L2.4: Le châteleir’s Principle

• Le Châteleir’s Principle

  ● When a chemical system at equilibrium is disturbed, it reattains equilibrium by undergoing a net reaction that reduces the effect of the disturbance

  ○ A system is disturbed when a change in conditions forces it temporarily out of equilibrium.
• Le Châteleir’s Principle

  ● The system responds to a disturbance by a shift in the equilibrium position.

  ○ A shift to the left is a net reaction from product to reactant.

  ○ A shift to the right is a net reaction from reaction to product.

  ● If a chemical system at equilibrium is disturbed or stressed, the system will react in a direction that counteracts the disturbance or relieves the stress.
• The Effect of a Change in Concentration

  ● If the concentration of A increases, the system reacts to consume some of it.

  ○ If a reactant is added, the equilibrium position shifts to the right.

  ○ If a product is added, the equilibrium position shifts to the left.
• The Effect of a Change in Concentration

  ● If concentration of B decreases, the system reacts to consume some of it.

  ○ If a reactant is removed, the equilibrium position shifts to the left.

  ○ If a product is removed, the equilibrium position shifts to the right.

  REMEMBER: Only substances that appear in the expression for Q can have an effect. A change in concentration has no effect on the value of K.
• The Effect of a Change in Pressure (Volume)

  Changes in pressure affect equilibrium systems containing gaseous components

  ● Changing the amount of a gas shifts the equilibrium, but adding an inert gas doesn’t (if volume stays the same), changing volume shifts equilibrium if gas moles change, and K stays the same no matter what.
• The Effect of a Change in Pressure (Volume)

  ● When pressure increases, equilibrium shifts in the direction that decreases the number of moles in order to decrease pressure.

  ● When pressure decreases, equilibrium shifts in the direction that increases the number of moles in order to increase pressure.
• The Effect of a Change in Temperature

  ● To determine the effect of a change in temperature. Heat is considered a component of the system.

  ○ Heat is a product in an exothermic reaction.

  ○ Heat is a reactant in an endothermic reaction.

  ○ An increase in temperature adds heat, which favors endothermic reactions.

  ○ A decrease in temperature removes heat, which favors exothermic reactions.
• The Effect of a Change in Temperature

  ● When temperature is increased, the reaction that absorbs heat is favored.

  ○ An endothermic reaction absorbs heat, so increasing temperature favors the forward reaction.

  ○ Conversely, when temperature is decreased, the reaction that adds heat is favored.
• Endothermic

  • Heat enters
  • Positive H
  • Heat is a reactant

  Exothermic

  • Heat exits
  • Negative H
  • Heat is a product