Question 3
(a) Give one reason for each of the following activities carried out in the laboratory:
(i) acidifying the test solution with dilute hydrochloric acid in the confirmation test for SO42-;
(ii) addition of starch indicator to iodine solution near the end-point in the titration with thiosulphate solution.
[2 marks]
(b) State one chemical reagent that could be used to distinguish between
(i) ethane and ethene;
(ii) ethanol and ethanoic acid;
(iii) a terminal alkyne and a non-terminal alkyne.
[3 marks]
(c) (i) Name two reaction vessels used in the laboratory preparation of gases.
(ii) Which of the vessels named in 3(c)(i) is preferred when heating is required?
Observation
The performance of majority of the candidates in this question was below average.
In part (a), majority of the candidates could not give one reason why acidifying the test solution with dilute hydrochloric acid in the confirmation test for SO42-.
In part (b), majority of the candidates stated one chemical reagent that could be used to distinguish between ethane and ethene. However, they could not state a chemical reagent to distinguish between a terminal alkyne and a non-terminal alkyne.
In part (c), majority of the candidates could not name two reaction vessels used in the laboratory preparation of gases.
The expected answers include:
