Energy changes and reversible reactions

Exothermic reactions transfer energy to the surroundings. Endothermic reactions take in energy from the surroundings.

Reversible reactions are where the products can react to remake the original reactants. If the forward reaction is exothermic, the reverse reaction is endothermic.

Exothermic reactions

When a chemical reaction occurs, energy is transferred to or from the surroundings - and there is often a temperature change.

Exothermic reactions transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. Some examples of exothermic reactions are:

• Combustion (burning)
• Many oxidation reactions, for example rusting
• Neutralisation reactions between acids and alkalis

When a flame burns it transfers heat to its surroundings.

Exothermic reactions can be used for everyday purposes. For example, hand warmers and self-heating cans for drinks (such as coffee) use exothermic reactions.