1. Le Chatelier’s Principle Chapter 11

2. Le Chatelier’s Principle states that a system at equilibrium will stay at equilibrium UNLESS it is acted upon externally.
If there is an external action, then the equilibrium has to be re-established.

3. Let us examine the following reaction: aA + bB cC + dD
At equilibrium, there is a certain amount of A, B, C, D present.
These amounts will remain unchanged UNLESS some external stress is applied.
Let’s see how equilibrium is affected when a stress is applied.

4. aA + bB cC + dD Addition of some B
If you add more B, then there will be more molecules of B able to react.
The forward reaction will be favored and equilibrium will move towards the right.
With time, there will be less A, more B, more C and more D present than at the original equilibrium.

5. aA + bB cC + dD Removal of some A
If you remove some A this will cause the reaction to shift towards the left (reactant side).
The amount of A will go down, the amount of B will go up and the amount of C and D will also go down.

6. aA + bB cC + dD Increasing or Decreasing the pressure
Pressure for a gas depends on the number of gas particles present.
An increase or decrease in pressure will have an effect on equilibrium only when there is a different number of gas particles on either side of the balanced equation.

7. An increase in pressure will favor the side with the fewer gas particles.
A decrease in pressure will favor the side with the larger number of gas particles.

8. aA + bB cC + dD + Energy
Increasing or Decreasing the temperature
For an exothermic reaction, an increase in temperature will favor the reverse reaction. (equilibrium shifts to the left)
A decrease in temperature will have the opposite effect.

9. aA + bB + Energy cC + dD
For an endothermic reaction, an increase in temperature will favor the forward reaction. (equilibrium shifts to the right)
A decrease in temperature will have the opposite effect

10. Pressure (due to decreased volume): increase in pressure favors side with fewer molecules. shift right
Catalysts: do not influence the reaction