Le Chatelier's Principles on Equilibrium

What Are Le Chatelier’s Principles?

Le Chatelier’s principles, also known as the equilibrium law, are used to predict the effect of some changes on a system in chemical equilibrium (such as the change in temperature or pressure). The principle is named after the French chemist Henry Louis Le Chatelier.

Le Chatelier discovered that equilibrium adjusts the forward and backward reactions in such a way as to accept the changes affecting the equilibrium conditions.

Table of Contents

• Effect of Concentration Changes
• Effect of Change of Volume, Pressure
• Effect of Temperature Changes
• Effect of Catalyst

When factors like concentration, pressure, temperature, and inert gases that affect equilibrium are changed, the equilibrium will shift in that direction, where the effects caused by these changes are nullified.

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

Le Chatelier’s Principle – Video Lesson

Effect of Concentration Changes on Equilibrium and Product Formation

As per Le Chatelier’s principles, the only way for equilibrium to accept more reactants is to increase product formation. The forward reaction is favoured when the concentration of the reactant is increased. The equilibrium of the reaction shift towards the use of reactants in the reaction, which decreases the concentration of the reactants.

Similarly, the addition of product (concentration/pressure) shall increase the backward reaction to decrease the product concentration. The backward reaction is favoured when the concentration of the reactant decreases, the equilibrium of the reaction shift towards the production of reactants, and the concentration of the reactants is more.

Example:

Consider a reaction between oxygen and sulfur dioxide to produce sulfur trioxide.

2SO_2(g) + O_2(g) ⇋ 2SO_3(g)

If the concentration of the reactant increases, then

• Equilibrium will shift towards the decrease in the concentration of the reactants.
• More favoured for the forward reaction.
• Some of the  SO₂ or O₂ form SO_3.
• Equilibrium of the reaction shift towards the right.

If the concentration of the reactant decreases, then

• Equilibrium will shift towards the increase in the concentration of the reactants.
• More favoured for the backward reaction.
• Some of the SO₃ would change to SO₂ or O₂.
• Equilibrium of the reaction shift towards the left.

If the product decreases, then

• The equilibrium of the reaction shift to increase the concentration of the sulfur trioxide.
• Increase in the forward reaction rate.
• Some of the  SO₂ or O₂ form SO_3.
• Equilibrium of the reaction shift towards the right.

If the product increases, then

• The equilibrium of the reaction shifts to decrease the concentration of the sulfur trioxide.
• Increase in the reverse reaction rate.
• Some of the SO₃ would change to SO₂ or O₂.
• Equilibrium of the reaction shift towards the left.

Effect of Change in Concentration – Video Lesson

Also Read:

• Chemical Equilibrium
• Ionic Equilibrium – Degree of Ionization and Dissociation
• Equilibrium Constant – Characteristics and Applications

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

K_p = K_c (RT)^Δn = K_c (p/v)^Δn

Change of volume, pressure or inert gases has no effect on the reactions of liquids and solids. They may have an effect 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:

As per Le Chatelier’s principles, there will be no effect on Equilibrium and Product Formation by changing the volume, pressure or inert gas.

When ∆n = +ve:

An increase in pressure or decrease in volume will decrease the formation of the product. A decrease in pressure or increase in volume shall have the opposite effect of increasing the product formation.

Inert gases do not take part in the reaction and shall increase the volume or pressure only.

• At constant pressure, the addition of inert gas increases the volume, so it increases the product formation.
• At constant volume, inert gas is added, which does not take part in the reaction; the equilibrium remains undisturbed because the addition of inert gas at constant volume does not change the partial pressures or the molar concentrations of the substance involved in the reaction. The reaction quotient changes only if the added gas is a reactant or product involved in the reaction.

PCl₅ ⇌ PCl₃ + PCl₂

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

An increase in pressure or decrease in volume decreases the decomposition of PCl₅

• At constant pressure, the addition of inert gas increases the PCl₅ formation.
• At constant volume, the addition of inert gas decreases the PCl₅ formation.

When ∆n = -ve:

As per Le Chatelier’s principle, an increase in pressure or decrease in volume will increase the formation of the product.

• At constant pressure, the addition of inert gas increases the volume, so it decreases the product formation.
• At constant volume, the equilibrium remains undisturbed. It is because adding an inert gas at constant volume does not change the partial pressures or the molar concentrations of the substance involved in the reaction. The reaction quotient changes only if the added gas is a reactant or product involved in the reaction.

N₂ + 3H₂ ⇌ 2NH₃

In the formation of ammonia, ∆n = -2. The increase of pressure or decrease in volume increases the formation of ammonia_.

• At constant pressure, the addition of inert gas decreases ammonia formation.
• At constant volume, the addition of inert gas increases ammonia formation.

Effect of Change in Volume of Equilibrium Mixture – Video Lesson

Effect of Addition of Inert Gas – Video Lesson

Effect of Change of Temperature on Equilibrium and Product Formation

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

According to Le Chatelier’s Principles,

• 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 the product formation, and a decrease in temperature decreases the product formation

Le Chatelier’s Principle on Change of Temperature

As per the Van’t Hoff equation, for an exothermic equilibrium, ∆H will be negative. An increase in temperature shall decrease K_2, or a decrease in temperature increases K₂. The opposite is true for an endothermic reaction.

Example:

Consider a reaction

N_2(g)+ 3H_2(g) ⇋ 2NH_3(g)                                 ΔH=−92kJ

Increase in temperature

• This prefers an endothermic reaction because it takes energy.
• An endothermic reaction is a reverse reaction, and it is favoured.
• The yield of the product (NH₃) decreases.

Decrease in temperature

• This prefers an exothermic reaction because it gives energy.
• An exothermic reaction is a forward reaction, and it is favoured.
• The yield of the product (NH₃) increases.

Effect of Catalyst on Equilibrium and Product Formation

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 reaction rate is the same for both forward and backward reactions.

The ratio of the reaction rates remains the same, and so is the equilibrium constant. According to 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.