Le-Chatelier's Principle

Le-Chatelier's Principle

Le-Chatelier's Principle

Le-Chatelier's Principle

According to this principle, if a system at equilibrium is subjected to a change of concentration, pressure or temperature then the equilibrium is shifted in such a way as to nullify the effect of change.
Le-Chatelier's principle is applicable for both chemical and physical equilibrium.

Chemical Equilibrium

Reactant(R) ⇌ Product(P)

1. 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).

2. 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 and Product Formation on changing the pressure.

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.

3. 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. For exothermic reactions, increase in temperature will favour the backward reaction.

4. Effect of Catalyst

A catalyst is a substance that changes the rate of reactions (increase or decrease) without quantitatively taking part in the reaction.
In a reversible reaction, the change in rate of reaction is the same for both forward and backward reactions.
According to Le Chatelier's principle, the presence of the catalyst may speed up or slow down the attainment of equilibrium but will not affect the equilibrium concentration.

5. Effect of Addition of Inert Gas

At constant pressure, if an inert gas is added 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.
At constant volume, if an inert gas is added the relative molar concentration of the substance will not change. Hence. the equilibrium position of the reaction remains unaffected.

Physical Equilibrium

Those reaction in which change in only and only physical states (solid, liquid and gas) of substance takes place without any chemical change, is called physical reaction.

1. Ice-Water System (melting of ice)

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.

2. Water-Water Vapour System (Vapourisation of water)

Vapourisation of water is an endothermic and condensation of vapour into water is an exothermic.
Favourable conditions for conversion of water into vapour are high tempeprature and low pressure.
When the temperature is increased, the equilibrium shifts towards right side. So rise in temperature will increase the vapour.
When the pressure is increased, The equilibrium shifts towards left side. So increase in pressure will favour the rate of condensation of vapour into water.

3. 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.