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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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

Model answer (4 marks)

Increasing the concentration of Na₂S₂O₃ means the particles are closer together.

This increases the collision frequency between Na₂S₂O₃ and HCl.

With more collisions per unit time, there are more successful collisions.

Hence the rate of reaction is faster and the yellow precipitate appears more quickly.

Examiner tips

  • Use the word "increase" to link concentration to collision frequency. Show the chain: concentration → collision frequency → successful collisions → rate. Mention the product (yellow precipitate) as evidence of faster rate.

Common mistakes

  • Confusing concentration with temperature. Failing to explain that higher concentration leads to more collisions. Using vague terms like "more reactants" without linking to collision theory.

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 in this question

concentration · rate of reaction

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