1. Le Châtelier’s Principle
When a system is at equilibrium:
If any stress is placed on the system, equilibrium will shift to overcome the stress.
Stressors can be:
Adding products
Adding reactants
Changing temperature
Etc.

2. Le Châtelier’s Principle
To predict the shift, look at which side has gained something from the initial stress.
The reaction shifts to the side opposite the side that gained from the stress.

3. Le Châtelier’s Principle
How do you predict changes?
After the change, determine which side has more substance.
The reaction shifts to the opposite side.

4. Le Châtelier’s Principle
To shift a reaction to the reactant side ():
Add products
Remove reactants
If exothermic:
Raise temperature
If endothermic:
Lower temperature

5. Le Châtelier’s Principle
To shift a reaction to the product side ():
Add reactants
Remove products
If endothermic:
Raise temperature
If exothermic:
Lower temperature

6. Le Châtelier’s Principle
2SO2(g) + O2(g) ⇄ 2SO3(g)
∆H = -198 kJ
Predict how this reaction shifts when the following variables are changed:
Add O2
Add SO3
Raise the temperature
Shifts to the products
Shifts to the reactants

7. Special Note about Temperature Changes
Temperature changes also affect Keq values.
If a temperature change shifts the equilibrium to the right (), then Keq will INCREASE.
If a temperature change shifts the equilibrium to the left (), then Keq will DECREASE.

8. Le Châtelier’s Principle
Special Scenario for gases:
If pressure increases, the equilibrium shifts to the side with less total moles of gas.
If pressure decreases, the equilibrium shifts to the side with more total moles of gas.

9. Adding a Catalyst
No affect on equilibrium

10. Le Châtelier’s Principle
2SO2(g) + O2(g) ⇄ 2SO3(g)
∆H = -198 kJ
Predict how this reaction shifts when the following variables are changed:
Cool down the system
Increase the pressure
Remove SO3
Shifts to the products
Shifts to the products
Shifts to the products

11. Review 2SO2(g) + O2(g) ⇄ 2SO3(g) ∆H = -198 kJ
How does the reaction shift when:
SO2 is added
The temperature is raised
The volume is increased
A catalyst is added
Shifts to the products
Shifts to the reactants
Shifts to the reactants
No effect

12. Predicting Shift At Non-Equilibrium Conditions
If concentrations are not at equilibrium, the equilibrium constant is labeled as Q.
Compare the Kc value to the Q value to predict shift:
If Kc > Q, the equilibrium shifts to the product side.
If Kc < Q, the equilibrium shifts to the reactant side.
If Kc = Q, the system is at equilibrium.

13. Q & A
The reaction 2A + B ⇄ 3C has an equilibrium constant value of Kc = If you add the following solutions to a flask, which way will the reaction shift to reach equilibrium?
[A] = M
[B] = M
[C] = M
[C]3 Q =
[A]2
[B]
[0.200 M]3 Q =
[0.100 M]2
[0.200 M] Q =
4.00
Because Kc > Q, the reaction shifts to the products side.

14. More Q &A H2(g) + F2(g) ⇄ 2 HF(g) Kc = 115
What is the value of Kc for the reverse reaction?
If the equation was written as
2H2(g) + 2F2(g) ⇄ 4 HF(g)
what would the value of Kc be?
[HF]2
[H2]
[F2] 1 If
115 =
Then = =
8.70  10-3
[H2]
[F2]
[HF]2
115
[HF]2
[HF]4 If
115 =
Then =
(115)2 =
13200
[H2]
[F2]
[H2]2
[F2]2