1
CHE 116: General Chemistry
CHAPTER
SIXTEEN
Copyright © Tyna L. Heise 2001-2002
All Rights Reserved
Prof. T. L. Heise
CHE
116
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Acids and Bases: Review
Properties of Acids
sour taste
change with litmus
Properties of Bases
bitter taste
change with litmus
Prof. T. L. Heise
CHE
116
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Acids and Bases: Review
1830 - scientists have recognized that all acids
contain hydrogen, but not all hydrogen
bearing compounds are acids
Svante Arrhenius - linked acid behavior with
the presence of an H+ and base behavior
with the presence of an OH-
Prof. T. L. Heise
CHE
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Bronsted-Lowry Acids and Bases
Arrhenius’ definition is useful, but restricts
acid base reactions to aqueous conditions
Johannes Bronsted and Thomas Lowry
proposed a more general definition which
involves the transfer of an H+ ion from one
molecule to another
Prof. T. L. Heise
CHE
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Bronsted-Lowry Acids and Bases
H+ ion is simply a proton with no surrounding
valence electron.
Small particle interacts strongly with the
nonbonding pairs of water molecules to
form hydrated hydrogen ions
chemists use H+ and H3O+
interchangeably
Prof. T. L. Heise
CHE
116
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Bronsted-Lowry Acids and Bases
Fig 16.1
Prof. T. L. Heise
Demonstrates the
interconnections
possible between
hydrogenated water
CHE
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Bronsted-Lowry Acids and Bases
Definitions:
Acid - any compound which transfers an H+
to another molecule
Base - any compound which accepts a
transfer of an H+ from another molecule
* an acid and base always work together
Prof. T. L. Heise
CHE
116
8
Bronsted-Lowry Acids and Bases
Definitions:
Amphoteric - some substances can be an
acid or a base depending on reaction
Conjugate Acid Base pairs - two compounds
that differ only in the presence of an H+.
The molecule with the extra H is the
acid.
Prof. T. L. Heise
CHE
116
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Bronsted-Lowry Acids and Bases
Fig 16.7, 16.8
Prof. T. L. Heise
CHE
116
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Bronsted-Lowry Acids and Bases
Sample Exercise: Write the formula for the
conjugate acid of each of the following:
HSO3FPO43CO
Prof. T. L. Heise
CHE
116
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Bronsted-Lowry Acids and Bases
Sample Exercise: Write the formula for the
conjugate acid of each of the following:
HSO3FGiven a base,
bases accept H+,
3PO4
so add an H+ to
each molecule.
CO
Prof. T. L. Heise
CHE
116
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Bronsted-Lowry Acids and Bases
Sample Exercise: Write the formula for the
conjugate acid of each of the following:
H2SO3
HSO3FGiven a base,
bases accept H+,
3PO4
so add an H+ to
each molecule.
CO
Prof. T. L. Heise
CHE
116
13
Bronsted-Lowry Acids and Bases
Sample Exercise: Write the formula for the
conjugate acid of each of the following:
H2SO3
HSO3HF
FGiven a base,
bases accept H+,
3PO4
so add an H+ to
each molecule.
CO
Prof. T. L. Heise
CHE
116
14
Bronsted-Lowry Acids and Bases
Sample Exercise: Write the formula for the
conjugate acid of each of the following:
H2SO3
HSO3HF
FGiven a base,
bases accept H+,
23HPO4
PO4
so add an H+ to
each molecule.
CO
Prof. T. L. Heise
CHE
116
15
Bronsted-Lowry Acids and Bases
Sample Exercise: Write the formula for the
conjugate acid of each of the following:
H2SO3
HSO3HF
FGiven a base,
bases accept H+,
23HPO4
PO4
so add an H+ to
each molecule.
HCO+
CO
Prof. T. L. Heise
CHE
116
16
Bronsted-Lowry Acids and Bases
Sample Exercise: When lithium oxide, Li2O,
is dissolved in water, the solution turns basic
from the reaction of the oxide ion, O2-, with
water. Write the reaction that occurs, and
identify the conjugate acid base pairs.
Prof. T. L. Heise
CHE
116
17
Bronsted-Lowry Acids and Bases
Sample Exercise: When lithium oxide, Li2O,
is dissolved in water, the solution turns basic
from the reaction of the oxide ion, O2-, with
water. Write the reaction that occurs, and
identify the conjugate acid base pairs.
O2- + H2O  OH- + OHProf. T. L. Heise
CHE
116
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Bronsted-Lowry Acids and Bases
Sample Exercise: When lithium oxide, Li2O,
is dissolved in water, the solution turns basic
from the reaction of the oxide ion, O2-, with
water. Write the reaction that occurs, and
identify the conjugate acid base pairs.
O2- + H2O  OH- + OHProf. T. L. Heise
CHE
116
19
Bronsted-Lowry Acids and Bases
Sample Exercise: When lithium oxide, Li2O,
is dissolved in water, the solution turns basic
from the reaction of the oxide ion, O2-, with
water. Write the reaction that occurs, and
identify the conjugate acid base pairs.
acid
base
O2- + H2O  OH- + OHbase
Prof. T. L. Heise
acid
CHE
116
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Bronsted-Lowry Acids and Bases
Relative Strengths of Acids and Bases:
the more readily a substance donates an
H+, the less readily it’s conjugate base will
accept one
 the more readily a substance accepts an
H+, the less readily it’s conjugate acid will
donate one
the stronger one of the substances is, the
weaker it’s conjugate
Prof. T. L. Heise
CHE
116
21
Bronsted-Lowry Acids and Bases
Relative Strengths of Acids and Bases:
strong acids completely transfer their
protons to water, leaving no undissociated
molecules
 weak acids are those that only partly
dissociate in aqueous solution and
therefore exist in the solution as a mixture
of acid molecules and component ions
Prof. T. L. Heise
CHE
116
22
Bronsted-Lowry Acids and Bases
Relative Strengths of Acids and Bases:
negligible acids are those that have
hydrogen but do not donate them at all,
their conjugate bases would be extremely
strong, reacting with water to complete
their octet and leaving OH- behind.
