Post on 31-Dec-2015
1
18 Ionic Equilibria: Acids and Bases
2
Chapter Goals
1. A Review of Strong Electrolytes
2. The Autoionization of Water
3. The pH and pOH Scales
4. Ionization Constants for Weak Monoprotic Acids and Bases
5. Polyprotic Acids
6. Solvolysis
7. Salts of Strong Bases and Strong Acids
3
Chapter Goals
8. Salts of Strong Bases and Weak Acids
9. Salts of Weak Bases and Strong Acids
10.Salts of Weak Bases and Weak Acids
11.Salts That Contain Small, Highly Charged Cations
4
A Review of Strong Electrolytes
• This chapter details the equilibria of weak acids and bases.– We must distinguish weak acids and bases from strong
electrolytes.
• Weak acids and bases ionize or dissociate partially, much less than 100%.– In this chapter we will see that it is often less than 10%!
• Strong electrolytes ionize or dissociate completely.– Strong electrolytes approach 100% dissociation in
aqueous solutions.
5
A Review of Strong Electrolytes
• There are three classes of strong electrolytes.
1 Strong Water Soluble AcidsRemember the list of strong acids from Chapter 4.
3(aq)(aq)%100
)3(
3(aq)(aq)3100%
)(2)3(
NOHHNO
or
NOOH OHHNO
6
A Review of Strong Electrolytes
3(aq)(aq)%100
)3(
3(aq)(aq)3100%
)(2)3(
NOHHNO
or
NOOH OHHNO
7
A Review of Strong Electrolytes
2 Strong Water Soluble BasesThe entire list of these bases was also introduced in
Chapter 4.
-(aq)
2(aq)
100% OH2(s)
-(aq)(aq)
100% OH(s)
OH 2Sr Sr(OH)
OHKKOH
2
2
8
A Review of Strong Electrolytes
3 Most Water Soluble SaltsThe solubility guidelines from Chapter 4 will help you
remember these salts.
3(aq)2(aq)
100% OHs23
-(aq)(aq)
100% OH(s)
NO 2Ca )Ca(NO
ClNaNaCl
2
2
9
A Review of Strong Electrolytes
• The calculation of ion concentrations in solutions of strong electrolytes is easy.
• Example 18-1: Calculate the concentrations of ions in 0.050 M nitric acid, HNO3.
0.050 0.050 050.0
NOOHOHHNO 3(aq)(aq)3100%
)(2)3(
MMM
10
A Review of Strong Electrolytes
• Example 18-2: Calculate the concentrations of ions in 0.020 M strontium hydroxide, Sr(OH)2, solution.
You do it!
M
MMM
0.040
0.0202 0.020 020.0
OH 2SrSr(OH) -(aq)
2(aq)
OH2(s)
2
11
The Autoionization of Water
• Pure water ionizes very slightly.– The concentration of the ionized water is less than
one-millionth molar at room temperature.
12
The Autoionization of Water
• We can write the autoionization of water as a dissociation reaction similar to those previously done in this chapter.
• Because the activity of pure water is 1, the equilibrium constant for this reaction is:
-(aq)(aq)3)(2)(2 OHOHOH OH
Kc H3O+ OH
13
The Autoionization of Water
• Experimental measurements have determined that the concentration of each ion is 1.0 x 10-7 M at 25oC.– Note that this is at 25oC, not every temperature!
• We can determine the value of Kc from this information.
Kc H3O+ OH
1.0 x 10-7 1.0 x 10-7 1.0 x10 14
14
The Autoionization of Water
• This particular equilibrium constant is called the ion-product for water and given the symbol Kw.
– Kw is one of the recurring expressions for the remainder of this chapter and Chapters 19 and 20.
Kw H3O+ OH
1.0 x10 14
15
The Autoionization of Water• Example 18-3: Calculate the concentrations of
H3O+ and OH- in 0.050 M HCl.
HCl + H2O H3O+ Cl
0.050M 0.050M 0.050M
Thus the H3O+ 0.050M.
The H3O+ and Kw will allow us to calculate [OH -].
16
The Autoionization of Water
• Use the [H3O+] and Kw to determine the [OH-].
You do it!
H3O+ OH 1.0 10 14
OH 1.0 10 14
H3O+
1.0 10 14
5.0 10 2
OH 2.0 10 13M
17
The Autoionization of Water
• The increase in [H3O+] from HCl shifts the equilibrium and decreases the [OH-].– Remember from Chapter 17, increasing the product
concentration, [H3O+], causes the equilibrium to shift to the reactant side.
– This will decrease the [OH-] because it is a product!
. 0.050 10 1.0 0.050 ]O[H overall The
.101.0 is K from ]O[H The
OH OH OH OH
0.050 is HCl from ]O[H The
7-
3
7-
w3
-
322
3
MM
M
M
18
The Autoionization of Water
• Now that we know the [H3O+] we can calculate the [OH-].
You do it!You do it!
M
M
M
13-
14--
3
14--
3
-143w
100.2]OH[
] [0.050
101]OH[
]O[H
101]OH[
. 0.050 ]O[H Since
101][OH-]O[HK
19
The pH and pOH scales
• A convenient way to express the acidity and basicity of a solution is the pH and pOH scales.
