# Explain how increasing the concentration of sodium thiosulfate solution increases the rate of its reaction with hydrochloric acid.

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

> Sodium thiosulfate reacts with hydrochloric acid to produce a yellow precipitate of sulfur. The reaction can be followed by measuring how quickly the precipitate forms. When the concentration of sodium thiosulfate is increased, the reaction is observed to happen more quickly.

## Mark scheme (5 marks)

1. Increasing concentration means more particles (of sodium thiosulfate) in the same volume
2. This leads to more frequent collisions between sodium thiosulfate particles and hydrochloric acid particles
3. More collisions are successful / more collisions result in a reaction
4. Reference to activation energy — collisions must have energy equal to or greater than the activation energy to be successful
5. Therefore the rate of reaction increases

## Key terms

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

## Related

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