@<;:7J]

(c) DDDD can be used in titration with dilute sulfuric acid to produce the substances listed in the

diagram above. What could be a possible name of DDDD? [1]

Potassium hydroxide Potassium hydroxide Potassium hydroxide Potassium hydroxide

[L<<7=: ;9M N?6O= I PMJ?6Q@J7 R ;BG;B@. F6 A;?G8 H6? D;87]

dilute sulfuric acid

salt + BBBB

+ water

AAAA FFFF

a blue solution

forms DDDD

salt

+ water

CCCC EEEE

salt

+ hydrogen a red

solution forms



Office Tel: 63540340635403406354034063540340



2. The names and symbols of isotopes of some elements are given in the table below.

name fluorine-19 carbon-12 iodine-

131 strontium-

90 neon-20 carbon-14 magnesium-24

symbol WXYX Z[Y\ ]^_Y_Y `a_bXc deYc\c Z[Yf ghY\\f Use the isotopes from the table to answer the questions below. (a) State the names of two isotopes that have the same number of protons in their atoms. [1]

CarbonCarbonCarbonCarbon----12 and carbon12 and carbon12 and carbon12 and carbon----14141414

(b) State the names of two isotopes that have the same number of neutrons in their atoms. [1]

FluorineFluorineFluorineFluorine----19 and neon19 and neon19 and neon19 and neon----20 20 20 20 (c) State the names of two isotopes that have 7 electrons in the valence shell of their atoms. [1]

FluorineFluorineFluorineFluorine----19 and iodine19 and iodine19 and iodine19 and iodine----131 131 131 131 [F6 A;?G8 H6? 8:;:@9k 8MAD6B 6? P;l@9k ;9M 6H :P7 m @86:6=78 >?69k] (d) Which two isotopes form stable compounds that have ions of charge 2+? [1]

StrontiumStrontiumStrontiumStrontium----90 and magnesium90 and magnesium90 and magnesium90 and magnesium----24 24 24 24 [L<<7=: 8MAD6B8 ;8 ;98>7?] 3. Oxides of nitrogen in the stratosphere cause damage to the ozone layer and engines of aircraft are a

source of oxides of nitrogen. (a) (i) Explain how oxides of nitrogen can be formed in the engine of aircraft. [2]

From From From From internal combustioninternal combustioninternal combustioninternal combustion of of of of nitrogen in airnitrogen in airnitrogen in airnitrogen in air inside the engine under inside the engine under inside the engine under inside the engine under high high high high

temperaturetemperaturetemperaturetemperature....

(ii) Suggest a natural source of oxides of nitrogen in air. [1]

During During During During lightning activiteslightning activiteslightning activiteslightning activites

[F6 A;?G8 H6? 8=7BB@9k 7??6? 6H B@kP:9@9k] (b) Nitrogen monoxide also damages the ozone layer through a two-step reaction. Step 1: dp + p_ → dp\ + p\ Step 2: dp\ + p_ → dp + 2p\ (i) State which element is oxidised in step 1 using its oxidation states. [2]

element : nitrogen / Nnitrogen / Nnitrogen / Nnitrogen / N

change in oxidation state : from +2 to +4from +2 to +4from +2 to +4from +2 to +4

s5678 96: ;<<7=: 6Q@J;:@69 8:;:7 ;8 ′m+t5678 96: ;<<7=: 7B7A79: ;8 ′Fu′ v



Office Tel: 63540340635403406354034063540340



(b) (ii) Each nitrogen monoxide molecule can destroy thousands of ozone molecules. Using equations from both steps, explain this phenomenon. [2]

Each NO molecule in step 1 reacts to form one molecule of Each NO molecule in step 1 reacts to form one molecule of Each NO molecule in step 1 reacts to form one molecule of Each NO molecule in step 1 reacts to form one molecule of Fum which in turnwhich in turnwhich in turnwhich in turn

reactsreactsreactsreacts to form back one molecule of NO. As such, each molecule of NO reacts withto form back one molecule of NO. As such, each molecule of NO reacts withto form back one molecule of NO. As such, each molecule of NO reacts withto form back one molecule of NO. As such, each molecule of NO reacts with

2 molecules of 2 molecules of 2 molecules of 2 molecules of uw and gets formed back into NO to further react continuously withand gets formed back into NO to further react continuously withand gets formed back into NO to further react continuously withand gets formed back into NO to further react continuously with

more more more more uw ....

sxy R J78<?@D@9k :P7 7zO;:@698; xy R 8Okk78:@9k 8;A7 A6B7<OB7 6H Fu <;9 ?7;<: <69:@9O6O8BM v

(c) Oxides of nitrogen are removed in car exhaust by the usage of catalytic converters by

reacting nitrogen monoxide with carbon monoxide. (i) Write a balanced equation for the reaction. [1]

mFu + m|u → Fm + m|um

(ii) Gases that are harmful to the environment are still emitted from cars fitted with

catalytic converters. Name one such gas and describe the problem it causes. [1]

Carbon dioxide. It is a Carbon dioxide. It is a Carbon dioxide. It is a Carbon dioxide. It is a greenhousegreenhousegreenhousegreenhouse gas which causes gas which causes gas which causes gas which causes global warmingglobal warmingglobal warmingglobal warming and the and the and the and the

melting of polar capsmelting of polar capsmelting of polar capsmelting of polar caps which results to the which results to the which results to the which results to the rise in searise in searise in searise in sea----levelslevelslevelslevels, causing , causing , causing , causing floods.floods.floods.floods.

