# A student investigates the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid. The reaction produces a yellow precipitate of sulfur. In one experiment, the student uses sodium thiosulfate solution at a concentration of 0.10 mol/dm³. In a second experiment, everything is kept the same except the concentration of sodium thiosulfate solution is increased to 0.40 mol/dm³. Explain why the rate of reaction is faster in the second experiment.

> Pearson Edexcel International GCSE Chemistry (4CH1) — 3.2 Rates of reaction · Explain · 4 marks

> Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l)

## Mark scheme (4 marks)

1. Increasing the concentration means the particles (sodium thiosulfate / reacting particles) are closer together
2. The particles collide more often / collision frequency increases
3. There are more successful collisions (per unit time) / a higher rate of successful collisions
4. Therefore the rate of reaction increases / product is formed faster

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Chemistry (4CH1)](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-student-investigates-the-rate-of-973e9485) · Published by Druglandscape Ltd.