1. Chapter 15
Chemical Equilibrium

2. Liquid Gas
phase equilibrium
Figure: 11-24a,b

3. Note: equilibrium is dynamic!
Two opposite processes have reached the same rate.
Note: equilibrium is dynamic!

4. Reactants → Products
Reactants ← Products
Reversible Reaction
Reactants ⇌ Products

5. N2O4(g) ⇌ 2NO2(g) rf = kf[N2O4] rr = kr[NO2]2
Both directions are elementary reactions
What are the differential rate laws?
rf = kf[N2O4]
rr = kr[NO2]2

6. N2O4(g) ⇌ 2NO2(g) rf = kf[N2O4] rr = kr[NO2]2
Suppose we start with a flask of pure N2O4 gas
rf = kf[N2O4]
rr = kr[NO2]2
At equilibrium, the forward and reverse reactions are proceeding at the same constant rate. rf = rr

7. N2O4(g) ⇌ 2NO2(g) rf = kf[N2O4] = rr = kr[NO2]2
Once equilibrium is achieved, the concentration of each reactant and product remains constant.

8. N2O4(g) ⇌ 2NO2(g) rf = kf[N2O4] rr = kr[NO2]2 kf[N2O4] = kr[NO2]2 K =

9. a A + b B ⇌ c C + d D K: equilibrium constant.
[ ]: concentration at equilibrium!
K: no unit.
K: independent of concentration,
dependent upon temperature.
recall d=m/V

10. K is independent of concentrations.
Chapter 15, Table 15.1
At one temperature: one K, infinite number of equilibrium concentrations.
K is independent of concentrations.

11. Write the equilibrium expression for K for the following reactions:
(a) 2O3(g) ⇌ 3O2(g)
(b) 2NO(g) + Cl2(g) ⇌ 2NOCl(g)
(c) Ag+(aq) + 2NH3(aq) ⇌ Ag(NH3)2+(aq)
a A + b B ⇌ c C + d D

12. What Does the Value of K Mean?

13. a A + b B ⇌ c C + d D
If K>>1, the reaction is product-favored; product predominates at equilibrium.
If K<<1, the reaction is reactant-favored; reactant predominates at equilibrium.

14. Some properties of K

15. a A + b B ⇌ c C + d D c C + d D ⇌ a A + b B na A + nb B ⇌ nc C + nd D
reverse
a A + b B ⇌ c C + d D
c C + d D ⇌ a A + b B
x n
na A + nb B ⇌ nc C + nd D

16. N2(g) + 3H2(g) ⇌ 2NH3(g)
Haber process

17. Anhydrous Ammonia is Injected into the Solid to Act as a Fertilizer

19. Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g)
The following equilibrium concentrations were observed for the Haber process
at 127 °C: [NH3] = 3.1 x 10−2 mol/L, [N2] = 8.5 x 10−1 mol/L,
[H2] = 3.1 x 10−3 mol/L.
Calculate the value of K at 127 °C for this reaction.
Calculate the value of the equilibrium constant at 127 °C for the reaction
2NH3(g) ⇌ N2(g) + 3H2(g)
3.8 x 104
2.6 x 10−5
c) Calculate the value of the equilibrium constant at 127 °C for the reaction
given by the equation
⇌ NH3(g)
190

20. Consider the following chemical equation and equilibrium
For practice 15.2
Consider the following chemical equation and equilibrium
constant at 25 °C:
2COF2(g) ⇌ CO2(g) + CF4(g) K = 2.2 x 106
Compute the equilibrium constant for the following reaction
at 25 °C:
2CO2(g) + 2CF4(g) ⇌ 4COF2(g) K’ = ?
2.1 x 10−13

21. + Equilibrium constant of a composite reaction
2 NOBr (g) ⇌ 2 NO (g) + Br2(g)
Br2 (g) + Cl2 (g) ⇌ 2 BrCl (g) +
2NOBr (g) + Cl2 (g) ⇌ 2NO (g) + 2BrCl (g)
Sum of reactions  Product of equilibrium constants

22. HF (aq) ⇌ H+ (aq) + F− (aq) K1 = 6.8 x 10−4
H2C2O4 (aq) ⇌ 2H+ (aq) + C2O42− (aq)
K2 = 3.8 x 10−6
2HF (aq) + C2O42− (aq) ⇌ 2F− (aq) + H2C2O4 (aq)
Determine the equilibrium constant for
K = 0.12
Method: take linear combination of known reactions to construct target reaction.

