Le Chteliers Principle

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Le Châteliers Principle

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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Le Châteliers Principle

• Reading Assignment
• Chang Chapter 14.5
• Dynamic equilibrium allows a system that is loses
  equilibrium to adjust to reestablish equilibrium.
• The response of an equilibrium system to a
  perturbing stress is described by Le Châtelier's
  Principle.

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Dynamic Equilibrium Allows a system that is
knocked out of equilibrium to re-establish
equilibrium. The response of an equilibrium
system to a stress is described by Le Châteliers
Principle.
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Perturbing a System at Equilibrium

• Addition or removal of reactant
• Addition or removal of product
• Addition of a non-reacting component with no
  change in volume
• Volume change
• Temperature change
• Le Châtelier's Principle

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Le Châtelier's Principle

• What will be the effect on the reactants and
  products as each of the following changes are
  made?
• Additional SO2 is added to the system.

initially up then down
down
up
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Le Châtelier's Principle

• What will be the effect on the reactants and
  products as each of the following changes are
  made?
• O2 is removed from the system.

initially down then up
up
down
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Le Châtelier's Principle

• What will be the effect on the reactants and
  products as each of the following changes are
  made?
• Additional SO3 is added to the system.

initially up then down
up
up
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Le Châtelier's Principle

• What will be the effect on the reactants and
  products as each of the following changes are
  made?
• Inert N2 is added to the system with no change in
  volume or temperature.

no change
no change
no change
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Le Châtelier's Principle

• What will be the effect on the reactants and
  products as each of the following changes are
  made?
• The volume of the system is increased with no
  change in temperature.

initially down then up
initially down then up
initially down then down
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Changing the Volume of aGaseous Equilibrium
System
high pressure small volumes
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Changing the Volume of aGaseous Equilibrium
System
less at larger volumes
more at larger volumes
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Changing the Volume of aGaseous Equilibrium
System
low pressure large volumes
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Pressure Effects

• Nitrogen Dioxide-Dinitrogen Tetroxide System

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Nitrogen Dioxide - Dinitrogen Tetroxide
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Nitrogen Dioxide - Dinitrogen Tetroxide
0 sec
1 sec
10 sec
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Nitrogen Dioxide - Dinitrogen TetroxideMass-Actio
n Expression
products
reactants
Why does the gas initially get darker and then
lighten?
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Temperature Effects

• Sodium thiosulfate dissolution
• Aqueous cobalt(II) ion and chloride ion
• equilibrium

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Sodium Thiosulfate Dissolution
RT water 22.3 ?C
Water Na2S2O3 18.0 ?C
Heated Water Na2S2O3
Is the dissolution reaction endothermic or
exothermic?
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Hexaaquacobalt(II) ion Tetrachlorocobaltate(II)
ion Equilibrium
Why does the equilibrium shift when heated?
Is the reaction endothermic or exothermic?
Is the equilibrium constant getting larger
or smaller?
cold
hot
pink
blue
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Gas Pressure Analysis
Expanding the volume of the system causes the
partial pressures of all the gases to decrease.
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Gas Pressure Analysis
As the volume increases, the percentage
reaction shifts toward the side with more total
moles of gas.
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Gas Pressure Analysis
If the equilibrium is shifted all the way RIGHT
or LEFT what would be the initial starting
conditions?
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Equivalent Starting Conditions

• This system could have been prepared by initially
  adding 5.72 atm of SO3 gas to the evacuated 1.00
  L cylinder, or
• by adding 5.72 atm of SO2 gas and 2.86 atm of O2
  gas to the evacuated 1.00 L cylinder, or
• several other possibilities.

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Gas Pressure Analysis
The stoichiometric relationships remain true SO2
and O2 ratio
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Moles of Gas Analysis
The stoichiometric relationships remain
true total moles of sulfur containing
compounds is constant.
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Moles of Gas Analysis
As the volume expands, the reactions shifts
left towards the side with more moles of gas.
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Mole Fraction Analysis
As the volume expands, the reactions shifts left.
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Mole Fraction Analysis
The stoichiometric relationships remain true SO2
and O2 ratio
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Please Pick Up

• Problem Set