DP Chemistry (first assessment 2025)

Question 19M.1A.SL.TZ2.13

Date	May 2019	Marks available	[Maximum mark: 1]	Reference code	19M.1A.SL.TZ2.13
Level	SL	Paper	1A	Time zone	TZ2
Command term		Question number	13	Adapted from	N/A

13.

[Maximum mark: 1]

19M.1A.SL.TZ2.13

Consider the following equations.

2Al (s) + $\frac{3}{2}$ O₂ (g) → Al₂O₃ (s) ΔH^Ɵ = −1670 kJ
Mn (s) + O₂ (g) → MnO₂ (s) ΔH^Ɵ = −520 kJ

What is the standard enthalpy change, in kJ, of the reaction below?

4Al (s) + 3MnO₂ (s) → 2Al₂O₃ (s) + 3Mn (s)

A. −1670 + 520

B. $\frac{3}{2}$ (−1670) + 3(520)

C. 2(−1670) + 3(−520)

D. 2(−1670) + 3(520)

[1]

Markscheme

Examiners report

It is pleasing that 83 % of the candidates were confident in applying Hess’s law to obtain an expression for the standard enthalpy change of reaction. The most commonly chosen distractor (C) had the correct coefficients but one of the signs was incorrect.

Syllabus sections

Reactivity 1. What drives chemical reactions? » Reactivity 1.2—Energy cycles in reactions » Reactivity 1.2.2—Hess’s law states that the enthalpy change for a reaction is independent of the pathway between the initial and final states. Apply Hess’s law to calculate enthalpy changes in multistep reactions.

Question 19M.1A.SL.TZ2.13

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