Explain why the combustion of methane is an exothermic reaction, in terms of the bonds broken and bonds formed during the reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methane burns in oxygen according to the equation: CH₄ + 2O₂ → CO₂ + 2H₂O
Model answer (5 marks)
Energy is required to break the C–H bonds in CH₄ and the O–O bonds in O₂.
Energy is released when the C=O bonds in CO₂ and the O–H bonds in H₂O are formed.
The energy released in forming the new bonds is greater than the energy absorbed in breaking the old bonds.
Hence there is a net release of energy to the surroundings.
This overall release of energy makes the reaction exothermic, raising the temperature of the surroundings.
Energy is released when the C=O bonds in CO₂ and the O–H bonds in H₂O are formed.
The energy released in forming the new bonds is greater than the energy absorbed in breaking the old bonds.
Hence there is a net release of energy to the surroundings.
This overall release of energy makes the reaction exothermic, raising the temperature of the surroundings.
Examiner tips
- Use the sequence: break bonds → form bonds → compare energies → conclude exothermic.
- Mention that the net enthalpy change is negative.
- Show the logic clearly in five short points.
Common mistakes
- Failing to state that breaking bonds requires energy.
- Confusing the sign of the enthalpy change (writing positive instead of negative).
- Omitting the step that the released energy exceeds the absorbed energy.
Mark scheme (5 marks)
- Energy is required to break bonds (in the reactants)
- Energy is released when bonds are formed (in the products)
- The energy released (making bonds) is greater than the energy absorbed (breaking bonds)
- Therefore there is an overall release of energy to the surroundings
- This overall release of energy means the reaction is exothermic (temperature of surroundings increases)
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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