Le Chatelier

1
Le Chateliers Principle

• When a stress is applied to a system at
  Equilibrium, the system readjusts so as to
  relieve or offset the stress.

2
What is meant by stress?

• Stress is any imposed factor which upsets the
  balance in rates between the forward and reverse
  reactions.
• Stress factors may be
• changes in concentrations
• changes in total pressure (with gases only)
• volume changes (which cause the pressure
  changes)
• temperature changes.

3
Stress and Changes in Concentration

• Increasing the concentration of the reactants
  will cause a stress which is relieved by an
  increase in the concentration of product,
  product.
• If the product is increased, the reactants
  will increase to offset the stress.
• A 2B ? C D What happens if we increase D?

4
Stress and Changes in total Volume (or related
pressure)

• eg.     N2(g) 3 H2(g) ltgt  2 NH3(g) 92.00
  kJ
• If we decrease the volume, we increase the
  pressure.
• This stress can be relieved if the system reduces
  the number of molecules.
• ie.    4 molecules ? 2 molecules
• The reaction shifts right.

5

• eg.     N2(g) 3 H2(g) ltgt  2 NH3(g) 92.00
  kJ
• If we increase the volume, we decrease the
  pressure.
• This stress can be relieved if the system
  increases the number of molecules.
• ie.    4 molecules ? 2 molecules
• The reaction shifts left.

6
SUMMARY

• If an equilibrium contains gases,
• an increase in pressure, (decrease in volume),
  will cause the equilibrium to shift to the side
  with fewer molecules.
• an decrease in pressure, (increase in volume),
  will cause the equilibrium to shift to the side
  with more molecules.
• Note There is no shift in the equilibrium if
  there are the same number of molecules on each
  side of the arrows.

7
Stress and Changes in Temperature

• In any equilibrium reaction,
• a temperature increase favours the reaction that
  absorbs heat. i.e. the endothermic reaction.
• a temperature decrease favours the reaction that
  produces heat. i.e. the exothermic reaction.
• eg. N2(g) 3 H2(g) ltgt  2 NH3(g) 92.00 kJ
• The forward reaction is exothermic.
• The reverse reaction is endothermic.
• If we increase the temperature, the reaction will
  shift left.
• If we decrease the temperature, the reaction
  will shift right.

8
Effect of a Adding a Catalyst

• A catalyst equally favours both the forward and
  reverse reactions. Therefore a catalyst does not
  shift the Equilibrium 's. It simply causes the
  reaction system to reach Equilibrium in a shorter
  period of time.

9
Example Problem

• 2 SO2(g) O2(g) ltgt  2 SO3(g) heat
• Indicate the favoured reaction either forward or
  reverse under the stress applied below.
• a) increase the SO2
• b) Partial pressure of SO3 is decreased (Some of
  the SO3 is removed fro the system).
• c) Decrease in temperature.
• d) A catalyst is added.
• e) He gas is added at constant volume so that the
  total pressure is increased.
• f) He gas is added, and the container is allowed
  to expand so that the total pressure is kept
  constant.

10
Problem

• heat 2 H2O(g) ltgt  2 H2(g) O2(g)
• Indicate the direction of the reaction most
  favoured by each of the following conditions.
• a) increase the H2
• b) the partial pressure of H2O is increased
• c) the O2 is increased
• d) increase the temperature
• e) decrease the volume of the container
• f) He gas is added