frequency factor

Meaning: In collision theory, a collision leads to reaction when the molecules meet with at least the activation energy and in a productive orientation. The frequency factor A bundles the collision frequency with the orientation (steric) requirement. A larger A gives a larger rate constant at the same temperature and activation energy. Over the range of temperatures the MCAT uses, A is treated as roughly constant, so the exponential term drives the change in k with temperature.

Equation: k = A e^(−Ea/RT), or in linear form ln k = ln A − Ea/(RT). Here k is the rate constant (units depend on reaction order, such as s⁻¹ for first order), A has the same units as k, Ea is activation energy (J/mol), R = 8.314 J/(mol·K), and T is absolute temperature (K).

On the MCAT: Passages give an Arrhenius plot of ln k versus 1/T. The slope is −Ea/R and the y-intercept is ln A. Questions may also ask you to compare two reactions with the same Ea but different orientation demands, where the one with the stricter geometry has the smaller A and so the smaller k.

Common confusion: A is not the fraction of molecules with enough energy. That fraction is e^(−Ea/RT). As T becomes very large, the exponential approaches 1 and k approaches A, so A is the upper limit on k.