# The contact process is used to manufacture sulfuric acid. One step involves the reversible reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The forward reaction is exothermic. A chemical engineer considers changing the temperature and pressure conditions to increase the yield of SO₃. Explain how increasing the temperature and increasing the pressure would each affect the position of equilibrium in this reaction.

> OCR A-Level Chemistry B: Salters (H433) — 5.1 Rates, equilibrium and pH · Explain · 5 marks

> The contact process is a key industrial process. The reversible reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) reaches dynamic equilibrium when carried out in a closed system. The forward reaction is exothermic.

## Mark scheme (5 marks)

1. Increasing the temperature shifts the equilibrium position in the direction of the endothermic reaction
2. Therefore increasing the temperature shifts the equilibrium to the left / towards the reactants, decreasing the yield of SO₃
3. This is because increasing temperature counteracts the change, according to Le Chatelier's principle / the system opposes the increase in temperature by absorbing heat via the endothermic (reverse) reaction
4. Increasing the pressure shifts the equilibrium towards the side with the smaller number of moles of gas
5. There are 3 moles of gas on the left and 2 moles of gas on the right, so increasing pressure shifts the equilibrium to the right / towards the products, increasing the yield of SO₃

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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