Chapter
15
Aqueous Equilibria:
Acids and Bases
Chemistry 4th Edition
McMurry/Fay
Acid–Base Concepts
01
Arrhenius Acid: Substance which dissociates in
water to form hydrogen ions (H+) in solution:
OLD
HA (aq)  H+ (aq) + A– (aq)
definitions
Ex:
HCl(aq) + H2O(l)  H3O+(aq) + Cl–(aq)
Arrhenius Base: Substance that dissociates in water,
or reacts with water, to form hydroxide ions (OH–):
MOH (aq)  M+ (aq) + OH– (aq)
Ex:
KOH(aq) 
K+(aq) + OH–(aq)
Acid–Base Concepts
NEW
definitions
02
Brønsted-Lowry Acid: Substance that can donate H+
Brønsted-Lowry Base: Substance that can accept H+
(often has a lone pair of electrons)
:
Compounds whose formulas differ only by a proton
are said to be conjugate acid–base pairs.
Brønsted–Lowry Definition
Give products, and identify the Brønsted–Lowry
acid, base, and conjugate acid/base.
base
..
HBr + H2O:
acid

acid
..base –
:NH2 + CH3OH
acid
HNO3
acid
..base
+ PH3
base
H2O + tBuO–
conj.
base
–
Br
conj.
acid
H3O+
+
conj.base

conj. acid
CH3O– + NH3

NO3–

tBuOH + HO–
+ PH4+
Acid–Base Concepts
03
Example
hydrochloric acid
Strong
Acid
HCl (aq)
no
HA
(strong electrolyte)
Weak
Acid
Very
Weak
Acid
acetic acid
CH3CO2H
water or
H2O
no
H3O+ A-
alcohol
CH3CH2OH
(non-electrolyte)
Acid–Base Concepts
04
Know these Acids and Bases!
Strong Acids
Strong Bases
HCl
Hydrochloric Acid
NaOH
Sodium Hydroxide
HNO3
Nitric Acid
KOH
Potassium Hydroxide
H2SO4 Sulfuric Acid
Weak Acids
HF
Hydrofluoric Acid
CH3CO2H Acetic Acid
Ca(OH)2 Calcium Hydroxide
Weak Bases
NH3
Ammonia
Dissociation of Water
Water act as both an acid and as a base:
2 H2O(l)
H3O+(aq) + OH–(aq)
01
What is the
position of
this
equilibrium?
This is called the auto-ionization of water.
H2O(l) + H2O(l)
 H3O+(aq) + OH–(aq)
H+(aq) + OH–(aq)
H2O(l)
Write the equilibrium expression for this reaction
( called the ion product constant for water ):
KKwc =
[H+]
[OH–]
= 1.0 x
10–14
at RT
only
The Kc for water called “Kw”. What is the equilibrium
constant value? (Are products or reactants favored?)
Kw is very small, so this reaction mixture will
be nearly ALL starting material.
Dissociation of Water
02
H2O(l)
H+(aq) + OH–(aq)
By the balanced equation, we see [H+] = [OH–].
So if:
[H+] [OH–] = 1.0 x 10–14
(at RT)
then
[H+] = 1.0 x 10-7
log [H+] = -7
-log [H+] = 7
We will call the negative log of the H+ concentration
the pH of the aqueous solution.
The pH of pure water is 7, which is the neutral point.
pH – A Measure of Acidity
The pH of a solution is the negative log of the
H+ concentration:
pH = –log [H+]
Neutral solutions: [H+] = 1.0 x 10–7 M, pH = 7.00
[OH-] = 1.0 x 10-7 M also
Acidic solutions: [H+] > 1.0 x 10–7 M,
Example: 0.001 M HCl [H+] = 1.0 x 10-3
pH < 7.00
[H+] < 1.0 x 10–7 M,
pH > 7.00
Basic solutions:
pH = 3
01
Acid/Base Concepts:
pH Scale
Fig on p. 621 of text
Remember that pH is a log scale.
If pH of coffee is 5 and pH of vinegar is 3,
then vinegar is 100 times more acidic
than coffee.
pH 5: [H+] = 10-5
pH 3: [H+] = 10-3
x 100
pH Calculations
03
1. The concentration of aqueous nitric acid (HNO3) at
RT is 0.045M. What is the pH of this solution?
pH = -log [H+] = -log (0.045) = 1.35
2. A basic solution has a pH of 10.5. What is [H+]?
[H+] = inv log (-10.5) = 3.16 x 10-11 M
strong
acid
pH Calculations
3. In a solution where the [H+] = 4.8 x 10-12M, find:
a) the OH– conc.
b) the pH
(Hint: use Kw which is an equilibrium constant.)
[H+] x [OH–] = 1.0 x 10-14
[OH–] = 1.0 x 10-14 / [H+]
= 1.0 x 10-14 / 4.8 x 10-12
= 2.08 x 10-3 M
pH = -log(4.8 x 10-12) = 11.3
03
pOH – Base Concentration
pH negative log of [H+]:
Define:
01
pH = –log [H+]
pOH negative log of [OH–]: pOH = –log [OH–]
Since
then
[H+] x [OH-] = 1.0 x 10-14 = Kw
pH + pOH = 14
If pH is 3.4, what is the pOH?
pH + pOH = 14
pOH = 11.6
If pH is 12.7, what is pOH?
12.7 + pOH = 14
pOH = 1.3
Dissociation of Water
03
The concentration of OH– ions in a certain household
ammonia cleaning solution is 0.0025 M. Calculate pH.
1. Use Kw to find the H+ concentration, then calc. pH.
- or -
1. [H+] x [OH-] = 1.0 x 10-14 = Kw
[H+] = 1x10-14/(0.0025) = 4.0 x 10-12
pH = -log(4.0x10-12) = 11.40
Dissociation of Water
03
The concentration of OH– ions in a certain household
ammonia cleaning solution is 0.0025 M. Calculate pH.
1. Use Kw to find the H+ concentration, then calc. pH.
- or 2. Calculate pOH, then find pH (pH + pOH = 14)
2. pOH = -log(0.0025) = 2.60
pH + pOH = 14
pH = 14 – pOH = 11.40
Acid / Base Review
Strong acids and bases ionize completely in water:
HA + H2O
A- (aq) + H3O+ (aq)
at equilib:
~0%
~100%
Weak acids and bases ionize only slightly in water:
HA + H2O
A- (aq) + H3O+ (aq)
at equilib:
98.5%
or
99.96%
1.5%
0.04%
Solutions of weak acids and bases contain ionized AND
non-ionized species.
Acid / Base Review
Write balanced equations for the dissociation of
each of the following Brønsted–Lowry acids.
(a) H2SO4
H2SO4 (aq) + H2O(l)
HSO4– (aq) + H3O+ (aq)
better
(b) HSO4–
HSO4–
(c) H4N+
H4N+ (aq) + H2O(l)
SO42– + H+
H3N (aq) + H3O+ (aq)
Acid / Base Review
Write balanced equations for the reaction of each
of the following Brønsted–Lowry bases.
(a) NaOH
NaOH (aq) + H+
(b) NH3
NH3 (aq) + H2O(l)
weak base
strong base
Na+ (aq) + H2O (l)
H4N+ (aq) + OH– (aq)
Strength of Acids and Bases
02
If an acid is strong (ex: HCl), its conjugate base (Cl–)
has no measurable base strength.
A weak acid (ex: HF) has a conjugate base (F–) that
is a weak base.
H3O+ is the strongest acid that can exist in aqueous
solution.
OH– ion is the strongest base that can exist in
aqueous solution.
Indicators – Color Depends on pH
Acid Dissociation Constants
01
Acid Dissociation Constant: the equilibrium
constant for the ionization of an acid.
H3O+(aq) + A–(aq)
HA(aq) + H2O(l)
Or simply:
H+(aq) + A–(aq)
HA(aq)


