# A chemical plant produces sulfuric acid using the Contact process. One stage involves the reversible reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The forward reaction is exothermic. The plant operates at a moderate temperature of around 450 °C rather than a very low temperature, and uses vanadium pentoxide as a catalyst. Explain how using a catalyst increases the rate of this reaction, and explain why the plant uses a moderate temperature rather than a very low temperature, even though a lower temperature would favour a greater yield of SO₃.

> OCR A-Level Chemistry A (H432) — 5.3 Transition elements · Explain · 5 marks

> The Contact process is used industrially to manufacture sulfuric acid. The key reversible reaction is: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The forward reaction is exothermic. Vanadium pentoxide is used as a catalyst at approximately 450 °C.

## Mark scheme (5 marks)

1. A catalyst provides a different reaction pathway with a lower activation energy
2. A greater proportion of particles now have at least the activation energy, so more collisions are successful and the rate increases
3. A lower temperature would shift the equilibrium position towards the exothermic (forward) reaction, giving a higher yield of SO₃
4. However, at a very low temperature the rate of reaction would be too slow
5. A moderate temperature is a compromise between a sufficient rate of reaction and an acceptable yield of SO₃

## Key terms

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

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-chemical-plant-produces-sulfuric-acid-b14647e9) · Published by Druglandscape Ltd.