Hydrogen peroxide solution decomposes slowly at room temperature into water and oxygen. When a small amount of manganese dioxide is added, the reaction speeds up and the surroundings feel warm. Explain what type of energy change is occurring and why the temperature of the surroundings increases. In your answer, state what happens to the bonds in the reactants and the bonds formed in the products.

AQA A-Level Chemistry (7405) — 3.1.4 Energetics · 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 into water and oxygen. When a small amount of manganese dioxide is added, the reaction speeds up and the surroundings feel warm.

Model answer (5 marks)

The decomposition of hydrogen peroxide is an exothermic reaction. Energy must be supplied to break the O–O bond in the H₂O₂ molecules. When the reaction proceeds, new bonds are formed – two O–H bonds in each water molecule and a triple bond in each O₂ molecule. The energy released in forming these bonds is greater than the energy required to break the O–O bonds. The excess energy is released to the surroundings, raising their temperature.

Examiner tips

  • Use the word "exothermic" to show you know the type of reaction.
  • Explain the energy flow: breaking bonds (endothermic) then forming bonds (exothermic).
  • Show that the energy released > energy absorbed, leading to heat transfer to surroundings.
  • Mention the specific bonds involved (O–O, O–H, O≡O).

Common mistakes

  • Saying the reaction is endothermic or ignoring the bond breaking step.
  • Failing to specify which bonds are broken and which are formed.
  • Claiming the temperature rises because the reaction is faster, not because of energy release.

Mark scheme (5 marks)

  1. The reaction is exothermic
  2. Energy is required / taken in to break bonds in the reactants (hydrogen peroxide)
  3. Energy is released when bonds are formed in the products (water and oxygen)
  4. More energy is released forming bonds in the products than is required to break bonds in the reactants
  5. The surplus energy is transferred to the surroundings, so the temperature of the surroundings increases

Key terms in this question

surroundings

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