1. In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. The activation energy (Ea) of a reaction is measured in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol). Simplified: Activation energy is the minimum energy barrier that reactant molecules must overcome to transform into products. A reaction occurs only if enough molecules have kinetic energy equal to or greater than this barrier, which usually requires sufficiently high temperature. The term "activation energy" was introduced in 1889 by the Swedish scientist Svante Arrhenius. == Other uses == Although less commonly used, activation energy also applies to nuclear reactions and various other physical phenomena. == Temperature dependence and the relation to the Arrhenius equation == The Arrhenius equation gives the quantitative basis of the relationship between the activation energy and the rate at which a reaction... from wikipedia.org