Le Chatelier's principle makes it possible to predict, in a qualitative way, the direction of the reaction (forward or reverse) that will be favoured when the conditions of a system at equilibrium are modified.
The state of equilibrium assumes that the forward and reverse reactions proceed at the same rate. However, this balance is fragile. If the experimental conditions under which the chemical reaction takes place are disturbed, the equilibrium is then broken and the system adapts to reach a new state of equilibrium. With Le Chatelier's principle, it is possible to predict, qualitatively, in which direction a system in equilibrium will evolve if its experimental conditions are modified.
Le Chatelier's principle, enunciated by Henry Le Chatelier in 1884, states that:
"If we tend to modify the conditions of a system at equilibrium, it reacts in such a way as to partially oppose the changes imposed on it until a new state of equilibrium is attained."
Factors that influence the state of equilibrium include the impact of temperature, concentration, and pressure.
Equilibrium can be changed in two ways.
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The forward reaction rate increases relative to that of the reverse reaction. The forward reaction is then temporarily favoured and the equilibrium shifts towards the formation of products.
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The rate of the reverse reaction increases relative to that of the forward reaction. The reverse reaction is then temporarily favoured and the equilibrium shifts towards the formation of reactants.
According to Le Chatelier's principle, the system will then react by partially opposing these changes.
The addition of a catalyst does not impact the chemical equilibrium.
In a reversible reaction, a catalyst increases both the rate of the forward reaction and that of the reverse reaction. Thus, it does not alter the equilibrium of the system. It only allows a faster attainment of equilibrium. Therefore, equilibrium is not influenced by the presence or absence of a catalyst.