Chapter
15
Aqueous Equilibria:
Acids and Bases
Chemistry 4th Edition
McMurry/Fay
Dr. Paul Charlesworth
Michigan Technological University
Acid–Base Concepts
01
Arrhenius Acid: A substance which dissociates to
form hydrogen ions (H+) in solution.
HA(aq)  H+(aq) + A–(aq)
Arrhenius Base: A substance that dissociates in, or
reacts with water to form hydroxide ions (OH–).
MOH(aq)  M+(aq) + OH–(aq)
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Chapter 15
Slide 2
Acid–Base Concepts
02
Brønsted–Lowry Acid: Substance that can donate H+
• Brønsted–Lowry Base: Substance that can accept H+
•
•
Chemical species whose formulas differ only by one
proton are said to be conjugate acid–base pairs.
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Chapter 15
Slide 3
Acid–Base Concepts
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Chapter 15
03
Slide 4
Acid–Base Concepts
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Chapter 15
04
Slide 5
Acid–Base Concepts
05
•
A Lewis Acid is an electron-pair acceptor. These
are generally cations and neutral molecules with
vacant valence orbitals, such as Al3+, Cu2+, H+, BF3.
•
A Lewis Base is an electron-pair donor. These are
generally anions and neutral molecules with
available pairs of electrons, such as H2O, NH3, O2–.
•
The bond formed is called a coordinate bond.
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Chapter 15
Slide 6
Acid–Base Concepts
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Chapter 15
06
Slide 7
Acid–Base Concepts
•
Write balanced equations for the dissociation of
each of the following Brønsted–Lowry acids.
(a) H2SO4
•
07
(b) HSO4–
(c) H3O+
Identify the Lewis acid and Lewis base in each of
the following reactions:
(a) SnCl4(s) + 2 Cl–(aq) æ SnCl62–(aq)
(b) Hg2+(aq) + 4 CN–(aq) æ Hg(CN)42–(aq)
(c) Co3+(aq) + 6 NH3(aq) æ Co(NH3)63+(aq)
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Chapter 15
Slide 8
Dissociation of Water
•
01
Water can act as an acid or as a base.
H2O(l) æ H+(aq) + OH–(aq)
•
This is called the autoionization of water.
H2O(l) + H2O(l)
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æ H3O+(aq) + OH–(aq)
Chapter 15
Slide 9
Dissociation of Water
•
02
This equilibrium gives us the ion product constant
for water.
Kw = Kc = [H+][OH–] = 1.0 x 10–14
•
If we know either [H+] or [OH–] then we can
determine the other quantity.
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Chapter 15
Slide 10
Dissociation of Water
•
03
The concentration of OH– ions in a certain household
ammonia cleaning solution is 0.0025 M. Calculate the
concentration of H+ ions.
•
Calculate the concentration of OH– ions in a HCl
solution whose hydrogen ion concentration is 1.3 M.
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Chapter 15
Slide 11
pH – A Measure of Acidity
•
01
The pH of a solution is the negative logarithm of the
hydrogen ion concentration (in mol/L).
pH = –log [H+]
pH + pOH = 14
Acidic solutions:
[H+] > 1.0 x 10–7 M,
pH < 7.00
Basic solutions:
[H+] < 1.0 x 10–7 M,
pH > 7.00
Neutral solutions: [H+] = 1.0 x 10–7 M,
pH = 7.00
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Chapter 15
Slide 12
pH – A Measure of Acidity
02
•
Nitric acid (HNO3) is used in the production of
fertilizer, dyes, drugs, and explosives. Calculate the
pH of a HNO3 solution having a hydrogen ion
concentration of 0.76 M.
•
The pH of a certain orange juice is 3.33. Calculate
the H+ ion concentration.
•
The OH– ion concentration of a blood sample is
2.5 x 10–7 M. What is the pH of the blood?
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Chapter 15
Slide 13
pH – A Measure of Acidity
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Chapter 15
03
Slide 14
Strength of Acids and Bases
•
01
Strong acids and bases: are strong electrolytes
that are assumed to ionize completely in water.
•
Weak acids and bases: are weak electrolytes that
ionize only to a limited extent in water.
•
Solutions of weak acids and bases contain ionized
and non-ionized species.
