1. IB Chemistry Power Points
Topic 7
Equilibrium
www.pedagogics.ca
Topic 7.2

2. The Position of Equilibrium
Consider
A +A
B B+D
C
A reacts with B in a sealed flask.
What happens to [A], [B], [C], and [D]?

3. The Position of Equilibrium
A +A
B B+D
C
As the reaction progresses
– [A] & [B] decrease to constant values,
– [C] & [D] increase from zero to
constant values
– When [A], [B], [C] and [D] are all
constant values, equilibrium is
achieved.

4. see simulation
3 scenarios for the reaction
N2O4 (g) 2NO2 (g)
equilibrium
equilibrium
equilibrium
Start with NO2 Start with N2O4 Start with NO2 & N2O4

5. Data for these (and other) scenarios might look like
[NO2]2
KC = = 4.63 x 10-3
[N2O4]

6. The Equilibrium Constant
BIG IDEA : At a given temperature, no matter the
starting composition of reactants and products, the
same ratio of concentrations is achieved at
equilibrium.
• For a general reaction
aA bB  cC dD

7. The Equilibrium Constant
the equilibrium constant expression is
c d
[C ] [D]
KC a b
[ A] [B]
where Kc is the equilibrium constant
and is dependent on temperature.

8. Homogenous equilibrium applies to reactions in
which all reacting species are in the same phase.
N2O4 (g) 2NO2 (g)
[NO2]2
Kc =
[N2O4]
CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq)
[CH3COO-][H3O+]
Kc‘ = [H2O] = does not change
[CH3COOH][H2O] significantly so is
omitted
[CH3COO-][H3O+]
Kc =
[CH3COOH]

9. Practice – homogeneous equilibrium
• Write the equilibrium expressions for the
following reactions:
H2 (g) + I2 (g) Ý 2 HI (g)
HF (aq) Ý H+ (aq) + F- (aq)

10. Practice – homogeneous equilibrium
• Write the equilibrium expressions for the
following reactions:
H2 (g) + I2 (g) Ý 2 HI (g)
2
[ HI ]
keq
[ H 2 ][ I 2 ]
HF (aq) Ý H+ (aq) + F- (aq)
[ H ][ F ]
keq
[ HF ]

11. Practice:
The equilibrium concentrations for the reaction between
carbon monoxide and molecular chlorine to form COCl2 (g)
at 740C are [CO] = 0.012 M, [Cl2] = 0.054 M, and [COCl2] =
0.14 M. Calculate Kc
CO (g) + Cl2 (g) COCl2 (g)
220 M-1

12. Heterogenous equilibrium applies to reactions in which
reactants and products are in different phases.
CaCO3 (s) CaO (s) + CO2 (g)
[CaO][CO2] [CaCO3] = constant
Kc‘ = [CaO] = constant
[CaCO3]
Kc = [CO2]
The concentration of solids and pure liquids do not
change hence they are not included in the expression
for the equilibrium constant.

13. CaCO3 (s) CaO (s) + CO2 (g)
[CO2] = Kc
[CO2] does not depend on the amount of CaCO3 or CaO

14. Practice – heterogeneous equilibrium
• Write the equilibrium expression for the
following reaction:
PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)

15. Practice – heterogeneous equilibrium
• Write the equilibrium expression for the
following reaction:
PbCl2 (s) Ý Pb2+ (aq) + 2 Cl- (aq)
keq [Pb2 ][Cl ]2

16. What does the Kc value mean?
Recall: for the reaction
aA + bB cC + dD
[C]c[D]d
K= a b
[A] [B]
Equilibrium Will
K >> 1 Lie to the right Favor products
K << 1 Lie to the left Favor reactants

17. Summary Writing Equilibrium Expressions
• The concentrations of the reacting species in the
condensed phase are expressed in M (mol dm3) In
the gaseous phase, the concentrations can be
expressed in M or in atm.
• The concentrations of pure solids, pure liquids and
other solvents do not appear in the equilibrium
constant expressions.
• In quoting a value for the equilibrium constant, you
must specify the balanced equation and the
temperature.

18. 7.2 Le Châtelier’s Principle
Le Chatelier’s Principle: if you disturb an
equilibrium, it will shift to undo the
disturbance.
Remember, in a system at equilibrium, the
concentrations will always change to restore
the same value for Kc (as long as there is a
constant temperature).

19. Changes in Concentration
N2 (g) + 3 H2 (g) 2 NH3 (g)
Equilibrium
Add
shifts left to
NH3
offset stress

20. Le Châtelier’s Principle
• Changes in Concentration continued
Add
aA + bB cC + dD
Change Shifts the Equilibrium
Increase concentration of product(s) left

21. Le Châtelier’s Principle
• Changes in Concentration continued
Remove
aA + bB cC + dD
Change Shifts the Equilibrium
Increase concentration of product(s) left
Decrease concentration of product(s) right

22. Le Châtelier’s Principle
• Changes in Concentration continued
Add
aA + bB cC + dD
Change Shifts the Equilibrium
Increase concentration of product(s) left
Decrease concentration of product(s) right
Increase concentration of reactant(s) right

23. Le Châtelier’s Principle
• Changes in Concentration continued
Remove
aA + bB cC + dD
Change Shifts the Equilibrium
Increase concentration of product(s) left
Decrease concentration of product(s) right
Increase concentration of reactant(s) right
Decrease concentration of reactant(s) left

24. Le Châtelier’s Principle
Changes in Pressure
A (g) + B (g) C (g)
Change Shifts the Equilibrium
Decrease pressure Side with most moles of gas
Increase pressure Side with fewest moles of gas

25. Le Châtelier’s Principle
Changes in Temperature
N2O4(g) 2 NO2(g) ΔH is +ive
Change Response
Increase temperature forward reaction favored - remove heat
Decrease temperature reverse reaction favored – absorb heat
Room temperature.
NO2 is brown colder hotter

26. Le Châtelier’s Principle
Changes in Temperature
N2O4(g) + heat 2 NO2(g)
Change Response
Increase temperature forward reaction favored - remove heat
Decrease temperature reverse reaction favored – absorb heat
Room temperature.
NO2 is brown colder hotter

27. Le Châtelier’s Principle
ONLY changes in temperature affect Kc
N2O4(g) 2 NO2(g) endothermic
Change Exothermic Rx Endothermic Rx
Increase temperature K decreases K increases
Decrease temperature K increases K decreases
Room temperature.
NO2 is brown colder hotter

28. Le Châtelier’s Principle
Catalysts
• does not change the value of Kc
• does not shift the position of an equilibrium system
• system will reach equilibrium sooner
uncatalyzed catalyzed
Catalyst lowers Ea for both forward and reverse reactions.
Catalyst does not change equilibrium constant or shift equilibrium

29. Example

30. Example
a) shifts left to favor products
b) water vaporizes – shift right to favor reactants
c) shifts right to favor reactants
d) no effect (solids not part of equilibrium expression)

31. Chemistry In Action: The Haber Process – see fact sheet and video
N2 (g) + 3H2 (g) 2NH3 (g) ΔH0 = -92.6 kJ/mol

32. Le Châtelier’s Principle (summary)
Change Equilibrium
Change Shift Equilibrium Constant
Concentration yes no
Pressure (g) yes no
Volume (g) yes no
Temperature yes yes
Catalyst no no