Activation Energy

Synonyms

E_a

Definition

In chemical kinetics, the activation energy (abbreviated E_a) is the energy barrier which must be overcome for a sufficient number of reactant molecules to acquire enough kinetic energy for a reaction to occur appreciably. The activation energy can generally be achieved by supplying energy, for example, in the form of heat, to the system or through the intervention of a catalyst. The Arrhenius equation gives the temperature dependence of the rate constant k: E_a is obtained from the slope of a plot of ln k versus 1/T, where T is the absolute temperature in Kelvin. The activation energy is usually measured in kJ/mol.

E_a is related to the rate (k) via the equation

$$ {E}_a=- RT\kern0.5em \ln \left(k/A\right) $$

where A is a constant called the frequency or pre-exponential factor, usually given in units of s⁻¹, and R is the universal gas constant.

Cross-References

• Arrhenius Plot