[5678 96: ;<<7=: ′|um′/<;O878 kB6D;B >;?A@9k] 4. The diagram below shows the structures of ethanoic acid and ethanoyl chloride. (a) Both compounds react with methanol to make the same ester. (i) Draw the structure of the ester. [1] Ans:Ans:Ans:Ans: [Allow: [Allow: [Allow: [Allow: ----COO to be sCOO to be sCOO to be sCOO to be slanted]lanted]lanted]lanted] (ii) State the formula of the other produtother produtother produtother produt formed when: [1] 1. methanol reacts with ethanoic acid: ~mu 2. methanol reacts with ethanoyl chloride: ~|B [Does not accept compound names][Does not accept compound names][Does not accept compound names][Does not accept compound names]

ethanoic acidethanoic acidethanoic acidethanoic acid ethanoyl chlorideethanoyl chlorideethanoyl chlorideethanoyl chloride



Office Tel: 63540340635403406354034063540340



(a) (iii) Give two uses of esters. [1]

As food flavourings and in the manufacturing of perfumes.As food flavourings and in the manufacturing of perfumes.As food flavourings and in the manufacturing of perfumes.As food flavourings and in the manufacturing of perfumes.

[Avoid short unclear answers such as ‘perfume and flavourings’][Avoid short unclear answers such as ‘perfume and flavourings’][Avoid short unclear answers such as ‘perfume and flavourings’][Avoid short unclear answers such as ‘perfume and flavourings’] (b) A homologous series contains a member known as ethanoyl chloride. (i) Give the name and structural formula of the next member of this homologous series. [2] name : propanoyl chloridepropanoyl chloridepropanoyl chloridepropanoyl chloride structural formula : [Does not accept [Does not accept [Does not accept [Does not accept |~w R |~m R |u|B]]]] (ii) What is the general formula of the members of this homologous series? [1]

|9~m9�x|u|B

(iii) As the size of the molecules in the homologous series increases, the physical

properties changes. Describe the trends in two properties as the sizes increase. [2]

As the molecular size increases, the As the molecular size increases, the As the molecular size increases, the As the molecular size increases, the melting and boiling pointsmelting and boiling pointsmelting and boiling pointsmelting and boiling points of the substancesof the substancesof the substancesof the substances

IncreasesIncreasesIncreasesIncreases. As the molecular size increases, the . As the molecular size increases, the . As the molecular size increases, the . As the molecular size increases, the density density density density of the substances of the substances of the substances of the substances increasesincreasesincreasesincreases....

Accept:Accept:Accept:Accept: ---- visvisvisviscosity increasescosity increasescosity increasescosity increases ---- less flammableless flammableless flammableless flammable [[[[Do not accept any chemical properties]Do not accept any chemical properties]Do not accept any chemical properties]Do not accept any chemical properties] State your two properties clearly and avoid joining two points into one sentence.State your two properties clearly and avoid joining two points into one sentence.State your two properties clearly and avoid joining two points into one sentence.State your two properties clearly and avoid joining two points into one sentence. 5. The diagram below shows a cooling pack that contains a small bag of solid ammonium nitrate

surrounded by a bag of water.

When the pack is squeezed, the small bag will break open and result in a reaction between ammonium nitrate and water. This reaction is endothermic and the bag becomes very cold.

(a) Given that the ∆� is positive, suggest why this endothermic change cause the water to

become cold. [1]

DuringDuringDuringDuring reaction, ammonium chloride reaction, ammonium chloride reaction, ammonium chloride reaction, ammonium chloride takes in heat from the surroundingtakes in heat from the surroundingtakes in heat from the surroundingtakes in heat from the surrounding which resultswhich resultswhich resultswhich results

in the in the in the in the decrease of temperature of waterdecrease of temperature of waterdecrease of temperature of waterdecrease of temperature of water, hence the bag becomes cold.., hence the bag becomes cold.., hence the bag becomes cold.., hence the bag becomes cold..

solid ammonium nitrate

water



Office Tel: 63540340635403406354034063540340



(b) The reaction produces ammonium nitrate solution as shown by the equation below. d�fdp_(�) + (��) → d�fdp_(��) ∆� � +26��/��

Complete the energy profile diagram using the axes provided, clearly indicating the reaction reaction reaction reaction enthalpy changeenthalpy changeenthalpy changeenthalpy change and the activation energyactivation energyactivation energyactivation energy. [3]

[1M[1M[1M[1M----correct shape with product written; 1Mcorrect shape with product written; 1Mcorrect shape with product written; 1Mcorrect shape with product written; 1M----activation energy shown with one arrowhead in activation energy shown with one arrowhead in activation energy shown with one arrowhead in activation energy shown with one arrowhead in the right direction; 1Mthe right direction; 1Mthe right direction; 1Mthe right direction; 1M----enthalpy change including the word ‘reaction’ as it appears in boldenthalpy change including the word ‘reaction’ as it appears in boldenthalpy change including the word ‘reaction’ as it appears in boldenthalpy change including the word ‘reaction’ as it appears in bold]]]]

(c) Ammonium nitrate, which is made from ammonia, is also used as a fertiliser. (i) Complete the table below. [2]

gas source method of production nitrogen liquefied air / airliquefied air / airliquefied air / airliquefied air / air fractional distillation hydrogen crude oil crackingcrackingcrackingcracking

[Note spelling of ‘liquefied’; does not accept fractional distillation for crude oil][Note spelling of ‘liquefied’; does not accept fractional distillation for crude oil][Note spelling of ‘liquefied’; does not accept fractional distillation for crude oil][Note spelling of ‘liquefied’; does not accept fractional distillation for crude oil] (ii) Crops need the nitrogen content found in fertilisers such as ammonium nitrate for

healthy growth. 50kg of ammonium nitrate was found in some fertilisers. Calculate the mass of nitrogen in this fertiliser. (1kg�1000g) [3]

