A student carries out two reactions. In Reaction A, zinc powder is added to dilute sulfuric acid at room temperature. In Reaction B, a large piece of zinc is added to the same volume of dilute sulfuric acid at the same temperature. The student notices that Reaction A produces bubbles much more quickly than Reaction B. Explain why Reaction A has a faster rate of reaction than Reaction B, and describe one other change the student could make to increase the rate of Reaction A even further, explaining how it works.

OCR A-Level Chemistry B: Salters (H433) — 5.2 Energy · Explain · 5 marks · View as Markdown

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

Zinc reacts with dilute sulfuric acid to produce zinc sulfate and hydrogen gas. The rate of reaction can be affected by several factors.

Model answer (5 marks)

1. Zinc powder has a much larger surface area to volume ratio than a large piece of zinc.
2. This means more zinc atoms are exposed to the acid at any one time.
3. Consequently there are more reacting particles in contact with the acid.
4. More contacts give more frequent successful collisions, so the overall rate of reaction is higher in Reaction A.
5. To increase the rate of Reaction A further, the student could raise the temperature of the acid. Higher temperature gives the reacting particles more kinetic energy, increasing the number of collisions that have sufficient energy to overcome the activation energy, thereby speeding up the reaction.

Examiner tips

  • Use the surface‑area argument first, then link to collision frequency and activation energy.
  • State a specific change (e.g. increase temperature) and explain its effect on kinetic energy and collision rate.
  • Keep the answer concise – each point should be a single sentence.
  • Use UK spelling and correct chemical formulae (Zn + H₂SO₄ → ZnSO₄ + H₂).

Common mistakes

  • Confusing surface area with volume; students say ‘larger volume’ instead of ‘larger surface area’. Failing to explain how the change (e.g. temperature) affects kinetic energy or collision frequency. Using vague terms like ‘more acid’ without specifying concentration or temperature.

Mark scheme (5 marks)

  1. Zinc powder has a greater surface area (to volume ratio) than a large piece of zinc
  2. There are more exposed reacting particles / more zinc particles in contact with the acid
  3. This leads to more frequent successful collisions, so the rate of reaction increases
  4. A valid change is stated, e.g. increase the temperature, increase the concentration of sulfuric acid, or add a catalyst
  5. Correct explanation linked to the chosen change, e.g. higher temperature gives particles more kinetic energy so more particles have at least the activation energy and collisions are more frequent; OR higher concentration means particles are closer together so collisions are more frequent; OR a catalyst provides a different reaction pathway with a lower activation energy

Key terms in this question

rate of reaction

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