Public Assessment of the HKDSE Chemistry Examination1. Public AssessmentThe public assessment of the Hong Kong Diploma of Secondary Education (HKDSE) Chemistry Exam consists of a public examination component and a school-based assessment component.

A. Public Examination componentIn Chemistry, the mark of the public examination component contributes 80% of the final subject mark.

Paper Section Question types Weighing Duration Curriculum

Paper 1Section A Multiple-choice Questions 18%

2 hours 30 minutes Compulsory Part

Section B Short Questions and Structured / Essay-type Questions 42%

Paper 2 – Structured Questions 20% 1 hour Elective Part (2 out of 3)

B. School-based Assessment componentIn Chemistry, students are assessed by their teachers on their performance of a wide range of skills involved in practical and non-practical related tasks throughout S.5 and S.6. The mark of the school-based assessment component contributes 20% of the final subject mark.

Year of examination Assessment types Weighing2012 and 2013 Practical related component 20%

Starting from 2014Practical related component 16%

Non-practical related component 4%

2. Standards-referenced ReportingThe HKDSE makes use of standards-referenced reporting, which means candidates’ levels of performance will be reported with reference to a set of standards as defined by cut scores on the variable or scale for a given subject. The following diagram represents the set of standards for a given subject:

Cut scores

U 1 2 3 4 5

Variable/scale

Within the context of the HKDSE there will be five cut scores, which will be used to distinguish five levels of performance (1–5), with 5 being the highest. The Level 5 candidates with the best performance will have their results annotated with the symbols ∗∗ and the next top group with the symbol ∗. A performance below the threshold cut score for Level 1 will be labelled as ‘Unclassified’ (U).

I

Exam StrategiesSection A Multiple-choice Questions

1. The statements in the options may be misleading. Do not spend time in reading the 4 options only.

To tackle the problem, follow the steps below:• Read the question (not options) twice or more.• Recall the related situation / knowledge.• Organize the memory.• Read the options. Cross-out the incorrect options if necessary.• Choose the correct answer.

Example:

Which of the following methods is used to test for oxygen?

A. Bubbling the sample gas into limewater.

B. Putting a glowing splint into a gas jar containing the sample gas.

C. Bubbling the sample gas into barium chloride solution.

D. Putting a burning splint into a gas jar containing the sample gas.

Ans: B

2. The given statements may be correct. However, each of them may or may not be a relevant explanation to the situation.

To tackle this kind of question, think critically about the relationship between the situation and each of the options.

Example:

Aluminium can be used to manufacture overhead power cables because it is

(1) cheap.

(2) corrosion resistant

(3) Non-poisonous

A. (1) only

B. (3) only

C. (1) and (2) only

D. (1), (2) and (3)

Ans: C

II

Fossil Fuels and Carbon Compounds

55 Fossil Fuels and Carbon Compounds

Fossil Fuels and Carbon Compounds

Fossil Fuels Homologous Series Burning Fuels Consequences of Using Fossil Fuels

Alkanes Alkenes

Fossil fuels•Coal,petroleumandnaturalgasarefossilfuels.

•Theyareformedfromdeadplantsandanimalsthatlivedmillionsofyearsago.

•Allfossilfuelsarenon-renewableandlimited.

Petroleum•Petroleum(crudeoil)isadarkbrownliquidcontainingmanyhundredsofdifferentsubstances.

•Petroleumcanbeseparatedintodifferentfractionsbyfractionaldistillation.

•Distilledfractionsofpetroleumhavemanyuses,e.g.,providingfuelforenergyproductionandtransportation.

Physical properties•Gradualchangeinphysicalproperties

•C1–C4:gases C5−C17:liquids C18orhigher:solids

Physical properties•Similartoalkanes•Non-polarandinsolubleinwater,butsolubleinorganicsolvents

Energy changes of reactionsA+B →C+D ∆H=-ve/+ve•Exothermicreaction (∆H=-ve)•Endothermicreaction (∆H=+ve)

Air pollutants•Carbonmonoxide•Unburnthydrocarbons•Suspendedparticulates•Sulphurdioxide•Nitrogenoxides

Energy sources•Commonlyusedfuelsincludepetrol,kerosene,dieseloil,LPGandtowngas.

