hkdse 2014 chemistry paper ii - hkeaa · calculations • q. 2(b) (iii) percentage of caco. 3. by...

HKDSE 2014 Chemistry Paper II

Focus areas (Last year) • Basic chemical knowledge • Number of decimal places / significant figures • Unit presented in the question • Experimental work • Calculations • Presentation • Question requirement • Misconceptions

Focus areas (This year) • Basic chemical knowledge • Number of decimal places / significant figures • Unit presented in the question • Experimental work • Calculations • Presentation • Question requirement • Misconceptions

Basic Chemical Knowledge

• Part (a) of ALL Sections • Expectations: – One or two sentences will do. – No detailed explanations.


• Question 1 (a): – Activation energy?

– Yeast function? Why failure at high T?

– Obtaining Vitamin C industrially?

– Chloroalkali industry: 2 important chemicals


• Question 1(a) Activation energy

• Expected: minimum energy possessed by reactant molecules to start a reaction

Version B answer

Version A answer


• Question 1(a) (ii) Yeast function? …

• Expected: Yeast provides enzyme / catalyst which will be denatured at high temperature.

> half of the candidates neglected that enzymes would be denatured at high temperature.


• Version A answer:

• Version B answer:


• Q. 1(a) (iii) Vitamin C industrially?

• Expected: To solve the problems of inadequate or shrinking supply of vitamin C.


• Q.1(a) (iii) • Version A:

• Version B:


• Q. 1(a) (iv) 2 products of chloroalkali industry?

• Expected: Any two: hydrogen, chlorine, sodium hydroxide

• BUT NOT sodium chloride


• Q.1(a) (iv) • Version A:

• Version B:


• Question 2 (a): – ccp, bcc?

– 2 structural features … liquid crystal behaviour?

– Thermoplastics vs. thermosets


• Q.2 (a) (i) ccp, fcp

• Expected:


• Version A:

Basic Chemical Knowledge • Q.2(ii) 2 structural features … liquid crystalbehaviour. • Expected: – Fairly rigid molecular backbone – Benzene rings – Rod‐like or disc‐like – Polar groups

• Version A


• Q.2(a)(iii) Thermoplastics vs. thermosetting plastics • Expected: – Thermoplastics: polyvinyl chloride, polystyrene

– Thermosetting plastics: urea‐methanal

About 1/4 of the candidates were not able to distinguish between thermoplastics and thermosetting plastics


• Version A

• Version B


• Question 3(a) – Chemical test for • HCl(g) • \

C = O

/

– Drying agent for ethyl ethanoate?


• Q.3(a) (i) Test for HCl(g)?

• Expected: – Place near NH3(g) – Dense white fume is observed.

• Not asking test for HCl(aq)


• Version A

• Version B


• Q.3(a) (ii) Test for carbonyl group?

• Expected: – Add 2,4‐dinitrophenylhydrazine. – Yellow / orange / red precipitate is formed.

• Not asking for ketone group.


• Version A

• Version B

Basic Chemical KnowledgeQuestion 3(a) Drying agent: CH3CO2CH2CH3 • Question 3(a) Drying agent: CH3CO2CH2CH3?

• Expected: Anhydrous magnesium sulphate

• Not asking for explanation

• Version A:

Focus areas • Basic chemical knowledge

• Experimental work

• Calculations • Presentation

• Question requirement • Misconceptions

Experimental work

• Q.1(b) Given: BrO3

‐ + 6I‐(aq) + 6H+(aq) 3I2(aq) + Br‐(aq) + 3H2O(l)

(i) Initial rate?

Expected: instantaneous rate at the start of the reaction

About three quarters of the candidates were not able to explain the term ‘initial rate’.

Experimental work

• Version A:

• Version B:

Experimental work • Q.1(b) Given: BrO3

‐ + 6I‐(aq) + 6H+(aq) 3I2(aq) + Br‐(aq) + 3H2O(l)

(ii) Method with justification to follow theprogress of I2(aq)? Expected: Follow colour intensity of the solution as the solution changes from colourless to brown.

