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{{Short description|Numerical constant in the Arrhenius equation in chemical kinetics}}
{{refimprove|date=June 2018}}
{{Main|Arrhenius equation}}
 
In [[chemical kinetics]], the '''preexponentialpre-exponential factor''' or '''A factor''' is the pre-exponential constant in the [[Arrhenius equation#Equation|Arrhenius equation]] (equation shown below), an empirical relationship between temperature and [[Reaction rate constant|rate coefficient]]. It is usually designated by A when determined from experiment, while Z is usually left for [[collision frequency]]. 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 <math>k</math> 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.<ref>{{cite web|url=https://socratic.org/questions/how-do-you-calculate-the-pre-exponential-factor-from-the-arrhenius-equation|title=How do you calculate the pre-exponential factor from the Arrhenius equation?|access-date=December 8, 2022}}</ref>
 
<math display="block">A=\frac{k}{e^{-\frac{E_a}{RT}}}=ke^{\frac{E_a}{RT}}</math>
For a first order reaction it has units of s<sup>-1</sup>, for that reason it is often called '''frequency factor'''.▼
 
▲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<sup>-1−1</sup>,. forFor that reason, it is often called '''frequency factor'''.
Values of A for some reactions can be found in [[Collision theory#Steric factor|here]].
 
According to [[collision theory]], 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 [[Collision theory#Steric factor|Collision theory]].
==References==▼
[http://www.iupac.org/goldbook/P04811.pdf IUPAC Goldbook definition of preexponential factor]▼
 
According to [[transition state theory]], A can be expressed in terms of the [[entropy of activation]] of the reaction.
[http://goldbook.iupac.org/P04811.html IUPAC Gold Book definition of pre-exponential factor]
 
▲==References==
[[Category:Chemical kinetics]]▼
{{Reflist}}
▲* [http://wwwgoldbook.iupac.org/goldbook/P04811.pdfhtml IUPAC GoldbookGold Book definition of preexponentialpre-exponential factor]
 
▲[[Category:Chemical kinetics]]
[[ru:Фактор частоты]]
 
 
{{chemistryReaction-stub}}
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