1. Le Chatelier’s Principle
Concentration and Temperature

2. Day 1: Le Chatelier’s Principle
If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.
Stresses include:
Change in concentration
Change in volume/pressure
Change in temperature

3. Le Chatelier Change in Concentration
PCl5(g) ⇆ PCl3(g) + Cl2(g) ΔH=-47kJ

4. Warm up #46 Ch.18B left left right right left right
Given PCl5(g) PCl3(g)+ Cl2(g) + heat
Which way will equilibrium shift given these stresses
Remove PCl5
Add heat
Increase PCl5
Decrease heat
Add Cl2
Remove PCl3
left
left
right
right
left
right

5. Day 2: Le Chatelier’s Principle Change in Volume/Pressure
Volume and pressure are related. As the volume goes up the pressure goes down and vice versa.
(Boyles Law; P1V1=P2V2)
Only gases are affected by a change in volume/pressure
You need to count the number of moles of gas on each side of the reaction
If you add a catalyst, it does not make the reaction favor one direction more than another. All it does is makes the reaction rate increase, which makes the reaction go to equilibrium faster.

6. 2H2O (g) ⇆ 2 H2(g) + O2(g)
The left side has 2 molecules of gas, while the right side has 3 molecules of gas
If the stress is pressure increasing, shift toward the side with the least number of moles of gas.
If the stress is pressure decreasing, shift toward the side with the most number of moles of gas.

7. Le Chatelier and Pressure - YouTube

8. Change in Volume/Pressure
Stress
PCl5(g) 
PCl3(g)
Cl2(g)
heat
 pressure
 volume
 heat
catalyst
H2(g)
I2(g)
2HI(g)
 pressure
 volume
 heat

9. Practice Stress CO(g) H2 (g)  CH4(g) H2O(g) pressure  volume  heat
 volume
 heat
catalyst
N2O4(g)
heat
2NO(g)
 pressure
 volume
 heat
2HBr(g)
B2(g)
H2(g)