 In every acid base reaction, the position
of the equilibrium favors transfer of H+
from stronger side to weaker side
Prof. T. L. Heise
CHE
116
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Bronsted-Lowry Acids and Bases
Fig. 16.4
Prof. T. L. Heise
CHE
116
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Bronsted-Lowry Acids and Bases
Sample exercise: For each of the following
reactions, use Fig. 16.4 to predict whether
the equilibrium lies predominantly to the left
or right:
a) PO43-(aq)+H2O(l)HPO42-(aq)+OH-(aq)
b) NH4+(aq)+OH-(aq)  NH3(aq)+H2O(l)
Prof. T. L. Heise
CHE
116
25
Bronsted-Lowry Acids and Bases
Sample exercise: For each of the following
reactions, use Fig. 16.4 to predict whether
the equilibrium lies predominantly to the left
or right:
a) PO43-(aq)+H2O(l)HPO42-(aq)+OH-(aq)
2 acids are: H2O and HPO422 bases are: PO43- and OHProf. T. L. Heise
CHE
116
26
Bronsted-Lowry Acids and Bases
Sample exercise: For each of the following
reactions, use Fig. 16.4 to predict whether
the equilibrium lies predominantly to the left
or right:
a) PO43-(aq)+H2O(l)HPO42-(aq)+OH-(aq)
2 acids are: H2O and HPO422 bases are: PO43- and OHred indicates strength
Prof. T. L. Heise
CHE
116
27
Bronsted-Lowry Acids and Bases
Sample exercise: For each of the following
reactions, use Fig. 16.4 to predict whether
the equilibrium lies predominantly to the left
or right:
a) PO43-(aq)+H2O(l)HPO42-(aq)+OH-(aq)
2 acids are: H2O and HPO422 bases are: PO43- and OHreverse reaction favored
Prof. T. L. Heise
CHE
116
28
Bronsted-Lowry Acids and Bases
Sample exercise: For each of the following
reactions, use Fig. 16.4 to predict whether
the equilibrium lies predominantly to the left
or right:
a) PO43-(aq)+H2O(l)HPO42-(aq)+OH-(aq)
2 acids are: H2O and HPO422 bases are: PO43- and OHshifts left
Prof. T. L. Heise
CHE
116
29
Bronsted-Lowry Acids and Bases
Sample exercise: For each of the following
reactions, use Fig. 16.4 to predict whether
the equilibrium lies predominantly to the left
or right:
b) NH4+(aq)+OH-(aq)  NH3(aq)+H2O(l)
Prof. T. L. Heise
CHE
116
30
Bronsted-Lowry Acids and Bases
Sample exercise: For each of the following
reactions, use Fig. 16.4 to predict whether
the equilibrium lies predominantly to the left
or right:
b) NH4+(aq)+OH-(aq)  NH3(aq)+H2O(l)
2 acids are: NH4+ and H2O
2 bases are: NH3 and OHProf. T. L. Heise
CHE
116
31
Bronsted-Lowry Acids and Bases
Sample exercise: For each of the following
reactions, use Fig. 16.4 to predict whether
the equilibrium lies predominantly to the left
or right:
b) NH4+(aq)+OH-(aq)  NH3(aq)+H2O(l)
2 acids are: NH4+ and H2O
2 bases are: NH3 and OHred indicates strength
Prof. T. L. Heise
CHE
116
32
Bronsted-Lowry Acids and Bases
Sample exercise: For each of the following
reactions, use Fig. 16.4 to predict whether
the equilibrium lies predominantly to the left
or right:
b) NH4+(aq)+OH-(aq)  NH3(aq)+H2O(l)
2 acids are: NH4+ and H2O
2 bases are: NH3 and OHfavors forward reaction
Prof. T. L. Heise
CHE
116
33
Bronsted-Lowry Acids and Bases
Sample exercise: For each of the following
reactions, use Fig. 16.4 to predict whether
the equilibrium lies predominantly to the left
or right:
b) NH4+(aq)+OH-(aq)  NH3(aq)+H2O(l)
2 acids are: NH4+ and H2O
2 bases are: NH3 and OHshifts right
Prof. T. L. Heise
CHE
116
34
The Autoionization of Water
One of the most important properties of water
is its ability to ac as either a Bronsted acid or
Bronsted base, depending on circumstances.
One water molecule can donate a proton
to another water molecule
Fig 16.10
Prof. T. L. Heise
CHE
116
35
The Autoionization of Water
The autoionization of water, although rapid
and weak, does exist as an equilibrium, and
therefore has an equilibrium constant
expression:
Keq = [H3O+][OH-]
[H2O]2
* because water is a liquid, it can be
excluded from the equation...
Prof. T. L. Heise
CHE
116
36
The Autoionization of Water
Keq[H2O]2 = [H3O+][OH-]
Kw = [H3O+][OH-] = 1.0 x 10-14
* this equation is not only applicable to
water, but to all aqueous solutions, and it is
upon this fact that the pH scale was built.
Prof. T. L. Heise
CHE
116
37
The Autoionization of Water
Sample exercise: Indicate whether each of the
following solutions is neutral, acidic, or
basic:
a) [H+] = 2 x 10-5
b) [OH-] = 3 x 10-9
c) [OH-] = 1 x 10-7
Prof. T. L. Heise
CHE
116
38
The Autoionization of Water
Sample exercise: Indicate whether each of the
following solutions is neutral, acidic, or
basic:
a) [H+] = 2 x 10-5 then [OH-] must
equal 1.0 x 10-14 which is
2 x10-5
[OH-] = 5.0 x 10-10
Prof. T. L. Heise
CHE
116
39
The Autoionization of Water
Sample exercise: Indicate whether each of the
following solutions is neutral, acidic, or
basic:
a) [H+] = 2 x 10-5 then [OH-] must
equal 1.0 x 10-14 which is
2 x10-5
[OH-] = 5.0 x 10-10
[H+] > [OH-] so acidic
Prof. T. L. Heise
CHE
116
40
The Autoionization of Water
Sample exercise: Indicate whether each of the
following solutions is neutral, acidic, or
basic:
b) [OH-] = 3 x 10-9 then [H+] must
equal 1.0 x 10-14 which is
3 x10-9
[H+] = 3.3 x 10-6
Prof. T. L. Heise
CHE
116
41
The Autoionization of Water
Sample exercise: Indicate whether each of the
following solutions is neutral, acidic, or
basic:
b) [OH-] = 3 x 10-9 then [H+] must
equal 1.0 x 10-14 which is
3 x10-9
[H+] = 3.3 x 10-6
[H+] > [OH-] so acidic
Prof. T. L. Heise
CHE
116
42
The Autoionization of Water
Sample exercise: Indicate whether each of the
following solutions is neutral, acidic, or
basic:
c) [OH-] = 1 x 10-7 then [H+] must
equal 1.0 x 10-14 which is
1 x10-7
[H+] = 1.0 x 10-7
Prof. T. L. Heise
CHE
116
43
The Autoionization of Water
Sample exercise: Indicate whether each of the
following solutions is neutral, acidic, or
basic:
c) [OH-] = 1 x 10-7 then [H+] must
equal 1.0 x 10-14 which is
1 x10-7
[H+] = 1.0 x 10-7
[H+] = [OH-] so neutral
Prof. T. L. Heise
CHE
116
44
The pH Scale
For convenience, we can use a logarithmic
version of concentration to turn the very
small concentrations of [H+] and [OH-] into
whole numbers.
p(anything) = - log[anything]
p(H) = -log[H+]
p(OH) = - log[OH-]
** pH + pOH = 14
Prof. T. L. Heise
CHE
116
45
The pH Scale
Prof. T. L. Heise
CHE
116
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The pH Scale
Common household
products and their
relative pH’s.
Prof. T. L. Heise
CHE
116
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The pH Scale
Sample exercise: In a sample of lemon juice
[H+] is 3.8 x 10-4 M. What is the pH?
Prof. T. L. Heise
CHE
116
48
The pH Scale
Sample exercise: In a sample of lemon juice
[H+] is 3.8 x 10-4 M. What is the pH?
pH = -log[H+]
Prof. T. L. Heise
CHE
116
49
The pH Scale
Sample exercise: In a sample of lemon juice
[H+] is 3.8 x 10-4 M. What is the pH?
pH = -log[H+]
pH = -log[3.8 x 10-4]
Prof. T. L. Heise
CHE
116
50
The pH Scale
Sample exercise: In a sample of lemon juice
[H+] is 3.8 x 10-4 M. What is the pH?
pH = -log[H+]
pH = -log[3.8 x 10-4]
pH = 3.42
Prof. T. L. Heise
CHE
116
51
The pH Scale
Sample exercise: A commonly available
window-cleaning solution has a [H+] is
5.3 x 10-9 M. What is the pH?
Prof. T. L. Heise
CHE
116
52
The pH Scale
Sample exercise: A commonly available
window-cleaning solution has a [H+] is
5.3 x 10-9 M. What is the pH?
pH = -log[H+]
Prof. T. L. Heise
CHE
116
53
The pH Scale
Sample exercise: A commonly available
window-cleaning solution has a [H+] is
5.3 x 10-9 M. What is the pH?
pH = -log[H+]
pH = -log[5.3 x 10-9]
Prof. T. L. Heise
CHE
116
54
The pH Scale
Sample exercise: A commonly available
window-cleaning solution has a [H+] is
5.3 x 10-9 M. What is the pH?
pH = -log[H+]
pH = -log[5.3 x 10-9]
pH = 8.28
Prof. T. L. Heise
CHE
116
55
The pH Scale
Sample exercise: A solution formed by
dissolving an antacid tablet has a pH of 9.18.
Calculate [H+].
Prof. T. L. Heise
CHE
116
56
The pH Scale
Sample exercise: A solution formed by
dissolving an antacid tablet has a pH of 9.18.
Calculate [H+].
pH = -log[H+]
Prof. T. L. Heise
CHE
116
57
The pH Scale
Sample exercise: A solution formed by
dissolving an antacid tablet has a pH of 9.18.
Calculate [H+].
pH = -log[H+]
9.18 = -log[H+]
Prof. T. L. Heise
CHE
116
58
The pH Scale
Sample exercise: A solution formed by
dissolving an antacid tablet has a pH of 9.18.
Calculate [H+].
pH = -log[H+]
9.18 = -log[H+]
10-9.18 = [H+]
Prof. T. L. Heise
CHE
116
59
The pH Scale
Sample exercise: A solution formed by
dissolving an antacid tablet has a pH of 9.18.
Calculate [H+].
pH = -log[H+]
9.18 = -log[H+]
10-9.18 = [H+]
6.61 x 10-10 = [H+]
Prof. T. L. Heise
CHE
116
60
The pH Scale
Measuring pH: a pH can be measured quickly
and accurately using a pH meter.
A pair of electrodes connected to a meter
capable of measuring small voltages
a voltage which varies with pH is
generated when the electrodes are placed
in a solution
calibrated to give pH
Prof. T. L. Heise
CHE
116
61
The pH Scale
Measuring pH: a pH can be measured quickly
and accurately using a pH meter.