• The pH of an aqueous solution is defined as:
pH = -log H3O+
20
The pH and pOH scales
• In general, a lower case p before a symbol is read as the ‘negative logarithm of’ the symbol.
• Thus we can write the following notations.
pOH = -log OH - pAg = -log Ag+
and so forth for other quantities.
21
The pH and pOH scales
• If either the [H3O+] or [OH-] is known, the pH and pOH can be calculated.
• Example 18-4: Calculate the pH of a solution in which the [H3O+] =0.030 M.
pH = -log H3O+
pH log 3.0 10 2 pH 1.52
22
The pH and pOH scales
• Example 18-5: The pH of a solution is 4.597. What is the concentration of H3O+?
You do it!You do it!
M53
4.597-3
3
3
3
1053.2]O[H
10]O[H
-4.597]Olog[H
]O-log[H4.597
]O-log[HpH
23
The pH and pOH scales
• A convenient relationship between pH and pOH may be derived for all dilute aqueous solutions at 250C.
• Taking the logarithm of both sides of this equation gives:
[H3O][OH ] 1.0 10 14
log H3O log OH 14.00
24
The pH and pOH scales
• Multiplying both sides of this equation by -1 gives:
• Which can be rearranged to this form:
-log H3O log OH 14.00
pH pOH 14.00
25
The pH and pOH scales
• Remember these two expressions!! – They are key to the next three chapters!
H3O OH 1.0 10 14
pH pOH 14.00
26
The pH and pOH scales
• The usual range for the pH scale is 0 to 14.
• And for pOH the scale is also 0 to 14 but inverted from pH.– pH = 0 has a pOH = 14 and pH = 14 has a pOH = 0.
H3O 1.0 M to H3O
1.0 10 14M
pH 0 to pH 14.00
OH 1.0 10 14M up to OH 1.0M
pOH 14.00 pOH 0
27
The pH and pOH scales
28
The pH and pOH scales
• Example 18-6: Calculate the [H3O+], pH, [OH-], and pOH for a 0.020 M HNO3 solution.
– Is HNO3 a weak or strong acid?
– What is the [H3O+] ?
70.1pH
100.2-logpH
100.2OH
0.020 0.020 020.0
NOOHOHHNO
2
23
-33
100%23
M
M
MMM
29
The pH and pOH scales
• Example 18-6: Calculate the [H3O+], pH, [OH-], and pOH for a 0.020 M HNO3 solution.
Kw H3O OH 1.0 10 14
OH 1.0 10 14
H3O
1.0 10 14
2.0 10 2 5.0 10 13M
pOH log 5.0 10 13 12.30
30
The pH and pOH scales• To help develop familiarity with the pH and pOH scale we
can look at a series of solutions in which [H3O+] varies between 1.0 M and 1.0 x 10-14 M.
[H3O+] [OH-] pH pOH
1.0 M 1.0 x 10-14 M 0.00 14.00
1.0 x 10-3 M 1.0 x 10-11 M 3.00 11.00
1.0 x 10-7 M 1.0 x 10-7 M 7.00 7.00
2.0 x 10-12 M 5.0 x 10-3 M 11.70 2.30
1.0 x 10-14 M 1.0 M 14.00 0.00
32
Ionization Constants for Weak Monoprotic Acids and Bases• Let’s look at the dissolution of acetic acid, a
weak acid, in water as an example.• The equation for the ionization of acetic acid is:
• The equilibrium constant for this ionization is expressed as:
-3323 COOCHOHOH COOHCH
Kc H3O
CH3COO CH3COOH H2O
33
Ionization Constants for Weak Monoprotic Acids and Bases• The water concentration in dilute aqueous
solutions is very high.• 1 L of water contains 55.5 moles of water.• Thus in dilute aqueous solutions:
M5.55OH2
34
Ionization Constants for Weak Monoprotic Acids and Bases• The water concentration is many orders of
magnitude greater than the ion concentrations.• Thus the water concentration is essentially that of
pure water.– Recall that the activity of pure water is 1.
Kc H2O H3O CH3COO
CH3COOH
K H3O
CH3COO CH3COOH
35
Ionization Constants for Weak Monoprotic Acids and Bases• We can define a new equilibrium constant for weak
acid equilibria that uses the previous definition.– This equilibrium constant is called the acid ionization
constant.
– The symbol for the ionization constant is Ka.
Ka H3O
CH3COO CH3COOH 1.8 10 5
for acetic acid
36
Ionization Constants for Weak Monoprotic Acids and Bases• In simplified formsimplified form the dissociation equation and
acid ionization expression are written as:
CH3COOH H CH3COO -
Ka H CH3COO
CH3COOH 1.8 10 5
37
Ionization Constants for Weak Monoprotic Acids and Bases• The ionization constant values for several acids
are given below.– Which acid is the strongest?
Acid Formula Ka value
Acetic CH3COOH 1.8 x 10-5
Nitrous HNO2 4.5 x 10-4
Hydrofluoric HF 7.2 x 10-4
Hypochlorous HClO 3.5 x 10-8
Hydrocyanic HCN 4.0 x 10-10
38
Ionization Constants for Weak Monoprotic Acids and Bases• From the above table we see that the order of
increasing acid strength for these weak acids is:
• The order of increasing base strength of the anions (conjugate bases) of these acids is:
HCN>HClO>COOHCH>HNO>HF 32
---3
-2
- CN<ClO<COOCH<NO<F
39
Ionization Constants for Weak Monoprotic Acids and Bases• Example 18-8: Write the equation for the
ionization of the weak acid HCN and the expression for its ionization constant.
