activation energy

activation energy, 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 transition-state theory, 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, $k=A{e}^{(-E_a/RT)}$,where A is the pre-exponential factor, which describes how often collisions occur and is different for each reaction; R is the universal gas constant, which is about 8.314 joules per kelvin per mole; and T is the temperature in kelvin. Activation energy is also represented by E_a in the expression for the diffusion coefficient D, $D=D_0{e}^{(-E_a/RT)}$,where D₀ is a constant.

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

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