Pre-exponential factor

In <a href="/facts/Chemical_kinetics/D8vyX4aE">chemical kinetics</a>, the pre-exponential factor or A factor is the pre-exponential constant in the <a href="/facts/Arrhenius_equation/TdAeo3Lf">Arrhenius equation</a> (equation shown below), an empirical relationship between temperature and <a href="/facts/Reaction_rate_constant/NcKvaTWP">rate coefficient</a>. It is usually designated by A when determined from experiment, while Z is usually left for <a href="/facts/Collision_frequency/0pE58y00">collision frequency</a>. 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 
 
 
 
 k
 
 
 {\displaystyle k}
 
 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.

 
 
 
 A
 =
 
 
 k
 
 e
 
 −
 
 
 
 E
 
 a
 
 
 
 R
 T
 
 
 
 
 
 
 
 =
 k
 
 e
 
 
 
 E
 
 a
 
 
 
 R
 T
 
 
 
 
 
 
 {\displaystyle A={\frac {k}{e^{-{\frac {E_{a}}{RT}}}}}=ke^{\frac {E_{a}}{RT}}}

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 <a href="/facts/Collision_theory/65DaYrTP">collision theory</a>, 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 <a href="/facts/Collision_theory/65DaYrTP">Collision theory</a>.
According to <a href="/facts/Transition_state_theory/s3MPBbpx">transition state theory</a>, A can be expressed in terms of the <a href="/facts/Entropy_of_activation/E9S09kBz">entropy of activation</a> of the reaction.

<ul><li><a href="http://goldbook.iupac.org/P04811.html">IUPAC Gold Book definition of pre-exponential factor</a></li></ul>

Pre-exponential factor open-in-new

Pre-exponential factor