Le Chateliers Principle 3.ppt

1. Le Chatelier's
Principle

2. Use Le Chatelier’s Principle to explain how the
position of a system at equilibrium is effected by:
 Changing concentration
 Changing temperature
 Changing pressure
 Adding a catalyst

3. Le Chatelier's Principle (1884)
When a system at equilibrium is subjected
to a stress, the system will adjust so as to
relieve the stress.
Remember: Kc value is constant.
BEFORE the stress, and AFTER the reaction
adjusts to the stress.

4. Types of Stress

5. 1. Concentration stress
Any change in concentration of products or
reactants by adding or removing to a balanced
system.
Reduction in stress due to increased collisions and
a redistribution of excess 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.
We say “shifts left”
We mean:
• Reverse is favoured, ↑ reactants, Kc the same.
Decrease [A]:

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

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

11. Temperature

12. 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

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

14. ∆H = -58 kJ
2 NO2 (g) N2O4 (g)
car exhaust smog

15. ∆H = -58 kJ
2 NO2 (g) N2O4 (g)
car exhaust smog

16. Volume/Pressure

17. 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

18. 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

19. 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)

20. 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)

21. 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.

22. Factors (stresses)
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 mass action expression - so they
cannot have an effect if altered.

25. Le Chatelier's
AND
life

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

27. Hb (aq) + O2 (g)  HbO2 (aq)
High altitudes - [O2] is very low - reverse reaction
favored.
Hb release O2, fewer Hb bind oxygen.
Result in exaggerated lack of oxygen to the tissues,
resulting in headache, nausea and fatigue.
Over time, body adjusts by producing more
haemoglobin molecules.
Increases [Hb] in the blood stream shifts
equilibrium right - more O2 bound and transported
to the tissue.

28. Appliance - NO energy - forward reaction favored
Energy release to run appliance.
Outlet (recharge) - high energy - reverse favored
Reforming the reactants, storing the energy for use.
Rechargable 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. THE HABER PROCESS

30. N2(g) + 3H2(g) 2NH3(g)
ΔH = -92.4 kJ mol-1
high pressure
medium temperature
- catayst
remove ammonia
high reactant concentrations