Le-Chatelier's Principle
Le Chatelier's Principle describes how a system at equilibrium responds to changes in concentration, temperature, pressure, or volume. According to this principle, if a system at equilibrium is disturbed by changing the concentration, pressure, temperature or volume then the equilibrium will shift to counter the change.
Let us consider the equilibrium reaction of ammonia formation-
N2(𝑔) + 3H2(𝑔) ⇌ 2NH3(𝑔) + heat
If the concentration of N2 or H2 increases, equilibrium shift to the right side and produce more NH3.
If the temperature increases, equilibrium shift to the left side and produce more N2 and H2.
If the pressure increases, equilibrium shift to the right side, as it has fewer gas molecules (4 moles on the left, 2 moles on the right).
Le-Chatelier's principle is applicable for both chemical and physical equilibrium.
Chemical Equilibrium
Reactant(R) ⇌ Product(P)Change in Concentration
In an equilibrium, on increasing the concentrations of reactants, equilibrium shift in favour of products (i.e. forward reaction) while on increasing the concentrations of the products equilibrium shift in favour of reactants (i.e. backward reaction).
Change in Pressure
Pressure has no effect on the reactions of liquids and solids. When the pressure on the gaseous system is increased, the volume decreases i.e. the total number of moles present per unit volume increases.
According to Le-Chatelier's principle, the equilibrium shifts in that direction in which there is decrease in number of moles. If there is no change in number of moles of gases in a reaction then a pressure change does not affect the equilibrium.
When ∆n = 0
As per Le Chatelier's principles, there will be no effect on Equilibrium.
When ∆n = +ve
An increase in pressure or decrease in volume will decrease the product formation. Decrease of pressure or increase of volume will increase the product formation.
When ∆n = -ve
An increase in pressure or decrease in volume will increase the formation of the product.
Change in Temperature
If the temperature of the system at equilibrium is increased then reaction will proceed in that direction in which heat can be used. Thus for endothermic reaction, increase in temperature will favour the forward reaction and for exothermic reactions, increase in temperature will favour the backward reaction.
Effect of Catalyst
According to Le Chatelier's principle, the presence of the catalyst may increase or decrease the attainment of equilibrium but will not affect the equilibrium concentration.
Effect of Addition of Inert Gas
If an inert gas is added at constant pressure, it will increase the volume of the system. Therefore, the equilibrium will shift in a direction in which there is an increase in the number of moles of gases.
If an inert gas is added at constant volume, the relative molar concentration of the substance will not change. Hence, the equilibrium position of the reaction remains unaffected.
Physical Equilibrium
The reaction in which change in only physical states (solid, liquid and gas) of substance takes place without any chemical change, is called physical reaction.
Melting of Ice (Ice-Water System)
Melting of ice is endothermic reaction (absorption of heat) and decrease in volume.
H2O(solid) ⇌ H2O(liquid)
Increase of temperature and pressure will favour the melting of ice into water.
Vapourization of Water (Water-Water Vapour System)
Vapourization of water is an endothermic and condensation of vapour into water is an exothermic. High temperature and low pressure is the favourable conditions for conversion of water into vapour.
When temperature increases, the equilibrium shifts towards right side. So increase in temperature will increase the vapour.
When pressure increases, The equilibrium shifts towards left side. So increase in pressure will favour the rate of condensation of vapour into water.
Solubility of Gases
Gas + H2O ⇌ Aqueous Solution
Solubility of gases increases with increasing pressure which dissolves in a solvent(H2O) with a decrease in volume.
