Le Chatelier's principle

In chemistry, Le Chatelier's principle can be used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henri Louis le Chatelier.

Le Chatelier's Principle can be summarized thus:

If a chemical system at equilibrium experiences a change in concentration, temperature or total pressure the equilibrium will shift in order to minimize that change.

The principle is used by chemists in order to manipulate the outcomes of reversible reactions, often to increase the yield of reactions.

Examples

Concentration

Changing the concentration of an ingredient will shift the equilibrium to the side that would reduce that change in concentration.

This can be illustrated by the equilibrium of carbon monoxide and hydrogen gas, reacting to form methanol.

Suppose we were to increase the concentration of CO in the system. Using Le Chatelier's principle we can predict that the amount of methanol will increase, decreasing the total change in CO.

Temperature

Let us take for example the reaction of nitrogen gas with hydrogen gas. This is a reversible reaction, in which the two gases react to form ammonia:

This exothermic reaction when creating ammonia. If we were to lower the temperature, the equilibrium would shift in such as way as to produce heat. Since this reaction is exothermic to the right, it would favour the production of more ammonia. This reaction is used in the Haber process, which is a good example of the way chemists use Le Chatelier's principle.

Total Pressure

Once again, let us refer to the reaction of nitrogen gas with hydrogen gas to form ammonia:

Note the number of moles of gas on the left hand side, and the number of moles of gas on the RHS. We know that gases at the same temperature and pressure will occupy the same volume. We can use this fact to predict the change in equilibrium that will occur if we were to change the total pressure.

Suppose we increase total pressure on the system, now by Le Chatelier's principle the equilibrium would move to decrease the pressure. Noting that 4 moles of gas occupy more volume than 2 moles of gas, we can deduce that the reaction will move to the right if we were to increase the pressure.