1. Chemical Equilbrium & Le Châtelier’s Principle

2. Le Châtelier’s Principle
Le Châtelier’s principle states that when a system at equilibrium is disturbed, the system adjusts in a way to reduce the change.
Stress causes a change in a system at equilibrium

3. Types of Stress Three kinds of stress:
When a stress is first applied to a system, equilibrium is disturbed and the rates of the forward and backward reactions are no longer equal.
Three kinds of stress:
changes in the concentrations of reactants or products
changes in pressure
changes in temperature

4. Changes in Concentration
If more of a substance is added to a reaction at equilibrium, the system will shift to remove the excess amount of that substance
If a substance is taken out of a reaction at equilibrium, the system will shift to produce more of the substance that was removed

5. Pencil’s down

6. Changes in Concentration
Pencils down
In a reaction of two colored complex ions:
pale blue blue-purple
When the reaction mixture in a beaker is pale blue, we know that chemical equilibrium favors the formation of reactants.
**If ammonia is added, the system responds by forming more product and the solution becomes blue-purple.

7. Pencils down
Predicting the Effect of a Change in Concentration on the Equil Position
Which way would the following equilibria shift in order to re-establish equilibrium:
1. CO(g) + Cl2(g) COCl2(g)
a. Remove COCl2
Answer:
b. Add chlorine gas
2. 2PbS(s) + 3O2(g) PbO(s) + 2SO2(g)
a. Remove O2
b. Add PbS
c. Add SO2
3. 2H 2S (g) + O2(g) S(s) + 2H2O (g)
a. Add sulfur
Answer:
b. Remove water vapor
c. Remove H2S

8. Pressure and Volume Changes
Reaction shifts to change the equilibrium position (concentrations), not the equilibrium constant.
A (g) + B (g) C (g)
Shifts the Equilibrium
Change
↑ pressure
Side with fewest moles of gas
↓ volume
Side with fewest moles of gas
↓ pressure
Side with most moles of gas
↑ volume
Side with most moles of gas

9. Pressure Changes cont’d
Pencils down
N2(g) +3 H2(g) <=> 2 NH3(g)
(a) A mixture of gaseous N2, H2, and NH3 at equilibrium.
(b) Predict what happens when the pressure is increased.
(c) Reaction occurs from left to right, decreasing the total number of gaseous molecules until equilibrium is re-established
(d) Which molecule would end up with the highest concentration?

10. An equilibrium reaction that has the same # of moles of gas on
both sides will not be affected by changes in pressure.

11. Predicting the Effect of a Change in Pressure on the Equilibrium Position
Pencils down
PROBLEM:
How would you change the pressure of each of the following reactions to increase the yield of products?
(a) CaCO3(s) CaO(s) + CO2(g)
(b) S(s) + 3F2(g) SF6(g)
(c) Cl2(g) + I2(g) ICl(g)
PLAN:
When gases are present a change in pressure will affect the concentration of the gas. If the pressure increases, the reaction will shift to fewer moles of gas and vice versa.
SOLUTION:

12. ***Temp—only factor that affects the value of Keq
Temperature Changes
H2(g) + I2(g) HI(g) + heat
Change
Exothermic Rx
Endothermic Rx
↑ temperature
Keq ↓ (less products)
Keq ↑
↓ temperature
Keq ↑ (more products)
Keq ↓
***Temp—only factor that affects the value of Keq

13. Predicting the Effect of a Change in Temperature on the Equilibrium Position
Pencils down
PROBLEM:
How does an increase in temperature affect the concentration of the underlined substance and Keq for the following reactions?
(a) CaO(s) + H2O(l) Ca(OH)2(aq) DH0 = -82kJ
(b) SO2(g) S(s) + O2(g) DH0 = 297kJ
PLAN:
Express the heat of reaction as a reactant or a product. Then consider the increase in temperature and its effect on Keq.
SOLUTION:
(a) CaO(s) + H2O(l) Ca(OH)2(aq)
(b) SO2(g) S(s) + O2(g)

14. Le Châtelier’s Principle
Change Equilibrium
Constant
Change
Shift Equilibrium
Concentration
yes no
Pressure
yes no
Volume
yes no
Temperature
yes
yes