Ans:Ans:Ans:Ans: Mr of Mr of Mr of Mr of F~�Fuw � 80� 80� 80� 80 Mass of nitrogen in Mass of nitrogen in Mass of nitrogen in Mass of nitrogen in F~�Fuw � � � � mQL? 6H F

y? 6H F~�Fuw� ��, ��k

� � � � mQx�

�� , ��k � 17,500g� 17,500g� 17,500g� 17,500g

[Accept answer as 17.5kg; marks deducted if working did not[Accept answer as 17.5kg; marks deducted if working did not[Accept answer as 17.5kg; marks deducted if working did not[Accept answer as 17.5kg; marks deducted if working did not indicate conversion of kg indicate conversion of kg indicate conversion of kg indicate conversion of kg to g; indicate clearly the formula]to g; indicate clearly the formula]to g; indicate clearly the formula]to g; indicate clearly the formula]

(c) (iii) Fertilisers that contain ammonium nitrate are slightly acidic, hence calcium carbonate

are usually added to soil to reduce the acidity. However, calcium hydroxide should not be used. Explain why. [2]

Calcium hydroxide, which is a Calcium hydroxide, which is a Calcium hydroxide, which is a Calcium hydroxide, which is a basebasebasebase, will , will , will , will react with react with react with react with ammonium nitrateammonium nitrateammonium nitrateammonium nitrate and resultand resultand resultand result

in in in in the the the the reduction of the composition by mass of nitrogenreduction of the composition by mass of nitrogenreduction of the composition by mass of nitrogenreduction of the composition by mass of nitrogen in the form of in the form of in the form of in the form of ammoniaammoniaammoniaammonia

released into the surroundingreleased into the surroundingreleased into the surroundingreleased into the surrounding....

[1M[1M[1M[1M----suggest reaction in tsuggest reaction in tsuggest reaction in tsuggest reaction in the form of ammonium compound + base; 1Mhe form of ammonium compound + base; 1Mhe form of ammonium compound + base; 1Mhe form of ammonium compound + base; 1M----nitrogen content nitrogen content nitrogen content nitrogen content decrease due to ammonia produced and escaped]decrease due to ammonia produced and escaped]decrease due to ammonia produced and escaped]decrease due to ammonia produced and escaped]

enthalpy

progress of reaction

d�fdp_(�) + (��) d�fdp_(��)

activation energy

reaction enthalpy change, ∆� � +26��/��



Office Tel: 63540340635403406354034063540340



6. The table below shows the trends of the melting points and boiling points of Group I and Group VII elements.

element melting point/°Z boiling point/°Z

Group I

lithium 181 1347

sodium 98 883

potassium 64 774

Group VII

chlorine R101 R34

bromine R7 59

iodine 114 184

(a) (i) The elements in Group I and Group VII have different trends in melting points and

boiling points. Describe the trends down each group. [2]

The melting point and boiling point The melting point and boiling point The melting point and boiling point The melting point and boiling point decreasesdecreasesdecreasesdecreases down down down down Group IGroup IGroup IGroup I, but the melting point, but the melting point, but the melting point, but the melting point

and boiling point and boiling point and boiling point and boiling point increasesincreasesincreasesincreases down down down down Group VIIGroup VIIGroup VIIGroup VII.... (ii) The melting point and boiling point of sodium is higher than that of chlorine. Explain

this phenomenon using concept from bonding. [3]

Sodium is a metal with Sodium is a metal with Sodium is a metal with Sodium is a metal with metallic structuremetallic structuremetallic structuremetallic structure where where where where positive metal ions and ‘sea of positive metal ions and ‘sea of positive metal ions and ‘sea of positive metal ions and ‘sea of

delocalised' electronsdelocalised' electronsdelocalised' electronsdelocalised' electrons are held by are held by are held by are held by strong electrostatic forcesstrong electrostatic forcesstrong electrostatic forcesstrong electrostatic forces and a and a and a and a huge amounthuge amounthuge amounthuge amount ofofofof

energy is required to overcomeenergy is required to overcomeenergy is required to overcomeenergy is required to overcome to change state. However, chlorine has to change state. However, chlorine has to change state. However, chlorine has to change state. However, chlorine has simplesimplesimplesimple

molecular structuremolecular structuremolecular structuremolecular structure where where where where moleculesmoleculesmoleculesmolecules are held by are held by are held by are held by weak intermolecular forcesweak intermolecular forcesweak intermolecular forcesweak intermolecular forces andandandand

only a only a only a only a small amount of energy small amount of energy small amount of energy small amount of energy is required to overcome to change state. Hence, is required to overcome to change state. Hence, is required to overcome to change state. Hence, is required to overcome to change state. Hence,

sodium has a higher melting and boilingsodium has a higher melting and boilingsodium has a higher melting and boilingsodium has a higher melting and boiling point than chlorine.point than chlorine.point than chlorine.point than chlorine.

[To include [To include [To include [To include –––– structure/bonding(forces of attraction)/energy required; deduction of at least structure/bonding(forces of attraction)/energy required; deduction of at least structure/bonding(forces of attraction)/energy required; deduction of at least structure/bonding(forces of attraction)/energy required; deduction of at least 1M for any missing component.]1M for any missing component.]1M for any missing component.]1M for any missing component.]

(b) The densities of chlorine and bromine at room temperature and pressure is provided.

element density (g/cm3) chlorine 0.03 bromine 3.12

A student states that: “The difference in the molecular mass of chlorine and bromine is insufficient to account for the difference in their densities.”

(i) Explain why the student is correct. [2]

Density, Density, Density, Density, which is the mass per unit volume of a substance, is also dependent on which is the mass per unit volume of a substance, is also dependent on which is the mass per unit volume of a substance, is also dependent on which is the mass per unit volume of a substance, is also dependent on

volume. Hence, for chlorine is a volume. Hence, for chlorine is a volume. Hence, for chlorine is a volume. Hence, for chlorine is a gasgasgasgas but bromine a but bromine a but bromine a but bromine a liquidliquidliquidliquid at at at at room temperature androom temperature androom temperature androom temperature and

pressure, pressure, pressure, pressure, therefore the molecules take up different volume in therefore the molecules take up different volume in therefore the molecules take up different volume in therefore the molecules take up different volume in different statesdifferent statesdifferent statesdifferent states, , , ,

resultiresultiresultiresulting ng ng ng in difference in densities.in difference in densities.in difference in densities.in difference in densities.