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Chapter 15
Slide 15
ACID
CONJ. BASE
HClO4
HI
HBr
HCl
H2SO4
HNO3
H3O+
HSO4–
ClO4–
I–
Br –
Cl –
HSO4 –
NO3 –
H2O
SO42–
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ACID
Increasing Acid Strength
Increasing Acid Strength
Strength of Acids and Bases
Chapter 15
HSO4–
HF
HNO2
HCOOH
NH4+
HCN
H2O
NH3
02
CONJ. BASE
SO42–
F–
NO2 –
HCOO –
NH3
CN –
OH –
NH2 –
Slide 16
Strength of Acids and Bases
•
03
Stronger acid + stronger base 
weaker acid + weaker base
•
Predict the direction of the following:
HNO2(aq) + CN–(aq) æ HCN(aq) + NO2–(aq)
HF(aq) + NH3(aq) æ F–(aq) + NH4+(aq)
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Chapter 15
Slide 17
Acid Ionization Constants
•
01
Acid Ionization Constant: the equilibrium
constant for the ionization of an acid.
HA(aq) + H2O(l) æ H3O+(aq) + A–(aq)
•
Or simply:
HA(aq) æ H+(aq) + A–(aq)


[H ][A ]
Ka 
[HA]
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Chapter 15
Slide 18
Acid Ionization Constants
ACID
HF
HNO2
C9H8O4 (aspirin)
HCO2H (formic)
C6H8O6 (ascorbic)
C6H5CO2H (benzoic)
CH3CO2H (acetic)
HCN
C6H5OH (phenol)
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Ka
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
Chapter 15
02
CONJ. BASE
Kb
F–
NO2 –
C9H7O4 –
HCO2 –
C6H7O6 –
C6H5CO2 –
CH3CO2 –
CN –
C6H5O –
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
Slide 19
Strength of Acids and Bases
03
(a) Arrange the three acids in order of increasing value of Ka.
(b) Which acid, if any, is a strong acid?
(c) Which solution has the highest pH, and which has the
lowest?
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Chapter 15
Slide 20
Acid Ionization Constants
04
• Initial
Change Equilibrium Table: Determine the pH
of 0.50 M HA solution at 25°C. Ka = 7.1 x 10–4.
+
HA (aq) æ H (aq) + A (aq)
0.50
Initial (M):
Change (M):
–x
Equilib (M): 0.50 – x
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Chapter 15
0.00
+x
x
0.00
+x
x
Slide 21
Acid Ionization Constants
•
05
pH of a Weak Acid (Cont’d):
1.
Substitute new values into equilibrium expression.
2.
If Ka is significantly (>1000 x) smaller than [HA] the
expression (0.50 – x) approximates to (0.50).
3.
The equation can now be solved for x and pH.
4.
If Ka is not significantly smaller than [HA] the quadratic
equation must be used to solve for x and pH.
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Chapter 15
Slide 22
Acid Ionization Constants
•
The Quadratic Equation:
•
The expression must first be rearranged to:
06
2
ax  bx  c  0
•
The values are substituted into the quadratic and
solved for a positive solution to x and pH.
2
 b  b  4ac
x
2a
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Chapter 15
Slide 23
Acid Ionization Constants
07
•
Calculate the pH of a 0.036 M nitrous acid (HNO2)
solution.
•
What is the pH of a 0.122 M monoprotic acid
whose Ka is 5.7 x 10–4?
•
The pH of a 0.060 M weak monoprotic acid is 3.44.
Calculate the Ka of the acid.
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Chapter 15
Slide 24
Acid Ionization Constants
•
08
Percent Dissociation: A measure of the strength
of an acid.

[H ]
% Dissociati on 
 100%
[HA]
•
Stronger acids have higher percent dissociation.
•
Percent dissociation of a weak acid decreases as
its concentration increases.
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Chapter 15
Slide 25
Base Ionization Constants
01
Base Ionization Constant: The equilibrium
constant for the ionization of a base.
• The ionization of weak bases is treated in the same
way as the ionization of weak acids.
•
B(aq) + H2O(l) æ BH+(aq) + OH–(aq)
•
Calculations follow the same procedure as used for
a weak acid but [OH–] is calculated, not [H+].
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Chapter 15
Slide 26
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.
