Le Chatelier's principle

• The position of the equilibrium can be altered by changing the concentration of solutions, the pressure of gases or the temperature.
• If a system is at equilibrium, and a change is made in any of the conditions, then the system responds to counteract the change as much as possible.

Increasing reactants:

• Equilibrium shifts to the right

Increasing products:

• Equilibrium shifts to the left

Decreasing reactants:

• Equilibrium shifts to the left

Decreasing products:

• Equilibrium shifts to the right

Increasing pressure:

• Equilibrium shifts to the side with fewer gas molecules
• Example:
  • CO(g) + 2H2(g)  ⇄  CH3OH(g)
  • 3 molecules 1 molecule
  • Equilibrium shifts to the right

Decreasing pressure:

• Equilibrium shifts to the side with more gas molecules
• Example:
  • CH4(g) + H2O(g)  ⇄  CO(g) + 3H2(g)
  • 2 molecules  4 molecules
  • Equilibrium shifts to the right

Increasing temperature:

• Equilibrium shifts in the direction of the endothermic reaction

Decreasing temperature:

• Equilibrium shifts in the direction of the exothermic reaction

Example:

• 2NO2(g)   ⇄  N2O4(g)
  • Forward reaction is exothermic
  • Backward reaction is endothermic
• Temperature increases, equilibrium shifts to the left
• Temperature decreases, equilibrium shifts to the right

• Catalysts do not change the position of the equilibrium, just the rate at which the equilibrium is established.