Sulfur trioxide is produced in the Contact process according to the following reversible reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = −196 kJ/mol Explain the effect on the position of equilibrium of each of the following changes: (i) increasing the pressure (ii) increasing the temperature (iii) removing SO₃ from the equilibrium mixture
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
(i) Increasing the pressure shifts the equilibrium to the right, towards the products. The reaction has 3 mol of gas on the left and only 2 mol on the right, so a rise in pressure favours the side with fewer moles.
(ii) Increasing the temperature shifts the equilibrium to the left, towards the reactants. The forward reaction is exothermic (ΔH = −196 kJ mol⁻¹), so adding heat favours the endothermic reverse reaction.
(iii) Removing SO₃ shifts the equilibrium to the right, towards the products. The system will produce more SO₃ to replace the amount removed, restoring the concentration of SO₃.
(ii) Increasing the temperature shifts the equilibrium to the left, towards the reactants. The forward reaction is exothermic (ΔH = −196 kJ mol⁻¹), so adding heat favours the endothermic reverse reaction.
(iii) Removing SO₃ shifts the equilibrium to the right, towards the products. The system will produce more SO₃ to replace the amount removed, restoring the concentration of SO₃.
Examiner tips
- Use the word ‘shifts’ and state the direction (right/left) clearly; explain the reason in one sentence.
- Mention the change in moles for pressure and the exothermic nature for temperature; for removal, note the system’s response to maintain concentration.
Common mistakes
- Saying the equilibrium shifts to the left when pressure increases (wrong direction).
- For temperature, claiming the equilibrium shifts to the right because the reaction is exothermic (opposite of Le Chatelier’s rule).
- Not explaining the removal of SO₃ as a decrease in concentration that drives the forward reaction.
Mark scheme (5 marks)
- (i) Increasing pressure shifts the equilibrium to the right / towards the products
- (i) Because there are fewer moles of gas on the right-hand side (2 moles) than the left (3 moles), so the system reduces pressure by shifting that way
- (ii) Increasing temperature shifts the equilibrium to the left / towards the reactants
- (ii) Because the forward reaction is exothermic, so increasing temperature favours the endothermic (reverse) reaction to absorb the extra heat energy
- (iii) Removing SO₃ shifts the equilibrium to the right / forward reaction is favoured, because the system acts to replace the removed SO₃ / restore the concentration of SO₃
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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