In 1884, the French Chemist Henry Le Chatelier proposed a general principle which applies to all systems in equilibrium. It may be stated as-

'When any change is brought about in a system in equilibrium, such as that of concentration, temperature and pressure (or volume), then the system moves itself in such a direction in which the effect of that change is nullified as far as possible'.

EFFECT OF CONC. CHANGE- When conc. of any of the reactants or products is changed, the equilibrium shifts in a direction so as to reduce the change in conc. that was made. A change in conc. can be affected by the addition or removal of that species. In a general reaction,

A   +   B       C

when a reactant , say, A. is added at equilibrium, its conc. is increased. According to le chatelier's principle, a new equilibrium will be established so as to reduce the conc. of A. Thus the addition of A causes the equi. to shift to right. This increases the conc. (yield) of the product C. Similarly, a decrease in the conc. of A by its removal from the equi. mix. will be undone by shift of equi. position to the left. This reduces the conc. (yield) of the product C.

EFFECT OF PRESSURE CHANGE- When pressure is increased on a gaseous equi reaction, the equi will shift in a direction which tends to decrease the pressure. The pressure of a gaseous reaction at equi. is determined by the total no. of molecules it contains. If the forward reaction proceeds by the reduction of molecules, it will be accompanied by a decrease of pressure of the system and vice versa. The combination of A and B produces a decrease of no. of molecules while decomposition of C into A and B results in increase in molecules. Therefore, by the increase of pressure on the equi., it will shift to right and give more C. A decreasein pressure will cause the opp. effect. The equi. will shift to the left when C will decompose to form more of A and B. Thus, the increase of pressure on the equi. shifts it in that direction in which the no. of molecules decreases and vice versa.

EFFECT OF TEMP. CHANGE- When temp is increased, the equi. shifts in a direction in which heat is absorbed. In an exothermic reaction, when temp. of the system is increased, heat is supplied to it from outside. So the equi. will shift to the left which involves absorption of heat. This would result in the increase of the conc. of the reactants. In an endothermic reaction, the increase of temp. will shift the equi. to the right as it involves the absorption of heat. athis increases the conc. of the product. Thus, the increase of temp. favours the reverse change in an exothermic reaction and the forward change in an endothermic reaction.

Le Chatelier's principle finds its application in the synthesis of ammonia by Haber's process, the manufacture of sulphuric acid by Contact process and the manufacture of nitric acid by Birkeland-Eyde process.