Le Chatelier's Principle
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, 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 (more reactants converted to 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 an exothermic equilibrium, an increase in temperature decreases product formation, and a decrease in temperature increases product formation.
In an endothermic reaction, 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, 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 will increase the volume of the system. By Le Chatelier's principle, the equilibrium shifts to the side where a larger number of gaseous molecules are present.
5. Effect of a Catalyst
There is no effect on the equilibrium composition of a reaction mixture.
This is because a catalyst increases the speed of both the forward and backward reactions to the exact same extent in a reversible reaction. It only helps the system achieve equilibrium faster.
Examples & Applications
1Reaction between Ferric Nitrate and Potassium Thiocyanate
The reaction between yellow-colored ferric nitrate and colorless potassium thiocyanate forms a red-colored iron thiocyanate complex.
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 more iron thiocyanate complex, shifting equilibrium to the right. On adding more iron thiocyanate, the intensity of the color decreases, indicating the equilibrium is shifted to the left.
2Haber Process (Formation of Ammonia)
The formation of ammonia is an exothermic process, while the reverse is an endothermic process.
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 favored. To maximize ammonia yield, the temperature must be decreased (cooling), because heat is evolved in the forward reaction.
3Formation of Nitric Oxide
The formation of nitric oxide by the reaction of nitrogen and oxygen is an endothermic reaction.
An increase in temperature will shift the equilibrium to the right (favoring the endothermic path). In contrast, a decrease in temperature will shift the equilibrium to the left to neutralize the effect of the change in temperature.
4Dissociation of Phosphorus Pentachloride
In the above reaction, the number of moles of the product (1 mole) is less than the reactants (2 moles). Therefore, an increase in pressure will shift the equilibrium to the right (towards fewer gaseous moles). A decrease in pressure shifts the equilibrium to the left.
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