# The Contact Process is used industrially to manufacture sulfuric acid. One key step involves the following reversible reaction carried out in a closed system: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)   The forward reaction is exothermic. Industrial chemists must choose conditions carefully to obtain a good yield of sulfur trioxide at a reasonable rate. Explain how changes in temperature and pressure each affect the position of equilibrium in this reaction, and state the direction each change would shift the equilibrium.

> OCR A-Level Chemistry A (H432) — 5.1 Rates, equilibrium and pH · Explain · 5 marks

> The Contact Process produces sulfur trioxide as an intermediate in the manufacture of sulfuric acid. The reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) is reversible and exothermic in the forward direction. It is carried out in a closed system so that dynamic equilibrium can be established.

## Mark scheme (5 marks)

1. States that increasing temperature shifts the equilibrium in the direction of the endothermic (reverse/backward) reaction
2. Links the temperature shift correctly to Le Chatelier's principle — the system counteracts the increase in temperature by absorbing heat via the endothermic reaction
3. Correctly counts the moles of gas on each side: 3 moles on the left (reactants) and 2 moles on the right (products)
4. States that increasing pressure shifts the equilibrium towards the side with fewer moles of gas — i.e. to the right, producing more SO₃
5. Links the pressure shift to Le Chatelier's principle — the system counteracts the increased pressure by reducing the number of moles of gas

## Key terms

- [closed system](https://www.gradenine.co.uk/glossary/closed-system)
- [reversible reaction](https://www.gradenine.co.uk/glossary/reversible-reaction)

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](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/the-contact-process-is-used-industrially-f019b1d7) · Published by Druglandscape Ltd.