Pre-exponential factor
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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.
Values of A for some reactions can be found in here.
[edit] References
IUPAC Goldbook definition of preexponential factor
IUPAC Gold Book definition of pre-exponential factor