10--
a
-
10 x 4.0HCN
CN HK
CN H HCN
40
Ionization Constants for Weak Monoprotic Acids and Bases• Example 18-9: In a 0.12 M solution of a weak
monoprotic acid, HY, the acid is 5.0% ionized. Calculate the ionization constant for the weak acid.
You do it!You do it!
HY H + Y
KH Y
HY
+ -
a
+ -
41
Ionization Constants for Weak Monoprotic Acids and Bases• Since the weak acid is 5.0% ionized, it is also 95% unionized.• Calculate the concentration of all species in solution.
MM
M
MM
11.0)12.0(95.0HY
100.6YH
0060.0)12.0(05.0YH3+
+
42
Ionization Constants for Weak Monoprotic Acids and Bases
• Use the concentrations that were just determined in the ionization constant expression to get the value of Ka.
4
a
33
a
a
103.3K
11.0
100.6 100.6K
HY
Y HK
43
Ionization Constants for Weak Monoprotic Acids and Bases• Example 18-10: The pH of a 0.10 M solution of a
weak monoprotic acid, HA, is found to be 2.97. What is the value for its ionization constant?
pH = 2.97 so [H+]= 10-pH
M3
97.2
101.1H
10H
44
Ionization Constants for Weak Monoprotic Acids and Bases• Use the [H3O+] and the ionization reaction to
determine concentrations of all species.
HA H A
Equil. []'s 0.10 -1.1 10 1.1 10 1.1 10
0.10
+ -
-3 -3 -3
45
Ionization Constants for Weak Monoprotic Acids and Bases• Calculate the ionization constant from this
information.
5a
-3-3
a
102.1K
0.10
101.1101.1
HA
AHK
46
Ionization Constants for Weak Monoprotic Acids and Bases• Example 18-11: Calculate the concentrations of the
various species in 0.15 M acetic acid, CH3COOH, solution.
• It is always a good idea to write down the ionization reaction and the ionization constant expression.
5
3
-33
a
-3323
108.1COOHCH
COOCHOHK
COOCHOH OHCOOHCH
47
Ionization Constants for Weak Monoprotic Acids and Bases• Next, combine the basic chemical concepts with
some algebra to solve the problem.
M0.15 [] Initial
COOCH OH OHCOOHCH -3323
48
Ionization Constants for Weak Monoprotic Acids and Bases• Next we combine the basic chemical concepts
with some algebra to solve the problem
xMxMxM
M
- Change
0.15 [] Initial
COOCH OH OHCOOHCH -3323
49
Ionization Constants for Weak Monoprotic Acids and Bases• Next we combine the basic chemical concepts
with some algebra to solve the problem
xMxM-x)M.(
xMxMxM
M
150 [] mEquilibriu
- Change
0.15 [] Initial
COOCH OH OHCOOHCH -3323
50
• Substitute these algebraic quantities into the ionization expression.
Ka H3O
CH3COO CH3COOH
x x
0.15 x 1.8 10 5
Ionization Constants for Weak Monoprotic Acids and Bases
51
Ionization Constants for Weak Monoprotic Acids and Bases• Solve the algebraic equation, using a simplifying assumption that is
appropriate for all weak acid and base ionizations.
52
52
108.115.0
108.115.0
xx
x
x
52
• Solve the algebraic equation, using a simplifying assumption that is appropriate for all weak acid and base ionizations.
Ionization Constants for Weak Monoprotic Acids and Bases
52
3a
52
52
108.115.0
[]. tocompared ignore enough to small is
.assumption thismake then 10 K If
108.115.0
108.115.0
x
x
xx
x
x
53
• Complete the algebra and solve for the concentrations of the species.
Ionization Constants for Weak Monoprotic Acids and Bases
x 2 2.7 10 6
x 1.6 10 3M H3O CH3COO
CH3COOH 0.15 1.6 10 3 M 0.15M
54
Ionization Constants for Weak Monoprotic Acids and Bases• Note that the properly applied simplifying assumption gives
the same result as solving the quadratic equation does.
2a
4acbb
c b a
0107.2108.1
108.115.0
2
652
5
x
xx
X
xx
55
Ionization Constants for Weak Monoprotic Acids and Bases
3-3
6255
101.6- and 106.1
12
107.214108.1108.1
x
x
56
Ionization Constants for Weak Monoprotic Acids and Bases• Let us now calculate the percent ionization for the
0.15 M acetic acid. From Example 18-11, we know the concentration of CH3COOH that ionizes in this solution. The percent ionization of acetic acid is
%1.1%10015.0
106.1ionization %
%100COOHCH
COOHCH= ionization %
3
original3
ionized3
M
M
57
Ionization Constants for Weak Monoprotic Acids and Bases• Example 18-12: Calculate the
concentrations of the species in 0.15 M hydrocyanic acid, HCN, solution.
