Given an initial balance of populations of species in two states, the equilibrium is perturbed and the system is in a non-equilibrium state. The reaction is first order in A, first order in B, and second order overall. This situation is a little more complicated than the process of dimerization. All reactions proceed toward an equilibrium, where the forward and reverse reaction rates are equal and no net change in the reactants' or products' concentration is observed. At equilibrium, there is no observable macroscopic change in the concentration of the products and the reactants.
Equilibrium is dynamic, though, and involves both the forward and reverse reactions microscopically.
Solving the differential equation for this process leads to a very complicated equation. Then, the backward reaction can be ignored since very little product is present at the time the measurement is made. The Biophysical Chemistry Virtual Classroom is maintained by Charles J. Russell.