DP Chemistry (last assessment 2024)

Question 22N.2.hl.TZ0.d.vii

Date	November 2022	Marks available	[Maximum mark: 2]	Reference code	22N.2.hl.TZ0.d.vii
Level	hl	Paper	2	Time zone	TZ0
Command term	Calculate	Question number	d.vii	Adapted from	N/A

d.vii.

[Maximum mark: 2]

22N.2.hl.TZ0.d.vii

Ammonium nitrate, NH₄NO₃, is used as a high nitrogen fertilizer.

Cold packs contain ammonium nitrate and water separated by a membrane.

Calculate the value of the equilibrium constant for the dissolution of ammonium nitrate at 298 K using the answer to question part (d)(vi) and section 1 of the data booklet.

NH₄NO₃(s) $⇌$ NH₄NO₃(aq)

If you did not obtain an answer in (d)(vi), use −7.84 kJ/mol, although this is not the correct answer.

[2]

Markscheme

« $Δ G ° = - R T \ln K$ »

« $- 6.70 k J \times \frac{1000 J}{k J} = - 8.31 J K^{- 1} (298 K) \ln K$ »

«ln K =» 2.71 ✔

«K = e^2.71 =» 15.0 ✔

Award [2] for correct final answer.

If –7.84 kJ is used then answer is 23.7.

Examiners report

About half the candidates managed to obtain the value of K corresponding to the free energy just calculated. Ensuring that the correct energy units (J) were being used was again the main problem, however some also appeared to have had difficulties with manipulations on the calculator.

Syllabus sections

Additional higher level (AHL)

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

Additional higher level (AHL) » Topic 15: Energetics/thermochemistry

Question 22N.2.hl.TZ0.d.vii

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