Models of Acids and Bases
Arrhenius Concept: Acids produce H+ in
solution, bases produce OH ion.
Brønsted-Lowry: Acids are H+ donors, bases
are proton acceptors.
HCl + H2O  Cl + H3O+
acid base
Conjugate Acid/Base Pairs
HA(aq) + H2O(l)  H3O+(aq) + A(aq)
conj
acid 1
conj
base 2
conj
acid 2
conj
base 1
conjugate base: everything that remains of
the acid molecule after a proton is lost.
conjugate acid: formed when the proton is
transferred to the base.
Acid Dissociation Constant (Ka)
HA(aq) + H2O(l)  H3O+(aq) + A(aq)
Ka 
H3O

HA
A


H

A
HA

Acid Strength
Strong Acid:
-
Its equilibrium position lies far to the right.
(HNO3)
-
Yields a weak conjugate base. (NO3)
Strong Acid - Weak Acid
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Before dissociation
HA
After dissociation,
at equilibrium
H+ A–
(a)
HA
HA
H+ A–
(b)
Strong Acids
H2SO4
Sulfuric acid
HCl
Hydochloric acid
HNO3
Nitric acid
HClO4
Perchloric acid
HBr
Hydrobromic acid
Acid Strength
(continued)
Weak Acid:
-
Its equilibrium lies far to the left.
(CH3COOH)
-
Yields a much stronger (it is relatively
strong) conjugate base than water.
(CH3COO)
Strong Acid
Weak Acid
14_1577
H+
A-
H+
+
A- H
A(a)
AH+
A-
HB
H+
H+
A-
A-
H+
HB
H+
H+
HB
HB
H+
A-
A-
HB
HB
A-
AH+
H+
H+
HB
A(b)
HB
HB
B-
HB
HB
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Relative
acid strength
Very
strong
Relative
conjugate
base strength
Very
weak
Strong
Weak
Weak
Strong
Very
weak
Very
strong
14_02T
Formula
HSO4
HClO2
HC2H2ClO2
HF
HNO2
HC2H3O2
[Al(H2O)6]3+
HOCl
HCN
NH4
HOC6H5
Values of Ka for Some Common Monoprotic Acids
Name
Hydrogen sulfate ion
Chlorous acid
Monochloracetic acid
Hydrofluoric acid
Nitrous acid
Acetic acid
Hydrated aluminum(III) ion
Hypochlorous acid
Hydrocyanic acid
Ammonium ion
Phenol
*The units of Ka are mol/L but are customarily omitted.
Value of
K a*
1.2 x 102
1.2 x 102
1.35 x 103
7.2 x 104
4.0 x 104
1.8 x 105
1.4 x 105
3.5 x 108
6.2 x 1010
5.6 x 1010
1.6 x 1010
Increasing acid strength
Table 14.2
Water as an Acid and a Base
Water is amphoteric (it can behave either as
an acid or a base).
H2O + H2O  H3O+ + OH
acid 1 base 2
conj
acid 2
conj
base 1
Kw = 1  1014 at 25°C
The pH Scale
pH  log[H+]
pH in water ranges from 0 to 14.
Kw = 1.00  1014 = [H+] [OH]
pKw = 14.00 = pH + pOH
As pH rises, pOH falls (sum = 14.00).
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[H+] pH
10–14 14
1 M NaOH
10–13 13
Basic
10–12 12
10–11 11
Ammonia
(Household
cleaner)
10–10 10
10–9
9
10–8
8
Neutral 10–7
7
10–6
6
10–5
5
10–4
4
10–3
3
10–2
2
10–1
1
1
0
Acidic
Blood
Pure water
Milk
Vinegar
Lemon juice
Stomach acid
1 M HCl
Solving Weak Acid Equilibrium
Problems
-
List major species in solution.
-
Choose species that can produce H+ and write
reactions.
-
Based on K values, decide on dominant
equilibrium.
-
Write equilibrium expression for dominant
equilibrium.
-
List initial concentrations in dominant
equilibrium.
Solving Weak Acid Equilibrium
Problems (continued)
-
Define change at equilibrium (as “x”).
-
Write equilibrium concentrations in terms of x.
-
Substitute equilibrium concentrations into
equilibrium expression.
-
Solve for x the “easy way.”
-
Verify assumptions using 5% rule.
-
Calculate [H+] and pH.
Percent Dissociation
(Ionization)
amount dissociated( M )
% dissociation 
 100%
initial concentration( M )
In-Class Problem
1. Calculate the pH of a 0.20 M solution of
Iodic acid (HIO3). Ka = 0.17
2. What is the percent dissociation for this
acid.
Bases
“Strong” and “weak” are used in the same
sense for bases as for acids.
strong = complete dissociation (hydroxide ion
supplied to solution)
NaOH(s)  Na+(aq) + OH(aq)
Strong Bases
LiOH
Lithium
Hydroxide
NaOH
Sodium
Hydroxide
KOH
Potassium
Hydroxide
RbOH
Rubidium
Hydroxide
Ce(OH)2
Cesium
Hydroxide
Ba(OH)2
Barium
Hydroxide
Sr(OH)2
Strontium
Hydroxide
Ca(OH)2
Calcium
Hydroxide
Bases
(continued)
weak = very little dissociation (or reaction
with water)
H3CNH2(aq) + H2O(l)  H3CNH3+(aq) + OH(aq)
Polyprotic Acids
. . . can furnish more than one proton (H+) to
the solution.
H 2CO3  H   HCO3
( Ka1 )
HCO3  H   CO32 
( Ka 2 )
Acid-Base Properties of Salts
Cation
neutral
neutral
Acidic
or Basic
neutral
basic
Anion
neutral
conj base of
weak acid
conj acid of
neutral
acidic
weak base
conj acid of conj base of depends on
weak base weak acid
Ka & Kb
values
Example
NaCl
NaF
NH4Cl
Al2(SO4)3
Structure and Acid-Base
Properties
Two factors for acidity in binary compounds:
-
Bond Polarity (high is good)
-
Bond Strength (low is good)
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Cl
O
H
Electron density
O
Cl
O
H
Electron density
O
Cl
O
H
O
Electron density
O
O Cl
O
O
H
Electron density
Oxides
Acidic Oxides (Acid Anhydrides):
-
OX bond is strong and covalent.
SO2, NO2, CrO3
Basic Oxides (Basic Anhydrides):
-
OX bond is ionic.
K2O, CaO
Lewis Acids and Bases
Lewis Acid: electron pair acceptor
Lewis Base: electron pair donor
Al3+ + 6 O
H
H
Al
H
3+
O
H
6