Le Chateliers Principle

1
Le Chateliers Principle

• Chem 12
• Chapter 13, Pg 526-532

2
LeChateliers Principle

• When a system at equilibrium is placed under
  stress, the system will undergo a change in such
  a way as to relieve that stress.

3
Le Chatelier Translated

• When you take something away from a system at
  equilibrium, the system shifts in such a way as
  to replace what youve taken away.
• When you add something to a system at
  equilibrium, the system shifts in such a way as
  to use up what youve added.

4
Le Chatelier Example 1
A closed container of ice and water at
equilibrium. The temperature is raised.
Ice Energy ? Water
The equilibrium of the system shifts to the
_______ to use up the added energy.
right
5
Le Chatelier Example 2
A closed container of N2O4 and NO2 at
equilibrium. NO2 is added to the container.
N2O4 (g) Energy ? 2 NO2 (g)
The equilibrium of the system shifts to the
_______ to use up the added NO2.
left
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Le Chatelier Example 3
A closed container of water and its vapor at
equilibrium. Vapor is removed from the system.
water Energy ? vapor
The equilibrium of the system shifts to the
_______ to replace the vapor.
right
7
Le Chatelier Example 4
A closed container of N2O4 and NO2 at
equilibrium. The pressure is increased.
N2O4 (g) Energy ? 2 NO2 (g)
The equilibrium of the system shifts to the
_______ to lower the pressure, because there are
fewer moles of gas on that side of the equation.
left
8
Pressure Changes to system

• If the volume decreases, the concentration
  increases, and there will be a shift to the side
  with the less amount of moles.
• If the volume increases, the concentration
  decreases, and there will be a shift to the side
  with the more amount of moles.

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Example

• If I increase the pressure, where is the shift?
• (right)
• If I decrease the pressure, where is the shift?
  (left)
• 2SO2 O2 lt--gt 2SO3
• (3moles) (2moles)

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Effect of Concentration

• If you add more reactant, it shifts to the right
  increasing the formation of product, using up the
  reactants.
• If you add product, it shifts to the left
• If you remove product, it shifts to the right,
  increasing the formation of product.
• If you remove reactant, it shifts to the left

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Effect of temperature

• Energy is treated as a reactant if endothermic
  equation, and as a product if exothermic
  equation.
• If cooling a system, then it shifts so more heat
  is produced.
• If heating a system, the shifts so extra heat is
  used up.

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Example for temp. changes for Endothermic Reaction

• Heating the below reaction causes the system to
  shift to the right more products, because you
  treat energy like a reactant.
• 2NaCl H2SO4 energy lt -- gt 2HCl Na2SO4
• Cooling the above reaction causes the system to
  shift to the left less reactants, so need to
  make up more

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Effect of temp change on exothermic reactions

• Heating the below reaction causes the system to
  shift to the left, to use up the extra heat.
• 2SO2 O2 lt--gt 2SO3 energy
• Cooling the above reaction causes the system to
  shift to the right, to make up for the lost heat.

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Homework

• Page 529 33-37