Chemical properties•Combustion Alkene+O2→CO2+H2O•Additionreaction

C Y+

C C

X Y

XC →

•Additionpolymerisation

C

H

H H

H X

H

H X

C

n

n C C →

Chemical properties•Combustion Alkane+O2→CO2+H2O•Substitutionreaction Alkane → Haloalkane + + X2 HX•Cracking–Thebreakdownofalargemoleculeintosmallmolecules

Acid rain•AcidrainhasapHvaluelowerthan5.6.

•Acidrainisformedwhensulphurdioxideandnitrogendioxidedissolveinwater.

•Acidraincandamagebuildings,makesoilinfertileandharmaquaticlife.

Methods to reduce air pollutants•Reducingairpollutantsfrommotorvehicles–Usingunleadedpetrol– Installingcatalyticconverters

–ReplacingdieselwithLPG•Reducingairpollutantsfromindustry– Installingelectrostaticprecipitators

– Installingscrubbers

Alternative energy sources•Nuclearpower•Solarenergy•Hydroelectricpower•Geothermalenergy•Tidalenergy•Windpower•Energyfrombiomass

Global warming•TheCO2producedfromtheburningoffossilfuelsleadstoglobalwarming.

•ThetemperaturerisemaycausegreatchangesontheEarth.

4

Chemistry: Key Notes & Exambuilder

FossilFuels

Coal,petroleumandnaturalgas

• Fossil fuelsaresubstancesthatwereformedfromdeadplantsandanimals

whichlivedmillionsofyearsago.

• Coal,petroleum(crudeoil)andnaturalgasarethemajorfossilfuels.

• Fossilfuelsareveryimportantinourdailylives.Thefollowingfiguresshow

howweusefuelsasenergysourcetocookfood,generateelectricityandget

fromplacetoplace:

Barbecue

Bus

LPGtaxi

PowerplantFigure 5.1

5.1

A

• Doyouknowthatcoal,petroleumandnaturalgasarefossilfuels?

• Doyouknowthatpetroleumisamixtureofhydrocarbons,andcanbeseparatedintousefulfractionsbyfractionaldistillation?

• Whatarethemajorusesofdistilledfractionsofpetroleum?

• Howdoesthenumberofcarbonatoms inthemoleculesofvariousfractionsrelatetothegradationoftheirproperties?

HKEP

12

Chemistry: Key Notes & Exambuilder

Functionalgroups

• Afunctionalgroupisanatomoragroupofatomsresponsibleformostof

thechemicalreactionsofthememberofthehomologousseries.

• Thefollowingtablelistssomeexamplesoffunctionalgroups:

Homologous series Alkene Alkanol Alkanoicacid

Functional groupC C OH C

O

OH

Table 5.5

• Differenthomologousserieswithdifferentfunctionalgroupsareshownin

thefollowingtable:

Table 5.6

Homologous series

General formula Functional group

Structural formula of the first member of the

homologous series

Alkane CnH2n+2 — H

H H

H

C

Methane

Alkene CnH2n C C H H

H HC C

Ethene

Alkanol CnH2n+1OH OH H

H OH

H

C

Methanol

Alkanoicacid CnH2n+1COOH O

OHC

O

OHCHMethanoicacid

Ester R COOR ′ O

OC

O

OCH

H

HC

HMethylmethanoate

C

The ‘−COOH ’ group is

responsible for the acidic

properties of alkanoic acid.

The discussion of detecting

various functional groups

in carbon compounds is in

the elective part ‘Material

Chemistry’.

Alkenes must contain at

least two carbon atoms,

otherwise they would not

have C = C double bonds.

• ‘CnH2n+1’ is sometimes

simplified as R.

• R and R’ can be the

same or different.

