# A student is investigating the contact process, used to manufacture sulfuric acid. One step involves the reversible reaction between sulfur dioxide and oxygen to form sulfur trioxide. The reaction reaches dynamic equilibrium in a sealed container. Explain what is meant by dynamic equilibrium and describe how increasing the pressure affects the position of equilibrium in this reaction, which produces a greater volume of gas on the left-hand side than on the right-hand side.

> Edexcel GCSE Chemistry (1CH0) — 5.3 Dynamic equilibria, calculations involving volumes of gases (Chem only) · Explain · 4 marks

> The reversible reaction in the contact process: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). There are 3 moles of gas on the left-hand side and 2 moles of gas on the right-hand side.

## Mark scheme (4 marks)

1. At dynamic equilibrium, the rate of the forward reaction equals the rate of the backward reaction.
2. At dynamic equilibrium, the concentrations of reactants and products remain constant.
3. Increasing the pressure shifts the position of equilibrium to the right / towards the products.
4. Because the equilibrium shifts to the side with fewer moles of gas, which reduces the pressure.

## Key terms

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

## Related

- [Revision notes for Edexcel GCSE Chemistry (1CH0)](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-student-is-investigating-the-contact-64f6a37d) · Published by Druglandscape Ltd.