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The pre-exponential factor multiplies the exponential function in the Arrhenius equation. It has the same units as the rate constant, so for a first-order reaction both have units ${\mathrm{s}}^{-1}$. Its physical interpretation depends on the reaction mechanism; an assumed constant $A$ should not be identified with a universal collision frequency. Comparing activation energies alone does not rank absolute rate constants when the pre-exponential factors differ. See the pre-exponential factor.