activation energy

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.

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▪ chemistry

      in chemistry, the minimum amount of energy that is required to activate atoms or molecules to a condition in which they can undergo chemical transformation or physical transport. In terms of the transition-state theory (q.v.), the activation energy is the difference in energy content between atoms or molecules in an activated or transition-state configuration and the corresponding atoms and molecules in their initial configuration. The activation energy is usually represented by the symbol E_a in mathematical expressions for such quantities as the reaction-rate constant, k = Aexp(-E_a/RT), and the diffusion coefficient, D = D_oexp(-E_a/RT).

      Activation energies are determined from experimental rate constants or diffusion coefficients that are measured at different temperatures.

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