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).