Le Chatelier's Principle Chemistry Tutorial

Key Concepts

In 1884, the French Chemist Henri Le Chatelier suggested that equilibrium systems tend to compensate for the effects of perturbing influences (or disturbances).

When a system at equilibrium is perturbed (or disturbed), the equilibrium position will shift in the direction which tends to minimise, or counteract, the effect of the disturbance.

For the system: reactants ⇋ products     ΔH = ? kJ mol^-1

If the equilibrium is perturbed (disturbed) by an increase in

• reactant concentration in a solution:

  equilibrium position shifts right, more products produced

• product concentration in a solution:

  equilibrium position shifts left, more reactants produced

• volume available to a gas mixture (decrease in pressure):

  equilibrium position shifts to side with most gas molecules

• temperature when ΔH is positive (endothermic)

  equilibrium position shifts right, more products produced

• temperature when ΔH is negative (exothermic)

  equilibrium position shifts left, more reactants produced

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Changes in Concentration of Aqueous Solutions

Consider the following system at equilibrium at a constant temperature:

All the species, that is all reactants and products, are in aqueous solution.

At equilibrium, the rate at which Fe³⁺(aq) and SCN^-(aq) react to produce FeSCN²⁺(aq) is the same as the rate at which FeSCN²⁺(aq) breaks apart to produce Fe³⁺(aq) and SCN^-(aq).

The equilibrium position will be determined by

• the concentration of each reactant, [Fe³⁺(aq)] and [SCN^-(aq)], and the product, [FeSCN²⁺(aq)]
• the temperature of the system

The equilibrium position will NOT be effected by

• changes in volume or pressure because no gas species are present.
• addition of a catalyst (a catalyst speeds up the rate of the forward and reverse reactions equally and does not change the equilibrium position)

Now consider what happens when the equilibrium position is perturbed (or disturbed) by changing the concentration of reactants or products while maintaining a constant temperature:

Gaseous Solutions and Changes in Concentration

Consider the following gaseous system at equilibrium at constant temperature and constant volume:

All the species, reactant and product species, are gases.

At equilibrium, the rate at which NO₂ molecules break apart to form N₂O₄ molecules is the same as the rate at which N₂O₄ break apart to make NO₂ molecules.

The equilibrium position will be determined by the

• concentration of the gases (the amount of each gas in a given volume)
• temperature
• volume of the vessel since concentration is the amount of gas per unit volume, changing the volume occupied by a gas will change its concentration
  (Note that changing the volume of the vessel holding the gases will also change the pressure inside the vessel since volume is inversely proportional to pressure by Boyle's Law)

The equilibrium position will NOT be effected by the addition of

• a catalyst because a catalyst will speed up the rate of the forward and reverse reactions equally
• an inert gas while maintaining constant volume because this does not effect the concentration of gaseous reactants and gaseous products

Now consider what happens when the equilibrium position is perturbed (disturbed) by changing the concentration of reactants or products at constant temperature and volume:

Changes in Volume of a Gaseous System at Equilibrium

Consider the following gaseous system at equilibrium:

All the species, reactant and product species, are gases.

At equilibrium, the rate at which NO₂ molecules break apart to form N₂O₄ molecules is the same as the rate at which N₂O₄ break apart to make NO₂ molecules.

The equilibrium position will be determined by the

• concentration of the gases (the amount of each gas in a given volume)
• temperature
• volume of the vessel since concentration is the amount of gas per unit volume, changing the volume occupied by a gas will change its concentration
  (Note that changing the volume of the vessel holding the gases will also change the pressure inside the vessel since volume is inversely proportional to pressure by Boyle's Law)

The equilibrium position will NOT be effected by the addition of

• a catalyst because a catalyst will speed up the rate of the forward and reverse reactions equally
• an inert gas while maintaining constant volume because this does not effect the concentration of gaseous reactants and gaseous products (see section above)

Now consider what happens when the equilibrium position is perturbed (disturbed) by changing the volume of the vessel containing the gaseous reaction mixture while maintaining a constant temperature:

Changes in Temperature

In an endothermic reaction, energy can be considered as a reactant of the reaction.

In an exothermic reaction, energy can be considered as a product of the reaction.

Le Chatelier's Principle predicts that the equilibrium position will shift in order to:

• consume more heat if the reaction mixture is heated
  (that is, the endothermic reaction is favoured when the reaction mixture is heated)
• produce more heat if the reaction mixture is cooled
  (that is, the exothermic reaction is favoured when the reaction mixture is cooled)

Endothermic Reactions at Equilibrium

Consider the following reaction at equilibrium:

This reaction can also be written with the energy term incorporated into the equation on the side with the reactants:

When the system is at equilibrium, the rate at which H₂(g) combines with I₂(s) to produce HI(g) is the same as the rate at which HI(g) breaks apart to form H₂(aq) and I₂(s).

The equilibrium position will be determined by the

• concentration of H₂(g) and HI(g)
• temperature
• volume of the reaction vessel (or total pressure since volume is inversely proportional to pressure by Boyle's Law) since gaseous species are present

The equilibrium position will NOT be effected by the

• addition of a catalyst (a catalyst speeds up the rate of the forward and reverse reactions equally and does not change the equilibrium position)
• addition of an inert gas while maintaining constant volume because this has no effect on the concentration of gaseous reactants and products.
• amount of I₂(s) because the equilibrium position is not dependent on amounts of species present but rather on the concentration of species present.

Now consider what happens when the equilibrium position is perturbed (disturbed) by changing the temperature of the system:

Exothermic Reactions at Equilibrium

Consider the following system at equilibrium:

This reaction can also be written with the energy term incorporated into the equation on the side with the the products:

When the system is at equilibrium, the rate at which Ag⁺(aq) combines with Cl^-(aq) to produce a precipitate of AgCl(s) is the same as the rate at which AgCl(s) breaks apart to form Ag⁺(aq) and Cl^-(aq).

The equilibrium position will be determined by the

• concentration of Ag⁺(aq) and Cl^-(aq)
• temperature

The equilibrium position will NOT be effected by the

• addition of a catalyst (a catalyst speeds up the rate of the forward and reverse reactions equally and does not change the equilibrium position)
• addition of an inert gas while maintaining constant volume, or change in volume of vessel because no gas species are present
• amount of AgCl(s) because the equilibrium position is not dependent on amounts of species present but rather on the concentration of species present

Now consider what happens when the equilibrium position is perturbed (disturbed) by changing the temperature of the system:

Summary Table

The following symbols are used in the table:

see also Solubility and Le Chatelier's Principle

see also Henry's Law (gas solubility)