[Suggestion of volume as a factor and the difference in states at r.t.p][Suggestion of volume as a factor and the difference in states at r.t.p][Suggestion of volume as a factor and the difference in states at r.t.p][Suggestion of volume as a factor and the difference in states at r.t.p]



Office Tel: 63540340635403406354034063540340



(ii) What is the main reason for the difference in densities of the two substances? [1]

Chlorine, with its molecules much further apart than Chlorine, with its molecules much further apart than Chlorine, with its molecules much further apart than Chlorine, with its molecules much further apart than those of bromine due to the those of bromine due to the those of bromine due to the those of bromine due to the

difference in states, has a lower mass per unit volume.difference in states, has a lower mass per unit volume.difference in states, has a lower mass per unit volume.difference in states, has a lower mass per unit volume. (c) (i) Explain the meaning of the term ‘��’. [1]

It represents that a substance is made up of It represents that a substance is made up of It represents that a substance is made up of It represents that a substance is made up of 2 atoms2 atoms2 atoms2 atoms chemically combined chemically combined chemically combined chemically combined

together.together.together.together.

[Not accepted: missing ‘molecule’ or ‘chemically combined’][Not accepted: missing ‘molecule’ or ‘chemically combined’][Not accepted: missing ‘molecule’ or ‘chemically combined’][Not accepted: missing ‘molecule’ or ‘chemically combined’] (ii) Draw a dot-and-cross diagram of a chlorine molecule, showing only the valence shell. [1] (d) Sodium and chlorine react together to form sodium chloride. Describe what happens when

an atom of sodium and an atom of chlorine react to form ions. Your answer may include a diagram. [2]

Ans:Ans:Ans:Ans: Each atom of sodium Each atom of sodium Each atom of sodium Each atom of sodium loses its one valence electronloses its one valence electronloses its one valence electronloses its one valence electron to obtain to obtain to obtain to obtain stable noblestable noblestable noblestable noble----gas configurationgas configurationgas configurationgas configuration

and this is and this is and this is and this is transferredtransferredtransferredtransferred to an atom of chlorineto an atom of chlorineto an atom of chlorineto an atom of chlorine, such that the , such that the , such that the , such that the gaining of an electrongaining of an electrongaining of an electrongaining of an electron allows allows allows allows chlorine to obtain chlorine to obtain chlorine to obtain chlorine to obtain stable noblestable noblestable noblestable noble----gas configurationgas configurationgas configurationgas configuration to form to form to form to form a chlora chlora chlora chloride ionide ionide ionide ion....

End of Section AEnd of Section AEnd of Section AEnd of Section A

Key:Key:Key:Key: : electrons of : electrons of : electrons of : electrons of |B �: electrons of other : electrons of other : electrons of other : electrons of other |B

lose an electronlose an electronlose an electronlose an electron

NaNaNaNa |B



Office Tel: 63540340635403406354034063540340



SECTION B (30 marks)SECTION B (30 marks)SECTION B (30 marks)SECTION B (30 marks) 7. Industrial Electrolysis of BrineIndustrial Electrolysis of BrineIndustrial Electrolysis of BrineIndustrial Electrolysis of Brine

Brine is a concentrated solution of sodium chloride, containing about 25% by mass of sodium chloride. Electrolysis of brine produces chlorine, hydrogen and sodium hydroxide. Industrial electrolysis of brine used to be carried out in mercury cellsmercury cellsmercury cellsmercury cells but is now mostly carried out in membrane cellsmembrane cellsmembrane cellsmembrane cells. The diagrams below show how the two types of cells work.

The membrane cell allows sodium hydroxide and chlorine to be produced in the same cell. Without the membrane, the sodium hydroxide would not be pure because it would contain chloride ions. One other problem that the membrane cell solves is that it keeps the chlorine gas and hydroxide ions separate. Chlorine and hydroxide ions react together which would reduce the amount of chlorine gas made and create more impurities in the sodium hydroxide.

The table below shows some information of the two types of cell.

mercury cell membrane cell overall energy consumption (kWh per tonne of chlorine) 1 tonne = 1,000,000g

3360 2750

purity of sodium hydroxide produed high purity high purity concentration of sodium hydroxide produced

50% concentration 35% concentration

other points mercury is toxic and must be

removed from spent brine low maintenance

costs

(a) (i) Explain why it is important that the negative ions do not pass through the membrane in the membrane cell. [3]

Ans:Ans:Ans:Ans: During brine electrolysis, one of the key products is During brine electrolysis, one of the key products is During brine electrolysis, one of the key products is During brine electrolysis, one of the key products is chlorinechlorinechlorinechlorine. Allowing hydrox. Allowing hydrox. Allowing hydrox. Allowing hydroxide ions to ide ions to ide ions to ide ions to pass throughpass throughpass throughpass through decreases the concentration of decreases the concentration of decreases the concentration of decreases the concentration of |B¢ ionsionsionsions at the positive terminal, hence at the positive terminal, hence at the positive terminal, hence at the positive terminal, hence increasing the chances of discharge of increasing the chances of discharge of increasing the chances of discharge of increasing the chances of discharge of u~¢ ionsionsionsions which forms which forms which forms which forms oxygen gasoxygen gasoxygen gasoxygen gas that that that that contaminatescontaminatescontaminatescontaminates the chlorine gas collected. Thus the chlorine gas collected. Thus the chlorine gas collected. Thus the chlorine gas collected. Thus purificationpurificationpurificationpurification of chlorine gasof chlorine gasof chlorine gasof chlorine gas is necessary. is necessary. is necessary. is necessary. Likewise, the presence of Likewise, the presence of Likewise, the presence of Likewise, the presence of u~¢ions further reduces the amount of chlorine gas forms ions further reduces the amount of chlorine gas forms ions further reduces the amount of chlorine gas forms ions further reduces the amount of chlorine gas forms due to reaction between chlorine and due to reaction between chlorine and due to reaction between chlorine and due to reaction between chlorine and u~¢ . With . With . With . With |B¢ions moving to the negative ions moving to the negative ions moving to the negative ions moving to the negative terminal, it terminal, it terminal, it terminal, it acts as impurityacts as impurityacts as impurityacts as impurity in sodium hydroxide as it results tin sodium hydroxide as it results tin sodium hydroxide as it results tin sodium hydroxide as it results to the formation of o the formation of o the formation of o the formation of sodium sodium sodium sodium chloridechloridechloridechloride. As sodium hydroxide is another . As sodium hydroxide is another . As sodium hydroxide is another . As sodium hydroxide is another key product of this electrolysiskey product of this electrolysiskey product of this electrolysiskey product of this electrolysis, further , further , further , further purificationpurificationpurificationpurification is required to obtain pure sodium hydroxide.is required to obtain pure sodium hydroxide.is required to obtain pure sodium hydroxide.is required to obtain pure sodium hydroxide.

