A student burns a piece of wood in a fireplace. The room feels warmer after the wood has been burning for a few minutes. Explain why the temperature of the room increases during this reaction, and describe what this tells us about the overall energy change of the reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When wood burns in a fireplace, a chemical reaction takes place between the wood and oxygen in the air. The room gradually becomes warmer.
Model answer (4 marks)
Burning wood is an exothermic reaction.
Energy is released when the C–H and C–O bonds in the wood are broken and new bonds are formed in the products (CO₂ and H₂O). The energy released in forming the products is greater than the energy required to break the reactants’ bonds.
The released energy is transferred to the surrounding air, raising the room’s temperature.
Thus the overall energy change of the reaction is negative – the products have less energy than the reactants.
Energy is released when the C–H and C–O bonds in the wood are broken and new bonds are formed in the products (CO₂ and H₂O). The energy released in forming the products is greater than the energy required to break the reactants’ bonds.
The released energy is transferred to the surrounding air, raising the room’s temperature.
Thus the overall energy change of the reaction is negative – the products have less energy than the reactants.
Examiner tips
- Use the term "exothermic" early. Show the energy flow: reactants → products → surroundings. Mention bond breaking/formation. State the overall ΔE is negative.
Common mistakes
- Calling the reaction endothermic. Forgetting to explain energy transfer to the room. Using vague phrases like "heat is produced" without linking to bond energy.
Mark scheme (4 marks)
- Burning wood is an exothermic reaction
- Energy is transferred to the surroundings (the room / air)
- The energy released when bonds are formed in the products is greater than the energy needed to break bonds in the reactants
- The overall energy change of the reaction is negative / the products have less energy than the reactants
Key terms in this question
overall energy change of the reaction
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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