Le Chatelier's Principle: Temperature, Pressure, Concentration, Catalyst

Le chatelier principle is based on anticipating the effects on a chemical equilibrium due to certain changes made in the system. It is also known as the equilibrium law. This principle states that changes in the temperature, pressure, volume, or concentration of a system will lead to the predictable and opposing changes in it so as to attain a new equilibrium state.

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Le Chatelier's Principle: Effects of Concentration

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As indicated by le chatelier's rule, adding extra reactant to the product formation will move the equilibrium to one side. The equilibrium will move forward when the reactant's concentration is increased. The equilibrium of the reaction moves towards the usage of the reactants in the reaction which leads to a fall in the concentration of the reactants. 

In the same way, when we add products be it concentration or pressure it will lead to an increase in the backward reaction rate in order to lower the product concentration. The backward reaction takes place when the reactants' concentration decreases and then there is a shift in the equilibrium of the reaction towards the generation of reactants. Therefore there will be an increase in the concentrations of the reactants. 

N₂(g) + 3H₂(g) 2NH₃(g)

Note- this factor had a significant role to play only in case of gaseous reaction

Case 1: Increase in the concentration of the reactant:

• Equilibrium will move in the direction where there is a decrease in the concentration of the reactants.
• It is more suitable for the forward reaction.
• Some of the N₂ or H₂ becomes NH₃
• The equilibrium in this type of reaction shifts towards the right side.

Case 2: Decrease in the concentration of the reactant:

• Equilibrium will move to the direction where there is an increase in the concentration of the reactants.
• It is more suitable for the backward reaction.
• Some of the NH₃ will form into N₂ or H₂.
• The equilibrium in this type of reaction shifts towards the left side.

Case 3: Increase in the product:

• The equilibrium of the reaction moves to lower the concentration.
• Reverse reaction rate increases.
• Some of the NH₃ will form into N₂ or H₂.
• There will be a leftward shifting in the equilibrium of the reaction. 

Case 4: Decrease in the product:

• The equilibrium of the reaction moves to increase the concentration.
• Forward reaction rate increases.
• Some of the N₂ or H₂ becomes NH₃
• There will be a rightward shifting in the equilibrium of the reaction.

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Le Chatelier’s Principle: Effect of Temperature

The change of temperature alters the state of its equilibrium either by exothermic reactions where heat is released or by endothermic reactions where heat is absorbed.

Exothermic reaction ----- product side (heat release)

• Temperature and product formation is inversely related, with an increase in temperature there is a decrease in product formation and with a decrease in temperature there is an increase in product formation.

Endothermic reaction ----- reactant side (heat absorption)

• Temperature and product formation is directly related, with an increase in temperature there is an increase in product formation and with a decrease in temperature there is a decrease in product formation.

Le Chatelier’s Principle on Change of Temperature: log(K₁/K₂) = -ΔHº/R (1/T₂ - 1/T₁)

Example: N₂(g) + 3H₂(g) 2NH₃(g) ΔH = -92kJ

Case1 - Exothermic Reaction:

• In the case of an exothermic reaction, a decrease in temperature takes place since it releases energy. 
• It follows the forward reaction
• There is an increases the production of NH₃.

Case 2 - Endothermic Reaction:

• In the case of an endothermic reaction, an increase in temperature takes place since it absorbs energy. 
• It follows the reverse reaction. 
• There is a decrease in the production of NH₃. 

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Le Chatelier’s Principle: Effect of Pressure, Volume and Inert Gas

This factor has a significant role to play only in case of gaseous reaction. They don't have any effect on the liquids or solids. Even in the case of gases they have an effect only when there is a difference in the total of the number of reactants and product molecules is not equal to zero. 

When Δn = 0; According to Le Chartilier’s principle there will not be any effect on the equilibrium and formation of the product on altering the volume, pressure or inert gas. 

When Δn = +ve; The rise in pressure or reduction in volume will lower the product formation. On the other hand, reduction in pressure or rise in volume will increase the product formation. 

According to ideal gas equation

I.e., PV = nRT

Inert gases don't take part in reactions and only increase the pressure or the volume only. 

• At a fixed pressure the addition of inert gas expands the volume, hence there is an increase in the formation of the product. 
• At a fixed volume by adding inert gas the pressure can be increased along with a decrease in the formation of the product. 

Example: PCl₅ PCl₃ + PCl₂

During the decomposition of phosphorus pentachloride when Δn = +1:

• At a fixed pressure the addition of inert gas leads to an increase in PCl₅ production.
• At a fixed volume the addition of inert gas leads to a decrease in PCl₅ production. 

when Δn = -ve:

• At a fixed pressure the addition of inert gas expands the volume, hence there is a decrease in the formation of the product. 
• At a fixed volume by adding inert gas the pressure can be increased along with an increase in the formation of the product.

Example: N₂ + 3H₂2NH₃

During the formation of ammonia when Δn = -2:

• At a fixed pressure the addition of inert gas leads to a decrease in NH₃ production.
• At a fixed volume the addition of inert gas leads to an increase in NH₃ production. 

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Le Chatelier's Principle: Effect of Catalyst

Catalysts help in increasing or decreasing the rate of reaction in both directions without quantitatively taking part in the reaction. It may help to attain equilibrium faster or slower but would never alter its concentration. In case of a reversible reaction rate changes equally for both the reactions be it forward or reverse. Since the reaction rates ratio do not change, hence the equilibrium remains fixed. 

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Things to Remember

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• Le chatelier's principle is a part of CBSE class 11 second term chemistry syllabus unit 3 Equilibrium.
• Unit 3 carries a total of 12 periods and 4 to 5 marks. 
• Concentration: When the concentration increases the system moves to gain a certain added reactant or product and when the concentration decreases it moves to replace certain reactant or product which have been replaced.
• Temperature: When the temperature increases it moves to obtain certain added thermal energy and when the concentration decreases it moves to replace certain thermal energy which have been replaced.
• Volume: When volume increases, it moves towards the side that has a larger quantity of gases and when it decreases the system it moves towards the side that has a smaller quantity of gases.
• Catalysts and inert gases don't have any impact on the equilibrium.