A student investigates how temperature affects the rate of a chemical reaction between sodium thiosulfate solution and hydrochloric acid. The reaction produces a yellow precipitate of sulfur. As temperature increases, the reaction gets faster. Explain why increasing the temperature of the reactants causes the rate of this reaction to increase.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When sodium thiosulfate solution is mixed with hydrochloric acid, a yellow precipitate of sulfur slowly forms. The student repeats the experiment at different temperatures and observes that the reaction is much faster at higher temperatures.
Model answer (4 marks)
At higher temperatures the particles of the reactants possess more kinetic energy.
This causes them to collide more frequently, giving a greater number of collisions per second.
A larger proportion of these collisions have energy equal to or greater than the activation energy of the reaction.
Consequently, more collisions lead to product formation, so the rate of the reaction increases.
This causes them to collide more frequently, giving a greater number of collisions per second.
A larger proportion of these collisions have energy equal to or greater than the activation energy of the reaction.
Consequently, more collisions lead to product formation, so the rate of the reaction increases.
Examiner tips
- Use the word ‘cause’ or ‘therefore’ to link each point; show the logical flow from energy to collisions to rate.
- Mention ‘activation energy’ explicitly – examiners look for this term.
- Keep each point brief – 4 marks, so one sentence per point is sufficient.
Common mistakes
- Failing to mention activation energy; just saying ‘more collisions’ is incomplete.
- Using vague phrases like ‘more energy’ without linking to kinetic energy or collisions.
- Writing in a narrative style instead of concise, exam‑style sentences.
Mark scheme (4 marks)
- Particles have more kinetic energy at higher temperatures
- Particles collide more frequently / there are more collisions per second
- A greater proportion of collisions have energy equal to or greater than the activation energy
- Therefore more product is formed per unit time / the rate of reaction increases
Related
- All Eduqas GCSE Chemistry revision notes →
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- Decode the mark scheme abbreviations →
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