The Theory of Le Chatelier’s Principle

Le Chatelier’s principle is also called equilibrium law. They predict the effect of some changes on a system in chemical equilibrium (specially the change in temperature or pressure). Henry Louis Le Chatelier, the French chemist after which this principle is named .

In The Chatelier state, equilibrium adjusts the forward and backward reactions in such a way as to accept the changes affecting the equilibrium conditions.

Table of Content: –

• Effect of Concentration Changes

• Effect of Change of Volume, Pressure

• Effect of Temperature Changes

• Effect of Catalyst

 Concentration, pressure, temperature and inert gasses are the factors which affect the equilibrium, if they are changed, the equilibrium will shift in that direction where the effects caused by these changes are nullified.

Le Chatelier’s principles are used to manipulate reversible reactions in order to obtain suitable results (such as an improvement in yield).

Effect of Concentration Changes on Equilibrium and Product Formation

As per Le Chatelier’s principles, product formation is the only way to raise equilibrium to accept more reactants. The forward reaction is favored when the concentration of the reactant is rised. The equilibrium of the reaction shifts towards the use of reactants in the reaction and which in turns falls the concentration of the reactants.

Likely, the addition of product shall raise the backward reaction to fall the product concentration. The backward reaction is favored when the concentration of the reactant falls along with equilibrium of the reaction shift towards the production of reactants and the concentration of the reactants will be more.

Example:

Think about a reaction between oxygen and sulfur dioxide to produce sulfur trioxide.

2SO2(g) + O2(g) ⇋ 2SO3(g)

If the concentration of the reactant rises at that time, equilibrium will shift towards the fall in the concentration of the reactants. More favored for the forward reaction. A few of the SO2 or O2 to form SO3. Equilibrium of the reaction moves towards the right. 

If the concentration of the reactant lessens then equilibrium will shift towards the growth in the concentration of the reactants. More favored for the backward reaction. Some of the SO3 would change to SO2 or O2. Equilibrium of the reaction moves towards the left.

 If the product falls then equilibrium of the reaction shifts to raise the concentration of the sulfur trioxide. Rise in the forward reaction rate. Little of the SO2 or O2 to form SO3. Equilibrium of the reaction moves towards the right.

 If the product rises then equilibrium of the reaction shifts to fall the concentration of the sulfur trioxide. Rise in the reverse reaction rate. Some of the SO3 would change to SO2 or O2.Equilibrium of the reaction moves towards the left.

Effect of Change of Volume, Pressure, and Inert Gas on Equilibrium and Product Formation

Kp = Kc (RT)Δn = Kc (p/v)Δn

Change of volume, pressure and inert gasses has no effect on the reactions of liquids and solids. They may result in gaseous reactions and that too only when the difference in the sum of the number of reactant and product molecules (∆n) is not zero.

When ∆n = 0:

According to Chatelier’s principles, there will be no effect on Equilibrium and Product Formation on changing the volume, pressure or inert gas.

When ∆n = +ve:

A growth in pressure or lessen in volume will cause the formation of the product. Fall of pressure or rise of volume shall have the reverse effect of increasing the product formation.

Inert gasses do not participate in the reaction and shall raise the volume or pressure only.

At the same pressure, the addition of inert gas raises the volume, so rises the product formation.

At the same volume, the addition of inert gas raises the pressure, so falls the product formation.

PCl5 ⇌ PCl3 + PCl2

In the decay of phosphorus pentachloride ∆n = +1.

An rise of pressure or fall in volume falls the decaying of PCl5

At the same pressure, the addition of inert gas raises the PCl5 formation.

At the same volume, the addition of inert gas causes the PCl5 formation.

When ∆n = -ve:

According to the Chatelier’s principle, a rise of pressure or fall in volume will raise the formation of the product.

At the same pressure, the addition of inert gas raises the volume, so fall the product formation.

At same volume, the addition of inert gas rises the pressure, so rises the product formation

N2 + 3H2 ⇌ 2NH3

In the formation of ammonia ∆n = -2. The rise of pressure or fall in volume raises the formation of ammonia.

At the same pressure, the addition of inert gas causes ammonia formation.

At the same volume, the addition of inert gas raises ammonia formation.

Result of Change of Temperature on Equilibrium and Product Formation

The individual reaction in the equilibrium can be either endothermic or exothermic. Likewise, at equilibrium net energy involved may make the opposite reactions either endothermic or exothermic.

According to Le Chatelier’s Principles,

In exothermic equilibrium, a rise in temperature causes the product formation and a fall in temperature causes product formation.

In endothermic reactions, an rise in temperature rises the product formation and a fall in temperature falls the product formation

According to the Van’t Hoff equation, for an exothermic equilibrium, ∆H will be negative. A rise in temperature shall fall K2 or a fall in temperature rises K2. The reverse is true for an endothermic reaction.

Example:

Consider a reaction

 N2(g)+ 3H2(g) ⇋ 2NH3(g)     ΔH=−92kJ

Rise in Temperature

1. This prefers an endothermic reaction because it takes energy.

2. An endothermic reaction is a reverse reaction and it is favored.

3. The yield of the product (NH3) falls.

Fall in Temperature

1. This prefers an exothermic reaction because it gives energy.

2. An exothermic reaction is a forward reaction and it is favored.

3. The yield of the product (NH3) rises.

Result of Catalyst on Equilibrium and Product Formation

 Catalyst is a substance that changes the rate of reactions (rise or fall) without quantitatively taking part in the reaction.

According to reversible reactions, the change of reaction rate is the same for both forward and backward reactions.

The ratio of the reaction rates remains constant and so is the equilibrium same. By Le Chatelier’s principles, the presence of the catalyst may speed up or delay the attainment of equilibrium but will not affect the equilibrium concentration.

Conclusion on La chatelier’s principles_

La Chatelier’s principle can predict  the  effects on the change in temperature , pressure and concentration in a chemical reaction.It is very much applicable in industrial fields.

It states if the conditions in an equilibrium are changed then the system will counteract  the changes and reform the reaction.