In chemical kinetics

Chemical kinetics

Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...

, the preexponential factor or A factor is the pre-exponential constant in the Arrhenius equation

Arrhenius equation

The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of the reaction rate constant, and therefore, rate of a chemical reaction. The equation was first proposed by the Dutch chemist J. H. van 't Hoff in 1884; five years later in 1889, the Swedish...

, 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

Collision frequency

Collision frequency is defined in chemical kinetics, in the background of theoretical kinetics, as the average number of collisions between reacting molecules per unit of time...

.

For a first order reaction it has units of s^-1, for that reason it is often called frequency factor.

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 in here.