Chapter 13:
Acids and
Bases
13-1
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Questions for Consideration
1.
2.
3.
4.
5.
6.
How do acids and bases differ from other
substances?
How do strong and weak acids differ?
How can we compare the different strengths of
weak acids?
What causes aqueous solutions to be acidic or
basic?
How is pH related to acidity and basicity?
What is a buffered solution?
13-2
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Chapter 13 Topics
1.
2.
3.
4.
5.
6.
What Are Acids and Bases?
Strong and Weak Acids and Bases
Relative Strengths of Weak Acids
Acidic, Basic, and Neutral Solutions
The pH Scale
Buffered Solutions
13-3
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Chapter 13 Math Toolbox:
13.1 Log and Inverse Log Functions
13-4
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Acids and Bases Affect Our Lives
Figure 13.1
Figure 13.2
Figure 13.3
Figure from p. 536
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13-5
13.1 What are Acids and Bases?

Acids and bases have properties that differ
from other substances:





Acids taste sour.
Bases taste bitter and feel slippery.
Both change the color of some dyes.
Acids cause many metals to corrode.
Acids and bases combine to neutralize each
other.
CAUTION: Do not taste laboratory chemicals.
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13-6
Acid and Base Definitions

1800’s Arrhenius Model





An acid in water produces hydrogen (H+) ions.
A base in water produces hydroxide (OH) ions.
HCl(g)  H+(aq) + Cl(aq)
NaOH(s)  Na+(aq) + OH(aq)
Arrhenius earned the Nobel prize for his work
that showed that H+(aq) and OH(aq) ions are
important in acid-base chemistry.
13-7
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The Hydronium Ion

Problem with Arrhenius
Model:
 H+ does not exist
completely free in
aqueous solution. It
associates strongly with
other water molecules.
 Chemists recognize this
by representing an
aqueous H+ ion as
H3O+(aq), the hydronium
ion.
Figure 13.4
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13-8
Acid and Base Definitions

1923 Brønsted-Lowry definitions:


An acid donates an H+ ion to another
substance.
A base accepts an H+ from another substance.
HCl(g) + H2O(l)  H3O+(aq) + Cl(aq)
NH3(aq) + H2O(l)  NH4+(aq) + OH(aq)

In an acid-base reaction, the acid donates
(transfers) an H+ to a base.
13-9
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Activity: Brønsted-Lowry Acids and
Bases

For each reaction, identify the Brønsted-Lowry acid
and base reactants.
1.
2.
3.
OCl-(aq) + H2O(l)  HOCl(aq) + OH-(aq)
H2SO4(aq) + F-(aq) → HSO4-(aq) + HF(aq)
NH4+(aq) + H2O(l)  NH3(aq) + H3O+(aq)
13-10
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Activity Solutions: Brønsted-Lowry
Acids and Bases

For each reaction, identify the Brønsted-Lowry acid
and base reactants.
1.
OCl-(aq) + H2O(l)  HOCl(aq) + OH-(aq)
base
acid
The water (H2O) donates an H+ to the OCl- to form
HOCl. The water is an acid. The OCl- accepts an H+
from the water (H2O), therefore it is the base.
13-11
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Activity Solutions: Brønsted-Lowry
Acids and Bases
2.
H2SO4(aq) + F-(aq) → HSO4-(aq) + HF(aq)
acid
base
H2SO4, sulfuric acid, is the acid and it donates an H+
to the F-. F- accepts H+ from the sulfuric acid and is
therefore the base.
3.
NH4+(aq) + H2O(l)  NH3(aq) + H3O+(aq)
acid
base
NH4+ donates an H+ to the water and thus is the
acid. H2O accepts the H+ and is the base.
13-12
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Acid-base reactions can take place
without water:


NH3(g) + HCl(g)  NH4+(g) + Cl(g)
NH4+(g) + Cl(g)  NH4Cl(s)
Figure from p. 532
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13-13
Brønsted-Lowry Acids and Bases

Many anions from dissolved ionic compounds act
as bases:
Na2CO3(s)  2Na+(aq) + CO32(aq)

The carbonate ion acts as a base in water:
CO32(aq) + H2O(l)  HCO3(aq) + OH(aq)
base
acid
and with other acids:
HF(aq) + CO32(aq)  F(aq) + HCO3(aq)
acid
base

13-14
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Conjugate Acid-Base Pairs

Imagine the following reaction going in the reverse
direction. What would be the acid and what
would be the base?
HF(aq) + CO32(aq)  F(aq) + HCO3(aq)
We call the acid and base products the conjugates
of the base and acid that formed them.
13-15
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Conjugate Acid-Base Pairs
HF(aq) + CO32(aq)  F(aq) + HCO3(aq)
acid
base
conjugate conjugate
base
acid


A conjugate acid-base pair differs only by one
proton.
The conjugate base of H3PO4 is H2PO4-, not PO43-.
13-16
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Activity: Conjugate Acid-Base Pairs


Determine the formula of the conjugate base of each
acid.
Acid
Conjugate Base
H2CO3
HCO3
H3PO4
H2PO4HPO42PO43
NH4+
NH3
H2O
OH
13-17
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Activity: Conjugate Acid-Base Pairs


Determine the formula of the conjugate acid
of each base.
Base
Conjugate Acid
SO42
HCO3
NH2
ClO2
H2PO4 
HSO4
H2CO3
NH3
HClO2
H3PO4
13-18
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Activity: Conjugate Acid-Base Pairs

