1. Chemical
Equilibrium
A Balancing Act

2. Equilibrium
two opposing processes occurring at the same rate
a system at equilibrium is in balance

3. During a game, players enter and leave.
Always the same number of players on field.

4. H2O(l) H2O(g)

5. Photochromic
sunglasses
AgCl + light Ag + Cl
(transparent) (dark)

6. Reversible reaction
a chemical reaction that can occur in both the forward and the reverse directions
N2(g) + 3H2(g) NH3(g)

8. Chemical equilibrium
a state in which the forward and reverse reactions balance each other because they take place at equal rates
dynamic state; no net change

9. Law of chemical equilibrium
At a given temperature, a chemical system might reach a state in which a particular ratio of reactant and product concentrations has a constant value.

10. Equilibrium constant expression
ratio of molar concentrations of products to reactants; each raised to a power equal to coefficient in balanced equation

11. aA + bB cC + dD

12. Equilibrium constant (Keq)
numerical value of the ratio of product to reactant concentrations
constant only at a specific temperature

13. Value of equilibrium constant (Keq) shows
the extent to which reactants are converted into products.

14. Keq > 1: Products are favored at equilibrium
Keq < 1: Reactants are favored at equilibrium

15. Homogeneous equilibrium
all reactants and products in same phase

16. Write an equilibrium constant expression for:
N2(g) + 3H2(g) NH3(g)
[NH3]2
Keq =
___________
[N2] [H2]3

17. Heterogeneous equilibrium
reactants and products present in more than one physical state
H2O(l) H2O(g)

18. Heterogeneous equilibrium
Since concentrations of pure liquids and solids remain constant, these substances are omitted from the equilibrium constant expression.

19. Write an equilibrium constant expression for:
H2O(l) H2O(g)
Keq =
[H2O(g)]

20. Equilibrium constant expression Keq = . . .
Products over reactants raised to power of coefficient; leave out pure solids and liquids.

21. Equilibrium position
Equilibrium concentrations can vary from trial to trial.

22. H2(g) +I2(g) 2HI(g)

23. Each set of equilibrium concentrations represent an equilibrium position.
A system has only one value for Keq at a specific temperature, however there are unlimited number of equilibrium positions.

24. A system at equilibrium must:
take place in closed
system
temperature remain
constant
all reactants and
products are present
(both reactions can occur)

25. Write the equilibrium constant expression for
N2(g) + O2(g) 2NO(g)

26. Calculate the value of Keq if [N2] = 0.20 mol/L,
[O2] = 0.15 mol/L, and
[NO] = mol/L.
Keq = 4.1 x 10-4
What does the value of Keq tell you about the equilibrium?

27. What happens when a system is at equilibrium and you upset the balance?

28. Le Chatelier’s Principle
If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

29. Le Chatelier’s Principle
Used to predict how a equilibrium system will react to changes in concentration, pressure (volume) and temperature.

30. Changes in Concentration
Add reactant
1. Forward rxn speeds up.
2. Over time, forward
rxn slows down and
reverse rxn speeds up.

31. 3. At equilibrium,
forward and reverse
rxns occur at same
rate, new equilibrium
position established.
Equilibrium has
shifted right, value
of Keq unchanged.

32. Changes in Concentration
Remove reactant
1. Reverse rxn speeds up.
2. Over time, reverse
rxn slows down and
forward rxn speeds up.

33. 3. At equilibrium,
forward and reverse
rxns occur at same
rate, new equilibrium
position established.
Equilibrium has
shifted left; value of
Keq unchanged.

34. Changes in Pressure (Volume)
For a gas, decreasing
volume of container
increases pressure;
particles have less
space, collide more
frequently.

36. 2. System will respond
by trying to relieve
stress (decrease
pressure.)
3. Shifts to the side with
fewer moles of gas.

37. 4. Forward rxn speeds up.
5. Over time, forward
rxn slows down and
reverse rxn speeds up.

38. 6. At equilibrium,
forward and reverse
rxns occur at same
rate, new equilibrium
position established.
7. Equilibrium has
shifted right, value
of Keq unchanged.

39. Changes in Pressure (Volume)
If volume is decreased concentrations of all gaseous substances increases.
Note:
Side with fewer moles of gas increases more.

40. Changes in Temperature
Review:
If DH is negative
rxn is exothermic
heat written as product
If DH is positive
rxn is endothermic
heat written as reactant

41. Changes in Temperature
N2O4(g) 2NO2(g)
DH = 55.3 kJ
55.3 kJ + N2O4(g) 2NO2(g)
light dark

42. 55.3 kJ + N2O4(g) 2NO2(g)

43. Le Chatelier’s Principle
Used to predict how a equilibrium system will react to changes in concentration, pressure (volume) and temperature.
Chemists can change conditions to make rxn more productive.