7.4 Le Chatelier's Principle

When a chemical system at equilibrium is disturbed by a change in a property (such as concentration, temperature or pressure), the system in a way that opposes the change.

  • Any change in a system that results in a change in reactant and product concentration is known as an equilibrium shift.

Nature of Change

  • Adding or removing [R]
  • Adding or removing [P]
  • Increasing or decreasing pressure by increasing or decreasing volume
  • Adding or removing heat energy
  • Adding a catalyst or inert gas does not change equilibrium concentrations (catalysts make the system reach equilibrium faster, but result is the same)

Concentration

  • Add [R]
    • increase frequency of collisions between Reactants
    • increase in [P]
    • increase forward Rx
  • Remove [P]
    • decrease frequency of collisions between products
    • increase in [P]
    • increase forward Rx
  • Add [P]
    • increase frequency of collisions between products
    • increase in [R]
    • increase reverse Rx
  • Remove [R]
    • decrease frequency of collisions between reactants
    • increase in [R]
    • increase reverse Rx
  • Note: addition/removal of a substance in solid or liquid state does not change equilibrium

 When CO2 is added, reaction shifts right, favours forward reaction, products increase, reactants decrease.

Temperature

  • Exothermic reaction:
    • Reactants ↔ Products + HEAT
    • Adding heat → drives reverse reaction (adding product)
  • Endothermic reaction:
    • Reactants + HEAT ↔ Products
    • Adding heat → drives forward reaction (adding reactant)

 When temperature is decreased (energy removed), equilibrium shifts right, favours forward reaction, products increase, reactants decrease.

Pressure & Volume

 

  • By increasing the pressure, we drive the reaction to the side where there are fewer total particles, or smaller concentration (forward in this case)
  • By decreasing the pressure, we drive the reaction to the side where there are more total particles or larger concentration (reverse in this case)
  • Note: Pressure only affects gases as you cannot change volume of a liquid (does not affect aqueous states).

 

  • Remember that P and V are related inversely
  • When volume is decreased (pressure increased), reaction shifts in favour of the side with fewer moles
  • In this case, reaction shifts right, favours forward reaction, products increase, reactants decrease