Unit 17
Daily 2
What is Le Chatelier’s Principle?
Explore factors affecting equilibrium
2. Predict direction of equilibrium shift
3. Predict resulting changes in concentration
4. Make equilibrium serve your own evil purposes
1.
• Equilibrium is a RATE balance.
• Anything that can affect the rate of the forward or
reverse reaction can throw equilibrium off.
• What are the factors that affect reaction rate?
1)
2)
3)
4)
5)
Concentration
Pressure or Volume (gases)
Temperature
Surface Area
Catalysts/Inhibitors
 “If a system at equilibrium is subjected to an external
stress, the equilibrium will shift to minimize the
effects of that stress”
 The Steps of Le Chatelier’s Principle:
Stress: Adding or removing a reactant, adding or
removing a product, changing the pressure, volume, or
temperature.
2) Shift: The equilibrium will shift away from what is
added and towards what is removed.
3) Change in Concentration: The concentration of what is
being shifted towards increases, the concentration of
what is being shifted away from decreases.
1)
Fe3+(aq) + SCN-(aq)  FeSCN2+(aq)
(colorless)
(red)
If more Fe3+ is added to the system, what will happen?
 equilibrium shifts to the right
 equilibrium shifts to the product side
 the forward reaction is favored (forward shift)
How are concentrations affected?
 [Fe3+] increases, [SCN-] decreases, [FeSCN2+] increases
How is Keq affected?
 Keq does not change when changes in concentration cause a
shift in equilibrium
Fe3+(aq) + SCN-(aq)  FeSCN2+(aq)
(colorless)
(red)
If more FeSCN2+ is added to the system, what will
happen?
 equilibrium shifts to the left
 equilibrium shifts to the reactant side
 the reverse reaction is favored (reverse shift)
How are concentrations affected?
 [Fe3+] increases, [SCN-] increases, [FeSCN2+] increases
Fe3+(aq) + SCN-(aq)  FeSCN2+(aq)
(colorless)
(red)
If SCN- is removed from the system (by adding AgNO3 so
that AgSCN(s) precipitate forms), what will happen?
 equilibrium shifts to the left
 equilibrium shifts to the reactant side
 the reverse reaction is favored (reverse shift)
How are concentrations affected?
 [Fe3+] increases, [SCN-] increases (but also decreases
since it was removed), [FeSCN2+] decreases
 Only affect an equilibrium system if gases are present
 What is the relationship between moles of gas and
volume/pressure?
 P = (nRT)/(V)
 If pressure is increased (or volume decreased):
 equilibrium will shift towards the side with FEWER
moles of gas to bring the pressure back down
 If pressure is decreased (or volume increased):
 equilibrium will shift towards the side with MORE
moles of gas to bring the pressure back up
N2(g) + 3H2(g)  2NH3(g)
(4 moles)
(2 moles)
If pressure is increased (volume decreased) on the
system, what will happen?
 equilibrium shifts to the right
 equilibrium shifts to the product side
 the forward reaction is favored (forward shift)
How are concentrations affected?
 [N2] decreases, [H2] decreases, [NH3] increases
Notes:
 If the number of moles of gas are equal on both sides,
then the equilibrium can’t shift
H2(g) + I2(g)  2HI(g)
 Make sure to only count the moles of GAS when
considering pressure effects on equilibrium
If temperature is increased in the system, what will
happen?
 equilibrium will shift in the endothermic direction
 the endothermic direction is away from the side with
heat
If temperature is decreased in the system, what will
happen?
 equilibrium will shift in the exothermic direction
 the exothermic direction is towards the side with heat
H2O
H
OH
H2O
H
H
OH
Which way does the see saw have to shift to reach equilibrium again?
H2O
H
H
OH
Which way does the see saw have to shift to reach equilibrium again?
SHIFT LEFT
H
H2O
H
More H2O is produced due to the shift in equilibrium
OH
H2
H2
N2
H2
NH3
N2 (g) + 3H2 (g)  2NH3 (g)
NH3
SHIFT RIGHT
H2
N2
H2
NH3
NH3
H2
SHIFT will occur to side with the least number of MOLES
N2 (g) + 3H2 (g)  2NH3 (g)
H2
H2
N2
H2
NH3
N2 (g) + 3H2 (g)  2NH3 (g)
NH3
SHIFT RIGHT
H2
N2
H2
NH3
NH3
H2
SHIFT will occur to side with the least number of MOLES
N2 (g) + 3H2 (g)  2NH3 (g)
 Depends if reaction is Endothermic or Exothermic.
 Endothermic: Heat on LEFT
 Exothermic: Heat on RIGHT
HEAT
A
B
C
D
 Increase Temperature… which way will the see saw tip?
A
B
HEAT
C
Which way will the reaction shift?
D
A
HEAT
B
C
Reaction will SHIFT LEFT
D
HEAT
A
B
C
D
DecreaseTemperature… which way will the see saw tip?
HEAT
C
D
B
A
Which way will the reaction shift?
HEAT
C
B
A
Reaction will SHIFT Right
D
HEAT
A
B
C
D
 Increase Temperature… which way will the see saw tip?
HEAT
C
A
D
B
 Which way will the reaction SHIFT?
HEAT
C
A
B
 Reaction will SHIFT Right
D
HEAT
A
B
C
D
 Decrease Temperature… which way will the see saw
tip?
HEAT
A
B
C
 Which way will the reaction SHIFT?
D
HEAT
A
B
C
 Reaction will SHIFT Left
D
2NO2(g)  N2O4(g) + energy
(Dark Brown)
(Clear)