Le Chatelier's Principle Notes

Le Chatelier's Principle

Le Chatelier's principle states that if a stress is applied to a system at equilibrium, the equilibrium will shift in a direction to relieve that stress.

Types of Stress on Equilibrium

1. Modifying Concentration

• Adding Reactants:
  • The equilibrium shifts to the right (forward reaction speeds up).
  • The system consumes added reactants to form products and restore equilibrium.
• Adding Products:
  • The equilibrium shifts to the left (reverse reaction speeds up).
• Removing Components:
  • The equilibrium shifts to produce more of the removed species to re-establish balance.

2. Changing Temperature

• The effect depends on whether the reaction is exothermic or endothermic.
  • $\Delta H$ (change in enthalpy) determines if a reaction absorbs or releases energy.
• Exothermic Reactions ($\Delta H < 0$):
  • Heat is released (energy is a product).
  • Higher temperature: equilibrium shifts left to consume excess heat.
  • Cooling: equilibrium shifts right.
• Endothermic Reactions ($\Delta H > 0$):
  • Heat is absorbed (energy is a reactant).
  • Higher temperature: equilibrium shifts right, consuming heat.
  • Cooling: equilibrium shifts left.

3. Changing Volume or Pressure

• Consider a system involving gases in a container (e.g., a balloon).
• Decreasing Volume (Increasing Pressure):
  • According to Boyle's law: $P<em>1V</em>1 = P<em>2V</em>2$. Pressure increases as volume decreases.
  • The equilibrium shifts to the side with fewer moles of gas particles to reduce pressure.
    • Example: $X_2 \rightleftharpoons 2X$ (diatomic molecule to two monoatomic species).
    • Shifting left (towards $X_2$) reduces the number of particles and lowers pressure.
• Increasing Volume (Decreasing Pressure):
  • The equilibrium shifts to the side with more moles of gas particles to increase pressure.