2021 Examination Briefing - Chemistry and Combined Science

HKDSE 2021

Candidates’ Performance in Paper IB (Chem Q1 - 4)(CS Q5)

• Chemistry Paper 1B (Part I)

Questions 1 – 4

• CS(Chem) Section BQuestion 5

Q1 – Overall Good (CS same question)

1 (a) about half of the candidates gave correct electron diagram of C2H2

Common mistakes:

• Gave the electron diagram of C2H4

• Lone pairs on carbon atoms

1 (b) about half of the candidates gave the correct chemical equation for complete combustion of acetylene

Common mistake:

• Not able to balance the equation correctly

1 (c)(i) almost all candidates stated gas A is hydrogen

1 (c)(ii) about two-thirds of the candidates were able to explain why Reaction (I) is dangerous with the characteristics of gas A

1 (d) a third of the candidates stated the correct use of Ca(OH)2 in daily life.

Common mistakes:

• vague answers (e.g. for cleaning)

• Testing for CO2

Q2 – Overall Satisfactory (CS same question)

2 (a) two-thirds of the candidates recognised Br2 is formed at the graphite anode and stated the expected observation.

2 (b) two-thirds of the candidates gave the correct half equation for the reduction of Pb2+ at the graphite cathode.

Common mistake:

• Pb is oxidised to Pb2+ at the cathode.

2 (c) half of the candidates gave the correct half equation for the oxidation of OH– at Pt anode.

2 (d) two-thirds of the candidates stated H2 is formed at the Pt cathode.

(unbalanced equation)

2 (e) just below half of the candidates stated Cu2+ is formed at the copper anode

Common mistake: SO2 is formed at the anode.

2 (f) about half of the candidates stated correct expected observation at the Cu cathode

2 (g) about a quarter of the candidates gave correct justification for the reaction occurred at Cu cathode.

Common mistake: Incomplete answer

Q3 – Overall Satisfactory (CS same question)

Common mistake: Incorrect use of terminology (e.g. atoms of the same element with same protons but different neutrons.)

3 (a) about half of the candidates stated the correct meaning of the term “isotope”.

(incorrect handling of “%”)

3 (b) about two-thirds of the candidates correctly calculated the abundance of 29Si.

3 (c)(i) about two-thirds of the candidates correctly explained why silicon dioxide has a high melting point.

Common mistake:

SiO2 molecules are held by strong covalent bonds

3 (c)(ii) about half of the candidates gave a correct calculation.

Common mistake:

• Did not show the deduction for SiO2 is limiting.

• Wrongly considered Mg is limiting.

3 (d) about half of the candidates stated the correct name or formula of the mineral.

4 (a) about two-thirds of the candidates stated correct systematic name for X (Propene).

4 (b) about three-quarters of the candidates stated appropriate chemical test to distinguish X and butane, and gave the corresponding expected observations.

Common mistake:

• Stated the chemical test only• Gave incorrect observations

Q4 – Overall Good (4 (a) – (c) CS same question)

4 (c)(i) about three quarters of the candidates correctly explained why X can form a polymer.

4 (c)(ii) about half of the candidates gave the correct repeating unit of Z.

Common mistake:

• Gave the structure of the monomer• Included “n” in the answer

Common mistake:

• Did not mention the molecules are attracted by van der Waals’ force.

• Only compared butane and Y without mentioning decane.

(Chem Only )

4 (d)(i) about three quarters of the candidates were able to draw the correct structure of Y (2-methylpropane).

4 (d)(ii) about two-thirds of the candidates recogniseddecane has the highest boiling point, and gave a correct explanation.

HC(CH3)3

Q5 – Overall Very Poor (CS Only )

5 (a) only a very small number of the candidates were able to give the correct structures of both monomers that can form nylon-6,6.

5 (b) about a third of the candidates were able to give the correct meaning for the term “condensation”.

HKDSE 2021 化學卷一乙部第 5 至 8 題

2021組合化學卷乙部第 6 至 9 題

題 5 (=CS題 6) 表現令人滿意

(a) (只有少於1/3考生) 能精確地描述反應物共價鍵的斷裂是吸熱和生成物共價鍵的形成是放熱，以說明六胺的燃燒是放熱的。

(b)(i) (只有少於1/3考生) 能夠寫出所需的熱化學方程式，並包括標準生成焓變。

(ii) (多於2/3考生) 能夠計算六胺的標準燃燒焓變。

ΔHco = 6 × (–394) + 6 × (–286) + 4 × (+33) – 123

= – 4 071 kJ mol–1

(單位正確)

題 5 表現令人滿意

(c) (多於2/3考生) 能夠使用提供的實驗數據來計算六胺的燃燒焓變。

題 5 表現令人滿意

題 6 (=CS題 7) 表現令人滿意：

(a) (i) (只有極少數考生) 能夠提出「避免因凝結水倒流而令熾熱反應管破裂」。

(ii) (約2/3考生) 能夠提出氨／氧化鉛(II)

有毒。

(b) (約2/3考生)能夠正確書寫所需的化學方程式

3PbO(s) + 2NH3(g) → 3Pb(s) + N2(g) + 3H2O(l)