Identify the acid and base reactants and their
conjugate acid and base:

HCO3- + H3PO4  H2CO3 + H2PO4base
acid
conjugate conjugate
acid
base
13-19
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Amphoteric Substances


A substance that can act as either an acid or a base
Water is the most common amphoteric substance.
Another common amphoteric substance is the
bicarbonate ion, HCO3-:
HCO3-(aq) + OH-(aq) → CO32-(aq) + H2O(l)
acid
base
conjugate conjugate
base
acid
HCO3-(aq) + H3O+(aq) → H2CO3(aq) + H2O(l)
base
acid
conjugate conjugate
acid
base
13-20
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Acidic Hydrogen Atoms


If an acid has more
than one hydrogen
atom, we need to
determine which
hydrogen atoms are
acidic.
In oxoacids, the acidic
hydrogen atoms are
bonded to hydrogen
atoms:.
Figure 13.5
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13-21
13.2 Strong and Weak Acids and
Bases

Strong and weak acids and bases differ in
the extent of ionization.


Strong acids ionize completely.
Weak acids and bases ionize to only a small
extent – a small fraction of the molecules
ionize.
13-22
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Strong Acids

Strong acids are strong electrolytes.
Figure 13.6
13-23
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Strong Acids
Table 13.1
13-24
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Strong Bases

Strong bases are strong electrolytes.
Figure 13.7
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13-25
Strong Bases
Table 13.2
*Although the group IIA (2) metal hydroxides are not completely water soluble, they
are strong bases because the amount that dissolves dissociates almost completely.
13-26
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Weak Acids

Weak acids are weak electrolytes
Figure 13.8
13-27
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Common Weak Acids
Table 13.3
13-28
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Weak Bases

Weak bases are weak electrolytes
Figure 13.9
13-29
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Common Weak Bases
Table 13.4
13-30
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Strong and Weak Acids and Bases
HCl(aq)
HF(aq)
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13-31
Identify each as a strong or weak
acid or base:
A. weak base
B. weak acid
C. strong acid
Figure from p. 563
13-32
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Activity: Acids and Bases on the
Molecular Level

One of the diagrams below represents HClO4, and
the other represents an aqueous solution of HSO4-.
Which is which? Explain your reasoning.
Figure from p. 540
13-33
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Activity Solutions: Acids and Bases on
the Microscopic Level
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

The picture on the left (A) shows no acid molecules
and therefore shows an acid that has completely
dissociated.
Diagram A would then be a strong acid and of the
two choices, perchloric acid (HClO4) is the strong
one.
Diagram B (the picture on the right) shows acid
molecules that have not completely dissociated and
are therefore the weak acid, HSO4-.
13-34
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Activity: Strong vs. Weak Acids and
Bases

Identify each of the following as a strong acid, weak
acid, strong base, or weak base. Write an equation to
describe its reaction in water.
1. HI(aq)
2. NaCH3CO2(aq)
3. NH4+(aq)
4. NH3(aq)
13-35
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Activity Solutions: Strong vs. Weak
Acids and Bases
1.
HI(aq)
Hydroiodic acid is a strong acid (it is on the list of
strong acids in Table 13.1). Therefore, it
completely dissociates in water:
HI(aq) + H2O(l) → H3O+(aq) + I-(aq)
2.
NaCH3CO2(aq)
Sodium acetate is an ionic compound that partially
dissociates in water to form Na+(aq) and CH3CO2(aq). The CH3CO2- ion is the conjugate base of the
weak acid, HC2H3O2, so it is a weak base and does
not completely dissociate in water:
CH3CO2-(aq) + H2O(l)  CH3CO2H(aq) + OH-(aq)
13-36
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Activity Solutions: Strong vs. Weak
Acids and Bases
3.
NH4+(aq)
Ammonium ion is the conjugate acid of the weak
base, ammonia (NH3), so ammonium is therefore a
weak acid. It does not completely dissociate in
water.
NH4+(aq) + H2O(l)  NH3(aq) + H3O+(aq)
4.
NH3(aq)
Ammonia is a weak base, shown in Table 13.4,
and does not dissociate completely in water.
NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
13-37
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13.3 Relative Strengths of Weak Acids

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Acid strength depends on the degree of ionization,
which is a measure of the relative number of acid
molecules that ionize to hydronium ion and
conjugate base when dissolved in water.
Remember that Keq describes the relative amounts
of products to reactants.
The greater the degree of ionization, the greater
the ratio of products to reactants, and therefore the
greater the value of Keq for the acid ionization
reaction.
13-38
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Relative Strengths of Weak Acids


The strengths of weak acids can be compared by
examining the Keq values for their reactions with
water, which we label Ka, the acid ionization constant.
HCN(aq) + H2O(l)  H3O+(aq) + CN-(aq)
[H 3O+ ][CN- ]
Ka =
= 6.2  10-10
[HCN]

CH3CO2H(aq) + H2O(l)  H3O+(aq) + CH3CO2-(aq)
[H 3O+ ][CH 3CO 2- ]
Ka =
= 1.8  10-5
[CH 3CO2 H]

Which of these acids is stronger?
Which forms the greater concentration of H3O+(aq) when
equal concentrations of acids are added to water?

13-39
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