Electrodes come in a variety of shapes
and sizes
a set of electrodes exists that can be
placed inside a human cell
acid base indicators can be used, but are
much less precise
Prof. T. L. Heise
CHE
116
62
The pH Scale
Fig 16.7
Prof. T. L. Heise
CHE
116
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Strong Acids and Bases
Strong acids and bases are strong electrolytes,
existing in aqueous solution entirely as ions.
Strong Acids
HCl
HBr
HI
monoprotic
HNO3
HClO3
HClO4
H2SO4
diprotic
Prof. T. L. Heise
CHE
116
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Strong Acids and Bases
Strong acids and bases are strong electrolytes,
existing in aqueous solution entirely as ions.
Calculating the pH of a solution made up
entirely of ions means the [H+] is
proportional to [acid]
Prof. T. L. Heise
CHE
116
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Strong Acids and Bases
An aqueous solution of HNO3 has a pH of
2.66. What is the concentration of the acid?
Prof. T. L. Heise
CHE
116
66
Strong Acids and Bases
An aqueous solution of HNO3 has a pH of
2.66. What is the concentration of the acid?
pH = -log[H+]
Prof. T. L. Heise
CHE
116
67
Strong Acids and Bases
An aqueous solution of HNO3 has a pH of
2.66. What is the concentration of the acid?
pH = -log[H+]
2.66 = -log[H+]
Prof. T. L. Heise
CHE
116
68
Strong Acids and Bases
An aqueous solution of HNO3 has a pH of
2.66. What is the concentration of the acid?
pH = -log[H+]
2.66 = -log[H+]
10-2.66 = [H+]
2.2 x 10-3 = [H+]
Prof. T. L. Heise
CHE
116
69
Strong Acids and Bases
An aqueous solution of HNO3 has a pH of
2.66. What is the concentration of the acid?
pH = -log[H+]
2.66 = -log[H+]
10-2.66 = [H+]
2.2 x 10-3 = [H+]
2.2 x 10-3 M H+
1 mole HNO3
1 mole H+
Prof. T. L. Heise
CHE
116
70
Strong Acids and Bases
An aqueous solution of HNO3 has a pH of
2.66. What is the concentration of the acid?
pH = -log[H+]
2.66 = -log[H+]
10-2.66 = [H+]
2.2 x 10-3 = [H+]
2.2 x 10-3 M H+
1 mole HNO3
1 mole H+
2.2 x 10-3 HNO3
Prof. T. L. Heise
CHE
116
71
Strong Acids and Bases
The most soluble common bases are the ionic
hydroxides of the alkali and alkaline earth
metals.
Due to the complete dissociation of the base
into its ion components makes the pH
calculation equally straightforward
Prof. T. L. Heise
CHE
116
72
Strong Acids and Bases
Sample exercise: What is the concentration of
a solution of KOH for which the pH is 11.89?
Prof. T. L. Heise
CHE
116
73
Strong Acids and Bases
Sample exercise: What is the concentration of
a solution of KOH for which the pH is 11.89?
pH = -log[H+]
Prof. T. L. Heise
CHE
116
74
Strong Acids and Bases
Sample exercise: What is the concentration of
a solution of KOH for which the pH is 11.89?
pH = -log[H+]
11.89 = -log[H+]
Prof. T. L. Heise
CHE
116
75
Strong Acids and Bases
Sample exercise: What is the concentration of
a solution of KOH for which the pH is 11.89?
pH = -log[H+]
11.89 = -log[H+]
10-11.89 = [H+]
Prof. T. L. Heise
CHE
116
76
Strong Acids and Bases
Sample exercise: What is the concentration of
a solution of KOH for which the pH is 11.89?
pH = -log[H+]
11.89 = -log[H+]
10-11.89 = [H+]
1.29 x 10-12 = [H+]
Prof. T. L. Heise
CHE
116
77
Strong Acids and Bases
Sample exercise: What is the concentration of
a solution of KOH for which the pH is 11.89?
pH = -log[H+]
11.89 = -log[H+]
10-11.89 = [H+]
1.29 x 10-12 = [H+]
1.0 x 10-14 = [OH-]
1.29 x 10-12
Prof. T. L. Heise
CHE
116
78
Strong Acids and Bases
Sample exercise: What is the concentration of
a solution of KOH for which the pH is 11.89?
pH = -log[H+]
11.89 = -log[H+]
10-11.89 = [H+]
1.29 x 10-12 = [H+]
1.0 x 10-14 = [OH-] = 7.8 x 10-3
1.29 x 10-12
Prof. T. L. Heise
CHE
116
79
Strong Acids and Bases
Sample exercise: What is the concentration of
a solution of KOH for which the pH is 11.89?
1.0 x 10-14 = [OH-] = 7.8 x 10-3
1.29 x 10-12
7.8 x 10-3 M OH- 1 mol KOH
1 mol OH-
Prof. T. L. Heise
CHE
116
80
Strong Acids and Bases
Sample exercise: What is the concentration of
a solution of KOH for which the pH is 11.89?
1.0 x 10-14 = [OH-] = 7.8 x 10-3
1.29 x 10-12
7.8 x 10-3 M OH- 1 mol KOH
1 mol OH7.8 x 10-3 M KOH
Prof. T. L. Heise
CHE
116
81
Weak Acids
Most acids are weak acids and only partially
dissociate in aqueous solution.
The extent to which a weak acid dissociates
can be expressed using an equilibrium
constant for the ionization reaction
HX + H2O  H3O+ + XKa = [H3O+][X-]
[HX]
*the larger the Ka the stronger the acid
Prof. T. L. Heise
CHE
116
82
Weak Acids
Calculating Ka from pH: a much more
complicated calculation is required for the
determinations of weak acids, in many cases,
due to the extremely small magnitude of the
values, some simpler approximations can be
made.
