A student is investigating the rate of reaction between sodium thiosulfate solution and hydrochloric acid. The student repeats the experiment using a higher concentration of sodium thiosulfate solution. Explain why the reaction is faster when a higher concentration of sodium thiosulfate solution is used.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When sodium thiosulfate solution reacts with hydrochloric acid, a yellow precipitate of sulfur forms, making the mixture go cloudy. The student uses this cloudiness to judge when the reaction is complete.
Model answer (4 marks)
Increasing the concentration of sodium thiosulfate means the ions are closer together in solution. This increases the frequency of collisions between the β‑sulphate ions and the HCl molecules. With more collisions per unit time, there are more successful collisions that lead to the formation of elemental sulfur. Consequently the overall rate of the reaction rises.
Examiner tips
- Use the term ‘frequency of collisions’ and link it to concentration.
- Show the chain: higher concentration → closer particles → more collisions → more successful collisions → higher rate.
- Use ‘rate of reaction’ to finish the explanation.
Common mistakes
- Confusing concentration with temperature or pressure.
- Failing to mention collisions or successful collisions.
- Using vague phrases like ‘more reactants’ without explaining the collision theory.
Mark scheme (4 marks)
- Increasing concentration means particles are closer together (in solution)
- Particles collide more often / frequency of collisions increases
- There are more successful collisions per unit time / higher rate of successful collisions
- Therefore the rate of reaction increases
Key terms in this question
concentration · rate of reaction
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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