Example: For an association reaction (1) and its reverse (2) $$\mathrm{A}+\mathrm{B}⟶\mathrm{AB}(1)$$  $$\mathrm{AB}⟶\mathrm{A}+\mathrm{B}(2)$$ the forward velocity is ${\nu }_1=k_1[\mathrm{A}][\mathrm{B}]$, with $k_1$ being the rate constant for the association reaction. For the dissociation reaction 2, the velocity is ${\nu }_2=k_2[\mathrm{AB}]$. This is valid only for elementary reactions. Furthermore, the law of mass action states that when a reversible chemical reaction reaches equilibrium at a given temperature, the forward rate is the same as the backward rate. Therefore, the concentrations of the chemicals involved bear a constant relation to each other, described by the equilibrium constant, i.e., for $$\mathrm{A}+\mathrm{B}\leftrightarrows \mathrm{AB}$$ in equilibrium, ${\nu }_1=k_1[\mathrm{A}][\mathrm{B}]={\nu }_2=k_2[\mathrm{AB}]$ and one form of the equilibrium constant for the above chemical reaction is the ratio $$K_c=\frac{[\mathrm{AB}]}{[\mathrm{A}][\mathrm{B}]}=\frac{k_1}{k_2}$$

Note: First recognized in 1864 as the kinetic law of mass action by Guldberg and Waage, who first introduced the concept of dynamic equilibrium, but incorrectly assumed that the rates could be deduced from the stoichiometric equation. Only after the work of Horstmann and van’t Hoff a mathematical derivation of the reaction rates considering the order of the reaction involved was correctly made.