23. a A (g) + b B (g) ⇌ c C (g) + d D (g)
N2(g) + 3H2(g) ⇌ 2NH3(g)
= Kc
a A (g) + b B (g) ⇌ c C (g) + d D (g)
∆n = Sum of the coefficients of gaseous products
− Sum of the coefficients of gaseous reactants

24. Example 15.3
Kp for the following reaction is 2.2 x 1012 at 25 °C, calculate
the value of Kc.
2NO(g) + O2(g) ⇌ 2NO2(g)
5.4 x 1013
Try For Practice 15.3

26. Equilibrium Category based on Phase

27. Homogeneous Equilibrium
All species have the same phase
N2(g) + 3H2(g) ⇌ 2NH3(g)
Heterogeneous Equilibrium
Not all species have the same phase
2CO(g) ⇌ CO2(g) + C(s) K
The concentration of pure solid or liquid is not included in the
equilibrium constant expression for heterogeneous equilibria.

28. 2H2O(l) ⇌ 2H2(g) + O2(g)
2H2O(g) ⇌ 2H2(g) + O2(g)

29. Write the expression for K and Kp for the following processes:
PCl5(s) ⇌ PCl3(l) + Cl2(g)
CuSO4 • 5H2O (s) ⇌ CuSO4(s) + 5H2O(g)
blue
white

30. Hydrated Copper (II) Sulfate on the Left
Hydrated Copper (II) Sulfate on the Left. Water Applied to Anhydrous Copper (II) Sulfate, on the Right, Forms the Hydrated Compound

32. a A + b B ⇌ c C + d D [ ]: concentration at equilibrium!
reaction quotient
[ ]: concentration at a particular moment.

33. a A + b B ⇌ c C + d D
Q = K, system (mixture) is at equilibrium, rr = rf
Q > K → rr > rf → system will shift to left to
reach equilibrium
Q < K → rr < rf → system will shift to right to
reach equilibrium

34. Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g)
At 500 °C, K = 6.01 x 10−2 . Predict the direction in which the
system will shift to reach equilibrium in each of the following
cases:
[NH3]0=1.0 x 10−3 M; [N2]0 = 1.0 x 10−5 M; [H2]0 = 2.0 x 10−3 M
[NH3]0=2.00 x 10−4 M; [N2]0 = 1.50 x 10−5 M; [H2]0 =3.54 x 10−1 M
[NH3]0=1.0 x 10−4 M; [N2]0 = 5.0 M; [H2]0 = 1.0 x 10−2 M
a) Q = 1.3 x 107
b) Q = 6.01 x 10−2
c) Q = 2.0 x 10−3

35. value of Kc or Kp
equilibrium concentrations or pressures

36. I2(g) + Cl2(g) ⇌ 2ICl(g) Kp = 81.9
Example 15.11
Consider the following reaction:
I2(g) + Cl2(g) ⇌ 2ICl(g) Kp = 81.9
A reaction mixture at 25 C initially contains PI2  atm, PCl2  atm, and PICl = atm. Find the equilibrium partial pressures of I2, Cl2, and ICl at this temperature.
x = (atm)
PI2 = (atm), PCl2 = (atm), PICl = (atm)

37. Try Example 15.9

38. H2(g) + F2(g) ⇌ 2HF(g)
3.00 mol H2 and 6.00 mol F2 are mixed in a 3.00 L flask.
K at this temperature is 115. Calculate the equilibrium
concentration of each component.
x = 2.14 (M) or (M)

39. Try Example

40. Problem Set 1 Properties of K Relationship between Kc and Kp
Expression of K
K → equilibrium concentrations/pressures

42. value of Kc or Kp
equilibrium concentrations or pressures

43. Consider the following reaction: CO(g) + 2H2(g) ⇌ CH3OH(g)
Example 15.5
Consider the following reaction:
CO(g) + 2H2(g) ⇌ CH3OH(g)
A reaction mixture at 780 C initially contains [CO] = M and [H2] = 1.00 M. At equilibrium, the CO concentration is found to be 0.15 M. What is the value of the equilibrium constant?

