Upon completion of this topic, learners will be able to:

• state Le Châtelier’s principle.
• explain Le Châtelier’s principle as it relates to factors influencing equilibrium.
• state what happens to an equilibrium when a reactant or product is added or removed.
• describe the effect of a pressure change on chemical equilibrium.
• apply Le Châtelier’s principle when the temperature is altered.
• compare the effect of a catalyst on the rate of reaction with its effect on equilibrium.

Le Châtelier’s principle

We can qualitatively predict the effects of changes in concentration, pressure, and temperature on a system at equilibrium by using Le Châtelier’s principle, which states that if a system at equilibrium is disturbed by a change in temperature, pressure, or concentration of reactant or product, the system will shift its equilibrium position so as to nullify the effect of the disturbance.

Effect of change in reactant or product concentration

When the concentrations of species in the reaction are altered, the equilibrium shifts until a new state of balance is attained. Le Châtelier’s principle states that the shift is in the direction that minimizes or reduces the effect of the change. Therefore, if a chemical system is already at equilibrium and the concentration of any substance in the reaction mixture is increased (either reactant or product), the system reacts to consume some of that substance. Conversely, if the concentration of a substance is decreased, the system reacts to produce some of that substance.

Effects of volume and pressure changes

Basically, there are three ways to change the pressure of a reaction system involving gaseous components at a given temperature:

• Add or remove a gaseous reactant or product at constant volume.
• Add an inert gas (one not involved in the reaction) at constant volume.
• Change the volume of the container.

We have already considered the addition or removal of a reactant or product.

When an inert gas is added at constant volume, there is no effect on the equilibrium position. The addition of an inert gas increases the total pressure but has no effect on the concentrations or partial pressures of the reactants or products (assuming ideal gas behavior). Thus the system remains at the original equilibrium position.

A pressure change obtained by changing the volume can affect the yield of products in a gaseous reaction if the reaction involves a change in total moles of gas. When the volume of the container holding a gaseous system is reduced, the system responds by reducing its own volume. This is done by decreasing the total number of gaseous molecules in the system.

When the container volume is increased, the system will shift in the direction that increases its volume. An increase in volume in ammonia production will produce a shift to the left to increase the total number of gaseous molecules present.

Effect of change of temperature

The effect of change in temperature on an equilibrium reaction can be easily predicted by the following version of the Le Châtelier’s principle.

When temperature of a reaction is increased, the equilibrium shifts in a direction in which heat is absorbed.

Cooling a reaction has the opposite effect. As we lower the temperature, the equilibrium shifts in the direction that produces heat. Thus, cooling an endothermic reaction shifts the equilibrium to the left, decreasing K_eq, and cooling an exothermic reaction shifts the equilibrium to the right, increasing K_eq.

Effect of catalyst on the position of equilibrium

A catalyst has no effect on the equilibrium composition of a reaction mixture. It merely speeds up the attainment of equilibrium. This is due to the fact that the catalyst changes the rate of the forward reaction and the reverse reaction by the same extent so the equilibrium is not affected. We can therefore conclude that a presence of a catalyst does not alter the equilibrium constant, nor does it shift the position of an equilibrium system.