Le Chatelier's Principle and its Effects - SS3 Chemistry Lesson Note

Le Chatelier's Principle is a fundamental concept in chemistry that describes how a system at equilibrium responds to changes in temperature, pressure, or concentration of reactants or products. It states that if an external stress is applied to a system at equilibrium, the system will adjust in such a way as to partially offset the effect of the stress and restore equilibrium. This principle is named after the French chemist Henry Louis Le Chatelier, who formulated it in the late 19th century. Le Chatelier's Principle is a powerful tool for predicting the direction in which a reaction will shift when subjected to various perturbations.

Effect of Concentration Changes:

When the concentration of a reactant or product is changed, the system will adjust to counteract the change and restore equilibrium. The direction of the shift depends on whether the concentration is increased or decreased.

●     If the concentration of a reactant is increased, the system will shift in the direction of the products to consume the excess reactant.

●     If the concentration of a reactant is decreased, the system will shift in the direction of the reactants to compensate for the loss.

Conversely:

●     If the concentration of a product is increased, the system will shift in the direction of the reactants to reduce the excess product.

●     If the concentration of a product is decreased, the system will shift in the direction of the products to replace the lost product.

Effect of Pressure Changes (for Gaseous Reactions):

For gaseous reactions involving a change in the total moles of gas, changes in pressure can affect the position of the equilibrium. Le Chatelier's Principle predicts the following behaviour:

●     If the pressure is increased, the system will shift in the direction that reduces the total number of moles of gas.

●     If the pressure is decreased, the system will shift in the direction that increases the total number of moles of gas.

Effect of Temperature Changes:

Temperature changes can have different effects depending on whether the reaction is exothermic (heat is a product) or endothermic (heat is a reactant).

For exothermic reactions:

●     If the temperature is increased, the system will shift in the direction of the reactants to absorb the excess heat.

●     If the temperature is decreased, the system will shift in the direction of the products to generate more heat.

For endothermic reactions:

●     If the temperature is increased, the system will shift in the direction of the products to consume the additional heat.

●     If the temperature is decreased, the system will shift in the direction of the reactants to produce more heat.

Applications of Le Chatelier's Principle:

Le Chatelier's Principle has broad applications in various fields of chemistry:

●     In industrial processes, it is used to optimise reaction conditions and maximise yields of desired products.

●     In environmental chemistry, it helps to understand the effects of changes in environmental conditions on chemical equilibria.

●     In biological systems, it plays a role in understanding enzyme-catalysed reactions and the regulation of biochemical pathways.

Overall, Le Chatelier's Principle is a powerful tool for predicting and understanding how chemical systems respond to external perturbations. It enables chemists to manipulate reaction conditions to achieve desired outcomes and provides valuable insights into the behaviour of chemical equilibria.