Lesson 2: Le Chatelier's Principle

• Le Chatelier's Principle

  When a system at equilibrium is subjected to a change in one of the factors (concentration, temperature, pressure) determining the equilibrium, the system shifts in a direction that tends to counteract the change and reestablish the equilibrium
• Applying the Principle

  1. Initial equilibrium state

  2. Shifting non-equilibrium state

  3. New equilibrium state
• An equilibrium system will respond differently to different stresses so the principle reduces stress of chemical systems
• When the shift occurs, it is no longer in equilibrium, but it will establish a new equilibrium state since the reaction is reversible
• Changes in Concentration of Product or Reactant

  Increasing the concentration of a reactant shifts the equilibrium to the right, increasing the concentration of the product

  Increasing the concentration of a product shifts the equilibrium to the left, increasing the concentration of the reactants
• H2 (g) + I2 (g) ⇄ 2HI (g)

  • Increasing [H2 (g)] shifts right, decreasing [I2 (g)], increasing [HI (g)]

  Increasing [I2 (g)] shifts right, decreasing [H2 (g)], increasing [HI (g)]

  Increasing [HI (g)] shifts left, increasing [H2 (g)], increasing [I2 (g)]

  Removing [HI (g)] shifts right, decreasing [H2 (g)], decreasing [I2 (g)]

  Removing [H2 (g)] shifts left, increasing [I2 (g)], decreasing [HI (g)]

  Removing [I2 (g)] shifts left, increasing [H2 (g)], decreasing [HI (g)]
• If we change the concentration of reactants, the products will be made at a faster rate
• If we increase concentration of products, reactants will be made at a faster rate
• The Effect of Temperature on Equilibrium

  When energy in the form of heat is added to a system at equilibrium the temperature increases. This causes the equilibrium to shift in the direction that absorbs heat, and the value of the equilibrium constant, Keq, changes.
• Endothermic reactions

  • Adding heat shifts the reaction to the right, increasing the concentration of products

  Removing heat shifts the reaction to the left, decreasing the concentration of products
• Exothermic reactions

  • Adding heat shifts the reaction to the left, decreasing the concentration of products

  Removing heat shifts the reaction to the right, increasing the concentration of products
• Increasing the temperature increases the rates of both the forward and reverse reactions, but the endothermic reaction will be favoured because it "uses up" the excess energy
• Decreasing the temperature decreases the rates of both the forward and reverse reactions, but the exothermic reaction will be favoured because it produces more heat
• Changes in Volume or Pressure

  Decreasing the volume or increasing the pressure shifts the equilibrium to the side with fewer moles of gas

  Increasing the volume or decreasing the pressure shifts the equilibrium to the side with more moles of gas

  If there are equal numbers of gas molecules on each side, there is no change in the position of the equilibrium
• 2SO2 (g) + O2 (g) → 2SO3 (g)

  • Decreasing volume or increasing pressure shifts to the side with fewer moles of gas (2 + 1 = 3)

  Increasing volume or decreasing pressure shifts to the side with more moles of gas (2 + 1 = 3)
• H2 (g) + I2 (g) 2HI (g)

  • No effect on equilibrium since there are equal numbers of gas molecules on each side
• The volume in a gas is directly related to the pressure - decreasing volume increases pressure, and vice versa
• Decreasing volume causes more collisions, so the reaction shifts to the side with fewer moles of gas

  Increasing volume causes less collisions, so the reaction shifts to the side with more moles of gas
• A catalyst added to a reaction has no effect on equilibrium since it speeds up the rates of the forward and reverse processes equally
• Changing the surface area of a reactant or product has no effect on equilibrium
• Adding a catalyst or increasing surface area may affect the time it takes to reach equilibrium, but once equilibrium is established, neither factor can affect the position of the equilibrium
• decreasing the concentration of reactants will cause the equilibrium to shift to the left, causing the concentration of products to decrease

  decreasing the concentration of products will cause equilibrium to shift to the right, causing the concentration of reactants to decrease