# Bromine water decolourises when it reacts with propene. A student investigates how the initial concentration of bromine affects the initial rate of this reaction at constant temperature. When the concentration of bromine is doubled, the initial rate of reaction doubles. Explain what this tells the student about how bromine concentration affects the rate of this reaction, and describe one other factor, besides concentration, that the student would need to keep constant to make the investigation a fair test, explaining why it must be controlled.

> AQA A-Level Chemistry (7405) — 3.1.9 Rate equations (A-Level only) · Explain · 5 marks

> Bromine water decolourises when it reacts with propene. A student investigates how the initial concentration of bromine affects the initial rate of this reaction at constant temperature. When the concentration of bromine is doubled, the initial rate of reaction doubles.

## Mark scheme (5 marks)

1. The rate is directly proportional to the concentration of bromine / the reaction is first order with respect to bromine
2. When concentration doubles, the number of bromine particles per unit volume doubles / collision frequency between bromine and propene molecules doubles
3. This leads to a proportional increase in the rate of successful collisions / rate of reaction
4. Named control variable: temperature / concentration of propene / surface area (if propene is a liquid or solid form) — one acceptable named variable
5. Correct explanation of why the named variable must be controlled — e.g. increasing temperature increases the kinetic energy of particles / increases the proportion of particles with energy greater than or equal to the activation energy, which would also increase the rate, making results unreliable; OR changing propene concentration would change collision frequency independently of bromine concentration

## Key terms

- [rate of reaction](https://www.gradenine.co.uk/glossary/rate-of-reaction)
- [concentration](https://www.gradenine.co.uk/glossary/concentration)

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](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/bromine-water-decolourises-when-it-reacts-05654567) · Published by Druglandscape Ltd.