GCSE Chemistry: Energy Changes Guide

Exothermic reactions

An exothermic reaction releases energy to the surroundings, usually shown by a temperature increase — combustion and neutralisation reactions are common examples.

Endothermic reactions

An endothermic reaction takes in energy from the surroundings, usually shown by a temperature decrease — thermal decomposition is a common example.

Reaction profiles

A reaction profile diagram shows the energy change during a reaction. In an exothermic reaction, the products have less energy than the reactants; in an endothermic reaction, the products have more energy than the reactants.

Activation energy

Activation energy is the minimum energy needed for a reaction to start, shown as the "hump" on a reaction profile diagram between the reactants and the peak of the curve.

Common mistakes

• Mixing up which direction the energy changes for exothermic and endothermic reactions on a diagram.
• Assuming a temperature increase always means energy has been added, rather than released by the reaction itself.
• Forgetting activation energy is needed even for exothermic reactions to begin.