Prof. T. L. Heise
CHE
116
83
Weak Acids
Sample exercise: Niacin, one of the B
vitamins, has the following molecular
structure:
C
O
H
O
N
A 0.020 M solution of niacin has a pH of 3.26
a) What % of the acid is ionized in this
solution?
b) What is the acid-dissociation constant?
Prof. T. L. Heise
CHE
116
84
Weak Acids
Sample exercise: A 0.020 M solution of niacin
has a pH of 3.26
a) What % of the acid is ionized in this
solution?
pH = -log[H+]
3.26 = -log[H+]
e-3.26 = [H+]
5.5 x 10-4 = [H+]
Prof. T. L. Heise
CHE
116
85
Weak Acids
Sample exercise: A 0.020 M solution of niacin
has a pH of 3.26
a) What % of the acid is ionized in this
solution?
C6H4NOOH 
C6H4NOO- + H+
initial
0.020
0
change -5.5 x 10-4 +5.5 x 10-4
equil
0.01945 5.5 x 10-4
Prof. T. L. Heise
0
+5.5 x 10-4
5.5 x 10-4
CHE
116
86
Weak Acids
Sample exercise: A 0.020 M solution of niacin
has a pH of 3.26
a) What % of the acid is ionized in this
solution?
% = part/total x 100
= 5.5 x 10-4/0.01945 x 100
= 2.8%
Prof. T. L. Heise
CHE
116
87
Weak Acids
Sample exercise: A 0.020 M solution of niacin
has a pH of 3.26
b) What is the acid-dissociation constant?
C6H4NOOH 
C6H4NOO- + H+
Ka = [C6H4NOO- ][H+]
[C6H4NOOH]
= (5.5 x 10-4 )(5.5 x 10-4 )
0.01945
= 1.55 x10-5
Prof. T. L. Heise
CHE
116
88
Weak Acids
Using Ka to calculate pH:
Using the value of Ka and knowing the initial
concentration of the weak acid, we can
calculate the concentration of H+(aq).
Example: Calculate the pH of a 0.30 M
solution of acetic acid.
HC2H3O2(aq)  H+(aq) + C2H3O2-(aq)
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CHE
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Weak Acids
Example: Calculate the pH of a 0.30 M
solution of acetic acid.
HC2H3O2(aq)  H+(aq) + C2H3O2-(aq)
From Table 16.2, Ka = 1.8 x 10-5
Ka = 1.8 x 10-5 = [H+][C2H3O2-]
[HC2H3O2]
set up data table of concentrations involved...
Prof. T. L. Heise
CHE
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90
Weak Acids
Ka = 1.8 x 10-5 = [H+][C2H3O2-]
[HC2H3O2]
set up data table of concentrations involved…
[HC2H3O2] [H+] [C2H3O2-]
Initial
0.30 M
0
0
Change
-x
+x
+x
Equilibrium 0.30 - x
x
x
Prof. T. L. Heise
CHE
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91
Weak Acids
Input concentrations in formula
Ka = 1.8 x 10-5 = [x][x]
[0.30 -x]
This will lead to a quadratic equation, but we
can simplify the problem a bit. All weak
acids dissociate so little that we can assume
the initial concentration of the acid remains
essentially the same, that means we can
rewrite the formula to read...
Prof. T. L. Heise
CHE
116
92
Weak Acids
Ka = 1.8 x 10-5 = [x][x]
[0.30]
1.8 x 10-5 (0.30) = x2
1.8 x 10-5 (0.30) = x
0.0023 = x
0.0023 = [H+] pH = -log [H+]
= -log[0.0023]
= 2.64
Prof. T. L. Heise
CHE
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93
Weak Acids
Sample Exercise: The Ka for niacin is
1.5 x 10-5. What is the pH of a 0.010 M
solution of niacin?
Prof. T. L. Heise
CHE
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94
Weak Acids
Sample Exercise: The Ka for niacin is
1.5 x 10-5. What is the pH of a 0.010 M
solution of niacin?
C6H4NOOH  C6H4NOO- + H+
Ka = 1.5 x10-5 = [C6H4NOO-][H+]
[C6H4NOOH]
Prof. T. L. Heise
CHE
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95
Weak Acids
Sample Exercise: The Ka for niacin is
1.5 x 10-5. What is the pH of a 0.010 M
solution of niacin?
C6H4NOOH C6H4NOO- H+
initial
0.010
change
-x
equilibrium 0.010 -x
Prof. T. L. Heise
0
+x
x
0
+x
x
CHE
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96
Weak Acids
Sample Exercise: The Ka for niacin is
1.5 x 10-5. What is the pH of a 0.010 M
solution of niacin?
C6H4NOOH  C6H4NOO- + H+
Ka = 1.5 x10-5 = [x][x]
[0.010]
Prof. T. L. Heise
CHE
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97
Weak Acids
Sample Exercise: The Ka for niacin is
1.5 x 10-5. What is the pH of a 0.010 M
solution of niacin?
1.5 x10-5 [0.010] = x2
1.5 x10-5 [0.010] = x
3.87 x 10-4 = x
Prof. T. L. Heise
CHE
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98
Weak Acids
Sample Exercise: The Ka for niacin is
1.5 x 10-5. What is the pH of a 0.010 M
solution of niacin?
3.87 x 10-4 = x
3.87 x 10-4 = [H+]
Prof. T. L. Heise
pH = -log[H+]
pH = -log[3.87 x 10-4]
pH = 3.41
CHE
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99
Weak Acids
The results seen in the two previous
examples are typical of weak acids. The
lower number of ions produced during the
partial dissociation causes less electrical
conductivity and a slower reaction rate with
metals.
Percent ionization is a good way to discover
the actual conductivity, however...
