Le Chatelier's principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change to reestablish equilibrium. If a chemical reaction is at equilibrium and experiences a change in pressure, temperature, or concentration of products or reactants, the equilibrium shifts in the opposite direction to offset the change.

1. Effect of change in concentration

If the concentration of a substance is changed, the equilibrium will shift to minimize the effect of that change.

when the concentration of one of the substances in, a system at equilibrium is increased, then the equilibrium will shift so as to use up the substance added. Suppose at equilibrium one of the reactants is added, the equilibrium will shift in the direction that consumes reactants, i.e., the forward direction. The more of the reactants would be converted into products. On the other hand, if one of the products is added the equilibrium will shift in the backward direction because it consumes the products.

2. Effect of change in temperature

In exothermic equilibrium, an increase in temperature decreases product formation and a decrease in temperature increases product formation. In endothermic reactions, an increase in temperature increases product formation, and a decrease in temperature decreases product formation.

3. Effect of change in Pressure :

When there is an increase in pressure, the equilibrium will shift towards the side of the reaction with fewer moles of gas. When there is a decrease in pressure, the equilibrium will shift towards the side of the reaction with more moles of gas

4. Effect of the addition of inert gas :

When an inert gas is added to the equilibrium system at constant volume, then the total pressure will increase. But the concentration of reactants and products (ratio of their moles to the volume of the container) will not change. Hence, the position of the equilibrium will be unaffected

whereas the addition of inert gas at equilibrium and at constant pressure and temperature will increase the volume of the system. by Le Chatelier's principle, the equilibrium shifts to the side where large no. of gaseous molecular are present.

5. Effect of a Catalyst:

There is no effect on the equilibrium composition of a reaction mixture. This is because the catalyst increases the speed of both forward and backward reactions to the same extent in a reversible reaction.

Examples :

1. The reaction between yellow-colored ferric nitrate and colorless potassium thiocyanate forms a red-colored iron thiocyanate complex.

$F e^{3 +} (aq) + SC N^{-} (aq) ⇌ {[Fe (SCN)]}^{2 +} (aq)$

By adding a small amount of ferric chloride to the equilibrium mixture, the intensity of the red color will increase. This indicates that ferric ion has combined with thiocyanate ions to give an iron thiocyanate complex, and equilibrium has shifted to the right. On adding iron thiocyanate, the intensity of the color decreases indicating the equilibrium is shifted to the left.

2. The formation of ammonia is an exothermic process, while the reverse is an endothermic process.

$N_{2} (g) + 3 H_{2} (g) ⇌ 2N H_{3} (g); Δ H=-92 .5 kJmo l^{-1}$

Suppose the temperature of the reaction is increased, the equilibrium shifts to the left to neutralize or undo the effect of an increase in temperature. The backward reaction is accomplished by decreasing temperature or cooling. Similarly, if the temperature is increased, then the equilibrium will shift to the right because heat is evolved in the forward reaction.

3. The formation of nitric oxide by the reaction of nitrogen and oxygen is an endothermic reaction.
$N_{2} (g) + O_{2} (g) ⇌ 2NO (g); Δ H= + 180 .0 kJmo l^{-1}$

The increase in temperature will shift the equilibrium to the right. In contrast, the decrease in temperature will shift the equilibrium to the left to neutralize or undo the effect of the change in the temperature.

4.

$PC l_{3} (g) + C l_{2} (g) ⇌ PC l_{5} (g)$

In the above reaction, the number of moles of the product is less than the reactants. Therefore, the increase in pressure will shift the equilibrium to the right and decrease pressure when shifting the equilibrium to the left.

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