Le Chatelier's Principle

Overview

• Definition: If a stress is induced on a system at equilibrium, the equilibrium will shift to relieve that stress.

Types of Stresses on a System at Equilibrium

1. Modifying Concentration

• Adding Reactants:
  • Shifts equilibrium to the right (forward reaction speeds up) to consume added reactants and restore balance.
• Adding Products:
  • Shifts equilibrium to the left to use up excess products.
• Removing Components:
  • Equilibrium shifts towards the removed species to produce more and restore balance.

2. Changing Temperature

• Thermal Considerations:
  • Exothermic Reactions:
    • Negative H (change in enthalpy) indicates energy is released.
    • Heat can be considered a product.
  • Endothermic Reactions:
    • Positive H indicates energy is absorbed.
    • Heat can be considered a reactant.
• Effects of Temperature Change:
  • Increasing Temperature: Shifts equilibrium to use up excess heat (shift to the side that absorbs heat).
  • Decreasing Temperature: Shifts equilibrium to produce heat (shift to the side that releases heat).

3. Changing Volume or Pressure

• Boyle’s Law:
  • Decreasing volume increases pressure; increasing volume decreases pressure.
• Effect on Gas Equilibria:
  • Decreasing Volume: Shifts equilibrium towards the side with fewer moles of gas (reducing pressure).
  • Increasing Volume: Shifts equilibrium towards the side with more moles of gas (increasing pressure).

Example: Diatomic vs. Monoatomic Equilibrium

• An equilibrium involving a diatomic molecule and two monoatomic species:
  • Right side has more particles.
  • Shifting left leads to fewer particles, reducing pressure.

Conclusion

• Understanding how these stresses affect equilibrium is crucial for predicting the behavior of chemical reactions.
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