DP Chemistry (first assessment 2025)

Question 19M.2.HL.TZ2.2g(ii)

Date	May 2019	Marks available	[Maximum mark: 3]	Reference code	19M.2.HL.TZ2.2g(ii)
Level	HL	Paper	2	Time zone	TZ2
Command term	Deduce	Question number	g(ii)	Adapted from	N/A

g(ii).

[Maximum mark: 3]

19M.2.HL.TZ2.2g(ii)

(g(ii))

Dinitrogen monoxide has a positive standard enthalpy of formation, ΔH_f^θ.

Deduce, giving reasons, whether altering the temperature would change the spontaneity of the decomposition reaction.

[3]

Markscheme

exothermic decomposition
OR
ΔH_{(decomposition)} < 0 [✔]

TΔS^θ > ΔH^θOR
ΔG^θ «= ΔH^θ – TΔS^θ» < 0 «at all temperatures» [✔]

reaction spontaneous at all temperatures [✔]

Examiners report

Though the question asked for decomposition (in bold), most candidates ignored this and worked on the basis of a the ΔH of formation. However, many did write a sound explanation for that situation. On the other hand, in quite a number of cases, they did not state the sign of the ΔH (probably taking it for granted) nor explicitly relate ΔG and spontaneity, which left the examiner with no possibility of evaluating their reasoning.

Syllabus sections

Reactivity 1. What drives chemical reactions? » Reactivity 1.4—Entropy and spontaneity (Additional higher level) » Reactivity 1.4.3—At constant pressure, a change is spontaneous if the change in Gibbs energy, ΔG, is negative. Interpret the sign of ΔG calculated from thermodynamic data. Determine the temperature at which a reaction becomes spontaneous.

Question 19M.2.HL.TZ2.2g(ii)

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