Prof. T. L. Heise
CHE
116
100
Weak Acids
As the concentration of a weak acid
increases, the % ionized decreases.
Prof. T. L. Heise
CHE
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101
Weak Acids
As the concentration of a weak acid
increases, the % ionized decreases.
Prof. T. L. Heise
CHE
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102
Weak Acids
Sample Exercise: Calculate the % of niacin
molecules ionized in
a) the previous exercise
b) a 1.0 x 10-3 M solution
Prof. T. L. Heise
CHE
116
103
Weak Acids
Sample Exercise: Calculate the % of niacin
molecules ionized in
a) the previous exercise
Our approximation was good so
% = part x 100
total
= 3.87 x 10-4 x 100 = 3.9%
0.010
Prof. T. L. Heise
CHE
116
104
Weak Acids
Sample Exercise: Calculate the % of niacin
molecules ionized in
b) a 1.0 x 10-3 M solution
Ka = 1.5 x10-5 = [x][x]
[1.0 x 10-3]
1.5 x10-5 (1.0 x 10-3) = x2
1.2 x 10-4
Prof. T. L. Heise
CHE
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105
Weak Acids
Sample Exercise: Calculate the % of niacin
molecules ionized in
b) a 1.0 x 10-3 M solution
Ka = 1.5 x10-5 = [x][x]
[1.0 x 10-3] but, 1.2 x 10-3 x 100
1.5 x10-5 (1.0 x 10-3) = x2
1.0 x 10-3
1.2 x 10-3
Prof. T. L. Heise
is greater than 5%
so use quadratic...
CHE
116
106
Weak Acids
Sample Exercise: Calculate the % of niacin
molecules ionized in
b) a 1.0 x 10-3 M solution
Ka = 1.5 x10-5 = [x][x]
[1.0 x 10-3 - x]
-1.5 x 10-5(1.0 x10-3) + 1.5 x 10-5x + x2 = 0
Prof. T. L. Heise
CHE
116
107
Weak Acids
Sample Exercise: Calculate the % of niacin
molecules ionized in
b) a 1.0 x 10-3 M solution
-1.5 x 10-5(1.0 x10-3) + 1.5 x 10-5x + x2 = 0
x = -b ± b2 - 4ac
2a
x=
Prof. T. L. Heise
CHE
116
108
Weak Acids
Sample Exercise: Calculate the % of niacin
molecules ionized in
b) a 1.0 x 10-3 M solution
-1.5 x 10-5(1.0 x10-3) + 1.5 x 10-5x + x2 = 0
x = -b ± b2 - 4ac
2a
x = 1.1 x 10-4 or -1.3 x 10-4
Prof. T. L. Heise
CHE
116
109
Weak Acids
Sample Exercise: Calculate the % of niacin
molecules ionized in
b) a 1.0 x 10-3 M solution
x = 1.1 x 10-4
1.1 x 10-4 x 100
1.0 x 10-3
Prof. T. L. Heise
CHE
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110
Weak Acids
Sample Exercise: Calculate the % of niacin
molecules ionized in
b) a 1.0 x 10-3 M solution
x = 1.1 x 10-4
1.1 x 10-4 x 100 = 11%
1.0 x 10-3
Prof. T. L. Heise
CHE
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111
Weak Acids
Polyprotic Acids: many acids have more than
one ionizable H atom.
H2SO3(aq)  H+(aq) + HSO3-(aq) Ka1
HSO3-(aq)  H+(aq) + SO3-2(aq) Ka2
The Ka are labeled according to which proton
is dissociating.
-it is always easier to remove the first
proton than the second
Prof. T. L. Heise
CHE
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112
Weak Acids
Prof. T. L. Heise
CHE
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113
Weak Acids
If Ka values differ by 103 or more, only use
Ka1 to determine calculations.
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CHE
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114
Weak Acids
Sample Exercise: Calculate the pH and
concentration of oxalate ion, [C2O42-] in a
0.020 M solution of oxalic acid.
Prof. T. L. Heise
CHE
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115
Weak Acids
Sample Exercise: Calculate the pH and
concentration of oxalate ion, [C2O42-] in a
0.020 M solution of oxalic acid.
H2C2O4  HC2O4- + H+
Ka = 5.9 x 10-2 = [HC2O4- ][H+]
[H2C2O4]
Prof. T. L. Heise
CHE
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116
Weak Acids
Sample Exercise: Calculate the pH and
concentration of oxalate ion, [C2O42-] in a
0.020 M solution of oxalic acid.
H2C2O4  HC2O4- + H+
strong acid so 100% dissociation
0.020 M H+
Prof. T. L. Heise
CHE
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117
Weak Acids
Sample Exercise: Calculate the pH and
concentration of oxalate ion, [C2O42-] in a
0.020 M solution of oxalic acid.
HC2O4-  C2O4-2 + H+
I
0.020
0 0.020
D
-x
+x
+x
E 0.020 - x
x 0.020 + x
Prof. T. L. Heise
CHE
116
118
Weak Acids
Sample Exercise: Calculate the pH and
concentration of oxalate ion, [C2O42-] in a
0.020 M solution of oxalic acid.
Ka = 6.4 x 10-5 = [0.020 + x ][x]
[0.020 - x]
use your assumption
Prof. T. L. Heise
CHE
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119
Weak Acids
Sample Exercise: Calculate the pH and
concentration of oxalate ion, [C2O42-] in a
0.020 M solution of oxalic acid.
Ka = 6.4 x 10-5 = [0.020 ][x]
[0.020]
6.4 x 10-5 = x = [C2O42-]
Prof. T. L. Heise
CHE
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120
Weak Acids
Sample Exercise: Calculate the pH and
concentration of oxalate ion, [C2O42-] in a
0.020 M solution of oxalic acid.
[H+] = 0.020
Prof. T. L. Heise
CHE
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121
Weak Acids
Sample Exercise: Calculate the pH and
concentration of oxalate ion, [C2O42-] in a
0.020 M solution of oxalic acid.
pH = -log[H+]
pH = - log[0.020]
pH = 1.70
Prof. T. L. Heise
CHE
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122
Weak Bases
Many substances behave as weak bases in
water. Such substances react with water,
removing protons from water, leaving the
OH- ion behind.
