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Date	November 2018	Marks available	1	Reference code	18N.2.hl.TZ0.5
Level	HL	Paper	2	Time zone	TZ0
Command term	State	Question number	5	Adapted from	N/A

Question

This reaction is used in the manufacture of sulfuric acid.

2SO₂ (g) + O₂ (g) $\rightleftharpoons$ 2SO₃ (g) K_c = 280 at 1000 K

State why this equilibrium reaction is considered homogeneous.

[1]

Predict, giving your reason, the sign of the standard entropy change of the forward reaction.

[1]

Calculate the standard Gibbs free energy change, ΔG^Θ, in kJ, for this reaction at 1000 K. Use sections 1 and 2 of the data booklet.

[2]

Predict, giving your reasons, whether the forward reaction is endothermic or exothermic. Use your answers to (b) and (c).

[2]

0.200 mol sulfur dioxide, 0.300 mol oxygen and 0.500 mol sulfur trioxide were mixed in a 1.00 dm³ flask at 1000 K.

Predict the direction of the reaction showing your working.

[3]

Markscheme

all «species» are in same phase ✔

Accept “all species are in same state”.

Accept “all species are gases”.

negative AND fewer moles/molecules «of gas» in the products ✔

ΔG^Θ =«–RT ln K_c =» –8.31 J K^–1 mol^–1 × 1000 K × ln 280

ΔG^Θ = – 4.7 × 10⁴ «J» ✔

«ΔG^Θ =» – 47 «kJ» ✔

Award [2] for correct final answer.

ΔG^Θ < 0/spontaneous AND ΔS^Θ < 0/unfavourable ✔

exothermic AND ΔH^Θ «must be» negative/favourable ✔

«reaction quotient/Q =» $\frac{{{{\left[ {{\text{S}}{{\text{O}}_3}} \right]}^2}}}{{{{\left[ {{\text{S}}{{\text{O}}_2}} \right]}^2}\left[ {{{\text{O}}_2}} \right]}}{\text{/}}\frac{{{{0.500}^2}}}{{{{0.200}^2} \times 0.300}}{\text{/}}20.8$ ✔

reaction quotient/Q/20.8/answer < K_c/280

mixture needs more product for the number to equal Kc ✔

reaction proceeds to the right/products ✔

Do not award M3 without valid reasoning.

Examiners report

[N/A]

Syllabus sections

Core » Topic 1: Stoichiometric relationships » 1.1 Introduction to the particulate nature of matter and chemical change

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