Le Chatelier's Principle

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Le Chatelier's Principle

• Chapter 17 sections 17.8-17.10

2
Review

• What is equilibrium?
• What is an equilibrium constant?
• Describe the demo with the two gases, what
  happened when we change the temperature of the
  gas.

3
Le Chateliers Principle

• Scientists like to be able to control equilibria.
• For example when manufacturing Ammonia (N2 3H2
  ? 2NH3) they definitely want the equilibria to
  tend more toward the product side of the reaction
• So they can control the shift of equilibrium in a
  chemical reaction

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Le Chateliers Principle cont

• Scientists discovered how various changes in
  conditions affect the equilibrium position of a
  reaction system
• They use Le Chateliers principle
• When a change is imposed on a system at
  equilibrium, the position of equilibrium shifts
  in a direction that tends to reduce the effect of
  the change

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Le Chateliers Principle cont

• There are three things that can effect
  equilibrium
• Concentration changing the concentration of one
  of the components of the reaction
• Volume changing the pressure on the system
• Temperature changing the temperature at which
  the reaction is run.

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The effect of change of concentration

• Lets go back to the ammonia synthesis reaction
• N2 (g) 3H2 (g) ?? 2NH3 (g)
• Lets say I have .399 M of N2, 1.197 M of H2 and
  .203 M of NH3.
• What would happen if we inject 1.000 M of N2 into
  the reaction?
• Remember at equilibrium the rates of the forward
  and reverse reaction EXACTLY balance

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The effect of change of concentration cont

• N2 (g) 3H2 (g) ?? 2NH3 (g)
• So when a lot of N2 is added, there are suddenly
  more collisions between the N2 and H2 molecules.
• This makes the rate of the forward reaction go
  WAY up
• N2 (g) 3H2 (g) ?? 2NH3 (g)

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8
The effect of change of concentration cont

1. Initial equilibrium mixture.
2. Addition of N2.
3. New equilibrium position.

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The effect of change of concentration cont

• At equilibrium 1 N2 .399 M, H2 1.197 M, and
  NH3 .203 M
• At equilibrium 2 N2 .1.348 M, H2 1.044 M,
  and NH3 .304 M
• Note the H2 went down and the NH3 went up. (and
  of course the N2 went up, we increased it) so the
  equilibrium shifted to the right!
• Find the K for each of these
• Answer both are the same .0602!

10
The effect of change of concentration cont

• If ammonia had been added instead of Nitrogen
  then the system would have shifted to the left
• So we can say Le Chateliers principle also
  states
• When a reactant or product is added to a system
  at equilibrium, the system shifts ways from that
  added component.
• Or
• When a reactant or product is removed from a
  system the system shifts toward the removed
  component.
• If we would have taken nitrogen away the amount
  of ammonia would have been reduced!

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The effect of change of concentration cont

• Example
• 2SO2 (g) O2 (g) ?? 2SO3 (g)
• Which way would the reaction shift if I
• Added SO2?
• Shifts to the right
• Added SO3?
• Shifts to left
• Removed O2?
• Shifts to left

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The effect of change of volume

• When the volume of a gas is decreased the
  pressure goes up.
• This leads to more collisions
• So in order to elevate pressure (or collisions)
  according to Le Chateliers Principle, the system
  will shift to reduce pressure

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The effect of change of volume cont

• Lets look at
• CaCO3 (s) ?? CaO (s) CO2 (g)
• If this reaction took place in a container when I
  could increase the pressure, This would cause the
  CO2 molecules to collide more often, get used up
  and shift to the left
• The system shifts in the direction that leads to
  a smaller number of gas molecules

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System initially at equilibrium.
15
Piston is pushed in.
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The effect of change of volume cont

• Lets go back to
• N2 (g) 3H2 (g) ?? 2NH3 (g)
• These are all gases!
• But lets look There are 4 gas molecules on the
  right and only two on the left
• So the system will shift to the right, because it
  has lass gas!

