# A chemist is investigating the following reversible reaction, which takes place in a closed system at a fixed temperature: SO₂(g) + NO₂(g) ⇌ SO₃(g) + NO(g). The chemist adds extra NO₂ gas to the system once dynamic equilibrium has been established. Explain what happens to the position of equilibrium and why, and state what happens to the concentration of NO once the system reaches equilibrium again.

> OCR GCSE Chemistry A: Gateway Science (J248) — C5.3 Equilibria · Explain · 4 marks

> The reaction SO₂(g) + NO₂(g) ⇌ SO₃(g) + NO(g) reaches dynamic equilibrium in a closed system. Extra NO₂ is then added to the system.

## Mark scheme (4 marks)

1. Adding extra NO₂ increases the concentration of a reactant
2. The position of equilibrium shifts to the right / towards the products (to counteract the change / according to Le Chatelier's principle)
3. The rate of the forward reaction increases (until it equals the rate of the backward reaction again)
4. The concentration of NO increases (as more NO is produced)

## Key terms

- [dynamic equilibrium](https://www.gradenine.co.uk/glossary/dynamic-equilibrium)
- [closed system](https://www.gradenine.co.uk/glossary/closed-system)
- [reversible reaction](https://www.gradenine.co.uk/glossary/reversible-reaction)
- [concentration](https://www.gradenine.co.uk/glossary/concentration)

## Related

- [Revision notes for OCR GCSE Chemistry A: Gateway Science (J248)](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/a-chemist-is-investigating-the-following-c628c1e0) · Published by Druglandscape Ltd.