DP Chemistry (first assessment 2025)

Question 19N.2.HL.TZ0.4

Date	November 2019	Marks available	[Maximum mark: 8]	Reference code	19N.2.HL.TZ0.4
Level	HL	Paper	2	Time zone	TZ0
Command term	Apply, Calculate, Comment, Identify, Justify, Outline, State	Question number	4	Adapted from	N/A

[Maximum mark: 8]

19N.2.HL.TZ0.4

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)

(a(i))

Identify a conjugate acid–base pair in the equation.

[1]

Markscheme

C₆H₈O₇ AND C₆H₇O₇⁻
OR
H₂O AND H₃O⁺ ✔

(a(ii))

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

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

[1]

Markscheme

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

(a(iii))

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

[2]

Markscheme

(a(iv))

Calculate the standard Gibbs free energy change, $\Delta {G^\theta }$ , in kJ mol⁻¹, for the first dissociation of citric acid at 298 K, using section 1 of the data booklet.

[1]

Markscheme

« $\Delta {G^\theta }$ = −RT ln K = −8.31 J K^–1 mol^–1 × 298 K × ln(5.01 × 10^–4) ÷ 1000 =» 18.8 «kJ mol^–1» ✔

(a(v))

Comment on the spontaneity of the reaction at 298 K.

[1]

Markscheme

non-spontaneous AND $\Delta {G^\theta }$ positive ✔

(b)

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

[2]

Markscheme

Any two 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 ✔

NOTE: Accept “add universal indicator AND HCl more red/pink” for M2.

Accept any acid reaction AND HCl greater rise in temperature.