In chemical kinetics, the pre-exponential factor or A factor is the pre-exponential constant in the Arrhenius equation (equation shown below), an empirical relationship between temperature and rate coefficient.
The pre-exponential factor can be thought of as a measure of the frequency of properly oriented collisions.
It is typically determined experimentally by measuring the rate constant
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.
According to transition state theory, A can be expressed in terms of the entropy of activation of the reaction.