Ka= 4.0 x 10-10 for HCN
You do it!You do it!
58
Ionization Constants for Weak Monoprotic Acids and Bases
MMx
Mx
x
x
xx
MxMxMx
MxMxMx
M
15.0 15.0HCN
CNH107.7
100.6
100.415.0
HCN
CN HK
-0.15 mEquilibriu
+ + - Change
0.15 Initial
CN OH OH HCN
6
112
10a
-32
59
Ionization Constants for Weak Monoprotic Acids and Bases• The percent ionization of 0.15 M HCN solution is
calculated as in the previous example.
%0051.0%10015.0
107.7ionization %
%100HCN
HCN = ionization %
6
original
ionized
M
M
60
Ionization Constants for Weak Monoprotic Acids and Bases• Let’s look at the percent ionization of two weak acids
as a function of their ionization constants. Examples 18-11 and 18-12 will suffice.
• Note that the [H+] in 0.15 M acetic acid is 210 times greater than for 0.15 M HCN.
Solution Ka [H+] pH % ionization
0.15 M acetic acid
1.8 x 10-5 1.6 x 10-3 2.80 1.1
0.15 M
HCN
4.0 x 10-10 7.7 x 10-6 5.11 0.0051
61
Ionization Constants for Weak Monoprotic Acids and Bases• All of the calculations and understanding we have
at present can be applied to weak acids and weak bases!
• One example of a weak base ionization is ammonia ionizing in water.
62
Ionization Constants for Weak Monoprotic Acids and Bases• All of the calculations and understanding we have at
present can be applied to weak acids and weak bases!
• Example 18-13: Calculate the concentrations of the various species in 0.15 M aqueous ammonia.
xMxM-x)M.(
xMxMxM
M
150 [] mEquilibriu
- Change
0.15 [] Initial
OH NH OH NH -423
63
Ionization Constants for Weak Monoprotic Acids and Bases
MM
M
Mxx
x
xx
x
xx
x
xx
xMxM-x)M.(
xMxMxM
M
15.0101.615.0NH
101.6OHNH
101.6 and 107.2
108.1)15.0(
108.115.0
15.0
0.15 x - 0.15 thus0.15x
valid.is assumption gsimplifyin The
108.115.0
NH
OH NHK
150 [] mEquilibriu
- Change
0.15 [] Initial
OH NH OH NH
3-3
3--4
3-62
52
5
5
3
-4
b
-423
64
Ionization Constants for Weak Monoprotic Acids and Bases• The percent ionization for weak bases is
calculated exactly as for weak acids.
% ionization NH3 ionized
NH3 original
100%
1.6 10 3M
0.15M100%
1.1%
65
Ionization Constants for Weak Monoprotic Acids and Bases• Example 18-14: The pH of an aqueous
ammonia solution is 11.37. Calculate the molarity (original concentration) of the aqueous ammonia solution.
You do it!You do it!
66
Ionization Constants for Weak Monoprotic Acids and Bases
M
M3
4
363.2pOH-
103.2NH
103.21010OH
2.63 11.37-14.00pH - 14.00=pOH
pOH. thederivecan we14.00, = pOH + pH From
11.37=pH
67
Ionization Constants for Weak Monoprotic Acids and Bases• Use the ionization equation and some algebra to
get the equilibrium concentration.
NH 3 H2O NH 4 OH -
Initial[] xM
Change - 2.310-3 +2.310-3 +2.310-3
Equilibrium[] x - 2.310-3 M +2.310-3 +2.310-3
68
Ionization Constants for Weak Monoprotic Acids and Bases• Substitute these values into the ionization
constant expression.
Kb NH4
OH NH3 1.8 10 5
1.8 10 5 2.310 3 2.310 3
x 2.310 3
69
Ionization Constants for Weak Monoprotic Acids and Bases• Examination of the last equation suggests that
our simplifying assumption can be applied.• In other words (x-2.3x10-3) x.
– Making this assumption simplifies the calculation.
2.310 3 2
x1.8 10 5
x 0.30M NH3
70
Polyprotic Acids• Many weak acids contain two or more acidic hydrogens.
– Examples include H3PO4 and H3AsO4.
• The calculation of equilibria for polyprotic acids is done in a stepwise fashion.– There is an ionization constant for each step.
• Consider arsenic acid, H3AsO4, which has three ionization constants.1 Ka1 = 2.5 x 10-4
2 Ka2 = 5.6 x 10-8
3 Ka3 = 3.0 x 10-13
71
Polyprotic Acids
• The first ionization step for arsenic acid is:
H3AsO4 H H2AsO4
-
Ka1 H H2AsO4
H3AsO4 2.5 10 4
72
Polyprotic Acids
• The second ionization step for arsenic acid is:
H2AsO41- H HAsO4
2-
Ka2 H HAsO4
2 H2AsO4
1- 5.6 10 8
73
Polyprotic Acids
• The third ionization step for arsenic acid is:
HAsO42- H AsO 4
3-
Ka3 H AsO4
3 HAsO4
2- 3.0 10 13
74
Polyprotic Acids
• Notice that the ionization constants vary in the following fashion:
• This is a general relationship.– For weak polyprotic acids the Ka1 is always > Ka2,
etc.
a3a2a1 KKK
75
Polyprotic Acids
• Example 18-15: Calculate the concentration of all species in 0.100 M arsenic acid, H3AsO4, solution.
