If conditions are altered once equilibrium has been reached then the reaction may become unbalanced. In order to regain equilibrium the system may move more one way or the other. The direction the system moves can be predicted using 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 constant.

Constraints include:

• adding a product or reactant
• removing a product or reactant
• changing the pressure
• changing the temperature
• adding a catalyst

Concentration

Le Chatelier’s principle predicts:

• an increase in a reactant’s or a product’s concentration will move the system to the right in order to decrease the concentration of the reactant or product
• a decrease in a reactant’s or a product’s concentration will move the system to the left in order to increase the concentration of the reactant or product

The rate of constants of the forward and reverse reactions are unaffected and, therefore, so is the equilibrium constant, K_c. This means that the effect of changing the concentration can be shown with reference to K_c.

Take the following equation: aA + bB ? cC + dD

K_c = ([C]^c [D]^d) / ([A]^a [B]^b)

In order for K_c to be maintained, as A or B increases so must [C] and [D]. Likewise, as the concentration of C or D increases so must [A] and [B].

Pressure

The pressure of a system is dictated by the number of gas molecules present. Therefore, Le Chatelier’s principle predicts:

• an increase in pressure will move the system to the side with the lowest number of gas moles
• a decrease in pressure will move the system to the side with the highest number of gas moles
• if the moles on each side are equal then the equilibrium position is not effected by a pressure change

As a change in pressure affects reactant concentrations and not rate constants, the equilibrium constant is not affected by a change in pressure.

Temperature

When the forward reaction is exothermic the system’s temperature increases in order for this reaction to take place. The reverse reaction is then endothermic, leading to a drop in the system’s temperature. Therefore, Le Chatelier’s principle predicts:

• an increase in temperature favours an endothermic reaction
• a decrease in temperature favours an exothermic reaction

If the forward reaction is exothermic:

• a temperature increase will cause the system to shift left
• a temperature decrease will cause the system to shift right

If the forward reaction is endothermic:

• a temperature increase will cause the system to shift right
• a temperature decrease will cause the system to shift left

When ? = 0, a change of temperature will not change the position of equilibrium.

As a change in temperature leads to a range on the rate constants for both the forward and reverse reactions, there is a high chance that the equilibrium constant value will also change:

• in an exothermic reaction the temperature increases so the value of K_c will decrease
• in an endothermic reaction the temperature decreases so the value of K_c will increase

Catalysts

The position of equilibrium is not changed by adding a catalyst. This is because the rate of the forward and reverse reactions is increased by the same amount. Therefore, a catalyst also has no effect on the equilibrium constant.