Hydrogen peroxide decomposes into water and oxygen when manganese(IV) oxide is added as a catalyst. A student measures the temperature of the hydrogen peroxide solution before and after the reaction and notices a significant rise in temperature. Explain why the temperature of the solution rises, using the terms activation energy, bond breaking, bond forming, and energy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrogen peroxide decomposes into water and oxygen when manganese(IV) oxide is added as a catalyst. A student measures the temperature of the hydrogen peroxide solution before and after the reaction and notices a significant rise in temperature.
Model answer (4 marks)
The decomposition of hydrogen peroxide is:
2 H₂O₂ → 2 H₂O + O₂
Breaking the O–O bonds in H₂O₂ requires energy – this is endothermic. The new H–O bonds in water and the O–O bond in O₂ are stronger, so forming them releases more energy than was absorbed in breaking the original bonds. Consequently the reaction is overall exothermic. The excess energy is transferred to the surrounding solution, raising its temperature.
2 H₂O₂ → 2 H₂O + O₂
Breaking the O–O bonds in H₂O₂ requires energy – this is endothermic. The new H–O bonds in water and the O–O bond in O₂ are stronger, so forming them releases more energy than was absorbed in breaking the original bonds. Consequently the reaction is overall exothermic. The excess energy is transferred to the surrounding solution, raising its temperature.
Examiner tips
- Use the exact terms: activation energy, bond breaking, bond forming, energy
- Show the energy flow: absorbed in breaking, released in forming, net exothermic
- Explain temperature rise as energy transfer to surroundings
- Keep answer concise – 4‑point structure
Common mistakes
- Confusing activation energy with the energy released in the reaction
- Saying the reaction is endothermic or ignoring bond forming energy
- Not linking the exothermic nature to the temperature rise
Mark scheme (4 marks)
- Energy is required to break bonds (in the reactants) / bond breaking is endothermic
- Energy is released when new bonds are formed (in the products) / bond forming is exothermic
- More energy is released in bond forming than is absorbed in bond breaking
- The overall reaction is exothermic, so energy is transferred to the surroundings / solution, causing the temperature to rise
Key terms in this question
bond breaking · bond forming · activation energy · energy
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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