1 Write the first ionization step and represent the concentrations.Approach this problem exactly as previously done.
xMxMMx 100.0
AsOHHAsOH 4243
76
Polyprotic Acids
2 Substitute the algebraic quantities into the expression for Ka1.
apply.not does assumption gsimplifyin thecase, In this
0105.2105.2
105.210.0
K
105.2AsOH
AsOH HK
542
4a1
4
43
42a1
xx
x
xx
77
Polyprotic Acids
3 Use the quadratic equation to solve for x, and obtain both values of x.
x 2.5 10 4 2.5 10 4 2
4 1 2.5 10 5 2 1
x 5.110 3M and x 4.9 10 3M
H H2AsO4 xM 4.9 10 3M
H3AsO4 0.100 x M 0.095M
78
Polyprotic Acids
4 Next, write the equation for the second step ionization and represent the concentrations.
yMyMMy
M
)10(4.9lly algebraica
)10(4.9 step1st ]from [
HAsO + H AsOH
3-
3-
-24
+-42
79
Polyprotic Acids
5 Substitute the algebraic expressions into the second step ionization expression.
Ka2 =H3O
HAsO 42
H2AsO4 5.6 10 8
Ka2 =4.9 10-3 y y
4.9 10-3 y
For this step we can apply the assumption.
80
Polyprotic Acids
Ka2 =H3O
HAsO42
H2AsO4 5.6 10 8
Ka2 =4.9 10-3 y y
4.9 10-3 y
In this case the assumption can be applied.
y 4.9 10-3
Thus, 4.9 10-3 y 4.9 10-3
Ka2 =4.9 10-3 y
4.9 10-3 5.6 10 8
y 5.6 10 8M H 2nd
HAsO42
Note that H 1st H 2nd
81
Polyprotic Acids
6 Finally, repeat the entire procedure for the third ionization step.
MzMzMz
MM
105.6 changes ] [ of tionsrepresenta algebraic
105.6104.9 105.6 sionization 2 and 1 from s]' [
AsO H HAsO
8-
8-3-8-ndst
-34
24
82
Polyprotic Acids
7 Substitute the algebraic representations into the third ionization expression.
Ka3 =H3O
AsO43
HAsO42 3.0 10 13
Ka3 =4.9 10 3 5.6 10 8 z z
5.6 10 8 z The assumption can be applied, z 5.6 10-8.
83
Polyprotic Acids
• Use Kw to calculate the [OH-] in the 0.100 M H3AsO4 solution.
H OH 1.0 10 14
OH 1.0 10 14
H 1.0 10 14
4.9 10 3
OH 2.0 10 12M
4.9 10 3 z 5.6 10 8 3.0 10 13
z 3.4 10 18M H 3rd
AsO43
84
Polyprotic Acids• A comparison of the various species in 0.100 M
H3AsO4 solution follows.
Species Concentration
H3AsO4 0.095 M
H+ 0.0049 M
H2AsO4- 0.0049 M
HAsO42- 5.6 x 10-8 M
AsO43- 3.4 x 10-18 M
OH- 2.0 x 10-12 M
85
Solvolysis• This reaction process is the most difficult concept in this chapter.• Solvolysis is the reaction of a substance with the solvent in which
it is dissolved.• Hydrolysis refers to the reaction of a substance with water or its
ions.
• Combination of the anion of a weak acid with H3O+ ions from water to form nonionized weak acid molecules.
86
Solvolysis
• Hydrolysis refers to the reaction of a substance with water or its ions.– Hydrolysis is solvolysis in aqueous solutions.
• The combination of a weak acid’s anion with H3O+ ions, from water, to form nonionized weak acid molecules is a form of hydrolysis.
A H O HA H O
recall H O + H O H O OH
-3 2
2 2 3
87
Solvolysis
• The reaction of the anion of a weak monoprotic acid with water is commonly represented as:
mequilibriu water the
upsets OH of removal The
OH HA OH A+
3
-2
-
88
Solvolysis
• Recall that at 25oC• in neutral solutions:
[H3O+] = 1.0 x 10-7 M = [OH-]
• in basic solutions:[H3O+] < 1.0 x 10-7 M and [OH-] > 1.0 x 10-7 M
• in acidic solutions:[OH-] < 1.0 x 10-7 M and [H3O+] > 1.0 x 10-7 M
89
Solvolysis
• Remember from BrØnsted-Lowry acid-base theory:• The conjugate base of a strong acid is a very weak base.• The conjugate base of a weak acid is a stronger base.• Hydrochloric acid, a typical strong acid, is essentially
completely ionized in dilute aqueous solutions.
HCl H O H O Cl2 3 ~100%
90
Solvolysis
• The conjugate base of HCl, the Cl- ion, is a very weak base.– The chloride ion is such a weak base that it will not react with the hydronium ion.
• This fact is true for all strong acids and their anions.
Cl H O No rxn. in dilute aqueous solutions3
91
Solvolysis
• HF, a weak acid, is only slightly ionized in dilute aqueous solutions.• Its conjugate base, the F- ion, is a much stronger base than the Cl-
ion.
