# Explain why the rate of a gas-phase reaction increases when the pressure is increased at constant temperature.

> IB DP Chemistry Higher Level (2023 syllabus) — R2.2 How fast? The rate of chemical change · Explain · 4 marks

## Mark scheme (4 marks)

1. Increasing pressure decreases the volume, so the concentration (number of moles per unit volume) of the reactant molecules increases.
2. The increased concentration leads to more frequent collisions between reactant molecules per unit time.
3. Since temperature is unchanged, the fraction of molecules with energy greater than or equal to the activation energy remains the same.
4. Therefore, more collisions per unit time have sufficient energy (and correct orientation) to result in reaction, so the overall rate increases.

## Key terms

- [pressure](https://www.gradenine.co.uk/glossary/pressure)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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