NH3 + H2O  NH4+ + OHKb = [NH4+][OH-]
[NH3]
* Kb is the base dissociation constant
utilizing the [OH-]
Prof. T. L. Heise
CHE
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123
Weak Bases
* Kb is the base dissociation constant
utilizing the [OH-]
bases must contain one or more lone
pair to bond with the H+ from water.
as before, the larger the Kb the stronger
the base
stronger base have low pOH, but high
pH
Prof. T. L. Heise
CHE
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124
Weak Bases
Sample exercise: Which of the following
compounds should produce the highest pH
as a 0.05 M solution?
A) pyridine
B) methylamine
C) nitrous acid
Prof. T. L. Heise
CHE
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125
Weak Bases
Sample exercise: Which of the following
compounds should produce the highest pH
as a 0.05 M solution?
A) pyridine
Kb = 1.7 x 10-9
B) methylamine Kb = 4.4 x 10-4
C) nitrous acid
Kb = 2.2 x 10-11
Prof. T. L. Heise
CHE
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126
Weak Bases
Sample exercise: Which of the following
compounds should produce the highest pH
as a 0.05 M solution?
A) pyridine
Kb = 1.7 x 10-9
Kb = 1.7 x 10-9 = [x][x]
[0.05]
x = [OH-] = 9.2 x 10-6
pOH = 5.0 so pH = 9.0
Prof. T. L. Heise
CHE
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127
Weak Bases
Sample exercise: Which of the following
compounds should produce the highest pH
as a 0.05 M solution?
B) methylamine Kb = 4.4 x 10-4
Kb = 4.4 x 10-4 = [x][x]
[0.05]
x = [OH-] = 4.6 x 10-3
pOH = 2.32 so pH = 11.68
Prof. T. L. Heise
CHE
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128
Weak Bases
Sample exercise: Which of the following
compounds should produce the highest pH
as a 0.05 M solution?
C) nitrous acid
Kb = 2.2 x 10-11
Kb = 2.2 x 10-11 = [x][x]
[0.05]
x = [OH-] = 1.0 x 10-6
pOH = 5.97 so pH = 8.02
Prof. T. L. Heise
CHE
116
129
Weak Bases
Sample exercise: Which of the following
compounds should produce the highest pH
as a 0.05 M solution?
A) pyridine
pH = 9.0
B) methylamine pH = 11.68
C) nitrous acid
pH = 8.02
Prof. T. L. Heise
CHE
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130
Weak Bases
Identifying a Weak Base
Neutral substances that have an atom
with a nonbonding pair of electrons that
can serve as a proton acceptor.

Most of these are nitrogen atoms
Anions of weak acids
Prof. T. L. Heise
CHE
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Weak Bases
Sample exercise: A solution of NH3 in water
has a pH of 10.50. What is the molarity of
the solution?
Prof. T. L. Heise
CHE
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132
Weak Bases
Sample exercise: A solution of NH3 in water
has a pH of 10.50. What is the molarity of
the solution?
pH = 10.50 so pOH = 3.50
Prof. T. L. Heise
CHE
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133
Weak Bases
Sample exercise: A solution of NH3 in water
has a pH of 10.50. What is the molarity of
the solution?
pOH = 3.50
NH3 + H20  NH4+ + OH[OH-] = 3.16 x 10-4
Prof. T. L. Heise
CHE
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134
Weak Bases
Sample exercise: A solution of NH3 in water
has a pH of 10.50. What is the molarity of
the solution?
NH3 + H20  NH4+ + OHx
0
0
-3.16 x 10-4
+3.16 x 10-4 +3.16 x 10-4
x - 3.16 x 10-4
3.16 x 10-4 3.16 x 10-4
Prof. T. L. Heise
CHE
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135
Weak Bases
Sample exercise: A solution of NH3 in water
has a pH of 10.50. What is the molarity of
the solution?
NH3 + H20  NH4+ + OHKb = 1.8 x 10-5 = [3.16 x 10-4][3.16 x 10-4]
[x - 3.16 x 10-4]
Prof. T. L. Heise
CHE
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136
Weak Bases
Sample exercise: A solution of NH3 in water
has a pH of 10.50. What is the molarity of
the solution?
NH3 + H20  NH4+ + OHKb = 1.8 x 10-5 = [3.16 x 10-4][3.16 x 10-4]
[x - 3.16 x 10-4]
x = 0.0058 M
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CHE
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Relationship Between Ka and Kb
When two reactions are added to give a
third reaction, the equilibrium constant for
the third reaction is equal to the product of
the equilibrium constants for the two added
reactants.
NH4+(aq)  NH3(aq) + H+(aq)
NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
H2O(l)  H+(aq) + OH-(aq)
Prof. T. L. Heise
CHE
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138
Relationship Between Ka and Kb
NH4+(aq)  NH3(aq) + H+(aq)
NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
H2O(l)  H+(aq) + OH-(aq)
K a x Kb = K w
pKa + pKb = pKw = 14
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CHE
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139
Relationship Between Ka and Kb
Sample exercise: Which of the following
anions has the largest base-dissociation
constant?
A) NO2B) PO43C) N3-
Prof. T. L. Heise
CHE
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140
Relationship Between Ka and Kb
Sample exercise: Which of the following
anions has the largest base-dissociation
constant?
