Question

In theory, what could be done experimentally to change the value of A (frequency factor in Arrhenius equation)? Explain the effect in molecular terms.

Answer 1

Arrhenius equation,

Frequency factor (A) has been ignored because it is not directly involved in relating temperature and activation energy, which is the main practical use of the equation.

However, because A multiplies the exponential term, its value clearly contributes to the value of the rate constant and thus of the rate. Recall that the exponential part of the Arrhenius equation expresses the fraction of reactant molecules that possess enough kinetic energy to react, as governed by the Maxwell-Boltzmann law. This fraction can run from zero to nearly unity, depending on the magnitudes of Ea and of the temperature.

If this fraction were 0, the Arrhenius law would reduce to k=A.

In other words, A is the fraction of molecules that would react if either the activation energy were zero, or if the kinetic energy of all molecules exceeded Ea — admittedly, an uncommon scenario. The most obvious factor would be the rate at which reactant molecules come into contact.