During chemical reactions, energy is transferred to or from the surroundings. This tells us whether a reaction is exothermic or endothermic.

What is an exothermic reaction?

• An exothermic reaction is one that releases energy to the surroundings
• The temperature increases
• Energy is transferred as heat
• The products have less energy than the reactants

Examples of exothermic reactions:

• Combustion (burning fuels)
• Neutralisation (acid + alkali)
• Some oxidation reactions (e.g. iron + oxygen)

💡 Exothermic = exit (energy leaves)

What is an endothermic reaction?

• An endothermic reaction is one that takes in energy from the surroundings
• The temperature decreases
• The products have more energy than the reactants

Examples of endothermic reactions:

• Thermal decomposition
• Photosynthesis
• Reactions in cold packs

💡 Endothermic = enter (energy enters)

Energy profile diagrams

• Exothermic: reactants start high, products lower, energy goes out
• Endothermic: reactants start low, products higher, energy is taken in

Each diagram should:

• Show activation energy
• Indicate whether energy is released or absorbed

Uses of these reactions