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The effect of change of volume cont

1. A mixture of NH3(g), N2(g), and H2(g) at
   equilibrium.
2. The volume is suddenly decreased.
3. The new equilibrium position.

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The effect of change of volume cont

• Lets try one
• Predict what would happen if I increased the
  pressure on the following reaction
• P4 (s) 6 Cl2 (g) ?? 4 PCl3 (l)
• Answer Shifts to the right
• What would happen if I decreased pressure
• Answer Shifts to left

19
The effect of change of temperature

• It is important to remember that the previous two
  examples of shifting equilibrium alter the
  position of equilibrium NOT the equilibrium
  constant (K)
• Temperature is different
• Because the value of K changes with temperature
• So we use Le Chateliers principle to predict
  changes in K

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The effect of change of temperature cont

• We first need to decide whether a reaction is
  exothermic (produced energy) or endothermic
  (needs energy)
• N2 (g) 3H2 (g) ?? 2NH3 (g) 92 KJ
• This reaction will produce energy (exothermic)
• CaCO3 (s) 556 kJ ?? CaO (s) CO2 (g)
• This reaction needs energy (endothermic)

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The effect of change of temperature cont

• For Le Chateliers principle you simply need to
  treat heat (or energy) as a reactant or product
  to predict which way a reaction will shift
• N2 (g) 3H2 (g) ?? 2NH3 (g) 92 KJ
• If I add heat this reaction will shift to the left

h
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The effect of change of temperature cont

• Example
• Predict which way the reaction will shift
• C2H2 (g) 2Br2 (g) ?? C2H2Br4 (g) exothermic
• If I added heat
• It would shift to the left
• If I took away heat
• It would shift to right
• http//www.mhhe.com/physsci/chemistry/essentialche
  mistry/flash/lechv17.swf

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Applications using K

• You know how to find K, but you can actually tell
  a lot from what number K is.
• If K is larger than 1.0 it means at equilibrium
  the reaction will have more products than
  reactants
• Example
• A (g) ?? B (g)
• K B/ A and K 10,000
• That means that the concentration of B is 10,000
  times greater than A

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Applications using K cont

• On the other hand if K is less than 1 that means
  there are more reactants than products
• We also can use K to calculate the concentrations
  of products and reactants.

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Applications using K cont

• Example
• PCl5 (g) ?? PCl3 (g) Cl2 (g)
• If I know that K .0896 , and concentration of
  PCl5 .00670 M and PCl3 .300 M, can I
  calculate the concentration of Cl2?
• K PCl3Cl2
• PCl5
• So plug in the number you know
• .0896 .300 Cl2
• .00670 M
• Solve for Cl2
• Answer .00200 M

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Solubility Equilibria

• Solubility is a very important concept in
  chemistry
• Remember that certain ionic solids disassociate
  in water to form ions
• We can actually write their dissolving as a
  chemical reaction
• NaCl (s) ? Na1 (aq) Cl1- (aq)

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Solubility Equilibria cont

• CaF2 (s) ? Ca2 (aq) 2 F1- (aq)
• However when we first add the salt no ions are
  present (the need to dissociate)
• After a lot of them form ions the ions start to
  collide and reform a solid
• Ca2 (aq) 2 F1- (aq) ? CaF2 (s)
• Ultimately equilibrium is reached, no more solid
  dissolves and we have a saturated solution

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Solubility Equilibria cont

• We can write the Ksp (solubility constant) as
  such
• Ksp Ca2F-2
• We of course left out the solid, because we
  cannot calculate the concentration of a pure solid

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Solubility Equilibria cont

• Write a balanced equation for dissolving the
  following salt in water
• PbCl2
• First write out the equation
• PbCl2 (s) ? Pb2 (aq) 2Cl- (aq)
• Then write the Ksp, leaving out any pure solids
• Ksp Pb2 Cl-2

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Solubility Equilibria cont

• Of course you can also plug in values and
  ACTUALLY calculate the Ksp
• But I will actually just let you read about that!

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Homework

• Read pages 554-567
• Problems 1-4 on 567, and problems 37 and 42 on 572