2018 Examination Briefing - Chemistry and Combined Science ... · •1. (c) (iv) According to … graph, calculate E act … •Expected: Give the equation to be used Make use of

HKDSE

Q.1 & Q.3

2018 Chemistry Paper 2

Focus areas

• Basic chemical knowledge

• Question requirement

• Experimental work

• Calculations and graph interpretation

• Others

Basic Chemical Knowledge • Question 1(a)(i)

2 half equations for the electrolysis of brine using membrane electrolytic cell?

Expected

2H+(aq) + 2e- H2(g)

[OR 2H2O(l) + 2e- 2OH-(aq) + H2(g)

2Cl-(aq) Cl2(g) + 2e-

Comments:

About two thirds of the candidates were able to write the two half equations.

Basic Chemical Knowledge • Question 1(a)(i)

Exemplars:

Version A:

Version B: [Better]

Membrane electrolytic cell???

Basic Chemical Knowledge

• 1. (a) (ii)

labelled diagram for Maxwell Boltzmann distribution curve?

• Expected:


• Comments:

– About half of the candidates were able to sketch a labelled diagram for a Maxwell-Boltzmann distribution curve.


Exemplars

• Version A:

• Version B:

Not correct

Not start at zero


Exemplars

• Version C: [較佳]

如能說明是動能更佳

Basic Chemical Knowledge • 1. (a)

(iii) raw material for the manufacture of Vitamin C in industry?

acetic acid; acetone, formaldehyde, glucose

Expected:

glucose

Comments:

Just under half of the candidates were able to select glucose as the raw material for manufacturing vitamin C.


• 1. (b) Reaction (I): CO(g) + 2H2(g) CH3OH(g)

(i) (1) suitable catalyst?

Expected:

Cu/ZnO/Al2O3

• Comments:

– Less than half of the candidates were able to suggest a suitable catalyst for the given reaction.


• Exemplars: Version A:

Version B:

Incorrect catalyst


• 1. (b) (i)

(2) … why the reaction would proceed slowly in the absence of catalyst

• Expected: – higher activation energy

• Comments: – About half of the candidates were able to suggest a

higher activation energy in the absence of a catalyst that would allow the reaction to proceed slowly.


• Exemplars:

• Version A:

• Version B [較佳]

正確陳述

Basic Chemical Knowledge • 1. (b) (i) (3) … why operation pressure … set at 100 atm

but not at atmospheric pressure? • Expected:

– Moles of gaseous products < Moles of gaseous reactants – Increase in P shifts the eqm position to right

• Comments: – About two thirds of the candidates were able to explain why

the operation pressure in industry for the reaction is set at 100 atm but not at atmospheric pressure in terms of principles of equilibrium and kinetics.


• Exemplars

Version A:

Version B: [Better]


• 1. (b) (iii) … methanol to ethanoic acid …

… chemical equation …

• Expected:

CH3OH + CO CH3CO2H

• Comments:

a quarter of the candidates were able to write a chemical equation for the industrial production of ethanoic acid from methanol.


• 3. (a) (i) … brick red flame … Suggest a cation …

• Expected:

– Calcium ion / Ca2+

• Comments:

– Nearly all candidates were able to state the presence of calcium ion in the compound.


• (a) (ii) … distinguish between K2SO3 and K2SO4

• Expected:

– Correct use of reagent with correct observation

• Comments:

– Just only half of the candidates were able to suggest a chemical test to distinguish between K2SO3(aq) and K2SO4(aq).


Reagent Observation

H+(aq)/acid Only SO32- gives a gas with pungent

smell

Cr2O72-(aq)/H+(aq) Only SO3

2- turns the solution from orange to green.

MnO4-(aq)/H+(aq) Only SO3

2- turns the solution from purple to colourless.

Br2(aq) Only SO32- turns the solution from

brown/orange to colourless.

I2(aq) Only SO32- turns the solution from

brown to colourless.

…


• Exemplars:

• Version A:

• Version B:

Acidify???

試劑不正確



正確試劑及觀察

Basic Chemical Knowledge • 3. (a) (iii) … Rf value …?

• Expected:

Rf value of a substance is the ratio between the migration distance of the substance and the migration distance of the solvent front during chromatography.

