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

  

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)

  

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