# Sulfur trioxide is produced in the Contact process according to the following reversible reaction:

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

At a certain temperature, the equilibrium constant expression for this reaction is:

Kc = [SO₃]²  /  ([SO₂]² × [O₂])

Explain what the value of Kc tells us about the position of equilibrium, and describe how the value of Kc changes when (i) the temperature is increased and (ii) a catalyst is added.

> Eduqas A-Level Chemistry — 3.8 Equilibrium constants · Explain · 5 marks

> The Contact process is used industrially to manufacture sulfuric acid. The forward reaction is exothermic.

## Mark scheme (5 marks)

1. A large value of Kc means the equilibrium lies to the right / products are favoured
2. A small value of Kc means the equilibrium lies to the left / reactants are favoured
3. Increasing temperature shifts equilibrium to the left (endothermic direction) because the forward reaction is exothermic
4. Increasing temperature decreases the value of Kc (for this exothermic reaction)
5. Adding a catalyst does not change the value of Kc

## Key terms

- [equilibrium constant](https://www.gradenine.co.uk/glossary/equilibrium-constant)
- [Kc](https://www.gradenine.co.uk/glossary/kc)
- [catalyst](https://www.gradenine.co.uk/glossary/catalyst)

## Related

- [Revision notes for Eduqas A-Level Chemistry](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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/sulfur-trioxide-is-produced-in-the-740bda49) · Published by Druglandscape Ltd.