Le Chatelier’s Principle (Higher Tier)

Le Chatelier’s Principle

Le Chatelier’s Principle states that if a system at dynamic equilibrium is subjected to a change in conditions, the system will respond to counteract that change and restore a new equilibrium.

Dynamic equilibrium occurs when the rates of the forward and reverse reactions are equal, so the concentrations of reactants and products remain constant, even though the reactions continue to happen.

At dynamic equilibrium, the forward and backward reactions occur at the same rate, so the concentrations of reactants and products remain constant. However, the reactions continue to happen, making the system dynamic.

When a change is made to the concentration, pressure, or temperature of the system, the position of equilibrium shifts to oppose the change:

• If concentration changes, the system shifts to use up the added substance or replace the removed one.
• If pressure changes (only for gases), the system shifts to the side with fewer or more gas molecules.
• If temperature changes, the system shifts to absorb or release heat depending on the reaction’s endothermic or exothermic direction.

This principle helps predict how equilibrium mixtures respond to changes, which is essential in industrial processes like the Haber process.

For example, consider the equilibrium:

If more nitrogen is added, the equilibrium shifts to the right to produce more ammonia, using up the extra nitrogen.

Effect of Concentration Changes

Changing the concentration of reactants or products affects the equilibrium position:

• Increasing reactant concentration shifts equilibrium to the right (towards products) to use up the extra reactants.
• Increasing product concentration shifts equilibrium to the left (towards reactants) to reduce the excess products.
• Decreasing reactant concentration shifts equilibrium to the left (towards reactants) to replace the lost reactants.
• Decreasing product concentration shifts equilibrium to the right (towards products) to replace the lost products.

This shift happens because the system tries to restore balance by favouring the reaction direction that opposes the change.

For instance, in the reaction:

If the concentration of $\text{H}_2$ is increased, the equilibrium shifts right to produce more HI.

For example, if the concentration of reactant $\text{H}_2$ is doubled, the equilibrium shifts to use up the extra $\text{H}_2$ by producing more product.

Effect of Pressure Changes

Pressure changes only affect equilibria involving gases. The system responds according to the number of gas molecules on each side of the equation:

• Increasing pressure shifts equilibrium towards the side with fewer gas molecules, reducing pressure.
• Decreasing pressure shifts equilibrium towards the side with more gas molecules, increasing pressure.

This happens because gases are compressible, and changing pressure alters their concentration and volume.

For example, in the Haber process reaction:

There are 4 moles of gas on the left (1 $\text{N}_2$ + 3 $\text{H}_2$) and 2 moles on the right ($\text{NH}_3$). Increasing pressure shifts equilibrium right to the side with fewer gas molecules, producing more ammonia.

Example: If pressure is increased from 1 atm to 5 atm in this system, the equilibrium will shift right, increasing ammonia yield.

Effect of Temperature Changes

Temperature changes affect equilibrium by favouring either the endothermic or exothermic direction:

• Endothermic direction absorbs heat.
• Exothermic direction releases heat.
• Increasing temperature shifts equilibrium in the endothermic direction to absorb the extra heat.
• Decreasing temperature shifts equilibrium in the exothermic direction to release heat.

This means the system adjusts to oppose the temperature change by favouring the reaction direction that uses or produces heat.

For example, in the Haber process:

The forward reaction is exothermic (releases heat). Increasing temperature shifts equilibrium left (endothermic direction) to reduce heat, producing less ammonia.

Example: Raising the temperature from 400°C to 500°C decreases ammonia yield because equilibrium shifts left.