1. This is the Equilibrium arrow
Le Chatelier’s Principle…
Some reactions only proceed in one direction, others change their minds
A + B C + D
This is the Equilibrium arrow

2. Le Chatelier’s Principle…
What does equilibrium mean in the context of chemistry?
No observable change in the system
But from this point on we will use the concept of a Dynamic Equilibrium

3. Le Chatelier’s Principle…
What does Dynamic Equilibrium mean?
A closed system in which the macroscopic concentrations of reactants and products do not change because the forward and reverse reactions occur at the same rate.

4. Le Chatelier’s Principle…
What does Dynamic Equilibrium mean?
A closed system in which the macroscopic concentrations of reactants and products do not change because the forward and reverse reactions occur at the same rate.

5. Le Chatelier’s Principle…
We can determine the position of the equilibrium which tells us whether more product or more reactant is present at equilibrium.

6. Le Chatelier’s Principle…
A + B C + D
Equilibrium is to the right
More Products
Equilibrium is to the left
More Reactants

7. Le Chatelier’s Principle…
Le Chatelier’s principle is used to work out what happens to the reactant/product concentrations when a condition of the system changes.
“When a system in dynamic equilibrium is subjected to a change, the equilibrium position will shift to minimise that change”

8. The equilibrium shifts right
Le Chatelier’s Principle…
A + B C + D
If we add extra A what happens to:
B = ?
C = ?
D = ?
The equilibrium shifts right

9. A + B C + D B = Decreases C = Increases D = Increases
Le Chatelier’s Principle…
A + B C + D
If we add extra A what happens to:
B = Decreases
C = Increases
D = Increases

10. A + B C + D Le Chatelier’s Principle…
Increasing the concentration of a reactant causes the equilibrium to shift right and form more products.

11. The equilibrium shifts left
Le Chatelier’s Principle…
A + B 2C + D
If we increase the pressure:
A = ?
B = ?
C = ?
D = ?
The equilibrium shifts left

12. A + B 2C + D A = Increases B = Increases C = Decreases D = Decreases
Le Chatelier’s Principle…
A + B 2C + D
If we the pressure:
A = Increases
B = Increases
C = Decreases
D = Decreases

13. A + B 2C + D Le Chatelier’s Principle…
Increasing the pressure of a system (of a reaction in the gaseous state) causes the equilibrium to shift left in order to reduce the number of moles in the system, giving more reactants.

14. The equilibrium shifts left
Le Chatelier’s Principle…
A + B C + D
∆H = -812kJmol-1
If we increase the temperature:
A = ?
B = ?
C = ?
D = ?
The equilibrium shifts left

15. A + B C + D A = Increases B = Increases C = Decreases D = Decreases
Le Chatelier’s Principle…
A + B C + D
∆H = -812kJmol-1
If we the pressure:
A = Increases
B = Increases
C = Decreases
D = Decreases

16. A + B 2C + D Le Chatelier’s Principle… ∆H = -812kJmol-1
Increasing the temperature of a system causes the equilibrium to shift left to favour the ENDOthermic reaction and decrease the temperature of the reaction, giving more reactants.

17. The equilibrium does not move
Le Chatelier’s Principle…
A + B C + D
If we add a catalyst:
A = ?
B = ?
C = ?
D = ?
The equilibrium does not move

18. A + B C + D A = Stays constant B = Stays constant C = Stays constant
Le Chatelier’s Principle…
A + B C + D
If we add a catalyst:
A = Stays constant
B = Stays constant
C = Stays constant
D = Stays constant

19. A + B C + D Le Chatelier’s Principle…
Adding a catalyst increases the rate of reaction but does not affect the equilibrium position as both the forward and reverse reactions are sped up by the same amount.

20. Le Chatelier’s Principle…
How to work out what will have an effect:
Concentration will ALWAYS affect the equilibrium position.
Catalysts will NEVER affect the equilibrium position.

21. Le Chatelier’s Principle…
How to work out what will have an effect:
Are the compounds gases?
YES = pressure has an effect
NO = pressure has no effect

22. Le Chatelier’s Principle…
How to work out what will have an effect:
Are you given enthalpy data?
YES = temperature has an effect
NO = temperature has no effect