Public Assessment of the HKDSE Chemistry Examination

Public Assessment of the HKDSE Chemistry Examination1.PublicAssessmentThe 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. PublicExaminationcomponentIn 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-basedAssessmentcomponentIn 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-referencedReportingThe 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).

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Exam StrategiesSectionA Multiple-choiceQuestions

1. Thestatementsintheoptionsmaybemisleading.Donotspendtimeinreadingthe4optionsonly.

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. Thegivenstatementsmaybecorrect.However,eachofthemmayormaynotbearelevantexplanationtothesituation.

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

Planet Earth

Planet Earth

Planet Earth

Composition of airThe present atmosphere contains nitrogen, 78%; oxygen, 21%; noble gases, 0.9%; carbon dioxide, 0.04% and other gases.

Rocks as the source of minerals• An ore is a rock that has a lot of

metal compounds.• Ores are usually metal oxides

and sulphides which are mixed with impurities.

Separation of oxygen and nitrogen from liquid air• Air can be separated into oxygen, nitrogen

and other gases.• The steps of fractional distillation of liquid air:

– air is liquefied by repeated cooling and compression; and

– the liquid air is then separated by fractionating column.

• Nitrogen boils as a gas first (−196ºC), and then oxygen (−183ºC).

Extraction of common salt from sea water• Filtration, evaporation

and crystallization of sea water

Properties of oxygen• Physical properties

– Colourless and odourless gas– Dissolves in water slightly– Supports combustion

• Chemical properties– Metals react with oxygen to form basic

oxides.– Non-metals react with oxygen to form

acidic oxides.

Test for the ions in a sample of common salt• Test for sodium ions:

Use flame test to test for sodium ions, a golden yellow flame is observed.

• Test for chloride ions: Use acidified silver nitrate to test for chloride ions, white precipitate is formed.

Test for the presence of calcium ions and carbonate ions in a sample of limestone, chalk or marble• Use flame test to test for calcium

ion in the sample which give a brick-red flame.

• Add dilute hydrochloric acid to the sample containing carbonate ion, the gas evolves which turns limewater milky.

Test for the presence of water in a sample• Use cobalt chloride

paper (blue to pink) and anhydrous copper(II) sulphate (white to blue) to test for the presence of water.

• Use melting point or boiling point tests to test whether the water sample is pure or not.

Test for oxygenRelights a glowing splint which can be used to test for oxygen gas.

Isolation of useful materials from minerals• Mine the ore• Concentrate the ore• Extract the metal from the

concentrated ore• Purify the impure metal

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Isolation of pure water from sea waterSimple distillation is used to separate common salt from water.

Limestone, chalk and marbleCalcium carbonate is the major component of limestone, chalk and marble.

Weathering and erosion of rocks• Chemical means• Physical means

Thermal decomposition of calcium carbonateCalcium carbonate → Calcium oxide + Carbon dioxide

Test for carbon dioxideBubble carbon dioxide through limewater, it turns limewater milky.

Atmosphere Oceans

Composition of sea waterThe most common ions in sea water are sodium and chloride ions.

Electrolysis of sea water and the uses of its products

Rocks and Minerals

4

Chemistry: Key Notes & Exambuilder

TheAtmosphere

Compositionofair

• Airisamixtureofgaseswhichmakeuptheatmosphere.

• Theearlyatmospherewasmadeupofcarbondioxide,nitrogen,methane,

watervapour,ammoniaandcarbonmonoxide,butwithoutoxygen.

Figure 1.1

• Thepresentatmosphereismainlymadeupofnitrogenandoxygen.

• Thepresentatmospherecontains:

– nitrogen,78%;

– oxygen,21%;

– noblegases,0.9%

– carbondioxide,0.04%;and

– othergases.

• TheEarth isuniqueamongallplanets in theSolarSystemas ithasan

abundanceoffreeoxygen.

GuidedExample1

Theearlyatmosphereof theEarthcontainedmethaneandcarbonmonoxide,but

verylittleoxygen.Atthattime,temperaturewashighandthereweremanystorms

andalotoflightning.

The1.1

A

majorcomponentsinair

• Whatisthecompositionofair?