[Focus on importance of obtaining pure chlorine and sodium hydroxide. Avoid fully [Focus on importance of obtaining pure chlorine and sodium hydroxide. Avoid fully [Focus on importance of obtaining pure chlorine and sodium hydroxide. Avoid fully [Focus on importance of obtaining pure chlorine and sodium hydroxide. Avoid fully lllliiiiffffttttiiiinnnngggg ooooffffffff ffffrrrroooommmm ppppaaaassssssssaaaaggggeeee]]]]

The sodium and mercury mixture flows

into another tank which is not shown

where sodium reacts with water to make sodium hydroxide and hydrogen.

Positive sodium ions move through the

membrane but negative ions and water molecules cannot.

F;�

|B¢ u~¢

mercury cellmercury cellmercury cellmercury cell membranemembranemembranemembrane cellcellcellcell



Office Tel: 63540340635403406354034063540340



(a) (ii) It is an advantage that negative ions do not pass through the membrance. Describe other advantages of using membrane cell instead of mercury cell. [2]

For the For the For the For the same mass of chlorine producedsame mass of chlorine producedsame mass of chlorine producedsame mass of chlorine produced, membrane cell consumed , membrane cell consumed , membrane cell consumed , membrane cell consumed 610k610k610k610kWhWhWhWh

energy energy energy energy lesserlesserlesserlesser than that of mercury cell. Also, membrane cell does not contain anythan that of mercury cell. Also, membrane cell does not contain anythan that of mercury cell. Also, membrane cell does not contain anythan that of mercury cell. Also, membrane cell does not contain any

toxic substances such as mercurytoxic substances such as mercurytoxic substances such as mercurytoxic substances such as mercury which requires additional process of removal.which requires additional process of removal.which requires additional process of removal.which requires additional process of removal.

The membrane cell has a The membrane cell has a The membrane cell has a The membrane cell has a lowerlowerlowerlower maintenance cost.maintenance cost.maintenance cost.maintenance cost.

[Deduction of marks if only [Deduction of marks if only [Deduction of marks if only [Deduction of marks if only one advantage is given; separate each point clearly]one advantage is given; separate each point clearly]one advantage is given; separate each point clearly]one advantage is given; separate each point clearly] [Not accepted: It uses lesser energy and has low maintenance costs][Not accepted: It uses lesser energy and has low maintenance costs][Not accepted: It uses lesser energy and has low maintenance costs][Not accepted: It uses lesser energy and has low maintenance costs] (iii) Give a disadvantage of using membrane cell over mercury cell. [1]

The sodium hydroxide obtained is The sodium hydroxide obtained is The sodium hydroxide obtained is The sodium hydroxide obtained is more dilutedmore dilutedmore dilutedmore diluted, giving, giving, giving, giving rise to only 35% rise to only 35% rise to only 35% rise to only 35%

concentration as compared to the 50% concentration in mercury cell.concentration as compared to the 50% concentration in mercury cell.concentration as compared to the 50% concentration in mercury cell.concentration as compared to the 50% concentration in mercury cell.

(b) Calculate the energy consumption of the membrane cell per mole of chlorine gas produced. [2]

AnsAnsAnsAns:::: No. of moles of chlorineNo. of moles of chlorineNo. of moles of chlorineNo. of moles of chlorine = = = = x.��x�£k

y? 6H |Bm

= = = = x.��x�£

¤x

= = = = x. �� x�� mol M1mol M1mol M1mol M1 Energy consumed per moleEnergy consumed per moleEnergy consumed per moleEnergy consumed per mole = 2750= 2750= 2750= 2750 ¥ (x. �� x��) ==== 0.195 kWh (3sf) 0.195 kWh (3sf) 0.195 kWh (3sf) 0.195 kWh (3sf) M1M1M1M1

[Deduction of marks for wrong or missing units and wrong significance given][Deduction of marks for wrong or missing units and wrong significance given][Deduction of marks for wrong or missing units and wrong significance given][Deduction of marks for wrong or missing units and wrong significance given] (c) (i) Write the overall equation for the reaction in the membrane cell. [1]

mF;|B S m~mu → mF;u~ S |Bm S ~m

[[[[Not: Not: Not: Not: m|B¢(;z) S m~�(;z) → |Bm(k) S ~m(k) as in does not link to (ii); clues from as in does not link to (ii); clues from as in does not link to (ii); clues from as in does not link to (ii); clues from

paragraph 1 of questionparagraph 1 of questionparagraph 1 of questionparagraph 1 of question]]]] (ii) Calculate the maximum mass of sodium hydroxide that can be produced from 1tonne

of concentrated brine. [3]

