1. Le Chatelier's Principle

2. 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.
Additional KEY Terms

3. 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.

4. Types of Stress

5. Kc value is re-established after concentrations are adjusted
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.
Kc value is re-established after concentrations are adjusted

6. A + B C Kc = Kc = 1.35 [C] [A][B] Increase [C]: We say “shifts left”
We mean:
More C means increased rate of reverse collisions
Excess [C] used, [A] and [B] increase
Re-establish Kc

7. A + B C Kc = Kc = 1.35 [C] [A][B] Increase [B]: We say “shifts right”
We mean:
Forward reaction is favoured
Redistribute excess particles
Re-establish Kc

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

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

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

11. Temperature

12. *Different eqlbm at new temperature – SO…changes the Kc*
2. Temperature stress
Stress: a change in temperature by adding or removing heat.
Adjustment: change in collision rate and redistribution of particles.
Exothermic: A  B (- ∆H )
Endothermic: A  B (+ ∆H)
+ HEAT
HEAT +
*Different eqlbm at new temperature – SO…changes the Kc*

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

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

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

16. Volume/Pressure

17. Kc value is re-established after concentrations are adjusted
3. Volume stress
Stress: a change in pressure that only affects those systems with gaseous reactants and/or products.
Adjustment: change in collision rate and redistribution of particles.
Kc value is re-established after concentrations are adjusted

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

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

20. 2 NH3(g) N2(g) + 3 H2(g)
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

21. H2(g) + I2(g) 2 HI(g) :
Which way with the system shift IF the pressure is decreased?
Pressure has NO effect on this eqlbm
Same # of particles, same collision effects
No shift

22. Factors
that do not affect
Equilibrium Systems

23. 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.

24. 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.

25. Le Chatelier's
AND
life

26. Rechargeable Batteries
Electrical energy (like heat) is written in the reaction.
Lead-acid
PbO2 + Pb H SO42-  2 PbSO H2O + energy
Nickel-cadmium
Cd NiO(OH) H2O  2 Ni(OH) + Cd(OH)2 + energy
Appliance - NO energy - forward reaction favored
Energy released to run appliance.
Outlet (recharge) – HIGH energy - reverse favored
Reformes reactants, storing energy for use.

27. Haemoglobin AND Oxygen
Hb (aq) + O2 (g)  HbO2 (aq)
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

28. 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.