GCSE Chemistry · Edexcel 1CH0 · Topic 7B
Heat energy changes in chemical reactions
Key points
- Exothermic reactions transfer heat to the surroundings (temperature rises), e.g. combustion, neutralisation, displacement.
- Endothermic reactions take heat from the surroundings (temperature falls), e.g. thermal decomposition, dissolving ammonium nitrate.
- Core practical: measure temperature changes in a polystyrene cup when salts dissolve or acids are neutralised.
- Reaction profiles: in exothermic reactions the products are lower in energy than the reactants; in endothermic reactions they are higher. The hump is the activation energy.
- Breaking bonds is endothermic; making bonds is exothermic.
- Higher tier: energy change = energy needed to break bonds − energy released making bonds. A negative answer means exothermic.
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Every chemical reaction involves an energy change, because bonds are broken and new bonds are made. Whether the surroundings warm up or cool down tells you whether the reaction is exothermic or endothermic.
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