Ans:Ans:Ans:Ans: Mass of Mass of Mass of Mass of F;|B = 25% = 25% = 25% = 25% � (x. �� x�£k) = 0.25 = 0.25 = 0.25 = 0.25 � x�£k M1M1M1M1

No. of moles of No. of moles of No. of moles of No. of moles of F;|B = = = = �.m� � x�£k

y? 6H F;|B

= = = = �.m� � x�£k

��.�

= = = = �. m¤w� � x�w molmolmolmol From From From From equation,equation,equation,equation,

Max. mass of Max. mass of Max. mass of Max. mass of F;u~ = (= (= (= (�. m¤w� � x�w) ) ) ) � y? 6H F;u~ = (= (= (= (�. m¤w� � x�w) ) ) ) � �� = 1709= 1709= 1709= 170940404040 ¦ x¤x�� k (w8H)

F;|B :::: F;u~ ratioratioratioratio 2222 :::: 2222 molesmolesmolesmoles �. m¤w� � x�w molmolmolmol :::: �. m¤w� � x�w molmolmolmol M1M1M1M1

M1M1M1M1



Office Tel: 63540340635403406354034063540340



8. Nine experiments were carried out to investigate the effect of using different catalysts on the rate of reaction. The table below shows the time taken for the reaction to end when different metal compounds were used as catalysts.

experiment catalyst temperature at beginning

/°Z time taken for reaction to

end /second 1 d�Z� 19 45 2 WeZ�\ 20 22 3 Z�Z�\ 19 26 4 ghZ�\ 20 46 5 d�dp_ 19 45 6 We(dp_)\ 20 22 7 We(dp_)_ 19 15 8 Z�(dp_)\ 19 26 9 gh(dp_)\ 19 46

(a) Explain why it is important to record the temperature at the start of the reaction. [1]

To ensure the To ensure the To ensure the To ensure the variablevariablevariablevariable is is is is a constanta constanta constanta constant for a for a for a for a fair experimentfair experimentfair experimentfair experiment....

[Accept: for the same tem[Accept: for the same tem[Accept: for the same tem[Accept: for the same temperature experiments are used during comparison of results]perature experiments are used during comparison of results]perature experiments are used during comparison of results]perature experiments are used during comparison of results] (b) Using the information in the table, explain how it shows that metal compounds in Group I

and II are less effective as catalysts than those of transition metals. [2]

Ans:Ans:Ans:Ans: For the For the For the For the same stsame stsame stsame starting temperature of 19arting temperature of 19arting temperature of 19arting temperature of 19°|, compounds such as , compounds such as , compounds such as , compounds such as F;Fuw and and and and yk(Fuw)m require require require require 45s and 46s respectively45s and 46s respectively45s and 46s respectively45s and 46s respectively for complete reaction, but those of transition metal for complete reaction, but those of transition metal for complete reaction, but those of transition metal for complete reaction, but those of transition metal compounds such as compounds such as compounds such as compounds such as §7(Fuw)w and and and and |6(Fuw)m require only 15s andrequire only 15s andrequire only 15s andrequire only 15s and 26s respectively for 26s respectively for 26s respectively for 26s respectively for complete reaction, indicating that reactions complete reaction, indicating that reactions complete reaction, indicating that reactions complete reaction, indicating that reactions containing transition metal compounds as catalystcontaining transition metal compounds as catalystcontaining transition metal compounds as catalystcontaining transition metal compounds as catalyst are are are are fasterfasterfasterfaster, hence they are more effective catalysts than compounds of Group I and II., hence they are more effective catalysts than compounds of Group I and II., hence they are more effective catalysts than compounds of Group I and II., hence they are more effective catalysts than compounds of Group I and II.

[Data analysis: values and comparison of datas mu[Data analysis: values and comparison of datas mu[Data analysis: values and comparison of datas mu[Data analysis: values and comparison of datas must be clearly indicated. Accept using 20st be clearly indicated. Accept using 20st be clearly indicated. Accept using 20st be clearly indicated. Accept using 20°|]]]] (c) The experiments contain ions of two different iron.

(i) Give the formulae for the two ions. [1]

§7m� and and and and §7w� ionsionsionsions

(ii) Explain which ion of iron is the more effective catalyst. [1]

§7w� ionsionsionsions. By conversion, for the same temperature at 19. By conversion, for the same temperature at 19. By conversion, for the same temperature at 19. By conversion, for the same temperature at 19°|, , , , §7(Fuw)m takes takes takes takes

aaaa longer time for reaction to endlonger time for reaction to endlonger time for reaction to endlonger time for reaction to end as compared to that of as compared to that of as compared to that of as compared to that of §7(Fuw)w ....

[[[[§7(Fuw)m takes 20.9s to end reaction using temperatakes 20.9s to end reaction using temperatakes 20.9s to end reaction using temperatakes 20.9s to end reaction using temperature of 19ture of 19ture of 19ture of 19°| by conversion]by conversion]by conversion]by conversion] (d) A student wrote this conclusion: “The type of anion in the catalyst compound does not affect The type of anion in the catalyst compound does not affect The type of anion in the catalyst compound does not affect The type of anion in the catalyst compound does not affect

the rate of reactionthe rate of reactionthe rate of reactionthe rate of reaction” (i) Use the results in the table to explain whether you agree with the student’s conclusion. [2]

IIII agree. agree. agree. agree. |6|Bm and and and and |6(Fuw)m when used at a temperature at 19when used at a temperature at 19when used at a temperature at 19when used at a temperature at 19°| give rise togive rise togive rise togive rise to

the the the the same time taken of 26s, and same time taken of 26s, and same time taken of 26s, and same time taken of 26s, and F;|B and and and and F;Fuw when used at a when used at a when used at a when used at a

temperature of 19temperature of 19temperature of 19temperature of 19°| give rise to the same time taken of 45s. give rise to the same time taken of 45s. give rise to the same time taken of 45s. give rise to the same time taken of 45s.

