Activation Energy

Summary: The minimum energy that colliding particles must have for a successful reaction to occur. Tags: igcse chemistry definition energetics Created: 2026-07-14 Last Updated: 2026-07-14

Activation energy (Ea) is the minimum combined kinetic energy that colliding reactant particles must possess for their collision to result in a successful chemical reaction. On an energy profile diagram, it is represented as the vertical energy gap between the energy level of the reactants and the peak of the curve, known as the transition state or activated complex, where bonds are partially broken and formed. This concept explains why many exothermic reactions do not occur spontaneously at room temperature: although the products are more stable, the reactants must first absorb enough energy to overcome the activation energy barrier before they can rearrange into products. Catalysts are critically important in this context because they provide an alternative reaction pathway with a lower activation energy, meaning a greater proportion of colliding particles meet the energy requirement, dramatically increasing the reaction rate without the catalyst itself being consumed. For IGCSE exams, students must be able to label the activation energy on both exothermic and endothermic energy profile diagrams and must understand the key principle that a catalyst lowers the activation energy but does not alter the overall enthalpy change (ΔH) of the reaction.

Key term — IGCSE Chemistry (0620/0971) glossary.