Manganese(IV) oxide is used as a catalyst in the decomposition of hydrogen peroxide solution to produce water and oxygen. A student investigates how changing the mass of manganese(IV) oxide added affects the rate of this reaction. Explain how manganese(IV) oxide acts as a catalyst in this reaction and describe what happens to the mass of manganese(IV) oxide at the end of the reaction.

OCR A-Level Chemistry B: Salters (H433) — 5.3 Transition elements · Explain · 5 marks · View as Markdown

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

Hydrogen peroxide solution decomposes slowly at room temperature. Adding manganese(IV) oxide greatly speeds up this decomposition. The reaction can be represented as: 2H₂O₂(aq) → 2H₂O(l) + O₂(g)

Model answer (5 marks)

A catalyst supplies an alternative reaction pathway with a lower activation energy.

Because the activation energy is lower, a larger proportion of H₂O₂ molecules have sufficient energy to react, so the rate increases.

Manganese(IV) oxide is not consumed – its mass remains unchanged after the reaction.

Therefore, adding more MnO₂ gives more surface area and more active sites, so the rate of decomposition rises.

Examiner tips

  • Use the term "alternative reaction pathway" and "lower activation energy" – these are key phrases. Show the link between lower Ea, more molecules above the threshold and a higher rate. Mention that the catalyst is unchanged. Explain why more mass gives more surface area and thus more sites.
  • common_mistakes
  • :
  • Saying the catalyst is “used up” or “consumed”. Forgetting to link lower activation energy to a higher proportion of reacting molecules. Using vague wording such as “makes the reaction faster” without explaining the mechanism.

Mark scheme (5 marks)

  1. A catalyst provides a different reaction pathway / alternative reaction pathway
  2. The alternative pathway has a lower activation energy
  3. A greater proportion of particles have energy greater than or equal to the (lower) activation energy
  4. The manganese(IV) oxide is not used up / its mass remains the same at the end of the reaction
  5. Therefore increasing the mass of manganese(IV) oxide increases the rate of reaction because there is more surface area / more active sites for the reaction to occur

Key terms in this question

catalyst

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