[[[[x¨ ©ª« ′¬«®®′; for ‘disagree no marks is awardedfor ‘disagree no marks is awardedfor ‘disagree no marks is awardedfor ‘disagree no marks is awarded]]]] (ii) Predict the time taken when iron(III) chloride was used. [1]

15s15s15s15s



Office Tel: 63540340635403406354034063540340



EITHEREITHEREITHEREITHER 9. The order of reactivity of four metals WWWW, XXXX, YYYYand ZZZZ was determined by carrying out some

experiments as shown by the table below.

metal WWWW metal XXXX metal YYYY metal ZZZZ

solution of WWWW nitrate no experiment

carried out no reaction

occurs no reaction

occurs no reaction

occurs

solution of XXXX nitrate reaction occurs

no experiment carried out

reaction occurs reaction occurs

solution of YYYY nitrate reaction occurs

no reaction occurs


reaction occurs

solution of ZZZZ nitrate reaction occurs

no reaction occurs

no reaction occurs


(a) Place the metals in decreasing order of reactivity. [2]

WWWW, Z, Y, X, Z, Y, X, Z, Y, X, Z, Y, X

(b) State and explain what you would see when metal ZZZZ reacts with hydrochloric acid. [2]

Metal Z Metal Z Metal Z Metal Z decreases in sizedecreases in sizedecreases in sizedecreases in size while while while while effervescenceeffervescenceeffervescenceeffervescence forms. forms. forms. forms. Z is Z is Z is Z is more reactivemore reactivemore reactivemore reactive than than than than hydrogenhydrogenhydrogenhydrogen,,,,

hence it hence it hence it hence it displaces hydrogen from hydrochlodisplaces hydrogen from hydrochlodisplaces hydrogen from hydrochlodisplaces hydrogen from hydrochloric acidric acidric acidric acid....

[Not accepted: hydrogen gas forms][Not accepted: hydrogen gas forms][Not accepted: hydrogen gas forms][Not accepted: hydrogen gas forms] (c) More experiments were conducted to include metal MMMM into the list. The experiments included

reaction with dilute hydrochloric acid with samples of the metals. The conclusion was that metal MMMM is the fourth most reactive metal.

Describe the experiments that were carried out. Your answer should include: • the experiments that were done using dilute hydrochloric acid and the metals,

• the measurements taken, • how the results showed that metal MMMM is the fourth most reactive metal. [3]

Ans:Ans:Ans:Ans: PlacePlacePlacePlace metal W in excess to dilute hydrochloric metal W in excess to dilute hydrochloric metal W in excess to dilute hydrochloric metal W in excess to dilute hydrochloric

acid in a conical flask connected to a graduated acid in a conical flask connected to a graduated acid in a conical flask connected to a graduated acid in a conical flask connected to a graduated gas syringe as shown in the diagram to the left.gas syringe as shown in the diagram to the left.gas syringe as shown in the diagram to the left.gas syringe as shown in the diagram to the left.

The volume of gas collected is measured per The volume of gas collected is measured per The volume of gas collected is measured per The volume of gas collected is measured per second until thesecond until thesecond until thesecond until the reaction stops. The time in which reaction stops. The time in which reaction stops. The time in which reaction stops. The time in which volume of gas first remains constant is noted and volume of gas first remains constant is noted and volume of gas first remains constant is noted and volume of gas first remains constant is noted and recorded in a table below.recorded in a table below.recorded in a table below.recorded in a table below.

Repeat the process by changing the metals to X, Y, Repeat the process by changing the metals to X, Y, Repeat the process by changing the metals to X, Y, Repeat the process by changing the metals to X, Y, M and Z respectively.M and Z respectively.M and Z respectively.M and Z respectively.

hydrochloric acid added to:hydrochloric acid added to:hydrochloric acid added to:hydrochloric acid added to: WWWW XXXX YYYY MMMM ZZZZ Time taken when volume remains constant

10s R 22s 26s 12s

The The The The more reactive the metalmore reactive the metalmore reactive the metalmore reactive the metal, the , the , the , the shorter the time it takes to reach constant volume of gas producedshorter the time it takes to reach constant volume of gas producedshorter the time it takes to reach constant volume of gas producedshorter the time it takes to reach constant volume of gas produced. . . . Hence, from the table, W is the most reactive metal and Z, which does not react with the acid, is the Hence, from the table, W is the most reactive metal and Z, which does not react with the acid, is the Hence, from the table, W is the most reactive metal and Z, which does not react with the acid, is the Hence, from the table, W is the most reactive metal and Z, which does not react with the acid, is the least reactive metal. least reactive metal. least reactive metal. least reactive metal. The order of reactivity in decreasing order is as such: W, Z, Y, M and X where M The order of reactivity in decreasing order is as such: W, Z, Y, M and X where M The order of reactivity in decreasing order is as such: W, Z, Y, M and X where M The order of reactivity in decreasing order is as such: W, Z, Y, M and X where M is the is the is the is the fourth most reactive metalfourth most reactive metalfourth most reactive metalfourth most reactive metal....



Office Tel: 63540340635403406354034063540340



(d) The five metals above: WWWW, XXXX, YYYY, M M M M and ZZZZ can be extracted from their ores using the following methods:

Two of the metals are extracted by electrolysis. Metal MMMM and one other metal are extracted by carbon reduction. One of the metals occurs uncombined. (i) Suggest which is the other metal that is extracted by carbon reduction. Explain your

reasoning. [2] Ans:Ans:Ans:Ans: Metal Y. From reactivity, mMetal Y. From reactivity, mMetal Y. From reactivity, mMetal Y. From reactivity, metals which are etals which are etals which are etals which are less reactiveless reactiveless reactiveless reactive than carbonthan carbonthan carbonthan carbon can be obtained by can be obtained by can be obtained by can be obtained by

carbon reduction. For W and Z are the most reactive metals that can only be extracted carbon reduction. For W and Z are the most reactive metals that can only be extracted carbon reduction. For W and Z are the most reactive metals that can only be extracted carbon reduction. For W and Z are the most reactive metals that can only be extracted by electrolysiby electrolysiby electrolysiby electrolysis, Y and Ms, Y and Ms, Y and Ms, Y and M, the 3, the 3, the 3, the 3rdrdrdrd and 4and 4and 4and 4thththth most reactive metals in the list,most reactive metals in the list,most reactive metals in the list,most reactive metals in the list, must be below must be below must be below must be below carbon in the reactivity series.carbon in the reactivity series.carbon in the reactivity series.carbon in the reactivity series.