• The F- ions combine with H3O+ ions to form nonionized HF.– Two competing equilibria are established.
HF + H O H O F
only slightly
F + H O HF + H O
nearly completely
2 3+ -
-3
+2
92
Solvolysis
• Dilute aqueous solutions of salts that contain no free acid or base come in four types:
1. Salts of Strong Bases and Strong Acids
2. Salts of Strong Bases and Weak Acids
3. Salts of Weak Bases and Strong Acids
4. Salts of Weak Bases and Weak Acids
93
Salts of Strong Bases and Weak Acids• Salts made from strong acids and strong soluble
bases form neutral aqueous solutions.
• An example is potassium nitrate, KNO3, made from nitric acid and potassium hydroxide.
neutral. issolution theThus
OHOHupset oreaction t no is There
amounts. equalin present are HNO and KOH The
solution in are that ions The
OH OH OHOH
NOK KNO
-+3
3
HNOKOH
3-
22
3+OHin %100~
)(3
3
2
s
94
Salts of Strong Bases and Weak Acids• Salts made from strong soluble bases and weak acids hydrolyze to
form basic solutions.
– Anions of weak acids (strong conjugate bases) react with water to form hydroxide ions.
• An example is sodium hypochlorite, NaClO, made from sodium hydroxide and hypochlorous acid.
base?or acidstronger theisWhich
solution in ions Notice
OH + OH OH + OH
ClONaNaClO
HClONaOH
3-
22
-OHin %100~)(
2
s
95
Salts of Strong Bases and Weak Acids
Na ClO Na ClO
H O + H O OH + H O
ClO H O HClO H O
+ - in H O -
2 2-
3+
-3 2
2( )
~s
100%
• We can combine these last two equations into one single equation that represents the total reaction.
ClO H O HClO OH-2
96
Salts of Strong Bases and Weak Acids
Kb =HClO OH -
ClO -
• The equilibrium constant for this reaction, called the hydrolysis constant, is written as:
97
Salts of Strong Bases and Weak Acids• Algebraic manipulation of the previous expression give
us a very useful form of the expression.• Multiply the expression by one written as [H+]/ [H+].
H+/H+ = 1
Kb =HClO OH -
ClO - H H
Kb =HClO
H ClO - H OH -
1
98
Salts of Strong Bases and Weak Acids• Which can be rewritten as:
Kb =HClO
H ClO - H OH -
1
Kb =1
Ka for HClO
Kw
99
Salts of Strong Bases and Weak Acids• Which can be used to calculate the hydrolysis
constant for the hypochlorite ion:
K =1
KK
K =K
K=
1 10
3.5 10
K =HClO OH
ClO
ba for HClO
w
bw
a for HClO
-14
-8
b
2 9 10 7.
100
Salts of Strong Bases and Weak Acids• This same method can be applied to the anion of
any weak monoprotic acid.
A H2O HA OH -
Kb =HA OH
A KW
Ka for HA
101
Salts of Strong Bases and Weak Acids• Example 18-16: Calculate the hydrolysis constants for
the following anions of weak acids.
1. The fluoride ion, F-, the anion of hydrofluoric acid, HF. For HF, Ka=7.2 x 10-4.
F H O HF OH
KHF OH
F
KK
K
2
bw
a for HF
b
10 10
7 2 1014 10
14
411.
..
102
Salts of Strong Bases and Weak Acids• The cyanide ion, CN-, the anion of hydrocyanic acid, HCN.
For HCN, Ka = 4.0 x 10-10.
You do it!You do it!
CN H2O HCN +OH -
Kb HCN OH
CN Kw
Ka for HCN
Kb 1.0 10 14
4.0 10 10 2.5 10 5
103
Salts of Strong Bases and Weak Acids• Example 18-17: Calculate [OH-], pH and percent
hydrolysis for the hypochlorite ion in 0.10 M sodium hypochlorite, NaClO, solution. “Clorox”, “Purex”, etc., are 5% sodium hypochlorite solutions.
MMM 0.10 0.10 0.10
ClO Na NaClO OHin 100%~(s)
2
104
Salts of Strong Bases and Weak Acids • Set up the equation for the hydrolysis and the
algebraic representations of the equilibrium concentrations.
ClO + H O HClO + OH
Initial: 0.10
Change: - + +
At equil: 0.10 -
-2
-M M M
xM xM xM
x M xM xM
0 0
105
Salts of Strong Bases and Weak Acids• Substitute the algebraic expressions into the
hydrolysis constant expression.
Kb HClO OH
ClO 2.9 10 7
Kb x x
0.10 x 2.9 10 7
106
Salts of Strong Bases and Weak Acids• Substitute the algebraic expressions into the
hydrolysis constant expression.
10.23. pH get the we14.00 pOH pH From
3.77 pOH get the we][OH theFrom
OHClO107.1 becomesWhich
109.2 toreducesequation The
0.10 -0.10 and 0.10
case. in this made becan assumption gsimplifyin The
-
4
82
Mx
x
xx
107
Salts of Strong Bases and Weak Acids• The percent hydrolysis for the hypochlorite ion
may be represented as:
% hydrolysis =ClO- hydrolyzed
ClO- original
100%
% hydrolysis =1.7 10-4M
0.10M100% 0.17%
108
Salts of Strong Bases and Weak Acids• If a similar calculation is performed for 0.10 M NaF
solution and the results from 0.10 M sodium fluoride and 0.10 M sodium hypochlorite compared, the following table can be constructed.
