Le Chateliers Principle 2 chemistry grade 12

1. Le Chatelier's
Principle

2. Le Chatelier's Principle (1884)
When a system at equilibrium
is subjected to a stress, the system
will adjust to relieve the stress and
return to equilibrium.
Remember: Kc value is constant.
BEFORE the stress, and AFTER the reaction
adjusts.
It is like the “undo” button
on your computer!

3. Types of Stress

4. Factors that Affect
Equilibrium
Concentration
Temperature
Pressure
For gaseous systems only!
The presence of a catalyst

5. 1. Concentration Stress
Stress: a change in concentration
of products or reactants by adding or
removing.
Adjustment: change in collision rate and
redistribution of particles.
• [Add] – system shifts to use it up.
• [Remove] – system shifts to make more.

6. • More C means increased rate of reverse reaction.
Kc = [C]
[A][B]
C
B
A +
Kc = 1.35
We say “shifts left”
We mean:
• Excess C used up until ratio of product to reactant
concentrations is equal to Kc once again.
Increase [C]:

7. Kc = [C]
[A][B]
B C
A +
Kc = 1.35
• Forward reaction is favoured
We say “shifts right”
We mean:
• New concentrations re-establish Kc.
Increase [B]:

8. Kc = [C]
[A][B]
B C
A +
Kc = 1.35
Removing a particle is like decreasing [ ].
• Decreased rate of forward reaction collisions.
We say “shifts left”
We mean:
• Reverse is favoured, ↑ reactants, Kc the same.
Decrease [A]:

9. Concentration Changes
Add more reactant  Shift to
products
Remove reactants  Shift to
reactants
•Add more product  Shift to reactants
•Remove products  Shift to products

10. 2 NO2 (g) N2O4 (g)
car exhaust smog
Huge spike indicates that [ ] was changed by adding more particles.

11. 2 NO2 (g) N2O4 (g)
car exhaust smog
A huge spike indicates that [ ] was changed by removing particles.

12. Temperature

13. Temperature stress addressed the SAME way as
concentration by changing collision rates.
**Re-establishes new eqlbm (with new [ ]s)
at new temperature – SO…changes the Kc.
Exothermic: A B (- ∆H )
Endothermic: A B (+ ∆H)
HEAT +
+ HEAT
2. Temperature stress

14. Temperature increase / add heat
• Reaction shifts left.
• Endothermic collisions (reverse) favored.
Temperature decrease / removing heat
• Reaction shifts right.
• Exothermic collisions (forward) favored.
+ heat
heat
A B
+
A B
= [B]
[A]
= [B]
[A]
Kc
Kc

15. ∆H = -58 kJ
2 NO2 (g) N2O4 (g)
car exhaust smog
Initial drop in ALL rates can only occur through temperature decrease.

16. ∆H = -58 kJ
2 NO2 (g) N2O4 (g)
car exhaust smog
Initial spike in ALL rates can only occur through temperature decrease.

17. Volume/Pressure

18. Pressure Changes
Only affects equilibrium
systems with unequal
moles of gaseous
reactants and products.

19. Changing the pressure of a system only
affects those equilibria with gaseous
reactants and/or products.
3. Volume stress
Rates of collisions change with pressure and
effect all concentrations – BUT, Kc
will re-establish***.
A + 2 B  C

20. A + 2 B C
Volume increase – (↓P ):
A
B
B
C
Decreased rate of forward reaction.
(fewer collisions, in larger space)
Reverse rate favoured – shifts left
(pressure increases with more particles)
B
B
A

21. A + 2 B C
A
B
B
C
C
Volume decrease– (↑P ):
Increased rate of forward reaction.
(MORE collisions, in smaller space)
Forward rate favoured – shifts right
(pressure reduced with fewer particles)

22. Which way with the system shift IF the size of the
container is cut in half?
Reverse reaction favoured
(increased likelihood of collisions in a smaller space)
Shifts left
2 NH3(g) N2(g) + 3 H2(g)

23. Equilibrium position unchanged.
H2(g) + I2(g) 2 HI(g)
Which way with the system shift IF the pressure is
decreased?
1 + 1 : 2
Pressure changes have NO effect on this eqlbm –
Same # of particles, same collision effects.

24. Factors (stresses)
that do not affect
Equilibrium Systems

25. Catalysts
Lowers activation energy for both forward and
reverse reaction equally.
Equilibrium established more quickly, but position
and ratios of concentrations will remain the same.
K value remains the same.

26. Inert Gases (noble gases)
Unreactive – are not part of a reaction, therefore can
not affect equilibrium of a concentration-based
equation.
Catalysts, inert gases, pure solids or pure liquids do
NOT appear in the Equilibrium Law - so they have no
effect if altered.

27. Le Chatelier's
AND
life

28. Appliance - NO energy - forward reaction favored
Energy released to run appliance.
Outlet (recharge) – HIGH energy - reverse favored
Reformes reactants, storing energy for use.
Rechargeable Batteries
Lead-acid
PbO2 + Pb + 4 H+ + 2 SO4
2-  2 PbSO4 + 2 H2O + energy
Nickel-cadmium
Cd + 2 NiO(OH) + 2 H2O  2 Ni(OH) + Cd(OH)2 + energy
Electrical energy (like heat) is written in the reaction.

29. Haemoglobin protein used to transport O2 from lungs to
body tissue.
Lungs - [O2] is high - forward reaction favored
Haemoglobin binds O2
Tissue - [CO2] is high and [O2] is low - reverse
reaction favored. Hb releases O2
Hb (aq) + O2 (g)  HbO2 (aq)
Haemoglobin AND Oxygen

30. CAN YOU / HAVE YOU?
• Use Le Chatelier’s principle to predict and
explain shifts in equilibrium.
Include: temperature, pressure/volume,
reactant/product concentration, catalyst, inert gas
• Interpret concentration versus time graphs.
Include: temp, concentration, catalyst changes.
• Describe practical applications of Le Chatelier’s
principle.