Section 8.4

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Section 8.4Le Chateliers Principle
How can we push a reaction to make more products?
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Le Chateliers Principle
Le Chateliers Principle If a system at
equilibrium is disturbed, it will shift to
re-establish equilibrium
A system will try to un-do whatever youve done
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Increasing Concentrations
Reaction shifts to right (get rid of extra
reactants and make more products)
Q becomes too small Theres too many reactants
Adding a reactant
Reaction shifts to left (get rid of extra
products and make more reactants)
Q becomes too large Theres too many products
Adding a product
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Decreasing Concentrations
Reaction shifts to left (make more reactants)
Q becomes too large Theres too few reactants
Removing a reactant
Reaction shifts to right (make more products)
Q becomes too small Theres too few products
Removing a product
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Changes in Pressure
Reaction shifts to the side with least moles of
gas to decrease pressure
Pressure increases
Decrease volume
Reactions shifts to the side with the most moles
of gas to increase pressure
Pressure decreases
Increase volume
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Endo Exothermic
Endothermic Reaction The reaction takes in
energythe products have more energy than the
reactants
Energy is a reactant in the reaction
Exothermic Reaction The reaction gives off
energythe products have less energy than the
reactants
Energy is a product in the reaction
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Changing temperatureEndothermic
Reaction shifts to right (get rid of extra
reactants and make more products)
Increase temperature of endothermic reaction
Increasing a reactant
Reaction shifts to left (make more reactants)
Decrease temperature of endothermic reaction
Remove a reactant
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Changing TemperatureExothermic
Reaction shifts to left (get rid of extra
products and make more reactants)
Increase temperature of exothermic reaction
Increasing a product
Reaction shifts to right (make more products)
Decrease temperature of exothermic reaction
Remove a product
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Some changes have no effect!

• Adding a pure solid or liquid reactant or product
• Theyre not in the equilibrium constant
  expression
• Increasing pressure by adding an inert gas
• Theyre not in the equilibrium constant
  expression
• Changing the volume of a reaction with an equal
  number of moles of gas on each side of the
  reaction
• The system wont gain anything by shifting since
  both sides will cause the same pressure
• Adding a catalyst
• A catalyst will speed up how fast equilibrium is
  establishedbut not the number of reactants and
  products once its at equilibrium

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Examples
Example Which way will the reaction shift for
each of the following changes
NH4Cl (s) ? NH3 (g) HCl (g)
Removing some NH4Cl Adding HCl Adding Ne
(g) Decreasing volume
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Examples
Example Which way will the reaction shift for
each of the following changes
NH4Cl (s) ? NH3 (g) HCl (g)
Removing some NH4Cl Adding HCl Adding Ne
(g) Decreasing volume
No change (its a solid)
(Adding a product)
No change (its an inert gas)
(Goes to side with least gas moles)
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Lets Practice
Which way will the reaction shift for each of the
following changes
2 SO2 (g) O2 (g) ? 2 SO3 (g) an exothermic
reaction
Increasing volume Raising temperature Adding
O2 Removing SO2
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Lets Practice
Which way will the reaction shift for each of the
following changes
2 SO2 (g) O2 (g) ? 2 SO3 (g) an exothermic
reaction
(Goes to side with most gas moles)
Increasing volume Raising temperature Adding
O2 Removing SO2
(Energy is a product)
(Adding a reactant)
(Removing a reactant)