Chemical equilibrium is the equilibrium position of a reversible reaction, in which the concentrations of the reactants and products have no tendency to change. In this case, the free energy of the system is a
aA + bB cC + dD
there is an equilibrium constant K given approximately by
K = [C]c[D]d/[A]a[B]b
where [A] is the concentration of A, and so on. Chemical kinetics yields the same equation, since at equilibrium the rates of the forward and back reactions are equal, and so K is also given by the ratio of the rate constants of these two reactions (K = k1/k-1).