• Howtoseparateoxygenandnitrogenfromliquidair?

• Whatarethepropertiesofair?

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Propertiesofoxygen

(I) Physicalpropertiesofoxygen

• Oxygenisacolourlessandodourlessgas.

• Itisslightlydenserthanair.

• Itdissolvesslightlyinwater.

• Itdoesnotburnbutsupportscombustion.

(II) Chemicalpropertiesofoxygen

• Manyelementsreactwithoxygentoformoxides,forexample:

– metalsreactwithoxygentoformbasicoxides;and

– non-metalsreactwithoxygentoformacidicoxides.

• Allreactionsinvolvingoxygenwouldreleaseenergy(heat).

(III)Testforoxygen

• Oxygensupportscombustionwhichallowssubstancestoburnin it.

Wecanmakeuseofthispropertytotestwhetheranunknowngasis

oxygenornot.

Test for oxygen

• Procedure:

Figure 1.3

– Prepareoxygeninaboilingtube.

– Prepareaglowingsplint.

– Puttheglowingsplintintotheboilingtubeandobserveanychangeof

theglowingsplint.

• Resultanddiscussion:

– Theglowingsplintrelighted.

– Whenaglowingsplintisputintoatesttubecontainingthegastobe

tested,theglowingsplintwillrelightifthegasisoxygen.

C

A glowing splint is prepared

by lighting a wooden splint

and then blowing the flame

out.

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Atmosphere大氣層 Flame test焰色試驗

Bauxite鋁土礦 Fractional distillation分餾

Calcium carbonate碳酸鈣 HaematiteHaematite赤鐵礦赤鐵礦

Calcium hydrogencarbonate碳酸氫鈣 Limestone石灰石

Carbon dioxide二氧化碳 Limewater石灰水

Chalk白堊 Marble大理石

Cobalt chloride paper氯化鈷試紙 Mineral礦物質

Combustion燃燒 Mining採礦

Common salt食鹽 Nitrogen氮氣

Condensation凝結 Noble gas貴氣體

Crystallization結晶 Odourless無氣味的

Decomposition分解 Ore礦石

Disintegration分裂 Oxygen氧氣

Distillation蒸餾 Purification提純

Erosion侵蝕 Sodium chloride氯化鈉

Evaporation蒸發 Thermal decompositionThermal decomposition熱分解熱分解

ExtractionExtraction提取提取 Weathering風化

Filtration過濾

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Suggested Answers (Check Your Progress 2.1)

(a) Incorrect. It is because the number of outermost shell electrons would not be greater than 8.

(b) Correct. It is because ‘Mass number = Number of protons + Number of neutrons’.

(c) Correct. It is because they have the same atomic number but different mass number.

(d) Correct. They have the same chemical properties because they have the same electronic arrangement.

(e) Incorrect. They have the same chemical properties. Hence they cannot be separated by chemical methods.

(1) nucleus (2) same (3) different (4) same (5) mass

(6) average (7) 2, 8, 8, 2

2.1A. Determine whether each of the following statements is correct. Put a 3 in the appropriate box.

Correct Incorrect

(a) Theelectronicarrangementoftheelement3919Xis2,8,9.

(b) Theatomicnumberandmassnumberofamagnesiumatomare12and24respectively.Itsnucleuscontains12neutronsand12protons.

(c) 3517Xand37

17XaretheisotopesofelementX.

(d) 3517Xand37

17Xhavethesamechemicalproperties.

(e) 126Cand14

6Ccanbeseparatedbychemicalmethods.

B. Fill in the blanks.

• The protons and neutrons are located in the (1) .

• Isotopes are different atoms of the same element which have the (2) atomic number but (3) mass numbers.

• Isotopes have the (4) chemical properties because of the same electronic arrangement.

• Isotopes have different physical properties because of the difference in (5) .

• The relative atomic mass of an element is the (6) isotopic mass of the natural isotopes.

• The electronic arrangement of a calcium atom is (7) .

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

2.12.12.12.1

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Notes:

1. Theoppositechargedionsareheldtogetherbystrongionicbonding.

2. GroupIVnon-metalelementswouldnotformionsbecausetheyneed

togainorlose4electronstoattainanoblegasstructure.

