6.2.5 The Effect of Changing Concentration (HT only)

When the concentration of one of the reactants or products in a system at equilibrium is changed, the equilibrium is disturbed, and the concentrations of all the substances will adjust until a new equilibrium is reached. This phenomenon can be explained by Le Chatelier's Principle.

Le Chatelier's Principle and Concentration Changes

According to Le Chatelier's Principle, if the concentration of a reactant or product is increased, the equilibrium will shift in the direction that reduces the concentration. Conversely, if the concentration is decreased, the equilibrium will shift in the direction that increases the concentration.

• Effect of Increasing Reactant Concentration: If the concentration of a reactant is increased, the system will respond by favouring the forward reaction to consume the excess reactant. This leads to an increase in the concentration of products until a new equilibrium is established.
• Effect of Decreasing Product Concentration: If the concentration of a product is decreased, the system will respond by favouring the reverse reaction to produce more of the depleted product. This results in an increase in the concentration of reactants until a new equilibrium is reached.

Predicting the Effect of Concentration Changes

By examining the stoichiometry and balanced equation of a reaction, we can predict the effect of changing the concentration of a reactant or product on the equilibrium position. Analysing the reaction and considering the direction in which the equilibrium will shift allows for qualitative predictions.

Conclusion

By understanding the principles of Le Chatelier's Principle and the concept of equilibrium, we can analyse the effects of concentration changes on the position of equilibrium. Changing the concentration of an inert substance (one that does not participate in the reaction) will not affect the equilibrium since its concentration does not appear in the equilibrium expression.