Chapter 14
Acids and Bases
Arrhenius Concept: Acids produce H+ in
solution, bases produce OH ion.
Brønsted-Lowry: Acids are H+ donors, bases
are proton acceptors.

Cl + H3O+
conjugate conjugate
base
acid
Proton is transferred from the HCl molecule
to the water molecule.
HCl + H2O
acid base
Figure 14.1 The Reaction of HCI and H2O
Figure 14.2 The Reaction of an Acid with Water
Figure 14.3 The Reaction of NH3 with HCI to Form NH4+ and CI-
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.
A conjugate acid-base pair consists of two
substances related to each other by the
donating and accepting of a single proton.
Acid Dissociation Constant (Ka)
HA(aq) + H2O(l)  H3O+(aq) + A(aq)
Ka 
H3O

HA
A


H

A

HA
Where, Ka is the acid dissociation constant. In
dilute solution we can assume that the
concentration of liquid water remains
essentially constant when an acid is dissolved.
Acid Strength
The strength of an acid is defined by the
equilibrium position of its dissociation
(ionization) reaction:
HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
Strong Acid:
 Its equilibrium position lies far to the right.
(HNO3)
 Yields a weak conjugate base. (NO3)
 Common strong acids are H2SO4, HCl, HNO3,
HClO4
Figure 14.4 Graphic Representation of the Behavior of Acids of Different Strengths in Aqueous Solution
Figure 14.5 Acid Strength Versus Conjugate Base Strength
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)
 Common weak acids are H3PO4, HNO2,
HOCl, organic acids (-COOH).
Figure 14.6 A Strong Acid (a) and a Weak Acid (b) in Water
Monoprotic acid: One acidic proton (HCl)
HCl(aq)
H+(aq) + Cl-(aq)
Diprotic acid: Two acidic protons (H2SO4)
H2SO4(aq)
H+(aq) + HSO4-(aq)
HSO4- (aq)
H+(aq) + SO4 2-(aq)
Oxyacids: Acidic proton is attached to an
oxygen atom (H2SO4)
Organic acids: Those with a carbon atom
backbone, contain the carboxyl group (COOH). CH3-COOH, C6H5-COOH
Water as an Acid and a Base
A substance is said to be amphoteric if it can behave
either as an acid or as a base. Water is amphoteric (it
can behave either as an acid or a base).
H2O + H2O  H3O+ + OH
conj conj
acid 1 base 2 acid 2 base 1
Kw = [H3O+][OH-] = [H+][OH-] = 1  1014 at 25°C
Where, Kw is the ion-product constant or
dissociation constant for water.
[H+] = [OH-] = 1.0 x 10-7 M at 25oC in pure water.
Figure 14.7 Two Water Molecules React to Form H3O+ and OH-
The pH Scale
The pH scale provides a convenient way to
represent solution acidity. The pH is a log
scale based on 10.
pH  log[H+]
pH in water ranges from 0 to 14. The pH
decreases as [H+] increases.
Kw = 1.00  1014 = [H+] [OH]
pKw = -log Kw = 14.00 = pH + pOH
As pH rises, pOH falls (sum = 14.00).
pOH = -log [OH-]
Figure 14.8 The pH Scale and pH Values of Some Common Substances
Calculating the pH of Strong Acid Solutions
• Calculate the pH of 1.0 M HCl.
Since HCl is a strong acid, the major species in
solution are H+, Cl- and H2O
To calculate the pH we will focus on major species
that can furnish H+. The acid is completely dissociates
in water producing H+ and water also furnishes H+ by
autoionization by the equilibrium
H2O(l)
H+(aq) + OH-(aq)
In pure water at 25oC, [H+] is 10-7M and in acidic
solution even less than that. So the amount of H+
contributed by water is negligible compared with the
1.0M H+ from the dissociation of HCl.
pH = -log [H+] = -log (1.0) = 0
Solving Weak Acid Equilibrium
Problems
 List
major species in solution.
species that can produce H+ and write
reactions.
 Choose
 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
 Write
change at equilibrium (as “x”).
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)
It is useful to specify the amount of weak acid
that has dissociated in achieving equilibrium in
an aqueous solution. The percent dissociation is
defined as follows:
amount dissociated ( M )
% dissociation 
 100%
initial concentration( M )
For a given weak acid, the percent dissociation
increases as the acid becomes more dilute.
Figure 14.10 The Effect of Dilution on the Percent Dissociation and (H+) of a Weak Acid Solution
Bases
• Arrhenius concept: A base is a substance that
produces OH- ions in aqueous solution.
• Bronsted-Lowry concept: A base is a proton
acceptor.
• “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)
Bases
(continued)
weak = very little dissociation (or reaction
with water)
H3CNH2(aq) + H2O(l)  H3CNH3+(aq) + OH(aq)
H3CNH2 molecule accepts a proton and thus
functions as a base. Water is the acid in this
reaction. Methyl amine contains no hydroxide
ion, it still increases the concentration of
hydroxide ion to yield a basic solution.
Polyprotic Acids
. . . can furnish more than one proton (H+) to the
solution. A polyprotic acid always dissociates in
a stepwise manner, one proton at a time.

( Ka1 )

( Ka 2 )
H 2CO3  H  HCO3
HCO3

 H  CO32 
For a typical weak polyprotic acid,
Ka1 > Ka2 > Ka3
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
• When a substance is dissolved in water, it
produces an acidic solution if it can donate
protons and produces a basic solution if it can
accept protons.
• Two factors for acidity in binary compounds:
 Bond
Polarity (high is good)
 Bond
Strength (low is good)
Figure 14.11 The Effect of the Number of Attached Oxygens on the O-H Bond in a Series of of Chlorine Oxyacids
Oxides
• Acidic Oxides (Acid Anhydrides): When a
covalent oxide dissolves in water an acidic
solution forms.
 OX
bond is strong and covalent.
SO2, NO2, CO2, CrO3
• Basic Oxides (Basic Anhydrides): When an
ionic oxide dissolves in water a basic solution
results.
 OX
bond is ionic.
K2O, CaO
Lewis Acids and Bases
Lewis Acid: electron pair acceptor
Lewis Base: electron pair donor
Lewis acid has an empty atomic orbital that it
can use to accept an electron pair from a
molecule that has a lone pair of electrons.
Al3+ + 6 O
H
Lewis
acid
H
Al
H
Lewis
base
3+
O
H
6
Figure 14.13 The AI(H2O)63+ Ion