3. Thefollowingtableisusedtopredictthecharge(s)ofanion:

GroupPeriod I II III IV V VI VII 0

2 Li+ Be2+ N3- O2- F-

3 Na+ Mg2+ Al3+ S2- Cl-

4 K+ Ca2+ Br-

Table 2.15

GuidedExample5

Withthehelpofelectronicdiagram,describetheformationofcalciumfluoridefrom

theatomsoftherelevantelements.

Ans. Eachcalciumatomgives twoelectronsandeachfluorineatomgainsoneelectrontoformanioniccompound.

Figure 2.15

GuidedExample6

Drawtheelectronicdiagramsofthefollowingcompounds,showingelectronsinthe

outermostshellsonly:

(a) Sodiumhydride (b) Calciumoxide (c) Magnesiumchloride

(d) Potassiumoxide (e) Aluminiumoxide

Ans.

(a)

Figure 2.16

1. Postive charge(s) on

an ion = group number

(metals)

2. Negative charge(s) on

an ion = group number

(non-metals) - 8

3. G ro u p 0 e l e m e n t s

w o u l d n o t f o r m

i ons because t hey

have duplet or octet

structures.

theDo NOT d raw a l l t he

e lectron she l ls un less

requested.

56


Afterthecircuithasbeenclosedforsometime,awhitepatchisformedbetweenX

andY.

(a) Whatisthemainaimoftheexperiment?

(b) Whatisthewhitepatch?Explainyouranswer.

Ans. (a) Themainaimistoshowthationsexistinsilvernitrateandsodiumchloride.

(b)Silverchloride/AgCl• Ag+(aq)atXwillrepelthepositivepoleandmovetowardsYandCl-

(aq)atYwillalsorepelthenegativepoleandmovetowardsX.• Whiteprecipitateappearedduetotheformationofsilverchloride.

Ag+(aq)+Cl-(aq)→AgCl(s)

(V) Structure of ionic compounds

• The ioniccompoundsaremadeupofpositivelyandnegativelyby

chargedions.

• Theoppositecharged ionsareheld togetherby strongelectrostatic

attractiveforces.Theyarrangethemselvesinaregularpatterntoform

acrystallattice.

Figure 2.23

(VI) Properties of ionic compounds

• Theyarehardandhavehighmeltingandboilingpoints.

– This isbecausetheoppositechargedionsareheldtogetherbya

strongionicbonds orstrongelectrostaticattractiveforces.

• Conductivityofelectricity

– Theycannotconductelectricityinsolidstate.

– Theycanconductelectricityinmoltenoraqueousstatesbecause

theyhavemobileionsinthesetwostates.

• Ioniccompoundsareusuallysoluble inwaterbut insoluble innon-

aqueoussolutions.

• Twomaintypesof ioniccrystalsarerepresentedbysodiumchloride

andcaesiumchloride.

Solid lines should not be

used because solid l ine

represents covalent bond.

Solid lines should not be

used because solid l ine

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Microscopic World I

1. Which o f t he fo l lowing i s a co r rec trepresentation of the atomic structure ofanatom? Hint 1

Numberof Numberof Numberof protons neutrons electrons

A. 30 30 65

B. 35 35 30

C. 30 35 30

D. 35 30 35

2. Which of the following pairs of atomicnumbersrepresentstheelementswithsimilarchemicalproperties? Hint 2

A. 4and13

B. 8and17

C. 7and14

D. 11and19

3. Thetablebelowgivessomeinformationabouttwoelements,PandQ:

Element Relative atomic mass

3P 6.9

8Q 16

Table 2.26

ElementPcancombinewithelementQ toformacompoundR.Whatare the formulaandformulamassofR? Hint 3

FormulaofR FormulamassofR

A. PQ 22.9

B. PQ2 38.9

C. P2Q 29.8

D. P2Q3 61.8

4. Theelectronicarrangementsoffourchemicalparticlesareshownbelow: Hint 4

W:2,8,1

X-:2,8,8

Y+:2,8,8

Z: 2,8

Whichof theelementsW,X,YandZare inthesameperiodofthePeriodicTable?

