What is the difference between exothermic and endothermic reactions?
Short answer
An exothermic reaction gives out energy to the surroundings, usually as heat, so the temperature rises. An endothermic reaction takes in energy from the surroundings, so the temperature falls. Combustion and neutralisation are exothermic; thermal decomposition and photosynthesis are endothermic.
Step by step
- Measure the temperature of the surroundings before and after the reaction.
- Temperature rises: exothermic. Temperature falls: endothermic.
- Bond breaking takes in energy; bond making releases energy.
- If more energy is released making bonds than is used breaking them, the reaction is exothermic.
Worked example
Energy level diagrams
- Exothermic: products are lower than reactants, ΔH is negative
- Endothermic: products are higher than reactants, ΔH is positive
- Both diagrams show a hump: the activation energy
Common mistakes
- Saying bond breaking releases energy.
- Getting the sign of ΔH backwards. Exothermic is negative.
- Forgetting to label the activation energy on the diagram.
Useful for:GCSEWAECMatricIGCSE
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