Le Chateliers Principle

1
Le Chateliers Principle

• If an equilibrium is disturbed by-
• 1) Adding or removing reactants or products
• 2) Changing the pressure
• 3) Changing the temperature
• the equilibrium will shift to oppose the change.

2
1) Adding or Removing Reactants or Products

• chromate / dichromate equilibrium
• CrO42_(aq) 2H(aq) Cr2O72_(aq) H2O(l)
• Yellow orange
• Adding Acid disturbs the equilibrium, adding a
  reactant (H ions).
• The equilibrium position will shift to the right
  to oppose the change.
• This shift removes the excess H ions and
  converts chromate ions to dichromate ions (the
  solution changes from yellow to orange)
• Adding Alkali disturbs the equilibrium, removing
  a reactant ( H ions)
• The equilibrium position will shift to the left
  to oppose the change.
• This shift produces more H ions and converts
  dichromate ions to chromate ions (the solution
  changes from orange to yellow).

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2) Changing the Pressure

• An increase in pressure will shift the
  equilibrium in the direction that produces the
  least moles of gas.
• e.g. 2NO2 (g) N2O4 (g)
• Brown Colourless
• 2 moles 1 mole
• This equilibrium would shift to the right if the
  pressure was increased (turning colourless).
• This would oppose the increase in pressure,
  because there are less moles of gas on the right
  hand side of the equation.

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3) Changing the Temperature

• Increasing the temperature will shift the
  equilibrium in the direction that absorbs energy
  (endothermic).
• exothermic
• e.g. 2NO2 (g) N2O4 (g)
• Brown Colourless
• endothermic
• This equilibrium would shift to the left if the
  temperature was increased (turning brown).
• This would oppose the increase in temperature,
  because the reverse reaction absorbs energy
  (endothermic).

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Catalysts

• Catalysts dont shift an equilibrium one way or
  the other.
• They just increase the rate of attainment of
  equilibrium (they speed up the forward and
  reverse reactions).

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CO2 and the Oceans

• CO2 (g) aq CO2 (aq)
• CO2 (aq) H2O(l) HCO3_ (aq) H(aq)

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Le Chateliers Principle

• If an equilibrium is disturbed by-
• 1) Adding or removing reactants or products
• 2) Changing the pressure
• 3) Changing the temperature
• the equilibrium will shift to oppose the change.

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The Oscillating Reaction
3CH2(CO2H)2 (aq) 4BrO3-(aq)
4Br-(aq) 9CO2(g) 6H20(l)

Mn2 catalyst