A. WandXonly

B. XandYonly

C. YandZonly

D. W,XandYonly

5. What is theatomic structureof theX3- ioninwhichXhasatomicnumber15andmassnumber31? Hint 5

Numberof Numberof Numberof electrons protons neutrons

A. 15 15 16

B. 16 16 31

C. 18 15 16

D. 31 16 15

SectionA Multiple-choiceQuestions

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Microscopic World I

Section A Multiple-choice QuestionsHint 1 Recallthedefinitionsofatomicnumberandmassnumber.

Hint 2 Thinkabouttherelationshipbetweennumberofelectronsintheoutermostshellandchemicalproperties.

Hint 3 ThinkaboutthechargesofPandQ,andusecrossmethodtoworkouttheformula.

Hint 4 Sameperiodmeansthesamenumberofelectronshells.

Hint 5 X3-ionhasthreemoreelectronsthanX.

Hint 6 Recallthedefinitionofisotope.

Hint 7 FindthenumberofelectronsinatomsXandYfirst.

Hint 8 Thinkaboutthestatesofiodineandmercuryatroomtemperatureandpressure.

Section B Short Questions and Structured / Essay-type Questions

Hint 9 Notetheatomicnumbersoftheseatoms.

Hint 10 NooxideformulameansthatXwouldnotreactwithoxygen.

Hint 11 Covalentbondisformedbythecombinationofnon-metalsonly.Ionicbondisformedbythe combinationofmetalsandnon-metals.

Hint 12 ChecktheelementswhichareplacedinGroupIIandPeriod3inthePeriodicTable.

Hint 13 Themeltingpointdependsonthebondstrength.

Hint 14 Thepropertiescanbepredictedbyusingthesamegroupofcompoundwhichisalreadyknown.

Hint 15 Interactionmeansbondingorforces.

88


Metal Uses Reasons for use

Lead (Pb) Car batteries • Dense and heavy• Corrosion resistant• Unreactive

Tin (Sn) Tin plates (food can) • Non-poisonous• Unreactive• Malleable• Corrosion resistant

Table 3.2

Metals used in spacecraft and air craft

Aluminium is the most abundant metal and titanium is the ninth most abundant element in the Earth’s crust. Titanium can form an oxide with the formula ‘TiO2’. In extracting titanium from its oxide (TiO2), it is first converted to titanium(IV) chloride. This compound is then converted to titanium which is achieved by heating it with magnesium in an atmosphere of argon.

In spacecraft, titanium is used rather than magnesium. However, magnesium and its alloys are used in aircraft construction. In space, the temperature is very low, but on re-entering the atmosphere the surface temperature of the spacecraft becomes very high.

The following table summarizes the relevant information about magnesium and titanium:

Metal Melting point / °C Boiling point / °C Density in g / cm3

Magnesium High High 1.7

Titanium Very high Very high 4.5

Table 3.3

Suggested Answers (STSE Connections)

(a) Magnesium has quite a low melting point compared to titanium so it would melt on re-entering the atmosphere.

Spacecraft carries oxygen. In a fire, magnesium burns in oxygen to give magnesium oxide. This is very dangerous reaction in a spacecraft.

(b) Magnesium has a low density.

Use the information given in the above table and your chemical knowledge to

explain

(a) how TWO properties of magnesium makes it less suitable than titanium for

making spacecraft.

(b) how ONE property of magnesium makes it suitable for aircraft construction.

HKEP

185

Acids and Alkalis

Numberofmoles

=Massofsubstance(g)

Molarmass(g/mol)

=Numberofparticles

Avogadro’snumber

Molarityofasolution(Mormoldm-3)

=Numberofmolesofsolute

Volumeofsolution(dm3)

Concentrationofasolution(gdm-3)

=Massofasubstance(g)

Volumeofsolution(dm3)

Concept Check

Topics Common errors Correct concepts

Strengthofacid5MCH3COOH(aq) isastrongeracidthan1MHCl(aq).

Strengthofanaciddependsonthedegreeofionizationof theacid.So1MHCl(aq) isastrongeracidthan5MCH3COOH(aq).

Neutralisation

Whencarbondioxideisbubbledintosodiumhydroxide solution, there is no reactionbetweencarbondioxideandsodiumhydroxidesolutionbecausethereisnoobservablechange.

