A student investigates the combustion of methane. The student knows that during the reaction, bonds in the reactants are broken and new bonds in the products are formed. Explain why the combustion of methane is an exothermic reaction, referring to bond breaking and bond making in your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The combustion of methane can be represented by the following equation: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
Model answer (5 marks)
Bond breaking in CH₄ and O₂ requires energy, so the reactants absorb heat.
Bond making in CO₂ and H₂O releases more energy than was absorbed.
Because the energy released in forming the new bonds exceeds the energy taken to break the old bonds, the net change is negative.
The overall energy change is therefore exothermic – heat is transferred to the surroundings.
In an exothermic reaction the products have a lower internal energy than the reactants.
Bond making in CO₂ and H₂O releases more energy than was absorbed.
Because the energy released in forming the new bonds exceeds the energy taken to break the old bonds, the net change is negative.
The overall energy change is therefore exothermic – heat is transferred to the surroundings.
In an exothermic reaction the products have a lower internal energy than the reactants.
Examiner tips
- Use the exact terms ‘bond breaking is endothermic’ and ‘bond making is exothermic’.
- Show the energy flow: absorbed → released → net negative.
- Mention that the products have lower energy than the reactants.
Common mistakes
- Confusing the direction of heat flow (writing that heat is absorbed by the reaction).
- Failing to state that the products have lower energy than the reactants.
Mark scheme (5 marks)
- Bond breaking is an endothermic process / energy is taken in to break bonds in the reactants
- Bond making is an exothermic process / energy is released when bonds form in the products
- More energy is released in bond making than is taken in during bond breaking
- The overall energy change is negative / energy is transferred to the surroundings
- In an exothermic reaction the energy of the products is lower than the energy of the reactants
Key terms in this question
exothermic reaction · bond breaking · bond making
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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