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 In chemical kinetics a reaction rate constant or reaction rate coefficient, ''k'', quantifies the rate and direction of a chemical reaction.
For a reaction between reactants A and B to form product C
the reaction rate is often found to have the f ...
. It is usually designated by A when determined from experiment, while Z is usually left for
collision frequency
Collision frequency describes the rate of collisions between two atomic or molecular species in a given volume, per unit time. In an ideal gas, assuming that the species behave like hard spheres, the collision frequency between entities of specie ...
. 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
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.
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#Steric factor.
According to
transition state theory, A can be expressed in terms of the
entropy of activation of the reaction.
References
IUPAC Gold Book definition of pre-exponential factor
Chemical kinetics
{{chemistry-stub