Minimum amount of energy ( heat , electromagnetic radiation , or electrical energy) required to activate atoms or molecules to a condition in which it is equally likely that they will undergo chemical reaction or transport as it is that they will return to their original state.
Chemists posit a transition state between the initial conditions and the product conditions and theorize that the activation energy is the amount of energy required to boost the initial materials "uphill" to the transition state; the reaction then proceeds "downhill" to form the product materials. Catalysts (including enzymes ) lower the activation energy by altering the transition state. Activation energies are determined by experiments that measure them as the constant of proportionality in the equation describing the dependence of reaction rate on temperature, proposed by Svante Arrhenius . See also entropy , heat of reaction .