# Sulfur dioxide and oxygen react together in a closed container to form sulfur trioxide. The equation for the reaction is: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). At a certain temperature, the equilibrium concentrations are: [SO₂] = 0.20 mol/dm³, [O₂] = 0.10 mol/dm³, and [SO₃] = 0.40 mol/dm³. Write the expression for the equilibrium constant, Kc, for this reaction and explain what it tells us about the position of equilibrium at this temperature.

> WJEC A-Level Chemistry (Wales) — 3.8 Equilibrium constants · Explain · 5 marks

> The contact process uses this reversible reaction to produce sulfur trioxide, which is a key step in making sulfuric acid. The reaction is carried out in a closed system and reaches dynamic equilibrium.

## Mark scheme (5 marks)

1. Correct Kc expression with SO₃ squared in the numerator
2. Correct calculation or identification that Kc = 40 mol⁻¹dm³ (or equivalent large value shown by substitution)
3. Kc is a large number / greater than 1
4. A large Kc means the equilibrium lies to the right / favours the products
5. Kc only changes with temperature / concentrations do not change Kc

## Key terms

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

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](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/sulfur-dioxide-and-oxygen-react-together-cfc46dca) · Published by Druglandscape Ltd.