3.4 Le chateliers principle

Le chatelier's principle

A number of factors can influence the equilibrium of a reaction. These are:

1. concentration
2. temperature
3. pressure

Le Chatelier's Principle helps to predict what a change in temperature, concentration or pressure will have on the position of the equilibrium in a chemical reaction. This is very important, particularly in industrial applications, where yields must be accurately predicted and maximised.

Le Chatelier's Principle

If a chemical system at equilibrium experiences a change in concentration, temperature or total pressure the equilibrium will shift in order to minimise that change and a new equilibrium is established.

The effect of concentration on equilibrium

If the concentration of a substance is increased, the equilibrium will shift so that this concentration decreases. So for example, if the concentration of a reactant was increased, the equilibrium would shift in the direction of the reaction that uses up the reactants, so that the reactant concentration decreases and equilibrium is restored. In the reaction between nitrogen and hydrogen to produce ammonia:

$N_2\left(g\right)+3H_2\left(g\right)\rightleftharpoons 2N{H}_3\left(g\right)$

• If the nitrogen or hydrogen concentration was increased, Le Chatelier's principle predicts that equilibrium will shift to favour the forward reaction so that the excess nitrogen and hydrogen are used up to produce ammonia. Equilibrium shifts to the right .
• If the nitrogen or hydrogen concentration was decreased, the reverse reaction would be favoured so that some of the ammonia would change back to nitrogen and hydrogen to restore equilibrium.
• The same would be true if the concentration of the product (NH ${}_3$ ) was changed. If [NH ${}_3$ ] decreases, the forward reaction is favoured and if [NH ${}_3$ ] increases, the reverse reaction is favoured.

The effect of temperature on equilibrium

If the temperature of a reaction mixture is increased, the equilibrium will shift to decrease the temperature. So it will favour the reaction which will use up heat energy, in other words the endothermic reaction. The opposite is true if the temperature is decreased. In this case, the reaction that produces heat energy will be favoured, in other words, the exothermic reaction.

The reaction shown below is exothermic (shown by the negative value for $\Delta$ H). This means that the forward reaction, where nitrogen and hydrogen react to form ammonia, gives off heat. In the reverse reaction, where ammonia is broken down into hydrogen and nitrogen gas, heat is used up and so this reaction is endothermic.

An increase in temperature favours the reaction that is endothermic (the reverse reaction) because it uses up energy. If the temperature is increased, then the yield of ammonia (NH ${}_3$ ) decreases.

A decrease in temperature favours the reaction that is exothermic (the forward reaction) because it produces energy. Therefore, if the temperature is decreased, then the yield of NH ${}_3$ increases.

Experiment : le chatelier's principle

Aim:

To determine the effect of a change in concentration and temperature on chemical equilibrium