• Comments:

Just only half of the candidates were able to suggest the meaning of Rf value in a paper chromatogram.


• Exemplars:

• Version A:

• Version B: (Better)


• 3. (b) (ii) …. ClO3-(aq) reacted … I-(aq) …

presence of H+(aq) to form I2(aq) and Cl-(aq). … ionic equation …

• Expected:

ClO3-(aq) + 6I-(aq) + 6H+(aq)

Cl-(aq) + 3I2(aq) + 3H2O(l)


• Comments:

a third of the candidates were able to write the ionic equation for the reaction involved.

• Exemplars:

Version A:

Version B:

?????

Charge not balanced

Basic Chemical Knowledge • 3. (b) … added to the flask … resulting mixture was

titrated with … Na2S2O3(aq), adding starch solution …

(iii) … colour change at the end point

Expected:

[From Q. information: Na2S2O3 in the burette]

Blue to colourless

Comments:

a third of the candidates were able to write the colour change at the end point being from blue to colourless.


• Exemplars

• Version A:

• Version B:

I2???

Focus areas





• Others

Question Requirement • 1. (b) (ii) Reaction (I) CO(g) + 2H2(g) CH3OH(g) Reaction (II) CO2(g) + 3H2(g) CH3OH(g) + H2O(g) (1) … why Reaction (II) greener … ? Expected: • A comparative sense • Reaction (I) involves poisonous CO while Reaction (II)

does not.

Question Requirement

• Comments:

About a third of the candidates were able to comparatively show how Reaction (II) is greener than Reaction(I).


• Exemplars:

Version A:

Version B:

Not a comparative sense Better mention CO as the toxic gas

寫下CO是有毒如果作出比較更佳


• 1. (b) (ii) (2)

Reaction (II) CO2(g) + 3H2(g) CH3OH(g) + H2O(g)

… potential benefit of Reaction (II) …

environment.

Expected:

Reduce CO2

Alleviate global warming


Comments:

1/2 of the candidates were able to suggest there is a potential of alleviating global warming for Reaction (II).


• Exemplars:

• Version A:

• Version B: (較佳)

未能寫下對環境的潛在好處

Focus areas





• Others

Experimental Work – Making up of a solution • 3. (b) (i) … sample dissolved … made up … 250 cm3. • Expected: Place the dissolved sample in a volumetric flask. Add deionised water to the flask until reaching the

graduation mark. Shake thoroughly. • Comments: half of the candidates were able to describe how the

sample dissolved in making up a 250.0 cm3 solution.

Experimental Work – Making up of a solution • Exemplars:

• Version A:

Better mention: Shake thoroughly.

Experimental Work – Making up of a solution • Version B: [較佳]

Experimental work • 3. (c) b.p. X (81.4oC); Y (79.6oC)

(i) … why fractional distillation … not … to separate X from the mixture.

• Expected:

The boiling points of X and Y are too close.

• Comments:

A very high proportion of the candidates were able to suggest the small difference in boiling points between X and Y making fractional distillation not a suitable method to separate X from the mixture.

Focus areas





• Others

Calculations - order of a reaction

1. (c) (i) l1 and l2 .. same slope .. chemical kinetics?

Expected: Order of a reaction is not affected by temperature change.

Calculations – order of a reaction

• Comments:

About a third of the candidates were able to deduce that the order of the reaction is not affected by temperature change.

• Exemplars:

Version A:

Calculations – order of a reaction

• Exemplars:

Version B: [Better]

Calculations – Order of a reaction

• 1. (c) (ii) From l1, … order … N2O5(g).

• Expected:

Slope determination

Calculations – Order of a reaction

• Comments

two-thirds of the candidates were able to deduce the order of reaction with respect to N2O5.

Exemplars:

Version A:

未能寫下計算方法

Calculations - Order

• Exemplars:

Version B:

Make use of graph and Correct calculation [(y2-y1)/(x2-x1)]

Calculations – Rate constant

• 1. (c) (iii) From l2 … rate constant … at 345 K.

• Expected:

y-intercept

log k = -2

k = 0.01 s-1 (unit from data)

y-intercept

unit indication


• Comments:

< half of the candidates were able to deduce the rate constant for the reaction with the appropriate unit.


