Le Chatelier’s principle

AS

Le Chatelier’s principle states that if the conditions of a system in dynamic equilibrium are changed, the position of equilibrium will move to oppose the change.

This occurs if there is a change in concentration, pressure or temperature.

Change in concentration

Increasing the concentration of reactants increases the yield of products, while decreasing concentration of reactants decreases yield as the position of equilibrium moves to keep the ratio of concentrations equal

Change in pressure

At high pressure the position of equilibrium moves to the side of fewer moles to bring the pressure back down, and vice versa

Change in temperature

At high temperatures, the position of equilibrium is moved in the endothermic direction to bring the temperature back down

At low temperatures, the position of equilibrium moves in the exothermic direction

Catalysts have no effect on the position of equilibrium

They increase the rate of both the forward and backwards reactions by the same amount. This means equilibrium is attained faster, but there is no effect on the yield

University

Le Chatelier’s principle is just a consequence of the equilibrium constant, Kc:

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This must be constant at constant temperature, so the ratio of concentrations of products to reactions must also be constant. This is also true for pressures, since the equilibrium constant can be written in terms of partial pressures:

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pn is the product of the number of moles of n over total moles, and the total pressure:

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This can be substituted in, and some of the P terms cancel. If the remaining Ps change, so must the ratio of products and reactants.

The equilibrium constant changes at different temperatures, eg if the position of equilibrium moves to the right at higher temperature, then the equilibrium constant has increased.

Back to Contents: Chemistry: Equilibria

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