44. Example 15.6

46. a A + b B ⇌ c C + d D Q = K, system (mixture) is at equilibrium.
Q > K → rr > rf → system will shift to left to
reach equilibrium
Q < K → rr < rf → system will shift to right to
reach equilibrium

47. a A + b B ⇌ c C + d D Effect of a change in concentration
add reactants → reactant concentrations ↑ → Q < K
→ system shifts to right
remove reactants → reactant concentrations ↓ → Q > K
→ system shifts to left

48. a A + b B ⇌ c C + d D Effect of a change in concentration
add products → product concentrations ↑ → Q > K
→ system shifts to left
remove products → product concentrations ↓ → Q < K
→ system shifts to right

49. Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g)
concentration

50. Chapter 14, Figure 14.10A
Le Châtelier’s Principle: Changing Concentration

51. Chapter 14, Figure 14.10B
Le Châtelier’s Principle: Changing Concentration

52. CaCO3(s) ⇌ CaO(s) + CO2(g)
Example 15.14
Consider the following reaction at equilibrium:
CaCO3(s) ⇌ CaO(s) + CO2(g)
What is the effect of adding additional CO2 to the
reaction mixture?
What is the effect of adding additional CaCO3?

53. Homogeneous Equilibrium
All species have the same phase
N2(g) + 3H2(g) ⇌ 2NH3(g)
Heterogeneous Equilibrium
Not all species have the same phase
2CO(g) ⇌ CO2(g) + C(s) K
The concentration of pure solid or liquid is not included in the
equilibrium constant expression for heterogeneous equilibria.

54. Effect of a change in pressure
1) Add or remove a gaseous reactant or product.

55. Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g)
concentration

56. Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g)

57. Effect of a change in pressure
1) Add or remove a gaseous reactant or product.
2) Add an inert gas (one not involved in the reaction).
Equilibrium does not shift
3) Change the volume of the container.

58. N2(g) + 3H2(g) ⇌ 2NH3(g)
(a) A Mixture of NH3(g), N2(g), and H2(g) at Equilibrium (b) The Volume is Suddenly Decreased (c) The New Equilibrium Position for the System Containing More NH3 and Less N2 and H2 compared to the old equilibrium (a)

59. Effect of a change in pressure
1) Add or remove a gaseous reactant or product.
2) Add an inert gas (one not involved in the reaction).
Equilibrium does not shift
3) Change the volume of the container.
decrease volume → equilibrium shifts to the direction with less
moles of gases
increase volume → equilibrium shifts to the direction with more
moles of gases

60. Predict the shift of equilibrium position that will occur for
each of the following processes when the volume is reduced.
P4(s) + 6Cl2(g) ⇌ 4PCl3(l)
PCl3(g) + Cl2(g) ⇌ PCl5(g)
PCl3(g) + 3NH3(g) ⇌ P(NH2)3(g) + 3HCl(g)

61. Try Example 15.15 and For Practice 15.15.

62. Your grade up to midterm was calculated according to the
following formula described in the syllabus.
<Q> is the average of your top three quizzes
<L> is the average of your labs

63. Your grade of the course is calculated according to the
following formula described in the syllabus.
<Q> is the average of your top three quizzes
<L> is the average of your labs

65. a A + b B ⇌ c C + d D

66. Effect of a change in temperature
K is a function of temperature
Do experiments

67. (brown) 2NO2 ⇌ N2O4 (colorless)
∆H = − 58 kJ
T > 0 °C
T = 0 °C
∆H < 0, exothermic
∆H > 0, endothermic
decrease T → shifts to right → K increases
increase T → shifts to left → K decreases

68. Endothermic reactions
Exothermic reactions
decrease T → shifts to right → K increases
increase T → shifts to left → K decreases
Endothermic reactions
decrease T → shifts to left → K decreases
increase T → shifts to right → K increases

69. For each of the following reactions, predict which direction the
reaction will shift to and how the value of K will change as the
temperature is increased.
N2(g) + O2(g) ⇌ 2NO(g) ∆H = 181 kJ
2SO2(g) + O2(g) ⇌ 2SO3(g) ∆H = −198 kJ

70. Try Example 15.16 and For Practice 15.16.

71. Le Châtelier's Principle
If a change is imposed on a system at equilibrium,
the position of the equilibrium will shift in a direction
that tends to reduce that change.

72. Does catalyst shift an equilibrium?

73. A ⇌ B

74. A ⇌ B
∆Ea
Ef’
Er’
Catalyst does not shift equilibrium

75. For the reaction
PCl5(g) ⇌ PCl3(g) + Cl2(g) ∆H = 87.9 kJ
In which direction will the equilibrium shift when
Cl2(g) is removed,
The temperature is decreased,
The volume of the system is increased,
PCl3(g) is added?
In b) and d), will the equilibrium constant increase or
decrease?
In d), will the concentration of Cl2 and PCl5 increase or

76. Problem Set 2
Shift of chemical equilibrium
Le Châtelier's Principle

77. Review problem set 1