  The exponential factor of Arrhenius satisfactorily quantifies the energetic
restriction of chemical reactions but is still awaiting a rigorous basis.
Assuming that the Arrhenius equation should be based on statistical mechanics
and is probabilistic in nature, two structures for this equation are compared,
depending on whether the reactant energies are viewed as the mean values of
specific energy distributions or as particular levels in a global energy
distribution. In the first version, the Arrhenius exponential factor would be a
probability that depends once on temperature, while in the second it is a ratio
of probabilities that depends twice on temperature. These concurrent equations
are tested using experimental data for the isomerization of 2-butene. This
comparison reveals the fundamental structure of the Arrhenius law in isothermal
systems and overlooked properties resulting from the introduction of reactant
energies into the equation.
