Once dynamic equilibrium has been reached, different amounts can affect the forward and reverse reactions thereby affecting the balance achieved. If this occurs then the reactions will move more in one direction or the other in order to re-establish equilibrium. This direction can be predicted by Le Chatelier’s principle:

If a constraint is imposed on a system at equilibrium, then the system will respond in such a way as to counteract the effect of that constraint.

Constraints include:

• adding or removing one of the reactants (concentration)
• adding or removing one of the products (concentration)
• changing the pressure
• changing the temperature
• adding a catalyst

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Concentration

Le Chatelier’s principle predicts: when the concentration of a species increases, the system will attempt to remove that species (a shift to the right). Likewise, when the concentration of a species decreases, the system will attempt to replace that species (a shift to the left).

Consider the reaction: A + B ? C + D

If the concentration of A of B increases then the rate of the forward reaction increases. If the reverse reaction does not also increase then the equilibrium will move to the right. However, if the reverse reaction increases but not the forward reaction then the equilibrium will move left.

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Pressure

Le Chatelier’s principle predicts: when there is an increase in pressure the system will attempt to decrease the pressure by moving to the side with less moles. When there is a decrease in pressure the system will attempt to increase the pressure by moving to the side with more moles.

The pressure in a system is dictated by how many gas moles are present in the system. If the gas moles stay the same on both sides then the pressure has no effect on the system’s equilibrium.

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Temperature

Le Chatelier’s principle predicts: if there is an increase in temperature then the system will move in the endothermic direction so that the temperature will decrease. If there is a decrease in temperature then the system will move in the exothermic direction so that the temperature will increase.

If the forward reaction is exothermic and takes place there will be a rise in the system’s temperature. In turn, the reverse reaction will be endothermic and, if it also takes place, will cause the temperature in the system to drop.

Therefore, if the forward reaction is exothermic an increase in temperature will mean the system shifts left and a decrease in temperature will mean the system shifts right. If the temperature is the same on both sides then a change in temperature will have no effect on the position of equilibrium.

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Catalysts

Le Chatelier’s principle predicts: with the addition of a catalyst both the forward and reverse reactions are increased by the same amountso there is no change in the position of equilibrium.

A catalyst works by lowering the energy required for activation. However, it lowers the activation energies for the forward and reverse reactions by equal amounts.