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Chapter 15
Slide 27
Diprotic & Polyprotic Acids
•
01
Diprotic and polyprotic acids yield more than one
hydrogen ion per molecule.
•
One proton is lost at a time. Conjugate base of first
step is acid of second step.
•
Ionization constants decrease as protons are
removed.
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Chapter 15
Slide 28
Diprotic & Polyprotic Acids
ACID
H2SO4
HSO4–
C2H2O4
C2HO4–
H2SO3
HSO3–
H2CO3
HCO3–
H2S
HS–
H3PO4
H2PO4–
HPO42–
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Ka
CONJ. BASE
HSO4 –
SO4 2–
C2HO4–
C2O42–
HSO3 –
SO3 2–
HCO3–
CO3 2–
HS–
S 2–
H2PO4–
HPO42–
PO43–
Very Large
1.3 x 10 –2
6.5 x 10 –2
6.1 x 10 –5
1.3 x 10 –2
6.3 x 10 –8
4.2 x 10 –7
4.8 x 10 –11
9.5 x 10 –8
1 x 10 –19
7.5 x 10 –3
6.2 x 10 –8
4.8 x 10 –13
Chapter 15
02
Kb
Very Small
7.7 x 10 –13
1.5 x 10 –13
1.6 x 10 –10
7.7 x 10 –13
1.6 x 10 –7
2.4 x 10 –8
2.1 x 10 –4
1.1 x 10 –7
1 x 10 –5
1.3 x 10 –12
1.6 x 10 –7
2.1 x 10 –2
Slide 29
Molecular Structure
and Acid Strength
01
•
The strength of an acid depends on its tendency to
ionize.
•
For general acids of the type H–X:
•
1.
The stronger the bond, the weaker the acid.
2.
The more polar the bond, the stronger the acid.
For the hydrohalic acids, bond strength plays the
key role giving: HF < HCl < HBr < HI
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Chapter 15
Slide 30
Molecular Structure
and Acid Strength
•
02
The electrostatic potential maps show all the hydrohalic
acids are polar. The variation in polarity is less
significant than the bond strength which decreases
from 567 kJ/mol for HF to 299 kJ/mol for HI.
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Chapter 15
Slide 31
Molecular Structure
and Acid Strength
03
•
For binary acids in the same group, H–A bond strength
decreases with increasing size of A, so acidity increases.
•
For binary acids in the same row, H–A polarity increases
with increasing electronegativity of A, so acidity increases.
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Chapter 15
Slide 32
Molecular Structure
and Acid Strength
•
04
For oxoacids bond polarity is more important. If we
consider the main element (Y):
Y–O–H
•
If Y is an electronegative element, or in a high
oxidation state, the Y–O bond will be more covalent
and the O–H bond more polar and the acid stronger.
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Chapter 15
Slide 33
Molecular Structure
and Acid Strength
•
05
For oxoacids with different central atoms that are
from the same group of the periodic table and that
have the same oxidation number, acid strength
increases with increasing electronegativity.
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Chapter 15
Slide 34
Molecular Structure
and Acid Strength
06
•
For oxoacids having the same central atom but
different numbers of attached groups, acid strength
increases with increasing central atom oxidation
number.
•
As shown on the next slide, the number of oxygen
atoms increases the positive charge on the chlorine
which weakens the O–H bond and increases its
polarity.
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Chapter 15
Slide 35
Molecular Structure
and Acid Strength
•
07
Oxoacids of Chlorine:
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Chapter 15
Slide 36
Molecular Structure
and Acid Strength
•
08
Predict the relative strengths of the following
groups of oxoacids:
a) HClO, HBrO, and HIO.
b) HNO3 and HNO2.
c) H3PO3 and H3PO4.
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Chapter 15
Slide 37
Acid–Base Properties of Salts
01
•
Salts that produce neutral solutions are those
formed from strong acids and strong bases.
•
Salts that produce basic solutions are those formed
from weak acids and strong bases.
•
Salts that produce acidic solutions are those
formed from strong acids and weak bases.
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Chapter 15
Slide 38
Acid–Base Properties of Salts
•
02
Calculate the pH of a 0.15 M solution of sodium
acetate (CH3COONa). What is the percent
hydrolysis?
•
Calculate the pH of a 0.24 M sodium formate
solution (HCOONa).
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Chapter 15
Slide 39