Le Chatelier's Principle Lecture Notes

Introduction to Le Chatelier's Principle

• Basic Concept:
  • When a change is imposed on a system at equilibrium, the system will adjust to counteract that change.
  • If raised, it will try to lower, if lowered, it will try to raise.

Chemical Equilibria

• Example Reaction:
  • Reactants: A (solid), B (gas)
  • Products: C (gas), D (liquid)
  • Coefficients: 1 A, 2 B, 1 C, 1 D

Dynamic Equilibrium

• Characteristics:
  • Rate of forward reaction equals rate of reverse reaction.
  • Concentrations of reactants and products remain constant.

Applying Le Chatelier's Principle

• Increase in Concentration of Reactant B:
  • Reaction shifts right to decrease B, increasing products.
• Decrease in Concentration of Reactant B:
  • Reaction shifts left to increase B (reactants).
• Increase in Concentration of Product C:
  • Reaction shifts left to decrease C (products).
• Decrease in Concentration of Product C:
  • Reaction shifts right to increase C (products).

Effects of Solid and Liquid Reactants and Products

• Changes in concentration of solids (A) or liquids (D) do not affect equilibrium position.
• Equilibrium constant K is unaffected by solids/liquids as they are not included in equilibrium expression.

Catalyst Effects

• Addition of a Catalyst:
  • Speeds up both forward and reverse reactions equally.
  • Does not change equilibrium position.

Practice Problems

• Shifts in Reaction Direction:
  • Remove Hydrogen Gas (H2): Shifts left to increase H2.
  • Increase Reactant Concentration (N2): Shifts right to decrease N2.

Problem Solving Strategies

• Remember:
  • Increase in reactant/product shifts away to decrease it.
  • Decrease in reactant/product shifts towards to increase it.
  • Catalysts only affect reaction rate, not equilibrium position.

Volume Changes and Equilibrium

• Increasing Volume:
  • Total pressure drops, system shifts to side with more moles of gas (left).
• Decreasing Volume:
  • Total pressure increases, system shifts to side with fewer moles of gas (right).
• Partial Pressure Consideration:
  • Increase in partial pressure shifts away from increased gas.
  • Decrease in partial pressure shifts towards decreased gas.

Summary

• Equilibrium shifts depend on changes in concentration, pressure, and volume.
• Solids and liquids do not affect equilibrium constants.
• Catalysts affect reaction rates but not equilibrium positions.