What Le Chatelier’s principle is

Le Chatelier’s principle is used to predict the effect of changes in conditions on the position of equilibrium of a reversible reaction.

The position of equilibrium refers to the relative amounts of products and reactants present at equilibrium.

Le Chatelier's principle states that if a chemical system at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will shift to counteract the change.

In other words, the equilibrium tries to reduce the stress and resist the changes.

• Shifting 'to the right' means more products form.
• Shifting 'to the left' means more reactants form.

Using Le Chatelier’s principle

We can use the following rules to predict how the position of equilibrium will change:

Concentration changes

• Increasing the concentration of a reactant - Equilibrium shifts towards the products (to the right) to use up the extra reactant.
• Increasing the concentration of a product - Equilibrium shifts towards the reactants (to the left) to use up the extra product.
• Decreasing the concentrations has the opposite effects.

For example, in the reaction:

N_2(g)_ + 3H_2(g)_ ⇌ 2NH_3(g)_

• If [H_2_] is increased, the equilibrium position will shift right towards NH_3_ to use up the extra H_2_.
• If [NH_3_] is decreased, the equilibrium position will shift right to produce more NH_3_.

Pressure changes

Pressure changes only apply to gaseous equilibria.

• Increasing the pressure - Equilibrium shifts towards the side with fewer gas molecules to lower the pressure.
• Decreasing the pressure - Equilibrium shifts towards the side with more gas molecules to raise the pressure.

For example, in the reaction:

2SO_2(g)_ + O_2(g)_ ⇌ 2SO_3(g)_

The left side has 3 gas molecules, and the right side has 2. So, raising the pressure moves the equilibrium to the right, where there are fewer gas molecules.

Temperature changes

• Increasing the temperature - Equilibrium shifts in the endothermic direction to absorb the excess heat.
• Decreasing the temperature - Equilibrium shifts in the exothermic direction to release heat.

For example, in the reaction:

N_2(g)_ + O_2(g)_ ⇌ 2NO_(g)_, ΔH = +180 kJ mol^-1^

• Heating this endothermic reaction shifts the equilibrium to the right to absorb the additional heat.
• Cooling this reaction shifts the equilibrium to the left, releasing more heat.

Effect of catalysts

• Adding a catalyst does not affect the position of equilibrium.
• A catalyst speeds up both the forward and reverse reactions equally, so the equilibrium constant remains unchanged.
• Catalysts lower the activation energy for the reaction, allowing equilibrium to be reached faster, but they do not alter the final equilibrium composition.

Temperature changes affect K

The equilibrium constant (K) is influenced solely by temperature. Changes in concentration or pressure have no impact on K.

When the position of equilibrium is altered due to a temperature shift:

• A decrease in the amount of product at equilibrium causes K to decrease.
• An increase in the amount of product at equilibrium causes K to increase.

This ensures that the value of K aligns with the new position of equilibrium.

Example:

2SO_2(g) + O_2(g) ⇌ 2SO_3(g), ΔH = $-$197 kJ mol^-1

Raising the temperature causes the equilibrium to shift left towards the reactants, absorbing heat and resulting in less SO₃ at the new equilibrium position.

With a decrease in SO₃:

• The proportion of products reduces.
• Consequently, K (the ratio of products to reactants) decreases.

The impact of temperature changes on exothermic and endothermic reactions is summarised in the table below:

Concentration and pressure changes do not affect K

While shifts in concentration or pressure can move the equilibrium position, altering the quantities present to counteract the change, the equilibrium constant (K) remains unchanged at a specific temperature.

For example:

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

• Adding more N_2_ leads to an increased formation of NH_3_ at the new equilibrium. However, the equilibrium ratio of [NH_3_]^2^ to [N_2_][H_2_]^3^ still remains equal to keep value of K constant.
• Thus, changes in concentration and pressure do not directly influence K - its value is only determined by temperature.

Catalysts do not change K or equilibrium position

Catalysts increase the rate of both the forward and reverse reactions, reducing the time needed to reach equilibrium following a change.

However, catalysts neither alter the equilibrium position nor the value of the equilibrium constant (K).

The table below summarises the effects of changing reaction conditions on the position of equilibrium and the value of K: