A student adds a small lump of manganese(IV) oxide to a solution of hydrogen peroxide. Bubbles of oxygen gas are produced rapidly. The student then removes the manganese(IV) oxide, dries it, and finds its mass is unchanged. Explain how the manganese(IV) oxide increases the rate of decomposition of hydrogen peroxide, and justify why its mass is unchanged at the end of the reaction.

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).

Hydrogen peroxide decomposes slowly at room temperature: H₂O₂(aq) → H₂O(l) + ½O₂(g). In the presence of manganese(IV) oxide the reaction is much faster.

Model answer (4 marks)

Manganese(IV) oxide acts as a catalyst for the decomposition of hydrogen peroxide.
It provides an alternative reaction pathway that has a lower activation energy than the uncatalysed reaction.
Because the activation energy is lower, a greater proportion of the hydrogen peroxide molecules have enough energy to react, so the number of successful collisions per unit time increases.
The catalyst is not consumed in the reaction, so its mass remains unchanged after the reaction.

Examiner tips

  • Use the word ‘catalyst’ first, then explain the lower activation energy and increased collisions, finish with the catalyst not being used up.
  • Show the logical sequence: catalyst → lower Ea → more successful collisions → faster rate, and that the catalyst is unchanged.
  • Include the specific reaction equation if space allows to show the decomposition.
  • Use UK spelling (e.g., ‘catalyst’).

Common mistakes

  • Failing to state that manganese(IV) oxide is a catalyst.
  • Confusing the catalyst with a reactant and saying it is consumed.
  • Not linking lower activation energy to increased collisions or rate.

Mark scheme (4 marks)

  1. Manganese(IV) oxide acts as a catalyst
  2. The catalyst provides a different (alternative) reaction pathway with a lower activation energy
  3. A lower activation energy means a greater proportion of colliding particles have sufficient energy to react, so there are more successful collisions per unit time / the rate of successful collisions increases
  4. The catalyst is not used up during the reaction, so its mass remains unchanged at the end

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