Solution Ka Kb [OH-] (M) pH%
hydrolysis
NaF 7.2 x 10-4 1.4 x 10-11 1.2 x 10-6 8.08 0.0012
NaClO 3.5 x 10-8 2.9 x 10-7 1.7 x 10-4 10.23
0.17
109
Salts of Weak Bases and Strong Acids• Salts made from weak bases and strong acids
form acidic aqueous solutions.
• An example is ammonium bromide, NH4Br, made from ammonia and hydrobromic acid.
base?or acidstronger theisWhich
aresolution in Ions
OH OH OHOH
Br NH BrNH
HBrOHNH
3-
22
-4
100%~OHs4
4
2
110
Salts of Weak Bases and Strong Acids
OH excess generates
OHNHOHNH
OH excess leavingsolution fromit
removingion OH with thereacts
,NH acid, strong relatively The
3
23-
4
3
-
4
• The reaction may be more simply represented as:
OHNH OHNH 3324
111
Salts of Weak Bases and Strong Acids
• The hydrolysis constant expression for this process is:
• Or even more simply as:
Ka NH3 H3O
NH4
or Ka NH3 H
NH4
HNH NH 34
112
Salts of Weak Bases and Strong Acids• Multiplication of the hydrolysis constant
expression by [OH-]/ [OH-] gives:
Ka NH3 H3O
NH4
OH - OH -
Ka NH3
NH4 OH -
H3O OH - 1
113
Salts of Weak Bases and Strong Acids• Which we recognize as:
K
KK1
KK
K
ab NH
w w
b NH
a
3 3
1
10 10
18 105 6 10
14
510.
..
114
Salts of Weak Bases and Strong Acids• In its simplest form for this hydrolysis:
115
Salts of Weak Bases and Strong Acids• Example 18-18: Calculate [H+], pH, and percent
hydrolysis for the ammonium ion in 0.10 M ammonium bromide, NH4Br, solution.
1. Write down the hydrolysis reaction and set up the table as we have done before:
NH H O NH H
Initial[]: 0.10 + +
Change: - + +
Equilibrium[]: 0.10
4+
2 3
M xM xM
xM xM xM
x M xM xM
116
Salts of Weak Bases and Strong Acids2. Substitute the algebraic expressions into the
hydrolysis constant.
0.10 - 0.10 thus0.10
.applicable is assumption The
105.6=10.0
K
106.5NH
H NHK
10-a
10
4
3a
xx
x
xx
117
Salts of Weak Bases and Strong Acids3. Complete the algebra and determine the
concentrations and pH.
12.5pH
107.5HNH
107.5=
106.5
106.510.0
6-3
6-
112
10
M
Mx
x
x
xx
118
Salts of Weak Bases and Strong Acids4. The percent hydrolysis of the ammonium ion in
0.10 M NH4Br solution is:
%
%.
%
hydrolysis =NH
NH
hydrolysis =7.5 10
hydrolysis = 0.0075%
4+
hydrolized
4+
original
-6
100%
010100%
MM
119
Salts of Weak Bases and Weak Acids• Salts made from weak acids and weak bases can
form neutral, acidic or basic aqueous solutions.
– The pH of the solution depends on the relative values of the ionization constant of the weak acids and bases.
1. Salts of weak bases and weak acids for which parent Kbase =Kacid make neutral solutions.
• An example is ammonium acetate, NH4CH3COO, made from aqueous ammonia, NH3,and acetic acid, CH3COOH.
Ka for acetic acid = Kb for ammonia = 1.8 x 10-5.
120
Salts of Weak Bases and Weak Acids• The ammonium ion hydrolyzes to produce H+
ions. Its hydrolysis constant is:
NH NH H
KNH H
NH
4+
3
a3
4+
5 6 10 10.
121
Salts of Weak Bases and Weak Acids• The acetate ion hydrolyzes to produce OH- ions.
Its hydrolysis constant is:
CH COO H O CH COOH OH
KCH COOH OH
CH COO
3 2 3
b3
3
5 6 10 10.
122
Salts of Weak Bases and Weak Acids• Because the hydrolysis constants for both ions
are equal, their aqueous solutions are neutral.• Equal numbers of H+ and OH- ions are produced.
solution!in formed are base and acidA weak
aresolution in Ions
OH OH OH OH
COOCHNHCOOCHNH
COOHCHOHNH
3-
22
34100%~OH
34
34
2
123
Salts of Weak Bases and Weak Acids2. Salts of weak bases and weak acids for
which parent Kbase > Kacid make basic solutions.
• An example is ammonium hypochlorite, NH4ClO, made from aqueous ammonia, NH3,and hypochlorous acid, HClO.
Kb for NH3 = 1.8 x 10-5 > Ka for HClO = 3.5x10-8
124
Salts of Weak Bases and Weak Acids• The ammonium ion hydrolyzes to produce H+
ions. Its hydrolysis constant is:
NH4 NH3 H
Ka NH3 H
NH4 5.6 10 10
125
Salts of Weak Bases and Weak Acids• The hypochlorite ion hydrolyzes to produce OH-
ions. Its hydrolysis constant is:
• Because the Kb for ClO- ions is three orders of magnitude larger than the Ka for NH4
+ ions, OH- ions are produced in excess making the solution basic.
