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In chemical kinetics, the preexponential factor or A factor is the pre-exponential constant in the Arrhenius equation, 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.

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.

References

IUPAC Gold Book definition of pre-exponential factor