57

Fossil Fuels and Carbon Compounds

acid rain 酸雨 fraction 餾分

addition reaction 加成反應 functional group 官能基

addition polymer 加成聚合物 geothermal energy 地能熱

addition polymerisation 加成聚合作用 global warming 全球增温

alcohol 酒精 greenhouse effect 温室效應

alkane 烷烴 greenhouse gases 温室氣體

alkanoic acid 烷酸 haemoglobin 血紅素

alkyl group 烷基 homologous series 同系列

biomass 生物量 hydrocarbon 烴 / 碳氫化合物

carbon monoxide 一氧化碳 hydroelectric power 水力發電

catalyst 催化劑 hydrolysis 水解作用

catalytic converter 催化轉化器 incinerator 焚化爐

catalytic cracking 催化裂解作用 kerosene 煤油 / 火水

coal 煤 lead 鉛

combustion 燃燒 liquefied petroleum gas / LPG 液化石油氣

condensation polymerisation 縮合聚合作用 man-made polymer 人造聚合物

cracking 裂解作用 naphtha 石腦油

crude oil 原油 natural gas 天然氣

diesel oil 柴油 natural polymer 天然聚合物

electrostatic precipitator 靜電沉積器 nitrogen oxides 氧化氮

endothermic reaction 吸熱反應 non-renewable energy source 不可更新的能源

ester 酯 nuclear power 核能

exothermic reaction 放熱反應 oil spill 石油洩漏

fossil fuel 化石燃料 organic compound 有機化合物

fractional distillation 分餾 perspex 有機玻璃

fractionating tower 分餾塔 petrol 汽油

102

Chemistry: Key Notes & Exambuilder

1. Whatis thebondangleof theB–ClbondinBCl3? Hint 1

A. 180°

B. 120°

C. 109.5°

D. 90°

2. Which of the following molecules has atrigonalplanarshape? Hint 2

A. SO3

B. SO2

C. H2O

D. I3-

3. Whichofthefollowingmoleculesisnon-polarbutpossessesapolarbond(s)? Hint 3

A. F2

B. H2O

C. HF

D. BF3

4. Trimethylamine, Me3N, reacts with borontrifluoride, BCl3, to form a compound offormulaMe3N BCl3.[Me=CH3]

How may th i s r eac t ion be wr i t t en interms of the shapes of the reactants andproducts? Hint 4

A.

Figure 6.108

B.

Figure 6.109

C.

Figure 6.110

D.

Figure 6.111

SectionA Multiple-choiceQuestions

114

Chemistry: Key Notes & Exambuilder

7.1A. Determine whether each of the following statements is correct. Put a 3 in the appropriate box.

Correct Incorrect

(a) Anoxidizingagentisasubstancewhichisoxidizedduringredoxreaction.

(b) Activemetalsarecommonstrongreducingagents.

(c) DiluteHCl(aq)isusedtoacidifyKMnO4andK2Cr2O7.

(d) Concentratedsodiumsulphatesolutionisastrongoxidizingagent.

(e) NH3+HNO3→NH4NO3isaredoxreaction.

(f ) TheoxidationnumberofsulphurinH2S2O7is+6.

B. Fill in the blanks.

• Both reduction and oxidation must occur together. This type of reaction is called a (1) reaction.

• Any redox reactions must involve change in (2) number.

• A / An (3) agent is a substance which can cause another substance to be oxidized.

• A / An (4) agent is a substance which can cause another substance to be reduced.

(For answers, see the bottom of the page.)

Suggested Answers (Check Your Progress 7.1)

(a) Incorrect. An oxidizing agent is reduced during redox reaction. It causes other substance to be oxidized.

(b) Correct.

(c) Incorrect. The Cl ions in HCl(aq) will be oxidized to Cl2(g) by KMnO4(aq).

(d) Incorrect. SO42- can be reduced to SO2 by concentrated sulphuric acid but not sulphate salt solution.

(e) Incorrect. There is no change in oxidation number.

(f) Correct.

