According to the collision theory for a bimolecular elementary reaction $A + B \rightarrow \text{Products}$, the rate of reaction can be expressed as: $\text{Rate} = Z_{AB}e^{-E_a/RT}$ In this equation, $Z_{AB}$ represents the collision frequency of reactants A and B, which is defined as the number of collisions per second per unit volume of the reaction mixture. The term $e^{-E_a/RT}$ corresponds to the fraction of molecules with energies equal to or greater than the activation energy ($E_a$). The steric factor (or probability factor), $P$, is introduced in modified equations to account for proper orientation, leading to Rate $= PZ_{AB}e^{-E_a/RT}$ .