A student adds a small piece of manganese(IV) oxide to a solution of hydrogen peroxide. Bubbles of gas are produced rapidly, but when the reaction is complete, the mass of manganese(IV) oxide is unchanged. Explain the role of manganese(IV) oxide in this reaction and describe how the rate of bubble production would change over time as the reaction proceeds.

Cambridge International IGCSE Chemistry (0620) — 6.2 Rate of reaction · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Manganese(IV) oxide acts as a catalyst, providing an alternative reaction pathway that lowers the activation energy for the decomposition of hydrogen peroxide. It is not consumed in the process, so its mass remains unchanged.

The catalytic action increases the rate of bubble (oxygen) production at the start of the reaction. As the reaction proceeds, the concentration of hydrogen peroxide falls, so the rate of bubble formation decreases.

Examiner tips

  • State that MnO₂ is a catalyst and that it is not consumed. Explain that the rate falls because H₂O₂ concentration decreases. Use the word "decreases" to show change over time.

Common mistakes

  • Claiming MnO₂ is a reactant or is used up. Failing to mention the decrease in rate. Using vague terms like "slow down" instead of "decreases".

Mark scheme (4 marks)

  1. Manganese(IV) oxide acts as a catalyst
  2. A catalyst increases the rate of reaction without being used up / without being chemically changed
  3. The rate of bubble production decreases over time
  4. Because the concentration of hydrogen peroxide decreases as it is used up

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