preexponential

The term preexponential refers to a factor in chemical kinetics that precedes the exponential term in rate laws. Specifically, it is part of the Arrhenius equation, which describes the temperature dependence of reaction rates. The Arrhenius equation is typically written as k = A * exp(-Ea/RT), where k is the rate constant, A is the preexponential factor, exp is the exponential function, Ea is the activation energy, R is the ideal gas constant, and T is the absolute temperature.

The preexponential factor, often denoted by A, is also known as the frequency factor. It represents the rate of collisions between reactant molecules that have the correct orientation to react, assuming sufficient energy. In simpler terms, it is the theoretical maximum rate constant if all collisions had enough energy to overcome the activation barrier. It is influenced by factors such as the frequency of molecular collisions and the probability that these collisions have the appropriate geometry for a reaction to occur. The units of the preexponential factor are the same as the units of the rate constant, which vary depending on the order of the reaction. The preexponential factor is generally considered to be largely independent of temperature, although in some more complex models, it can show a slight temperature dependence.