Le Chatelier’s Principle

Le Chatelier’s principle states:

The new equilibrium position differs from the old equilibrium position. The position of an equilibrium refers to the comparative concentrations of reactants and products.

Homogenous Equilibrium = All products and reactants are of the same state, e.g. all gaseous or aqueous.

Heterogenous Equilibrium = Products and reactants are in multiple states

Example

Take the following reaction as an example:

2\text{CrO}_4^{2-} (aq) + 2\text H^+ (aq) ⇌ \text {Cr}_2 \text 0_7^{2-} (aq) + \text H_2 \text O (l)

The chromate ion is yellow and the dichromate ion is orange.

Increasing the concentration of hydrogen ions is a change in the system. According to Le Chatelier’s principle, to counteract the change the hydrogen ion concentration is decreased by chromate ions reacting with some of the hydrogen ions to produce more dichromate ions. Thus the system becomes more orange. A new equilibrium position is established.

Gaseous Equilibria

In homogenous gaseous systems, changes in pressure (caused by changes in the volume of the vessel) will lead to changes in concentrations of reactants or products.

In general an increase in total pressure by volume reduction causes a gaseous equilibrium shift to the side of fewer gas molecules. If the number of particles is the same on each side of the equilibrium, then a change in total pressure has no effect.

Example

Take the following reaction as an example:

\text N_2 \text O_2 (g) ⇌ 2\text {NO}_2 (g)

N₂O₂ is colourless and NO₂ is brown.

If the volume of the reaction vessel is increased, total pressure decreases (Boyle’s law). According to Le Chatelier’s principle, the system counteracts the change by shifting to the right to increase the number of particles. The system will become more brown.

Heterogenous Equilibria

Example

A saturated sodium chloride solution is an example of a heterogenous equilibrium. Solid salt crystals are in equilibrium with ions in solution:

\text {NaCl} (s) ⇌ \text {Na}^+ (aq) + \text{Cl}^+ (aq)

If drops of concentrated hydrochloric acid are added to the saturated solution, the concentration of chloride ions increases. This disturbance shifts the equilibrium to the left to counteract the change. More NaCl crystals form.

If additional NaCl crystals are added to the saturated solution, no change is observed as the sodium and chloride ions are already at their maximum concentration in the water.

Adding water will dilute the concentration of the ions. The equilibrium shifts right as more salt crystals dissolve to restore the system to saturation.

Example

The thermal decomposition of solid calcium carbonate in a closed system is an example of a heterogenous reaction because the carbon dioxide produced is gaseous:

\text {CaCO}_3 (s) ⇌ \text {CaO} (s) + \text {CO}_2 (g)

The equilibrium is endothermic in the forward direction. If the system is heated to a higher temperature, the equilibrium shifts to the right (according to Le Chatelier’s principle) to counteract the change and the partial pressure of carbon dioxide increases.

The addition of more solid calcium oxide or solid calcium carbonate causes no change to the equilibrium position.

The addition of more carbon dioxide gas raises the gas pressure and causes the equilibrium to shift to the left to counteract the change. The partial pressure of carbon dioxide will decrease until the new equilibrium is established.