(ii) Suggest the name of metal MMMM. [1]

Iron / Tin (AIron / Tin (AIron / Tin (AIron / Tin (Avvvvoid oid oid oid choosing lead)choosing lead)choosing lead)choosing lead)

OROROROR 9. Fractions of crude oil can be cracked to form smaller molecules. There are 2 types of cracking

processes: steam cracking and catalytic cracking. (a) Steam cracking requires a temperature of about 700°Z. This process breaks larger molecules

into smaller ones. Dodecane is a straight chain alkene with the formula ZYc�\\. When one molecule of dodecane is cracked, two possible products are hexane, Z[�Yf and one other compound.

(i) Hexane is also a straight chain alkene. Draw the structure of hexane and give the name

and structure of the other compound. [2]

Ans:Ans:Ans:Ans: Hexane structureHexane structureHexane structureHexane structure Other productOther productOther productOther product nnnnameameameame : butene: butene: butene: butene structure : structure : structure : structure : (ii) Describe a simple test that can be used to distinguish between hexane and the other

product, [2]

Add Add Add Add aqueous bromineaqueous bromineaqueous bromineaqueous bromine into each of the two substances separately in the into each of the two substances separately in the into each of the two substances separately in the into each of the two substances separately in the absence ofabsence ofabsence ofabsence of

ultraviolet raysultraviolet raysultraviolet raysultraviolet rays. The . The . The . The redbrown redbrown redbrown redbrown solution solution solution solution decolourisesdecolourisesdecolourisesdecolourises in in in in butanebutanebutanebutane, but solution , but solution , but solution , but solution

remains redbrownremains redbrownremains redbrownremains redbrown in in in in hexanehexanehexanehexane....

[Must include ‘redbrown solution decolourises’][Must include ‘redbrown solution decolourises’][Must include ‘redbrown solution decolourises’][Must include ‘redbrown solution decolourises’] (b) Catalytic converter can be carried out at a lower temperature of approximately 500°Z. The

catalysts used are usually powdered alumina and silica. This process produces branched chain alkanes. Petrol containing branched alkanes burns more smoothly and leads to more complete combustion.



Office Tel: 63540340635403406354034063540340



(i) Draw the structure of a branched chain isomer of hexane. [1] Accept any:Accept any:Accept any:Accept any: (b) (ii) Write an equation for the complete combustion of hexane. [1]

m|£~x� S x±um → xm|um S x�~mu

(iii) Explain why incomplete combustion in car engines is undesirable. [1]

It prIt prIt prIt produces oduces oduces oduces carbon monoxidecarbon monoxidecarbon monoxidecarbon monoxide which, when inhaled, which, when inhaled, which, when inhaled, which, when inhaled, binds with haemoglobinbinds with haemoglobinbinds with haemoglobinbinds with haemoglobin in in in in

red blood cellsred blood cellsred blood cellsred blood cells and and and and preventspreventspreventsprevents blood from blood from blood from blood from transporting oxygentransporting oxygentransporting oxygentransporting oxygen to body, resulting into body, resulting into body, resulting into body, resulting in

brain damage and even deathbrain damage and even deathbrain damage and even deathbrain damage and even death.... (iv) Explain why the catalysts are made to be in powdered form. [1]

Powdered catalysts have Powdered catalysts have Powdered catalysts have Powdered catalysts have smaller volume, smaller volume, smaller volume, smaller volume, hence hence hence hence larger total surface arealarger total surface arealarger total surface arealarger total surface area for for for for higherhigherhigherhigher

chance of chance of chance of chance of contactcontactcontactcontact with with with with reactant particlesreactant particlesreactant particlesreactant particles such that such that such that such that more particles can start more particles can start more particles can start more particles can start

reactionreactionreactionreaction at a lower activation energyat a lower activation energyat a lower activation energyat a lower activation energy....

[Not accepted: collision theory][Not accepted: collision theory][Not accepted: collision theory][Not accepted: collision theory] (c) There is a large demand for molecules in crude oil that can be used as petrol. Both cracking

processes are important to ensure that the demand is met. Explain how cracking helps to meet such demand for petrol. [2]

CrackingCrackingCrackingCracking breaks downbreaks downbreaks downbreaks down larger fractions larger fractions larger fractions larger fractions such as such as such as such as bitumenbitumenbitumenbitumen that has that has that has that has very little demand but very little demand but very little demand but very little demand but

high supplyhigh supplyhigh supplyhigh supply into into into into smaller moleculessmaller moleculessmaller moleculessmaller molecules like petrol that has a very high demand. As such, thelike petrol that has a very high demand. As such, thelike petrol that has a very high demand. As such, thelike petrol that has a very high demand. As such, the

the demand can be met the demand can be met the demand can be met the demand can be met without without without without the constant need to extract more crude oilthe constant need to extract more crude oilthe constant need to extract more crude oilthe constant need to extract more crude oil....

End of Section BEnd of Section BEnd of Section BEnd of Section B

2012 olevel pure chemistry paper 2 questions and answers

Documents

obtain stable

53176143j

straight chain

dilute sulfuric

dilute hydrochloric

molecular

reaction occurs

hydrochloric