(1) redox (2) oxidation (3) oxidizing (4) reducing

136

Chemistry: Key Notes & Exambuilder

chlorine氯 redox reaction氧化還原反應

electrochemical series電化次序 reducing agent還原劑

nitric acid硝酸 reduction還原作用

oxidation氧化作用 sulphur dioxide二氧化硫

oxidation number氧化數 sulphuric acid硫酸

oxidizing agent氧化劑

ConceptCheck

Topics Common errors Correct concepts

Acidifyingagent DiluteHCl(aq)isusedtoacidifyKMnO4(aq)The acidifiedKMnO4(aq)will oxidize theCl-(aq) ions to Cl2(g). Instead, di luteH2SO4(aq)shouldbeused.

Cracking

ThehalfequationforacidifyingK2Cr2O7(aq)is:

Cr2O72-(aq)+14H+(aq)+9e-C

Cr3+(aq)+7H2O(l)

Be careful that Cr2O72-(aq) will become

2Cr3+(aq)ionsinthereaction.

Thehalfequationis:

Cr2O72-(aq)+14H+(aq)+6e-C

2Cr3+(aq)+7H2O(l)

OxidizingagentMg(s)+2HCl(aq)→MgCl2(aq)+H2(g)

Theabovereactionisnotaredoxreaction.

It is a redox reaction as the O.N. of Mgchangesfrom0to+2andthatofHchangesfrom+1to0.

Usually,people regardHCl(aq) as an acid.Actually,itisalsoanoxidizingagent.

OxidizingagentSodiumionisastrongeroxidizingagentthanzincion.

Althoughsodiumatomisastrongerreducingagentthanzincatom,sodiumionisaweakeroxidizingagentthanzincionaccordingtotheE.C.S.

Oxidizingandreducingagent

Asubstancecannotbeoxidizedandreducedatthesametime.

Substancescanbeoxidizedandreducedatthesametime.Forexample:

Cl2(g)+2NaOH(aq)→

NaCl(aq)+NaOCl(aq)+H2O(l)

Itiscalledadisproportionationreaction.

148

Chemistry: Key Notes & Exambuilder

Comparisons of the reactivity of differentmetalsbyusingasimplechemicalcell

Comparing the reactivities of four metals: Mg, Zn, Pb and Ag

V

clips

voltmeter

voltmeter

metal stripunder test

copper sheet

filter paper moistenedwith sodium chloridesolution

=V

metal stripunder test

copper sheet

filter papermoistened withsodium chloridesolution

Figure 8.8

• Procedure:

– Cleanthemetalstripandcoppersheetwithsandpaper.

– Moistenapieceoffilterpaperwithsodiumchloridesolution.

– Putthemoistenedfilterpaperbetweenthemetalstripandthecopper

sheet.

– Connectthecoppersheettothepositiveterminalofthevoltmeterand

themetalstriptothenegativeterminalofthevoltmeter.

– Notethedirectionofelectronflowandthevoltage.

• Resultsanddiscussion:

1. Theresultsoftheexperimentareasfollows:

Metal under test

Direction of electron flow in the external circuit

Voltage recorded

Mg MgtoCu +1.42V

Zn ZntoCu +1.10V

Pb PbtoCu +0.46V

Ag CutoAg -0.17V

Table 8.2

Vmagnesium

copper

filter papermoistened withsodium chloridesolution

+−+1.42V

electron flow

Vsilver

copper

+−−0.17 V

electron flow

Figure 8.9

D

165

Chemical Cells

HKEP

Lithium ion battery

Alithiumionbattery is fromthefamilyofrechargeablebatterytypes. In the battery, lithium ions move from the negativeelectrode (anode) to thepositive electrode (cathode) throughthenon-aqueous electrolyte and separatordiaphragmduringdischarging,andfromthecathodetotheanodeduringcharging.Dependingon thekindsofmaterials a lithium ionbattery ismadeupof,thevoltage,capacity,lifeandsafetyofthebatterycanchangedramatically.

Lithium ionbatteries are one of themost popular types ofrechargeable batteries common used in different kinds ofconsumerelectronics. It isbecause theyhavehighenergydensities,nomemoryeffect,andaslow lossofchargewhennotinuse.Lithiumionbatteriesarealsogrowinginpopularityformilitary,electricvehicleandaerospaceapplications.

