1. Le Chatelier’s Principle
When a system at equilibrium is stressed by changes in its environment, the effects can be predicted using…..
Le Chatelier’s Principle

2. Equilibrium is reestablished by shifting to one side or the other side of the reaction.
Shift right/to the products  forward reaction
has a faster rate
A + B ↔ C + D
C and D (products) will increase
A and B (reactants) will decrease

3. Rates will remain unequal until equilibrium is reestablished
2. Shift left/to the reactants  reverse reaction
has a faster rate
A + B ↔ C + D
C and D (products) will decrease
A and B (reactants) will increase
Rates will remain unequal until equilibrium is reestablished

4. Factors that Affect Equilibrium
Concentration
Temperature
Pressure
For gaseous systems only!
Only when there is an unequal number of moles of gas on each side of the equation.

5. Concentration Changes
Add more reactant  Shift to products
Remove reactants  Shift to reactants
Add more product  Shift to reactants
Remove products  Shift to products

6. Temperature Changes Exothermic Reactions
Consider heat as a product in exothermic reactions.
Add heat 
Shift to reactants
Remove heat 
Shift to products
A + B  AB + Heat

7. Temperature Changes Endothermic Reactions
Consider heat as a reactant in endothermic reactions.
Add heat 
Shift to products
Remove heat 
Shift to reactants
A + B + heat  AB

8. Pressure Changes
Only affects equilibrium systems with unequal moles of gaseous reactants and products.

9. N2(g) + 3H2(g)  2NH3(g)
Increase Pressure (as a result of decreased volume)
There are 4 molecules of reactants vs. 2 molecules of products.
The reaction shifts to the product

10. PCl5(g)  PCl3(g) + Cl2(g)
Decrease Pressure (as a result of increased volume)
There are two product gas molecules vs. one reactant gas molecule.
The reaction shifts to the products.

11. Factors that do NOT affect Equilibrium
1. Catalyst -
Speeds up the reaction rate of both the
forward and reverse reactions equivalently,
therefore it has NO effect on equilibrium
Inert substance (ex. noble gas) -
Do not affect the rate of either reaction,

12. S8(g) + 12O2(g)  8 SO3(g) + HEAT Oxygen gas is added?
Given:
S8(g) O2(g)  8 SO3(g) + HEAT
What will happen when ……
Oxygen gas is added?
The reaction vessel is cooled?
The size of the container is increased?
Sulfur trioxide is removed?
A catalyst is added to make it faster?
Shifts to products 
Shifts to Products – to replace heat
V increases, Pressure decreases, shifts to more moles of gas – to reactants!
Shift to products to replace it!
No change!

13. When pressure is increased by decreasing the volume?
Given
N2(g) + O2(g) + HEAT  2NO(g)
What will happen
When pressure is increased by decreasing the volume?
To the amount of O2 if NO is removed?
To the amount of NO if heat is added?
What can be done to decrease the amount of N2?
no change b/c # of moles is the same
decrease
increase
remove NO, add O2, add heat