• Exemplar A:

Wrong log conversion


• Version B: [較佳]

Calculations – Activation energy

• 1. (c) (iv) According to … graph, calculate Eact …

• Expected:

Give the equation to be used

Make use of data from graph


• Comments:

a third of candidates were able to calculate the activation energy of the reaction with the correct unit and appropriate number of significant figures.


• Exemplar A:

Correct calculations Significant figures???

Calculations - Titration

• 3. (b) (iv) …

I2(aq) + 2S2O32-(aq) 2I-(aq) + S4O6

2-(aq)

… calculate the percentage by mass of NaClO3 in the sample.

Expected:


• Comments:

a third of the candidates were able to calculate the percentage by mass of NaClO3(s) in the sample.


• Exemplars:

• Version A:

?????


• Version B: [Better]

Focus areas





• Others

Others: Unknown deduction

• 3. (c) (ii) (1) … IR spectrum … one functional group that may be present.


• Expected:

about 1700 cm-1 – a C=O group

about 1650 cm-1 – a C=C group


• Comments:

About half of the candidates were able to deduce the functional group that may be present in X.

• Exemplars:

Version A:

不提較佳


• 3. (c) (ii) (2) … mass spectrum,

… m/z = 43 and 55.

• Expected:

43: CH3CO+ 55: CH2=CHCO+

• Comments: About a third of the candidates were able to suggest one chemical species for the corresponding peaks in the mass spectrum .


• Exemplars:

• Version A:

Presence of aldehyde group???

Incorrect deduction


• Version B: Correct deduction

Missing positive charge


• 3. (c) (ii) (3) … a possible structure of X.

• Expected:

CH2=CHCOCH3

• Comments:

a third of the candidates were able to draw CH2=CHCOCH3as a possible structure of X.


• Exemplars:

• Version A:

Aldehyde group???


• 3. (c) (iii) … Y … positive … 2,4-dinitrophenylhydrazine …, and a negative … Tollen’s … Deduce … Y …

• Expected:

(1) Deduction from 2,4-dinitrophenylhydrazine test

(2) Deduction from Tollen’s reagent test

(3) Identity of Y


Positive result with 2,4-dinitrophenylhydrazine test – presence of carbonyl group

Negative result for Tollen’s reagent test – not an aldehyde

Y : CH3CH2COCH3 / butanone [butan-2-one]

• Comments:

half of the candidates were able to deduce Y as butanone.


• Exemplars:

• Version A:

具有正確推論但未能說明Y是丁酮


• Version B:

Correct ketone deduction but did not specify clearly that it is butanone



Focus areas




• Calculations

• Others

HKDSE

第2題物料化學

2018 化學卷 2

討論範圍

• 基本化學知識： – 寫出（某物質）的名稱

– 寫出（某觀察／現象）的原因

– 辨別一些物質是否…

• 理解關係： – 解釋（某物質）是否具有（某性質）

– 預測…並解釋…

• 傳意： – 書寫（某些分子）的結構式／繪畫部分結構

– 使用簡短文句解說…

基本化學知識 (1)

• 綠色化學

Q2 (a) (i) 有約三分之一考生能夠運用合適的綠色化學原理提出解釋。

基本化學知識 (2)

• 微觀世界 II 的化學知識：

Q2 (a) (ii) (2)

理解關係 (1)：

• 液晶顯示屏的一個像素：

分子結構性質用途

Q2 (b) (ii) (2)

理解關係 (2)：

• Y的熱性質：

分子結構性質

Q2 (c) (ii) (2) 約四分之一的考生能評論 Y 的熱性質

傳意 (1)：

• 要求考生繪畫X聚合物的各個單體的結構：

Q2 (c) (i)

約四分之一考生能夠寫出 X 的單體結構

傳意 (2) ：

• 要求考生繪畫A的結構：

– Q2 (c) (ii) (1) 考生表現？

傳意 (3) ：

• 使用簡短文句解說：

Q2 (c) (ii) (3) 只有極少數考生能夠提出一個正確方法來控制 Y 的剛性

Documents

2018 Examination Briefing - Chemistry and Combined Science ... · •1. (c) (iv) According to … graph, calculate E act … •Expected: Give the equation to be used Make use of