{{Short description|Numerical constant in the Arrhenius equation in chemical kinetics}} {{refimprove|date=June 2018}} {{Main|Arrhenius equation}}
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. It is usually designated by A when determined from experiment, while Z is usually left for collision frequency. 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 <math>k</math> at a particular temperature and fitting the data to the Arrhenius equation. The pre-exponential factor is generally not exactly constant, but rather depends on the specific reaction being studied and the temperature at which the reaction is occurring.<ref>{{cite web|url=https://socratic.org/questions/how-do-you-calculate-the-pre-exponential-factor-from-the-arrhenius-equation|title=How do you calculate the pre-exponential factor from the Arrhenius equation?|access-date=December 8, 2022}}</ref>
<math display="block">A=\frac{k}{e^{-\frac{E_a}{RT}}}=ke^{\frac{E_a}{RT}}</math>
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<sup>−1</sup>. 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.
==References== {{Reflist}} * [http://goldbook.iupac.org/P04811.html IUPAC Gold Book definition of pre-exponential factor]
Category:Chemical kinetics
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