[H ][A ]
Ka 
[HA]
Base Dissociation Constants
01
Base Dissociation Constant (Kb): the equilibrium
constant for the reaction of a base w/ a proton.
NH4+(aq) + OH–(aq)
NH3(aq) + H2O(l)


[NH4 ][OH ]
Kb 
[NH3]
Weak Acid Dissociation Constants
ACID
Ka
HF
STRONGER
ACID
HNO2
C9H8O4 (aspirin)
HCO2H (formic)
C6H8O6 (ascorbic)
C6H5CO2H (benzoic)
CH3CO2H (acetic)
HCN
C6H5OH (phenol)
WEAKER
ACID
7.1 x 10 –4
4.5 x 10 –4
3.0 x 10 –4
1.7 x 10 –4
8.0 x 10 –5
6.5 x 10 –5
1.8 x 10 –5
4.9 x 10 –10
1.3 x 10 –10
CONJ. BASE
F– WEAKER
BASE
NO2 –
C9H7O4 –
HCO2 –
C6H7O6 –
C6H5CO2 –
CH3CO2 –
CN –
C6H5O –
STRONGER
CONJ. BASE
Kb
1.4 x 10 –11
2.2 x 10 –11
3.3 x 10 –11
5.9 x 10 –11
1.3 x 10 –10
1.5 x 10 –10
5.6 x 10 –10
2.0 x 10 –5
7.7 x 10 –5
Strength of Acids and Bases
04
Stronger acid + stronger base 
weaker acid + weaker base
Predict the direction of the following:
HNO2(aq) + CN–(aq)
HF(aq) + NH3(aq)
HCN(aq) + NO2–(aq)
F–(aq) + NH4+(aq)
Percent Dissociation
Concentration Dependence:
10
Base Ionization Constants
BASE
Kb
C2H5NH2 (ethylamine) 5.6 x 10 –4
CH3NH2 (methylamine) 4.4 x 10 –4
C8H10N4O2 (caffeine) 4.1 x 10 –4
NH3 (ammonia)
1.8 x 10 –5
C5H5N (pyridine)
1.7 x 10 –9
C6H5NH2 (aniline)
3.8 x 10 –10
NH2CONH2 (urea)
1.5 x 10 –14
CONJ. ACID
C2H5NH3+
CH3NH3+
C8H11N4O2+
NH4+
C5H6N+
C6H5NH3+
NH2CONH3+
02
Ka
1.8 x 10 –11
2.3 x 10 –11
2.4 x 10 –11
5.6 x 10 –10
5.9 x 10 –6
2.6 x 10 –5
0.67
Note that the positive charge sits on the nitrogen.