Pre-exponential factor

$${\displaystyle A=\frac{k}{e^{-\frac{E_a}{RT}}}=k{e}^{\frac{E_a}{RT}}}$$

The units of the pre-exponential factor A are identical to those of the rate constant and will vary depending on the order of the reaction. For a first-order reaction, it has units of s⁻¹. For that reason, it is often called frequency factor.

According to collision theory, the frequency factor, A, depends on how often molecules collide when all concentrations are 1 mol/L and on whether the molecules are properly oriented when they collide. Values of A for some reactions can be found at Collision theory.

According to transition state theory, A can be expressed in terms of the entropy of activation of the reaction.