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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
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

 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
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Example: Freon-12 (chlorofluorocarbon)
CCl
4(g)
+ 2 HF
(g)
⇋
CCl
2
F
2(g)
+ 2 HCl
(g)
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
Energy in a chemical equilibrium equation can be treated as though it were a reactant or a product
Endothermic Rxn: reactants + energy
⇋ products
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 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
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Example:
2 SO
2(g)
+ O
2(g)
⇋
2 SO
3(g)
+ energy
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
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: N
2(g)
+ 3 H
2(g)
⇋
2 NH
3(g)
(1 mol + 3 mol)
4 mol 2 mol
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


 if the if the volume is increased (decrease in pressure) equilibrium shifts toward the side with the larger total amount of gaseous entities 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
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N
2(g)
+ 3 H
2(g)
⇋
2 NH
3(g)
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
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
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
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
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The Reaction
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Le Châtelier's Principle
(worksheets)
AND p. 457 Practice
UC # 1, 2, 3, 4 and 5
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⇋
Stress Change
Add
CO
2(g)
Remove
CO
2(g)
Add
CO
(g)
Reason [CO
2(g)
] [CO
(g)
] [O
2(g)
]
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