At equilibrium, the concentration of the reactants and products does not change. The addition of more reactants or products to the reacting system at equilibrium causes an increase in their respective concentration.
According to Le – Chatelier’s principle, the effect of increase in concentration of a substance is to shift the equilibrium in a direction that consumes the added substance.
For example,
H2(g) + I2(g) ⇌ 2HI(g)
The addition of H2 or I2 to the equilibrium mixture, disturbs the equilibrium. In order to minimize the stress, the system shifts the reaction in a direction where H2 and I2 are consumed i.e., formation of additional HI would balance the effect of added reactant.
Hence the equilibrium shifts to the right (forward direction). i.e., the equilibrium is re – established.
Similarly, removal of HI (Product) also favours forward reaction. If HI is added to the equilibrium mixture, the concentration of HI is increased and system proceeds in the reverse direction to nullify the effect of increase in concentration of HI.
