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Date November 2019 Marks available 1 Reference code 19N.2.hl.TZ0.4
Level HL Paper 2 Time zone TZ0
Command term Calculate 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) C6H7O7 (aq) + H3O+ (aq)

Identify a conjugate acid–base pair in the equation.

[1]
a(i).

The value of Ka at 298 K for the first dissociation 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 Ka, of increasing the temperature.

[2]
a(iii).

Calculate the standard Gibbs free energy change, ΔGθ, in kJ mol−1, for the first dissociation of citric acid at 298 K, using section 1 of the data booklet.

[1]
a(iv).

Comment on the spontaneity of the reaction at 298 K.

[1]
a(v).

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

[2]
b.

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 Ka < 1 ✔

a(ii).

a(iii).

«ΔGθ = −RT ln K = −8.31 J K–1 mol–1 × 298 K × ln(5.01 × 10–4) ÷ 1000 =» 18.8 «kJ mol–1» ✔

a(iv).

non-spontaneous AND ΔGθ positive ✔

a(v).

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.

Accept specific examples throughout.

Do not accept “smell” or “taste”.

b.

Examiners report

[N/A]
a(i).
[N/A]
a(ii).
[N/A]
a(iii).
[N/A]
a(iv).
[N/A]
a(v).
[N/A]
b.

Syllabus sections

Additional higher level (AHL) » Topic 15: Energetics/thermochemistry » 15.2 Entropy and spontaneity
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Additional higher level (AHL) » Topic 15: Energetics/thermochemistry
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