Le Chatelier’s Principle

Statement: Whenever a reversible reaction at equilibrium is subjected to change in any condition like change in pressure, temperature or concentration etc. then the equilibrium shifts itself in such a direction to cancel out the effect and accordingly a new equilibrium state is established.

This can be explained taking the example of preparation of ammonia in Haber’s process. What happens when we increase or decrease the pressure or temperature of the reaction vessel or we add more reactant or when we add inert gas? Let us discuss one by one.

The reversible reaction is: N₂ + 3H₂ ⇌ 2NH₃

1.     1.    Effect of changing the pressure:

a.      When pressures is increased: If we increase the pressure inside a vessel by decreasing the volume, then the equilibrium tries cancel out the effect of change, i.e., the equilibrium tries to decrease the pressure. This is only possible if the total number of molecules inside the vessel decreases. Hence the equilibrium shifts in the forward direction as less number of moles of products are formed compared the numbers of moles of reactants.

b.      When pressures is decreased: If we decrease the pressure inside a vessel by increasing the volume, then the equilibrium tries cancel out the effect of change, i.e., the equilibrium tries to increase the pressure. This is only possible if the total number of molecules inside the vessel increases. Hence the equilibrium shifts in the backward direction as more number of moles of reactants are formed compared the numbers of moles of products.

As a conclusion, in increasing the pressure, the equilibrium shifts in the direction in which lesser number of moles is prepared. And there will be no effect of changing the pressure for a reaction in which Dn not equal to 0. H₂ + I₂ ⇌ 2HI

2.      2.    Effect of changing the Temperature: If a reversible reaction is exothermic in forward direction then it is endothermic in backward reaction. N₂ + 3H₂ ⇌ 2NH₃ , DH = — 92.4 kJ or we can write,

a.      When temperature is increased: If we increase the temperature of the reaction vessel by heating it, then the equilibrium tries to decrease the temperature by absorbing the heat. Thus the equilibrium prefers the direction which is endothermic. The reaction of ammonia is exothermic in forward and endothermic in the backward direction. Hence the equilibrium shifts in the backward direction when we increase the temperature.

b.      When temperature is decreased: If we decrease the temperature of the reaction vessel by refrigerating it, then the equilibrium tries to increase the temperature by producing heat. Thus the equilibrium prefers the direction which is exothermic. Hence the equilibrium shifts in the forward direction when we decrease the temperature.

As a conclusion, in increasing the temperature the equilibrium shifts in a direction which is endothermic and vice versa.

3.     3.     Effect of changing concentration: It has now been cleared that if we add more reactant or remove the product from the reaction vessel, then the equilibrium shifts in the forward direction and vice versa.

4.     4.     Effect of adding inert gas: Inert gas can be added with two conditions. They are at constant volume or at constant pressure.

a.      When added at constant volume: When inert gas is added at constant volume, only the total pressure increases but the partial pressures of the components of the reversible reaction do not change. Hence the equilibrium does not shift in any direction.

b.      When added at constant pressure: To keep the pressure constant while adding inert gas, we should increase the volume. This decreases the number of moles of each component per unit volume. To increase the total number of moles per unit volume the reaction shifts in the direction in which greater number of moles are prepared. Thus in ammonia preparation, adding inert gas at constant pressure shifts the equilibrium in the back ward direction.

As a conclusion, addition of inert gas at constant volume has no effect on the equilibrium whereas at constant pressure the equilibrium shifts towards larger number of moles.

5.      5.    Effect of catalyst: A catalyst increases both the rate of forward and backward reaction to the same extent. Thus it does not alter the position of the equilibrium nor does it changes the concentrations of any reactant or product rather it brings the equilibrium in earlier time than when it is not added.

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