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Date	November 2019	Marks available	1	Reference code	19N.2.sl.TZ0.4
Level	SL	Paper	2	Time zone	TZ0
Command term	State and Outline	Question number	4	Adapted from	N/A

Question

A molecule of citric acid, C₆H₈O₇, is shown.

The equation for the first dissociation of citric acid in water is

C₆H₈O₇ (aq) + H₂O (l) $\rightleftharpoons$ C₆H₇O₇⁻ (aq) + H₃O⁺ (aq)

Identify a conjugate acid–base pair in the equation.

[1]

a(i).

The value of the equilibrium constant for the first dissociation at 298 K is 5.01 × 10⁻⁴.

State, giving a reason, the strength of citric acid.

[1]

a(ii).

The dissociation of citric acid is an endothermic process. State the effect on the hydrogen ion concentration, [H⁺], and on the equilibrium constant, of increasing the temperature.

[2]

a(iii).

Outline one laboratory methods of distinguishing between solutions of citric acid and hydrochloric acid of equal concentration, stating the expected observations.

[1]

Markscheme

C₆H₈O₇ AND C₆H₇O₇⁻
OR
H₂O AND H₃O⁺ ✔

a(i).

weak acid AND partially dissociated
OR
weak acid AND equilibrium lies to left
OR
weak acid AND K_c/K_a<1 ✔

a(ii).

a(iii).

Any one of:
«electrical» conductivity AND HCl greater ✔
pH AND citric acid higher ✔
titrate with strong base AND pH at equivalence higher for citric acid ✔
add reactive metal/carbonate/hydrogen carbonate AND stronger effervescence/faster reaction with HCl ✔
titration AND volume of alkali for complete neutralisation greater for citric acid ✔
titrate with strong base AND more than one equivalence point for complete neutralisation of citric acid ✔
titrate with strong base AND buffer zone with citric acid ✔