Thetypesoflithium-ionbatteriescangenerallybecategorizedasfollows:

• Smallcylindrical(solidbodywithoutterminals,e.g.,laptopbatteries)

• Largecylindrical(solidbodywithlargethreadedterminals)

• Pouch(soft,flatbody,e.g.,cellphonesbatteries)

• Prismatic(semi-hardplasticcasewithlargethreadedterminals)

Figure 8.28

Suggested Answers (STSE Connections)

(a) There is no loss of material from the battery during the charging and recharging process.

(b) Li+ + e- → Li

(c) They have high energy densities.

OR They have no memory effect.

OR They have a slow loss of charge when not in use.

OR They have a wide variety of shapes and sizes.

OR They are lighter than most of the other energy-equivalent secondary batteries.

(Any four)

(a) Explainwhyalithiumionbatteryisrechargeable.

(b) Duringdischarging,write thehalfequationof thecathodicreaction fora

lithiumionbattery.

(c) SuggestFOURadvantagestousingalithiumionbattery.

HKEP

207

Electrolysis

Section A Multiple-choice QuestionsHint 1 Thechoiceofelectrodesshouldbeconsidered.

Hint 2 Asaltoftheelectroplatingmaterialisusedastheelectrolyteforelectroplating.

Hint 3 Considerthenatureoftheelectrodes.

Hint 4 Recalltheexperimentalset-upforthepurificationofcopper.

Hint 5 Whatchargedoescationcarry?

Section B Short Questions and Structured / Essay-type Questions

Hint 6 Issugaranelectrolyte?

Hint 7 Don’tgivethechemicalformulaonly.Thenameofthechemicalisrequiredhere.

Hint 8 Thenatureoftheelectrodesisafactorwhichaffectsthepreferentialdischargeofions.

Hint 9 Ioniccompoundshavemobileionswhentheyareinmoltenoraqueousstates.

Hint 10 Acidicandalkalinesolutionsturnlitmussolutiontoredandbluerespectively.

Hint 11 DoNOTgiveformulaeastheanswers.

Hint 12 Seawatercontainsmobileions.

Hint 13 Stateclearlythe+veand-vepoleofthea.c.supply,andgivethecontrolexperiment.

Hint 14 IfnoFe2+ionisformed,norustingwilloccur.

Hint 15 Considerthechangeofoxidationnumbers.

Hint 16 Payattentiontothereactivityoftheimpuritiescomparedtocopper.

Hint 17 Recall‘RedCat’.

Hint 18 Studentsshouldanswer‘true’or‘false’first.TheformulaofXNO3(aq)providestheinformationaboutthechangeintheXn+ion.

Hint 19 ThecolourofCuSO4(aq)solutionisblue.

Hint 20 Thisisanessay-typequestion,sopointformisNOTacceptable.

248

Chemistry: Key Notes & Exambuilder

RateofReaction4.8

This section is written in accordance with the HKDSE Combined Science (Chemistry) Examination Syllabus. Students studying the HKDSE Chemistry Examination Syllabus should skip this section and study Chapter 11 in Book 3.

Syllabus Reminder

Introduction to rate of reaction

• Therateofachemicalreactionistherateatwhichtheextentofthereaction

proceedswithtime.

• This is a positive quantitative expression that describes how the

concentrationofareactantorproductchangeswithtime.

• Rate

Fast

e.g., Precipitation,

Displacementreaction

e.g., Rusting,

Weatheringofmarblebyacid

Slow

Figure 1

• Thereasonsthatchemistsstudyratesofreactionaretoenhancetherateof

productionofachemicalandinvestigatethemechanismofthereaction.

• For any reaction, the concentrationsof the reactantsdecrease and the

concentrationsoftheproductsincreasewithtime.

• Therateofreactionisexpressedasthechangeofconcentrationofareactant

orproductinagiventimeinterval.

Rateofreaction=-

D[reactant]

Dtor

D[product]

Dt

unit:moldm-3s-1

• Initial rateisthereactionrateatthestartingpoint(t=0).

• Instantaneous rateisthereactionrateatanygiventimeinterval.

