1. Chemical Systems and Equilibrium
- Le Châtelier's Principle -

2. Le Châtelier’s Principle (pp. 450-456)
When a chemical system is disturbed by a change in property the system adjusts in a way that opposes the change – an equilibrium shift.
What is an equilibrium shift?
Equilibrium shift is the movement of a system at equilibrium resulting in a change in the concentration of reactants and products

3. Le Châtelier’s Principle
According to Le Châtelier’s principle, an equilibrium shift always OPPOSES the change in the system

4. Le Châtelier's Principle and Concentration Changes
predicts that if more reactant or product is added to a system at equilibrium, then that system will undergo an equilibrium shift
increasing the concentration of reactant or product will cause the equilibrium to shift to consume some of the added reactant or product
decreasing the concentration of reactant or product will cause the equilibrium to shift to replace some of the removed reactant or product

5. Le Châtelier's Principle and Concentration Changes
Example: Freon-12 (chlorofluorocarbon)
CCl4(g) + 2 HF(g) CCl2F2(g) + 2 HCl(g) ⇋

8. Le Châtelier's Principle and Temperature Changes
Energy in a chemical equilibrium equation can be treated as though it were a reactant or a product
Endothermic Rxn: reactants + energy products
Exothermic Rxn: reactants products + energy ⇋ ⇋

9. Le Châtelier's Principle and Temperature Changes
energy can be added to or removed from a system by heating or cooling the container
equilibrium will shift to minimize change
increase in temperature the system shifts to consume some of the added thermal energy
decrease in temperature the system shifts to replace some of the removed thermal energy

10. Le Châtelier's Principle and Temperature Changes
Example: 2 SO2(g) + O2(g) 2 SO3(g) + energy ⇋

11. Le Châtelier's Principle and Gas Volume Changes
Boyle’s Law, the concentration of a gas in a container is directly related to the pressure of the gas (i.e. decreasing the volume to half its original value, doubles the concentration of every gas in the container)
always consider the total number of moles of gas reactants and the total number of moles of gas products
Example: N2(g) + 3 H2(g) NH3(g)
(1 mol + 3 mol) mol mol ⇋

12. Le Châtelier's Principle and Gas Volume Changes
if the volume is increased (decrease in pressure) equilibrium shifts toward the side with the larger total amount of gaseous entities
if the volume is decreased (increase in pressure) equilibrium shifts towards the side with the smaller total amount of gaseous entities
system with equal number of gas molecules on each side of the equation will not shift after a change in volume
systems involving only liquids and solids are not affected appreciably – they are virtually incompressible – not affected by pressure

13. Le Châtelier's Principle and Gas Volume Changes⇋
N2(g) + 3 H2(g) NH3(g)

15. Changes That Do Not Affect the Position of Equilibrium Systems
Adding Catalysts
a catalyst decreases the time required to reach the equilibrium position, but does not affect the final position of equilibrium
lowers activation energy for both forward and reverse reactions by an equal amount – therefore occurs faster but equilibrium establishes in the same position as it would without the catalyst

18. Changes That Do Not Affect the Position of Equilibrium Systems
Adding Inert Gases
If a noble gas or a gas that cannot react with the entities in the system, the equilibrium position of the system will not change

19. Le Châtelier’s Principle at a High School Dance
The Reaction
MALE
FEMALE

20. (1) Is this system open or closed?

21. (2) What “reactants” and “products” are in equilibrium?

22. (3a) What would happen if a carload of boys from another school arrives? (addition of reactants)

23. (3b) What effects would many of the couples go outside to…
(3b) What effects would many of the couples go outside to…??? so dance floor is less crowded (removal of products)

24. (3c) What would happen if thermostat in the gym gets stuck and the gym gets very hot? (addition of heat)

25. (3d) What would happen if part of the gym is closed off
(3d) What would happen if part of the gym is closed off? (decrease in volume)

26. (3e) What would happen if a non-reacting species is added such as cows
(3e) What would happen if a non-reacting species is added such as cows? Nuns? or VP’s (increase in internal pressure).

27. Learning Checkpoint
Le Châtelier's Principle (worksheets) AND p. 457 Practice UC # 1, 2, 3, 4 and 5

28. 2 CO2(g) 2 CO(g) + O2(g) ∆H = -281 kJ⇋
2 CO2(g) CO(g) + O2(g) ∆H = -281 kJ
Stress
Change
Reason
[CO2(g)]
[CO(g)]
[O2(g)]
Add CO2(g)
Remove CO2(g)
Add CO(g)