Let us consider the reaction

If we start with A and B in a closed vessel, the forward reaction proceeds to form C and D. The concentrations of A and B decrease and those of C and D increase continuously. As a result the rate of forward reaction also decreases and the rate of the reverse reaction increases. Eventually, the rate of the two opposing reactions become equal and the system attains a state of equilibrium.

Figure 3. At equilibrium the forward reaction rate equals the reverse reaction rate.

Thus chemical equilibrium may be defined as: the state of a reversible reaction when the two opposing reactions occur at the same rate and the concentrations of reactants and products do not change with time.

Law of mass action

Science is fundamentally empirical – it is based on experiment. The development of the equilibrium concept is typical. From their observations of many chemical reactions, two Norwegian chemists, Cato Maximilian Guldberg (1836-1902) and Peter Waage (1833-1900), proposed in 1864 the law of mass action as a general description of the equilibrium condition.

The law of mass action is a relation that states the values of the equilibrium constant expression, K_eq are constant for a particular reaction at a given temperature, whatever equilibrium concentrations are substituted.