題 6 表現令人滿意：

(c) (多於1/3考生) 能夠使用氨的N原子氧化數的改變說明氨是還原劑

題 6 表現令人滿意：

(d)(i) (甚多考生) 能夠正確書寫所需的化學方程式

2PbO(s) + C(s) → 2Pb(s) + CO2(g)

(d)(ii)(1) (約1/2考生) 能夠繪畫本生燈及加上正確標示

(d)(ii)(2) (約1/4考生) 能夠寫出「坩堝 crucible 」的正確名稱

Q7 (=CS Q8) : Fair

(a) (Just below 1/2 candidates) can describe the correct procedure: … 250 cm3, volumetric flask, washings, graduation mark, …

Q7 Performance of candidates: Fair

(b) (Just below 2/3 candidates)

Colour change from yellow to orange.

(c) (About 1/4 candidates)

Reasonable average volume of hydrochloric acid used = 27.27 cm3

Q7 Performance of candidates: Fair

(d) (Just below 1/2 candidates) Concentration of hydrochloric acid (in g dm–3) =

Q8 (=CS Q9) : Poor• About 1/3 of the candidates were able to describe how to use

the materials and equipment provided to set up at least 2 experiments.

• Some candidates failed to make use of the expected experimental observations to confirm the order of reducing power of metals. Many of them simply recalled information from the metal reactivity series (no deduction).

• [Expt 1] Some candidates erroneously considered when Znand Cu strips are connected to positive and negative terminals of the multimeter respectively, a positive voltage reading will be recorded. Candidates had limited experience on the use of multimeter.

• [Expt 2] Some candidates considered when Ag and Cu strips are connected to positive and negative terminals of the multimeter respectively, a positive voltage reading will be recorded.

Q8

Q8

HKDSE 2021

Candidates’ Performance in Paper I (Q9 - 13)

• Chemistry Paper 1B (Part II)

Questions 9 – 13

Q9 – Overall Satisfactory

9 (a) a very high percentage of the candidates gave the correct expression for the equilibrium constant.

9 (b)(i) about two-thirds of the candidates gave the correct number of moles of CH4 at equilibrium.

9 (b)(ii) just below half of the candidates calculated the equilibrium constant correctly.

Common mistake:

• Failed to notice the volume is 2.0 dm3

• Gave incorrect unit.

(incorrect unit)

(omitted 2.0 dm3)

9 (b)(iii) about half of the candidates were able to state and explain the equilibrium constant would remain unchanged as Kc depends on temperature only.

Common mistake:

• Kc changes as the equilibrium position shifts.

Q10 – Overall Satisfactory

10 (a)(i) just below half of the candidates correctly explained why the deionised water used should be saturated with CO2 gas before the start of the experiment.

Common mistake:

• CO2 is a reagent to react with the tablet.

10 (a)(ii) about two-thirds of the candidates gave a correct labelled diagram for a gas syringe.

10 (b)(i) about half of the candidates performed the calculation correctly.

Common mistake:

• Used “50 dm3” in the calculation

• Used incorrect molar mass

10 (b)(ii) a very high percentage of the candidates gave a correct curve to show the experimental result if the tablet is ground into a powder.

Q11 – Overall Fair

11 (a) about a quarter of the candidates correctly stated the Markovnikov’s rule.

11 (b) two-thirds of the candidates gave the correct structure of X.

11 (c) about three quarters of the candidates gave the correct answer of U (NaOH / OH–).

11 (d)(i) about a third of the candidates gave the correct structure of Y.

(Tetrahedral shape not properly drawn)

11 (d)(ii) about a third of the candidates were able to state an optically active compound can rotate the plane of plane-polarised light.

(plane-polarised light)

• Some candidates only stated that Y has a chiral centre that is bonded to four different atoms or groups

11 (e) about half of the candidates were able to suggest an appropriate chemical test to distinguish Z and 2-methylbutan-2-ol.

Q12 – Overall Fair

12 (a)(i) about half of the candidates correctly stated why silicon dioxide is classified as an acidic oxide.

Common mistake:

• Silicon dioxide is soluble in water

12 (a)(ii) about half of the candidates correctly stated silicon dioxide is insoluble in water.

Common mistake:

• Silicon dioxide is a very weak acid, [H+] is very small

• Only stated [H+] = [OH–] in the solution.

12 (b) about a third of the candidates gave a correct explanation with an appropriate chemical equation.

Common mistake:

• Incorrect formula for phosphorus(V) oxide (P4O10)

• Phosphorus acid (H3PO3) is formed

• Incorrect/unbalanced chemical equation

12 (c) about a third of the candidates correctly stated how the reaction can demonstrate copper exhibits two characteristics of transition metals.

Common mistake:

• stated “the oxidation number of Cu is 0 in copper metal” as an example

• wrongly stated “transition metals have different colour”

Q13 – Overall Fair

• Only a small number of candidates were able to give a correct and complete answer.

• The majority of the candidates were able to state small molecules are eliminated.

• Only a few candidates explicitly stated the monomers should be bifunctional.

• About a third of the candidates gave the correct structures of both monomers.

• Some candidates gave the structure of the monomer with an amine group at one end, and a carboxyl group at the other end.