Hydrogen peroxide decomposes to form water and oxygen. The activation energy for this reaction is high. A student adds manganese(IV) oxide to hydrogen peroxide and observes that the reaction proceeds rapidly at room temperature. Explain, in terms of bond breaking and bond forming, why the decomposition of hydrogen peroxide is exothermic, and explain how the manganese(IV) oxide increases the rate of reaction without being used up.

AQA A-Level Chemistry (7405) — 3.1.8 Thermodynamics (A-Level only) · Explain · 5 marks · View as Markdown

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

Hydrogen peroxide, H₂O₂, decomposes according to the equation: 2H₂O₂(l) → 2H₂O(l) + O₂(g). The total energy required to break all bonds in the reactants is less than the total energy released when all bonds form in the products.

Model answer (5 marks)

The decomposition of H₂O₂ requires energy to break the O–O bond and the O–H bonds in the reactants. When the products form, new O–H bonds are made in water and an O=O double bond is formed in O₂. The energy released in forming these bonds is greater than the energy needed to break the bonds in H₂O₂, so the overall ΔH is negative – the reaction is exothermic.

Manganese(IV) oxide acts as a catalyst. It provides an alternative pathway in which the O–O bond in H₂O₂ is broken via a surface reaction, lowering the activation energy. Because the catalyst is not consumed in the reaction, it remains chemically unchanged and can continue to catalyse further decompositions.

Examiner tips

  • Use the term ‘exothermic’ and state that ΔH<0. Show the bond‑breaking vs bond‑forming energy comparison. Identify MnO₂ as a catalyst and explain the lower activation energy. Mention that the catalyst is unchanged after the reaction.
  • common_mistakes
  • :
  • Saying the reaction is ‘endothermic’ or ‘exothermic’ without linking to bond energies. Forgetting to state that the catalyst is not consumed. Using vague phrases like ‘makes the reaction faster’ without explaining the lower activation energy.

Mark scheme (5 marks)

  1. Energy is required to break bonds in the reactants (H₂O₂)
  2. Energy is released when new bonds form in the products (H₂O and O₂)
  3. The energy released forming bonds is greater than the energy required to break bonds, so the overall reaction is exothermic / releases energy to the surroundings
  4. Manganese(IV) oxide is a catalyst, which provides an alternative reaction pathway with a lower activation energy
  5. The catalyst is not used up / is chemically unchanged at the end of the reaction, so it can continue to catalyse further reactions

Key terms in this question

exothermic · bond breaking · bond forming · activation energy

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