Energy changes: exothermic and endothermic

Every reaction either gives out energy or takes it in. Exothermic reactions release heat (the surroundings warm up); endothermic reactions absorb it (the surroundings cool). The balance comes down to the energy of breaking bonds versus making them.

Exothermic and endothermic

In an exothermic reaction (combustion, neutralisation, most displacement) energy is released to the surroundings, which get warmer. In an endothermic reaction (such as thermal decomposition) energy is taken in, so the surroundings get cooler. A thermometer in the mixture tells you which is happening.

It's all about bonds

Breaking bonds takes in energy; making bonds releases it. If making the new bonds releases more than breaking the old ones took in, the reaction is exothermic overall; if less, it's endothermic. This bond-energy balance is the reason behind the temperature change.

Energy profiles

An energy-profile diagram plots energy against the progress of the reaction. Exothermic reactions finish lower than they start (products below reactants); endothermic ones finish higher. The hump in between is the activation energy — the minimum energy needed to get the reaction going. Our separate guide covers drawing and reading these diagrams.

In the exam

Expect to classify a reaction from a temperature change, explain it in terms of bond breaking and making, and sketch or interpret an energy profile. Keep the language precise: energy is transferred to or from the surroundings, and a catalyst lowers the activation energy without changing whether the reaction is exo- or endothermic.