Le Chatelier's

1
Le Chatelier's Principle
2
What if the conditions of the equilibrium changed?

• Le Chateliers principle states that if a closed
  system at equilibrium is subjected to a
  change(stress), things will happen to counteract
  that change (regain back balance).

3
The effect of Temperature changes

• Temperature affects heat terms in a reaction.
• If you increase temperature, you are increasing
  the heat term(energy). Therefore, the reaction
  will shift to use up this extra heat/energy and
  go back to original state.

4
The Effect of Temperature Changes A
B ? C Energy
Change increase Temp
reaction- shifts left
5
The Effect of Temperature Changes A
B Energy ? C
Change increase Temp
reaction- shifts right
6
The Effect of Temperature Changes A
B Energy ? C
Change decrease Temp
reaction- shifts left
7
The Effect of Concentration Changes
8
Adding a reactant or product shifts the reaction
in the direction that removes them/uses them
up. A B ? C Energy
reaction- shifts right
change- increase A
9
Adding a reactant or product shifts in the
direction that removes them. A
B ? C Energy
stress- increase C
reaction- shifts left
10
Removing a reactant or product shifts reaction to
make MORE. A B ? C
Energy
change- decrease A
reaction- shifts left
11
Removing a reactant or product shifts reaction to
make more. A B ? C
Energy
change- decrease C
reaction- shifts right
12
Effect of pressure changes
13
Usually we refer only to gases(g) when we talk
about pressure changes.Those pressure changes
we control by changing the volume.
A(g) B(g) ? AB(g)
2 mols
1 mol
more pressure less pressure
14
As you know

• If you decrease the volume, you increase the
  pressure.
• If you increase the volume, you decrease the
  pressure.
• Also, increasing pressure increases the
  concentrations of ALL gaseous species found.

15
Where to start?

• First identify where there is more pressure or
  where there is less pressure by counting up the
  molecules of gas you have on each side.

16
4HCl(g) 2O2(g) ? 2H2O(l) 2Cl2(g)
6 mols
2 mols
less pressure
more pressure
If we decreased the volume, pressure will
________ and so the reaction will go to low
pressure side.
17
2SO3(g) ? 2SO2(g) O2(g)
2 gas
3 gas
more pressure
less pressure
If we increased the volume, pressure will
_________, so the reaction responds
by increasing pressure again and goes to more
pressure side
18
Increasing the pressure by decreasing the volume
shifts to the eqlm fewest gas molecules. Only (g)
count for gas particles. 4HCl (g) 2O2
(g) ? 2H2O (l) 2Cl2 (g)
98kJ  
6 2
 We decrease the volume, we increase pressure!
The reaction responds by shifting to right to
decrease the pressure!
All concentrations go up!
19
5A(g) 2B(g) ? 4C(g) 5D(g)
7 9
We ? P
We ? V
The Reaction opposes and goes to low pressure
side? P
Shifts left
20
4A(g) 2B(g) ? 3C(g) 2D(g)
6 5
What volume change shifts right?
We ? P
We ? V
21
2A(g) 2B(g) ? 3C(g) 4D(g)
4 7
What volume change shifts right?
We ? P
We ? V
22
4HCl (g) 2O2 (g) ?
2H2O (l) 2Cl2 (g)
98kJ Describe the shift in the above
equilibrium system for each stress indicated.
Stress Shift HCl O2 Cl2

? HCl
?
?
?
?
? O2
?
?
?
?
?
?
?
?
Add water
?
?
?
?
? Cl2
?
?
?
?
? V
When the volume increases, pressure decreases and
all concentration go down
23
Adding a catalyst does not shift the equilibrium
Forward and reverse rates are increased by the
same amount
Equilibrium is attained faster
reactants
products
24
Adding an inert (non-reactive) gas does not shift
the equilibrium.
2NH3(g) ? N2(g) 3H2(g)
Adding Ne(g)
No shift!
25
Homework

• Need this to be done by next class
• Exercises 17-23 on page 54.
• Exercises 24-26, do NOT graph, only state what
  each change/stress would do to the equilibrium
  (ie where it shifts it)
• Have a fan-ta-bulous weekend!