HKDSE 2015

16, 23 November 2015

Candidates’ Performance in Paper I (Q5-13)

• Questions 5 – 8 Chemistry (Part I) / CS (Chem) (Common Questions)

Q5 - Fair

• About two thirds of the candidates were able to state that NH3 is a weak alkali because it ionises slightly in water, but some gave an incorrect equation.

Q5 - Fair

• About half of the candidates made conceptual mistakes in describing the experiment.

• Common mistakes include:

- comparing the rates of neutralisation between HCl and NH3 / NaOH

- comparing the amounts of HCl needed to neutralise a sample of NH3 / NaOH

Q5 - Fair

• Common mistakes include:

- comparing the voltages generated by the chemical cells constructed with NH3 / NaOH as the electrolyte

• About a third of the candidates were not able to state the appropriate conditions for carrying out a fair test.

Q6 - Fair

• Part (a):

- some candidates misspelt the answer ‘substitution’ as ‘subsituation’

• Part (e)(ii):

- about half of the candidates failed to correctly state that excess amount of methane was needed

Q6 - Fair

• Part (a):

- some candidates wrongly stated the electroplated object would be contaminated by oily dirt

• Part (b) – common mistakes:

- electrolytes can conduct electricity by allowing electrons to pass through.

- simply stated that electrolytes can conduct electricity but without any further elaboration.

Q7 - Fair

• Part (d):

- some candidates were confused about the relative oxidising power of copper(II) ions and hydrogen ions

Q7 - Fair

Q7 - Fair • Part (f) – common mistakes:

- stated the colour of the solution would become deeper or paler

- wrongly stated the expected changes at the electrodes without referring to the observable changes that would occur in the solution

Q7 - Fair

• Part (g) – common mistakes:

- about a third of them failed to recognise that it takes 2 moles of electrons to reduce 1 mole of Cu2+

Q8 - Fair

• Part (b)(i) – common mistakes:

- gave the chemical formulae (CO2, H2O) only without explicitly indicating the bonds formed or broken

- missed one or two species in their answers

- Failed to state the double bonds in O2 and CO2

Q8 - Fair

• Part (b)(ii) – common mistakes:

- without stating which process (bond breaking / bond forming) releases energy and which absorbs energy

- wrongly stated that energy is released in bond breaking and absorbed in bond forming

Q8 - Fair

• Part (b)(ii) – common mistakes:

• Questions 9 – 13 Chemistry (Part II)

Q9 - Poor

• Part (a)

- 2/3 candidates gave the correct answer

- some wrongly stated performing the experiment in micro-scale could increase the rate of the reaction

Q9 - Poor

• Part (b) – very poor

- Some of the candidates wrongly stated that hydrogen is less dense than air, so placing the tubing at the uppermost position of the inverted measuring cylinder could collect more pure hydrogen from the cylinder.

Q9 - Poor

• Part (b) – very poor

Q9 - Poor

• Part (e) – common mistakes

- Overlapped initial rates

- Curves at incorrect y- positions

Q10 - Satisfactory

• Part (a)(i) – common mistakes

- gave a “covalent-like” electron diagram for Na2O

- incorrect number of the outermost shell electrons for the O2– ion

- incorrect ionic charge for the O2– ion

Q10 - Satisfactory

• Part (a)(i) – common mistakes

Q10 - Satisfactory

• Part (a)(ii) – common mistakes

- omitted the lone pair electrons on the oxygen atom

- only stated whether Cl2O is soluble in water without mentioning its acidic property

- wrongly stated that the reaction of Cl2O with water would give HCl(aq) or a mixture of HCl(aq) and HOCl(aq).

Q10 - Satisfactory

• Part (a)(ii) – common mistakes

Q10 - Satisfactory

• Part (b) – common mistakes

- failed to give specific examples to illustrate their answers

- misspelt the term ‘Haber Process’ as ‘Habour Process’ or ‘Harbour Process’

• Part (a) – common mistakes

- failed to give the correct unit

- unable to recognise that the concentrations of H+ and OH– were equal in water

Q11 – Very Poor

• Part (a) – common mistakes

Q11 – Very Poor

• Part (b) – common mistakes

- Many of the candidates wrongly stated ‘water is only a solvent so [H2O] is a constant’.

Q11 – Very Poor

• Part (c) – common mistakes

- wrongly stated that as [H+] was still equal to [OH–] and the water was then still neutral, therefore, the pH would remain as 7

Q11 – Very Poor

• common mistakes

- omitted either the oxidation or reduction step in their answers

- omitted ‘acidified’ for potassium dichromate solution

- gave the incorrect catalyst (H+ or H2SO4(aq)) or omitted ‘heating’ for the esterification step

Q12 – Poor

Q12 – Poor

Q12 – Poor

• Common mistakes - unable to give the correct answer in a

systematic way and with appropriate / correct terminologies

- failed to recognise the correct meaning of the term ‘superimposable’

- failed to demonstrate an accurate understanding of the optical activities of chiral molecules

Q13 – fair

• Commonly encountered incorrect answers:

- ‘a molecule having a chiral carbon atom which connected to four different molecules’

- ‘the molecules rotate to different directions under plane-polarised light’

- ‘the molecule is superimposable with its mirror image’

Q13 – fair

Candidates are general weak in

General Comments

• calculations

• giving the appropriate units for numerical answers

• describe correct procedures / select appropriate apparatus and instruments for practical tasks

Candidates are general weak in

General Comments

• describing chemical concepts / explaining chemical phenomenon

• planning synthetic schemes

• did not read the questions carefully