8.6 Factors Affecting Equilibrium (The Le Chatelier's Principle)

This topic explains Le Chatelier's Principle and how changes in concentration, pressure, and temperature affect a system at equilibrium. It also describes the effect of adding a catalyst.

Definition

Le Chatelier's Principle: If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction which tends to reduce that change.

Main Idea

It is necessary to understand the factors that control the position of a chemical equilibrium. A knowledge of such factors helps industrial chemists to choose conditions that favour desired product as much as possible. We can predict the effect of various factors such as concentration, pressure, and temperature on a system at equilibrium by using Le Chatelier's principle.

At equilibrium, the forward and reverse reactions are driven by molecular collisions. When we change temperature, pressure, and concentration, the frequency and energy of collisions between reacting molecules increase or decrease. This in turn changes the reaction rates and causes a shift in the equilibrium position.

Core Concept

• Le Chatelier's Principle: System shifts to reduce imposed change.
• Concentration change: Equilibrium shifts to accommodate added/removed substance.
• Pressure change: Equilibrium shifts to side with fewer/more gas molecules.
• Temperature change: Equilibrium shifts in endothermic/exothermic direction.
• Catalyst: No effect on equilibrium position.

Detailed Explanation

Le Chatelier's Principle

"It states that if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction which tends to reduce that change."