• Average rateistheaveragereactionratemeasuredoveraperiodoftime.

A

• D[ ] means change of

concentration.

• The rate of reaction is

inversely proportional to

the length of time, i.e.,

rate ∝ 1

time.

• Rate of react ion is a

pos i t i ve quant i ta t i ve

e x p r e s s i o n . S o a

negat ive s ign should

be given to the change

of concentration of the

reactant.

285

Question Commands

Question CommandsQuestion command(s) Example(s)

What / Which ...

(Simple answer is usually required.)

What gas evolves?

Correct answer: Sulphur dioxide / SO2

What is the direction of electron flow in the external circuit?

Correct answer: From left to right

Which of the following compounds can be used to make an addition polymer?

H

C HC = C H2N NH2 OH

Correct answer: H

C HC = C

Suggest a formula ... The oxide of aluminium is insoluble in water, suggest the formula for this oxide.Correct answer: Al2O3

Incorrect answer: Aluminium oxideName ...

(Formula / Structure is NOT accepted.)

Name an element which is a metalloid.Correct answer: BoronIncorrect answer: B

Write the chemical equation ...

(Al though both chemical equation and ionic equation a r e a c c e p t e d , t h e b e s t answer should be a chemical equation.)

Write a chemical equation for the reaction when adding dilute hydrochloric acid to zinc granules.Correct answer: Zn + 2HCl → ZnCl2 + H2 (chemical equation)Poor answer: Zn + 2H+ → Zn2+ + H2 (ionic equation)

Write the chemical equation ... Write a chemical equation for the reaction between sodium and water. State symbols should be given.Correct answer: 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) (Score 2 marks)Poor answer: 2Na + 2H2O → 2NaOH + H2 (Score 1 mark only) (Remarks: 1 mark for equation and 1 mark for state symbols)

Write an ionic equation ... Write an ionic equation for the reaction when adding hydrochloric acid to sodium carbonate.Correct answer: 2H+ + CO3

2- → H2O + CO2

Incorrect answer: 2HCl + Na2CO3 → H2O + CO2 + 2NaCl

2

Chemistry: Key Notes & Exambuilder

Section A Multiple-choice Questions

1. A

2. B

Fossilfuelsincludecoal,petroleumandnaturalgas.

Coalissolidandwasformedfromdeadplants. So(1)and(2)areincorrect. Henceonly(3)mustbecorrect.

3. D

4. B

5. D

6. D

7. D

8. C

AlkenescandecolourizeKMnO4/H+butNOT

K2Cr2O7/H+.

9. D

10. B

11. B

Section B Short Questions and Structured/Essay-type Questions

1. H HH H H

C C CH HC C

H HH H H

1

Pentane 1

H HCH3 H

C C HH C C

H HH H

1

Methylbutane 1

H HCH3

CH3

C HH C C

H H

1

2,2-dimethylpropane/Dimethylpropane 1

2. (a) Brown fumesofBr2disappear. 1 CH3CH3+Br2→CH3CH2Br+HBr 1

(b) Acidifiedpotassiumpermanganatesolutionchanges from purple to colourless. 1

CH2=CH2+[O]+H2O→CH2-CH2 1

(c) Brominesolutionchanges from orangetocolourless. 1

CH3CH=CHCH3+Br2→CH3CHCHCH3 1 Br Br3. Treat propane and propenewithBr2inCH3CCl3

separatelyintheabsence of sunlight. 1 Propenereadilyturns Br2 inCH3CCl3from red-

orange to colourlessintheabsenceofsunlight. 1 Br2 can undergo addition reaction with propene

but cannot undergo any reaction with propaneintheabsence of sunlight. 1

CH3CH=CH2+Br2→CH3CHCH2 Br Br 1OR Treat propane and propenewithacidified

potassium permanganate solutionseparately. (1) Propene readily turns KMnO4 / H

+ from purple to colourless. (1)

KMnO4 / H+ can oxidize propene but not

propane. (1) CH3CH=CH2+[O]+H2O→CH3CHCH2 OHOH(1)

Chapter 5 Fossil Fuels and Carbon Compounds