# 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

> Edexcel A-Level Chemistry (9CH0) — 10.1 Dynamic equilibria and Le Chatelier · Explain · 5 marks

## Mark scheme (5 marks)

1. (i) Increasing pressure shifts the equilibrium to the right / towards the products
2. (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
3. (ii) Increasing temperature shifts the equilibrium to the left / towards the reactants
4. (ii) Because the forward reaction is exothermic, so increasing temperature favours the endothermic (reverse) reaction to absorb the extra heat energy
5. (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

- [position of equilibrium](https://www.gradenine.co.uk/glossary/position-of-equilibrium)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/sulfur-trioxide-is-produced-in-the-34fb0f86) · Published by Druglandscape Ltd.