Explain how increasing the concentration of sodium thiosulfate solution increases the rate of its reaction with hydrochloric acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
Increasing the concentration of sodium thiosulfate increases the number of σ‑SNa particles per unit volume.
1. More particles → higher probability of collisions with HCl molecules.
2. Collision frequency rises.
3. Each collision must have energy ≥ activation energy (E_a) to be successful.
4. With more collisions, the number of successful collisions per unit time increases.
5. Therefore the overall rate of formation of the yellow sulphur precipitate rises.
1. More particles → higher probability of collisions with HCl molecules.
2. Collision frequency rises.
3. Each collision must have energy ≥ activation energy (E_a) to be successful.
4. With more collisions, the number of successful collisions per unit time increases.
5. Therefore the overall rate of formation of the yellow sulphur precipitate rises.
Examiner tips
- Use the word "increases" and link concentration to collision frequency; mention activation energy; keep answer concise and structured; include all five points to match marks.
Common mistakes
- Confusing concentration with molarity; not mentioning activation energy; writing a long paragraph instead of clear points; omitting the link between collisions and reaction rate.
Mark scheme (5 marks)
- Increasing concentration means more particles (of sodium thiosulfate) in the same volume
- This leads to more frequent collisions between sodium thiosulfate particles and hydrochloric acid particles
- More collisions are successful / more collisions result in a reaction
- Reference to activation energy — collisions must have energy equal to or greater than the activation energy to be successful
- Therefore the rate of reaction increases
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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