ClO H O HClO OH
KHClO OH
ClO
2
b -
2 9 10 7.
126
Salts of Weak Bases and Weak Acids
3. Salts of weak bases and weak acids for which parent Kbase < Kacid make acidic solutions.
• An example is trimethylammonium fluoride,(CH3)3NHF, made from trimethylamine, (CH3)3N,and hydrofluoric acid acid, HF.
– Kb for (CH3)3N = 7.4 x 10-5 < Ka for HF = 7.2 x 10-4
127
Salts of Weak Bases and Weak Acids• Both the cation, (CH3)3NH+, and the anion, F-,
hydrolyze.
CH NH F CH NH F3 3H O~100%
3 32
128
Salts of Weak Bases and Weak Acids• The trimethylammonium ion hydrolyzes to
produce H+ ions. Its hydrolysis constant is:
(CH ) NH CH N H
KCH N H
(CH ) NH
KK
K
3 3+
3
a3
3 3+
w
b for CH N
a
3
( )
( )
.
..
( )
3
3
14
510
3
10 10
7 4 1014 10
129
Salts of Weak Bases and Weak Acids• The fluoride ion hydrolyzes to produce OH- ions.
Its hydrolysis constant is:
• Because the Ka for (CH3)3NH+ ions is one order of magnitude larger than the Kb for F- ions, H+ ions are produced in excess making the solution acidic.
F H O HF OH
KHF OH
F
KK
K
2
b -w
a for HF
b
10 10
7 2 1014 10
14
411.
..
130
Salts of Weak Bases and Weak Acids• Summary of the major points of
hydrolysis up to now.1. The reactions of anions of weak
monoprotic acids (from a salt) with water to form free molecular acids and OH-.
A + H O HA + OH
KK
K
-2
-
bw
a HA
131
Salts of Weak Bases and Weak Acids2. The reactions of anions of weak monoprotic
acids (from a salt) with water to form free molecular acids and OH-.
BH + H O B + H O
KK
K B = weak base
+2 3
+
aw
b B
132
Salts of Weak Bases and Weak Acids• Aqueous solutions of salts of strong acids and
strong bases are neutral.• Aqueous solutions of salts of strong bases
and weak acids are basic. • Aqueous solutions of salts of weak bases and
strong acids are acidic.• Aqueous solutions of salts of weak bases and
weak acids can be neutral, basic or acidic.The values of Ka and Kb determine the pH.
133
Hydrolysis of Small Highly-Charged Cations
• Cations of insoluble bases (metal hydroxides) become hydrated in solution.– An example is a solution of Be(NO3)3.
– Be2+ ions are thought to be tetrahydrated and sp3 hybridized.
Beaq
2+ Be(OH2 )4 2
1s 2s 2p
4 Be
molecules water dcoordinateon pairs e
)Be(OH
Be
2p 2s 1s
-
242
hybrids sp form
2
3
xxxxxxxx
134
Hydrolysis of Small Highly-Charged Cations• In condensed form it is represented as:
or, even more simply as:
Be(OH ) H O Be OH OH H O2 4 2 2 3 32
Be H O Be(OH) H2+2
135
Hydrolysis of Small Highly-Charged Cations• The hydrolysis constant expression for [Be(OH2)4]2+
and its value are:
or, more simply
Ka Be(OH2)3(OH)+ H3O
Be(OH2)4
2 1.0 10 5
Ka Be(OH)+ H
Be2+ 1.0 10 5
136
Hydrolysis of Small Highly-Charged Cations• Example 18-19: Calculate the pH and percent
hydrolysis in 0.10 M aqueous Be(NO3)2 solution.
1. The equation for the hydrolysis reaction and representations of concentrations of various species are:
xMxMMx 10.0
H Be(OH) OH Be 22
137
Hydrolysis of Small Highly-Charged Cations2. Algebraic substitution of the expressions into
the hydrolysis constant:
x x 0.10 x 1.0 10 5
The simplifying assumption applies.
x 0.10 and 0.10 - x 0.10
x x 0.10
1.0 10 5
x 2 1.0 10 6
x 1.0 10 3M
H Be2 hydrolyzed
1.0 10 3M
pH 3.00
138
Hydrolysis of Small Highly-Charged Cations
3 Calculate the percent hydrolysis of Be2+.
% hydrolyzed =Be2+ hydrolyzed
Be2+ original
100%
% hydrolyzed =1.0 10-3
0.10100% 1.0%
139
Hydrolysis of Small Highly-Charged Cations• This table is a comparison of 0.10 M Be(NO3 )2
solution and 0.10 M CH3COOH solution.
Solution [H3O+] pH
% hydrolysis or
% ionization0.10 M Be(NO3)2 1.0 x 10-3
M3.00 1.0%
0.10 M CH3COOH
1.3 x 10-3 M 2.89 1.3%
Notice that the Be solution is almost as acidic as the acetic acid solution.
140
18 Ionic Equilibria: Acids and Bases