Le-Chatelier’s Principle

In 1884, the French Chemist Henery Le-Chatelier and Braun proposed a generalization known as Le – Chatelier’s Principle which states that, “If a system under equilibrium be subjected to a change in temperature, pressure or concentration, then the equilibrium shifts itself in such a way so as to undo or neutralize the effect of the change.”

1.         Effect of Concentration

According to Le-Chatlier’s Principle, when the concentration of one of the substance in a system at equilibrium is increased, then the equilibrium will shift so as to use up the substance added. Suppose at equilibrium if one of the reactant is added, the equilibrium will shift in forward direction i.e. reactant is consumed. On the other hand, if concentration of product is increased, the equilibrium will shift in the backward direction, because it consumes the product.

Thus increase in concentration of any of the reactants shifts the equilibrium towards forward direction and increase in concentration of any of products shifts the equilibrium towards backward direction.

2.         Effect of Temperature

According to Le – Chatlier’s Principle, when temperature of the system is changed (increased or decreased), the equilibrium shifts in such a direction so as to undo the effect.

If the temperature is increased, the equilibrium will get shifted in direction which is accompanied by decrease in temperature i.e. endothermic reaction. If the temperature is decreased, the equilibrium shifts in that direction which is accompanied by increase in temperature i.e. exothermic reaction.

Thus, Increase in temperature, favours endothermic reaction.

Decrease in temperature, favours exothermic reaction.

3.         Effect of Pressure

According to Le – Chatlier’s Principle, increase of external pressure should effect the equilibrium in such a way as to reduce pressure. This means increase in pressure favours the reaction which is accompanied with decrease in number of moles. Whereas, decrease in pressure favours the reaction which is accompanied with increase in number of moles.

The change of pressure has no effect on those equilibria where number of moles of reactants and number of moles of products is same.