A) NO2- Ka = 4.5 x 10-4
Ka x Kb = 1.0 x 10-14
Kb = 1.0 x 10-14 = 2.2 x 10-11
4.5 x 10-4
Prof. T. L. Heise
CHE
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141
Relationship Between Ka and Kb
Sample exercise: Which of the following
anions has the largest base-dissociation
constant?
B) PO43- Ka = 4.2 x 10-13
Ka x Kb = 1.0 x 10-14
Kb = 1.0 x 10-14 = 2.4 x 10-2
4.2 x 10-13
Prof. T. L. Heise
CHE
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142
Relationship Between Ka and Kb
Sample exercise: Which of the following
anions has the largest base-dissociation
constant?
C) N3Ka = 1.9 x 10-5
Ka x Kb = 1.0 x 10-14
Kb = 1.0 x 10-14 = 5.2 x 10-10
1.9 x 10-5
Prof. T. L. Heise
CHE
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143
Relationship Between Ka and Kb
Sample exercise: Which of the following
anions has the largest base-dissociation
constant?
A) NO2- Kb = 2.2 x 10-11
B) PO43- Kb = 2.4 x 10-2
C) N3Kb = 5.2 x 10-10
Prof. T. L. Heise
CHE
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144
Relationship Between Ka and Kb
Sample exercise: The base quinoline has a pKa
of 4.90. What is the base dissociation
constant for quinoline?
Prof. T. L. Heise
CHE
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145
Relationship Between Ka and Kb
Sample exercise: The base quinoline has a pKa
of 4.90. What is the base dissociation
constant for quinoline?
pKa + pKb = 14
4.90 + x = 14
x = 9.1
Prof. T. L. Heise
CHE
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146
Relationship Between Ka and Kb
Sample exercise: The base quinoline has a pKa
of 4.90. What is the base dissociation
constant for quinoline?
pKb = 9.1
pKb = -log[Kb]
9.1 = -log[Kb]
Prof. T. L. Heise
[Kb] = 7.9 x 10-10
CHE
116
Acid Base Properties of Salt Soln’s
147
Salt solutions have the potential to be acidic or
basic.
Hydrolysis of a salt
acid base properties are due to the
behavior of their cations and anions
perform the necessary double
replacement reaction and examine the
products using your strength rules
Prof. T. L. Heise
CHE
116
Acid Base Properties of Salt Soln’s
148
If a strong acid and strong base are
produced, the resultant solution will be
neutral.
If a strong acid and weak base are produced,
the resultant solution will be acidic.
If a strong base and a weak acid are
produced, the resultant solution will be
basic.
Prof. T. L. Heise
CHE
116
Acid Base Properties of Salt Soln’s
149
If a weak acid and weak base are produced,
the resultant solution will be dependent on
the Ka values.
Prof. T. L. Heise
CHE
116
Acid Base Properties of Salt Soln’s
150
Sample exercise: In each of following, which
salt will form the more acidic solution.
A) NaNO3, Fe(NO3)3
NaNO3 + H2O  NaOH + HNO3
SB
Prof. T. L. Heise
SA
CHE
116
Acid Base Properties of Salt Soln’s
151
Sample exercise: In each of following, which
salt will form the more acidic solution.
A) NaNO3, Fe(NO3)3
Fe(NO3)3 + 3H2O  Fe(OH)3 + 3HNO3
WB
Prof. T. L. Heise
SA
CHE
116
Acid Base Properties of Salt Soln’s
152
Sample exercise: In each of following, which
salt will form the more acidic solution.
A) NaNO3, Fe(NO3)3
Fe(NO3)3 + 3H2O  Fe(OH)3 + 3HNO3
WB
Prof. T. L. Heise
SA
CHE
116
Acid Base Properties of Salt Soln’s
153
Sample exercise: In each of following, which
salt will form the more acidic solution.
B) KBr, KBrO
Prof. T. L. Heise
CHE
116
Acid Base Properties of Salt Soln’s
154
Sample exercise: In each of following, which
salt will form the more acidic solution.
B) KBr, KBrO
KBr + H2O  KOH + HBr
SB
Prof. T. L. Heise
SA
CHE
116
Acid Base Properties of Salt Soln’s
155
Sample exercise: In each of following, which
salt will form the more acidic solution.
B) KBr, KBrO
KBrO + H2O  KOH + HBrO
SB
Prof. T. L. Heise
WA
CHE
116
Acid Base Properties of Salt Soln’s
156
Sample exercise: In each of following, which
salt will form the more acidic solution.
B) KBr, KBrO
Prof. T. L. Heise
CHE
116
Acid-Base Behavior & Chem
Structure
157
How does the chemical structure determine
which of the behaviors will be exhibited?
Acidic
 a molecule containing H will transfer a
proton only if the H-X bond is polarized like
H -- X
the stronger the
bond the weaker
the acid and vice
versa
Prof. T. L. Heise
CHE
116
Acid-Base Behavior & Chem
Structure
158
How does the chemical structure determine
which of the behaviors will be exhibited?
Acidic
 oxyacids exist when the H is attached to an
oxygen bonded to a central atom
if the OH’s attached are equal in number
to the O’s present, acid strength
increases with electronegativity
Prof. T. L. Heise
CHE
116
Acid-Base Behavior & Chem
Structure
159
How does the chemical structure determine
which of the behaviors will be exhibited?
Acidic
 oxyacids exist when the H is attached to an
oxygen bonded to a central atom
the more O’s present compared to the
OH’s, the more polarized the OH bond
becomes and the stronger the acid is
Prof. T. L. Heise
CHE
116
Acid-Base Behavior & Chem
Structure
160
How does the chemical structure determine
which of the behaviors will be exhibited?
Acidic
 carboxylic acids exist when the functional
group COOH is present
the strength also increases as the number
of electronegative atoms in the molecule
increase
Prof. T. L. Heise
CHE
116