We call the product that results when a base accepts a proton the base’s conjugate acid. This species is an acid because it can give up a proton (and thus re-form the base):

We call the product that remains after an acid donates a proton the conjugate base of the acid. This species is a base because it can accept a proton (to re-form the acid):

Because an equilibrium state is achieved when the forward reaction rate equals the reverse reaction rate, under a given set of conditions there must be a relationship between the composition of the system at equilibrium and the kinetics of a reaction (represented by rate constants). We can show this relationship using the decomposition reaction of N2O4N2O4\ce{N_2O_4} to NO2NO2\ce{NO2}. Both the forward and reverse reactions for this system consist of a single elementary reaction, so the reaction rates are as follows:

The rate of dimerization of NO2NO2\ce{NO_2} (reverse reaction) decreases rapidly with time, as expected for a second-order reaction. Because the initial concentration of N2O4N2O4\ce{N_2O_4} is zero, the rate of the dissociation reaction (forward reaction) at t=0t=0t = 0 is also zero. As the dimerization reaction proceeds, the N2O4N2O4\ce{N_2O_4} concentration increases, and its rate of dissociation also increases. Eventually the rates of the two reactions are equal: chemical equilibrium has been reached, and the concentrations of N2O4N2O4\ce{N_2O_4} and NO2NO2\ce{NO_2} no longer change.