CO2 reacts with NaOH to form solublesubstanceandnogasevolves.Sonoobservablechange.Butheatisevolvedandhencethereisreaction.

TitrationThecolour changeof the endpointof thetitrationusingmethylorangeisfromyellowtoredorredtoyellow.

Thecolourchangeof theendpoint is fromyellowtoorangeorredtoorange.

PreparationofsaltAdding calcium carbonate to magnesiumsulphate solution, magnesium carbonatewouldbeprepared.

Calciumcarbonate is insoluble inwaterandsoithasnomobile(CO3

2-) ions.Therefore,noreactionoccurs.

PreparationofsaltToprepare copper(II) sulphate, add excessdilutesulphuricacidtocopper(II)oxideandwarmthemixture.

Excess copper(II) oxide should be usedbecauseitistoensurethattheproductisnotcontaminatedwithsulphuricacid.

Preparationofsalt

After crystallization, the crystals obtainedshouldbedriedbyheating.

Duringheating,thecrystalsobtainedwillbedecomposedor thewaterof crystallizationwillberemoved.Weshouldplacethecrystalsindesiccatorsoruse filterpaper todry thecrystals.

131313

Acids and Alkalis

1313131313131313

Chapter 4 Acids and Alkalis

Section A Multiple-choice Questions1. B • Orangejuiceandvinegarcontaincitricacid

andethanoicacidrespectively. SotheirpHshouldbelessthan7. • Milkofmagnesiaisanalkali,itspHisgreater

than7.

2. B • Hydrochloricacidreactswithmagnesium

hydroxidetogivemagnesiumchlorideandwateronly.

∴ (1)isincorrect. • Ethanoicacidreactswithsodiumcarbonate

togivecarbondioxidegas,sodiumethanoateandwater.

∴ (2)iscorrect. • Dilutesulphuricaciddoesnotreactwith

unreactivemetalssuchasAgandCu. ∴ (3)isincorrect.

3. B • Bothdilutehydrochloricacidanddilutenitric

acidreactwithmetalcarbonateandoxides. ∴ (1)and(2)cannotbeusedtodistinguish

betweenthem. ∴ (3)mustbetheonlyanswer.

4. D • Inactivecopperdoesnotreactwithacids. ∴ SoanswersAandBareincorrect. • Thereactionbetweencalciumcarbonateand

dilutesulphuricacidformsaprotectivelayerwhichpreventsfurtherreaction.

∴ AnswerCisincorrect. ∴ AnswerDiscorrect.

5. D AdrycitricacidsoliddoesnotcontainH+(aq)ions.

Thereforeitwillnotshowanyacidicproperties.

6. C • Ammoniasolutionwouldcontainunionized

ammoniamolecules,watermolecules,ammoniumionsandhydroxideions.

NH3(aq)+H2O(l)CNH4+(aq)+OH-(aq)

• Iron(III)ionswouldreactwithhydroxideionstoformyellowprecipitate,iron(III)hydroxide.

Fe3+(aq)+3OH-(aq)→Fe(OH)3(s)

7. A • InA,only0.01moleofH2SO4reactswith

NaOH.Thereare0.03moleofH2SO4

remaininginthesolution. • InD,theamountofsulphuricacidisjust

completelyreactedwithsodiumhydroxide. • Thetotalvolumeused(60cm3)amongthe

fouroptionsisthesame. ∴ Themaximumandminimumtemperaturerise

areinDandArespectively.

8. A • Hydrochloricacidismonobasicacidand

sulphuricacidisdiabasicacid. • NumberofH+ionsin1Msulphuricacidis

largerthanthatof1Mhydrochloricacid. ∴ (1)isincorrectand(2)iscorrect. • For(3),higherconcentrationofH+ionsreact

withmagnesiumribbonatafasterrate. ∴ (3)isincorrect.

9. C • Ca(OH)2+2HCl→CaCl2+2H2O 1mole: 2moles: 1mole: 2moles • Numberofmolesofhydrochloricacid

= ×1 501000

=0.05mol

Documents

Public Assessment of the HKDSE Chemistry Examination