Chapter 6: Chemical Equilibrium

1
Chapter 6Chemical Equilibrium
2
Equilibrium Constant
For the general reaction
aA bB cC dD

Equilibrium Constant
3
Standard States
1) solutes 1 M
e.g., Na(aq)
2) solids pure solid
e.g., AgCl(s)
3) liquids pure liquid
e.g., H2O(l) or Hg(l)
4) gases 1 atm
e.g., CO(g)
Concentrations of solids and liquids are omitted
from K expression because they are unity!
4
Manipulating K
K1
HA H A-
K1

5
Adding Reactions
aA bB cC K1
cC dD eE K2
aA bB dD eE K3

6
Meaning of K
K gt 1
Products Favored
Reactants Favored
K lt 1
7
Equilibrium Thermodynamics
K is related to the free energy difference, DG,
between the product and reactants

DG Gibbs free energy change of rxn R gas
constant 8.314 J K-1 mol-1 T temperature
8
What Decides if a Reaction Will Be Favored?
Gibbs free energy, DG

Reaction favored when DG lt 0
9
Le Chateliers Principle
States If a stress, such as a change in
concentration, pressure, temperature, etc. is
applied to a system at equilibrium, the reaction
will proceed in such a way as to reduce the
effects of the stress.
10
General Case
For the general reaction
aA bB cC dD

Increase A, would cause increase in C and D
K is constant at equilibrium!
11
Solubility Products (Ksp)
In general reaction
Ax By(s) xA(aq) yB(aq)
-x
y

AxBy is a solid!
12
Mercurous Azide
Hg2(N3)2 (s) Hg2 (aq) 2N3 (aq)
2
-

What is the concentration of mercuous ion, Hg22,
in a saturated solution?
Hg22 5.6 x 10-4 M
13
Separation by Precipitation
Suppose you wanted to separate an aqueous
solution of Ba2 and Ca2. How could you do it?
Look up Ksps in table to find anion that has
well separated Ksps for Ba2 and Ca2
You do that and you decide that fluoride ion is a
good candidate.
14
Separation by Precipitation
BaF2 (s) Ba2(aq) 2F-(aq) Ksp 1.7 x 10-6
CaF2 (s) Ca2(aq) 2F-(aq) Ksp 3.9 x 10-11
Reaction Quotient (Q) is useful in these type of
problems

Concentrations not at equilibrium
15
Separation by Precipitation
1) What F- is required to completely
separate (reduce Ca2 to 1 x 10-6 M)? The
initial concentration of each ion is 0.01 M.
2) Will F- cause Ba2 to precipitate?
3) At what F- will Ba2 precipitate?
4) At what F- will CaF2 start to precipitate?
16
Practical Considerations
1) Neglected OH-
2) Co-precipitation of Ba2 either adsorbed to
CaF2 crystal surface or incorporated into the
lattice.
3) Other reactions???
17
Precipitation Titration
Titration of 20.00 mL of 0.0825 M NaCl with 0.100
M AgNO3.
Ag converted to AgCl(s)
Cl- decreasing markedly
Ag is slowly increasing
18
Precipitation Titration
Titration of a mixture of 0.0150 M NaCl, 0.0100 M
NaBr, and 0.0250 NaI with 0.0500 M AgNO3.
Equivalence Points?
19
Common Ion Effect
A salt will be less soluble if one of its
constituent ions is already present in solution.
20
Plumbous Chloride
PbCl2 (s) Pb (aq) 2Cl (aq)
-
2

What is the molar solubility of Pb(II) chloride
in a 0.10 M NaCl solution?
S 1.7 x 10-3 M
Check approximations. Box 6-1
21
Complex Ion Formation
Complex ions (also called coordination ions)
Lewis Acid and Bases Definitions
Acid gt electron pair acceptor
metal
Base gt electron pair donator
ligand
Adduct gt product of Lewis-Base reaction
Coordinate covalent bond gt both electrons of
shared pair contributed by same atom
22
Complex Ion Formation
Stepwise Formation Constants
M X MX
K1
MX X MX2
K2
MX2 X MX3
K3
Cumulative Formation Constants
M X MX
b1
M 2X MX2
b2
M nX MXn
bn
23
Zinc Hydroxide
Zn(OH)2 (s) Zn (aq) 2OH (aq)
-
2

Zn(OH) b1 2.5 x 104 Zn(OH)3- b3 7.2 x
1015 Zn(OH)42- b4 2.8 x 1015
Zn(OH)4
2-
Calculate concentration of each Zn-containing
species in a solution saturated with Zn(OH)2 and
containing OH- fixed at 3.2 x 10-7 M
24
Acid Base Definitions
Arrenhius Definition
Acid gt increase H
Base gt increase OH-
Svante August Arrhenius Nobel Prize 1903
25
Anyone for Croquet?
26
Acid Base Definitions
Bronsted-Lowry Definition
Acid gt proton donor
Base gt proton acceptor
Thomas Lowry
27
Acid Base Definitions
Lewis Definition
Acid gt electron acceptor
Base gt electron donator
Gilbert Newton Lewis
28
Weak vs. Strong Acid
Strong gt equilibrium lies far to the right
K gtgt1
HCl H Cl-
NaOH Na OH-
Weak gt equilibrium more balanced
HCN H CN-
Ca(OH)2 Ca 2OH-
29
Conjugate Acid/Base Pairs
Related to each other by a gain or loss of one
proton (H)
NH3 HCN NH4 CN-
Base
Base
Acid
Acid
30
H2O
H2O may behave as an acid or base (amphiprotic)
H2O HCN H3O CN-
Base
H2O NH3 NH4 OH-
Acid
Also undergoes self-ionization called
autoprotolysis
H2O H2O H3O OH-
Base
Acid
31
Kw
H2O H2O H3O OH-
Also written as
H2O H OH-

Kw changes with temperature!!!
32
Autoprotolysis
What are the OH- and H in pure H2O at 25 C?
33
pH

34
pH Examples
1) What is the pH of pure water?
2) What is the pH of 0.1 M NaOH?
3) What is the pH of 0.1 M HCl?
35
Ka Kb
Ka and Kb are equilibrium constants for weak
acids and bases
Weak Acid
Weak Base
HA H A-
B H2O HB OH-


Acid Dissociation
Base Hydrolysis
36
pKa, pKb, pOH

pOH -logOH-
pKa -logKa
pKb -logKb
Larger pKa gt weaker the acid
Larger pKb gt weaker the base
37
Relationship of pH and pOH

Prove relationship
38
Relationship Ka, Kb, Kw
CH3COOH H CH3COO-
Ka
Kb
CH3COO- H2O CH3COOH OH-
H2O H OH-
Kw

39
Acetic Acid
What is the pKb for sodium acetate? The pKa of
acetic acid is 4.757
pKb 9.24