GCSE Chemistry · AQA 8462 · Topic 4.5
Energy changes
Key points
- Energy is conserved in reactions. Exothermic reactions transfer energy to the surroundings (temperature rises); endothermic reactions take in energy (temperature falls).
- Examples: combustion, neutralisation and many oxidation reactions are exothermic; thermal decomposition and citric acid with sodium hydrogencarbonate are endothermic.
- Reaction profiles show the energy of reactants and products and the activation energy, the minimum energy needed for particles to react.
- (Higher) Bond breaking takes in energy; bond making releases energy. Overall energy change = energy in to break bonds − energy out from forming bonds.
- Chemical cells (chemistry only) produce a voltage from two different metals in an electrolyte until a reactant runs out.
- Hydrogen fuel cells make electricity from hydrogen and oxygen, with water as the only product.
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Every chemical reaction involves an energy change. This topic explains why some reactions heat up their surroundings and others cool them down, and how reactions can be used to produce electricity directly.
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