Experimental work

• Version A:

• Version B:

Experimental work

• Q.3(c)(i) Experimental steps for purifying sample T containing organic acid as impurity by using NaHCO3(aq) and pentane?

Expected: T + pentane + NaHCO3 in a separating funnel. Shake. Collect the organic layer and carry out fractional distillation.

Experimental Work

• Version A:

• Version B:

Experimental work

• Version B:

Calculations • Question 1 (b) Order determination

Expected: Initial rate directly proportional to [BrO3

‐(aq)] Order = 1

Calculations • Version A:

• Version B:

Calculations • Q. 1(b) (iv) (1)

2

3

1084.11030.2

20.010.0

)30.0)(17.0()15.0)(17.0(

2rateinitial 1rateinitial

y Expected

Calculations • Version B:

Calculations • Q.1(b)(iv) (2) Trial 1: initial rate w.r.t. BrO3

‐(aq)?

Expected: Initial rate of consumption of BrO3

‐(aq) = 2.30 x 10‐3 x 1/3 = 7.67 x 10‐4 mol dm‐3 s ‐1


• Version B:

Calculations • Q. 2(b) (iii) Percentage of CaCO3 by mass? • Expected: Amount of CaC2O4 formed in step 6 = 2.374/128.1 = 0.01853 mol

Mass of CaCO3 in limestone sample = 0.01853 (100.1) = 1.855 g Percentage of CaCO3 by mass = 1.855 (100)/2.025 = 91.6%

About half of the candidates were not able to calculate the percentage of CaCO3 by mass as they wrongly calculated the formula masses of calcium carbonate and calcium oxalate as they were not familiar with the chemical formulae for these two compounds

Calculations • Version B:

Presentation

• 3 (c) (ii) Interpretation of IR spectrum + given information

Presentation

• (1) negative test with Tollen’s Reagent ???

• (2) strong absorption peak at 1730 cm‐1 ???

• (3) Absence of some important peaks ???

Presentation • Expected: • (1) negative test with Tollen’s Reagent

no aldehyde group • (2) strong absorption peak at 1730 cm‐1

presence of C=O with (1), ketone group present

• (3) Absence of some important peaks 2070 – 2250 cm‐1: no CC group 1610 – 1680 cm‐1: no C=C group 3230 – 3670 cm‐1: no –OH group

About three‐quarters of the candidates failed to identify the functional group that may be present in T.

Presentation

• Version A:

Presentation

• Version B:

Question requirement • 1 (c) (vi) … a lot of natural nitrate mines. … success of the Haber process … led to some impacts… Suggest one impact. • Expected:

The mining work was no longer profitable / mining work might be closed / it could lead to a high unemployment rate

Question requirement • Version A:

• Version B

Misconceptions • 1 (c) (v) One design to increase the overall conversion percentage of nitrogen?

Expected: reuse unreacted reactants recycle the reactants removing ammonia from the product mixture

Some wrongly suggested: “dissolving ammonia in water ”.

Misconceptions • Version A:

• Version B:

Misconceptions • Q.2(c) (i) moulding method?

• Expected: Blow moulding About two‐thirds of the candidates provided wrong answers: compression moulding

• Version A:

Misconceptions • Q.2(c) (ii) 2 common types of PE

• Expected: High density polyethene & Low density polyethene

Two‐thirds of candidates provided answers: PVC / PS / PP as types of PE

• Version A:

Misconceptions • Q.2(c) (iv) Given: PLA

• (1) structure of lactic acid

• Expected:

• Version A:

About two‐thirds drew incorrect structures.

Misconceptions • 3(c) (iii) mass spectrum of T, strong peaks at 43 and 91; parent peak : 134. m/z = 43 & m/z 134???

Expected: 43 : [CH3CO]+

134: [C7H7COCH3]+

Some candidates did not aware these species are positively charged.

Misconceptions • Version A:

• Version B:

hkdse 2014 chemistry paper ii - hkeaa · calculations • q. 2(b) (iii) percentage of caco. 3. by...

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