Transcript Le Chatelier

Chemical Equilbrium &
Le Châtelier’s Principle
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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
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Types of Stress
Three kinds of stress:
 changes in the concentrations of reactants or products
 changes in pressure
 changes in temperature

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.
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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
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Pencil’s down
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Changes in Concentration
Pencils down
• In a reaction of two colored complex ions:
2


[Cu(H2O)4 ]2 (aq)  4NH3 (aq ) 
[Cu(NH
)
]
(aq )  4H2O( l )

3 4
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.
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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
3. 2H 2S (g) + O2(g)
2S(s) + 2H2O (g)
Answer:
a. Add sulfur
b. Add chlorine gas
Answer:
Answer:
2. 2PbS(s) + 3O2(g)
2PbO(s) + 2SO2(g) b. Remove water vapor
a. Remove O2
Answer:
Answer:
c. Remove H2S
b. Add PbS
Answer:
Answer:
c. Add SO2
Answer:
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Pressure and Volume Changes
Reaction
shifts to change the equilibrium position
(concentrations), not the equilibrium constant.
A (g) + B (g)
Change
C (g)
Shifts the Equilibrium
↑ pressure
Side with fewest moles of gas
↓ volume
Side with fewest moles of gas
↓ pressure
Side with most moles of gas
Side with most moles of gas
↑ volume
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 reestablished
(d) Which molecule would end up with the highest
concentration?
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An equilibrium reaction that has the same # of moles of gas on
both sides will not be affected by changes in pressure.
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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)
PLAN:
CaO(s) + CO2(g)
(b) S(s) + 3F2(g)
SF6(g)
(c) Cl2(g) + I2(g)
2ICl(g)
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:
Temperature Changes
H2(g) + I2(g)
Change
↑ temperature
↓ temperature
2 HI(g) + heat
Exothermic Rx
Keq ↓ (less products)
Keq ↑ (more products)
Endothermic Rx
Keq ↑
Keq ↓
***Temp—only factor that affects the
value of Keq
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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)
(b) SO2(g)
PLAN:
Ca(OH)2(aq)
S(s) + O2(g)
DH0 = -82kJ
DH0 = 297kJ
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)
(b) SO2(g)
Ca(OH)2(aq)
S(s) + O2(g)
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Le Châtelier’s Principle
Change
Shift Equilibrium
Change Equilibrium
Constant
Concentration
yes
no
Pressure
yes
no
Volume
yes
no
Temperature
yes
yes
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