# Vanadium(V) oxide is used as a catalyst in the Contact process, which produces sulfur trioxide via the reversible reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The forward reaction is exothermic. Explain how the use of vanadium(V) oxide as a catalyst increases the rate of this reaction, and explain why increasing the temperature, whilst speeding up the reaction, is not favoured in terms of the equilibrium yield of sulfur trioxide.

> OCR A-Level Chemistry B: Salters (H433) — 5.3 Transition elements · Explain · 5 marks

> The Contact process is a key stage in the industrial manufacture of sulfuric acid. A compromise temperature of around 450 °C is chosen for the reaction, balancing rate of reaction against yield of sulfur trioxide.

## Mark scheme (5 marks)

1. The catalyst provides a different (alternative) reaction pathway
2. This alternative pathway has a lower activation energy
3. A greater proportion of colliding particles now have at least the activation energy, so the rate of reaction increases
4. Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction (the backward reaction), because the forward reaction is exothermic
5. This means less sulfur trioxide (product) is produced / the yield of SO₃ decreases at higher temperatures

## Key terms

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

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- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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