Section 16.1
Acids and Bases
Models of Acids and Bases
• Arrhenius: Acids produce H+ ions in solution,
bases produce OH– ions.
• Brønsted–Lowry: Acids are proton (H+) donors,
bases are proton acceptors.
HCl + H2O → Cl– + H3O+
acid base
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Section 16.1
Acids and Bases
Acid in Water
HA(aq) +
acid
•
•
H2O(l)
base
H3O+(aq) + A–(aq)
conjugate
conjugate
acid
base
Conjugate base is everything that remains of the acid
molecule after a proton is lost.
Conjugate acid is formed when the proton is transferred
to the base.
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Section 16.1
Acids and Bases
•
Water acts as a base accepting a proton from the acid.
•
Forms hydronium ion (H3O+).
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Section 16.1
Acids and Bases
Concept Check
Which of the following represent conjugate
acid–base pairs?
a)
b)
c)
d)
HCl, HNO3
H3O+, OH–
H2SO4, SO42–
HCN, CN–
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Section 16.2
Acid Strength
Strong Acid
• Completely ionized or completely dissociated
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Section 16.2
Acid Strength
Weak Acid
• Most of the acid molecules remain intact.
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Section 16.2
Acid Strength
Behavior of Acids of Different Strengths in Aqueous Solution
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Section 16.2
Acid Strength
•
A strong acid contains a
relatively weak
conjugate base.
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Section 16.2
Acid Strength
Ways to Describe Acid Strength
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Section 16.2
Acid Strength
Common Strong Acids
•
•
•
•
Sulfuric acid, H2SO4
Hydrochloric acid, HCl
Nitric acid, HNO3
Perchloric acid, HClO4
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Section 16.2
Acid Strength
•
•
Oxyacid – acidic proton is attached to an oxygen atom
Organic acids – have a carbon atom backbone and
commonly contain the carboxyl group:

Typically a weak acid
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Section 16.2
Acid Strength
Concept Check
Consider a 1.0 M solution of HCl.
Order the following from strongest to weakest
base and explain:
H2O(l)
A–(aq) (from weak acid HA)
Cl–(aq)
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Section 16.2
Acid Strength
Let’s Think About It…
• How good is Cl–(aq) as a base?
• Is A–(aq) a good base?
The bases from strongest to weakest are:
A–, H2O, Cl–
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Section 16.2
Acid Strength
Concept Check
Acetic acid (HC2H3O2) and HCN are both weak
acids. Acetic acid is a stronger acid than HCN.
Arrange these bases from weakest to strongest
and explain your answer:
H2O
Cl–
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CN–
C2H3O2–
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Section 16.2
Acid Strength
Let’s Think About It…
H3O+(aq) + OH–(aq)
conjugate conjugate
acid
base
• At 25°C, Kw = 1.0 x 10–14
• H2O(l) + H2O(l)
acid
base
The bases from weakest to strongest are:
Cl–, H2O, C2H3O2–, CN–
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Section 16.3
Water as an Acid and a Base
Water as an Acid and a Base
• Water is amphoteric:
 Behaves either as an acid or as a base.
• At 25°C:
Kw = [H+][OH–] = 1.0 × 10–14
• No matter what the solution contains, the
product of [H+] and [OH–] must always equal
1.0 × 10–14 at 25°C.
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Section 16.3
Water as an Acid and a Base
Three Possible Situations
• [H+] = [OH–]; neutral solution
• [H+] > [OH–]; acidic solution
• [H+] < [OH–]; basic solution
In each case, however,
Kw = [H+][OH–] = 1.0 × 10–14.
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Section 16.3
Water as an Acid and a Base
Concept Check
In an acidic aqueous solution, which statement
below is correct?
a)
b)
c)
d)
[H+] < 1.0 × 10–7 M
[H+] > 1.0 × 10–7 M
[OH–] > 1.0 × 10–7 M
[H+] < [OH–]
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Section 16.3
Water as an Acid and a Base
Exercise
In an aqueous solution in which [OH–] = 2.0 x 10–10 M, the
[H+] = ________ M, and the solution is ________.
a)
b)
c)
d)
2.0 × 10–10 M ; basic
1.0 × 10–14 M ; acidic
5.0 × 10–5 M ; acidic
5.0 × 10–5 M ; basic
[H+] = Kw/[OH–] = 1.0 × 10–14/2.0 × 10–10 = 5.0 × 10–5 M.
Since [H+] is greater than [OH–], the solution is acidic.
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Section 16.4
The pH Scale
•
•
•
•
pH = –log[H+]
A compact way to represent solution acidity.
pH decreases as [H+] increases.
Significant figures:
 The number of decimal places in the log is
equal to the number of significant figures in
the original number.
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Section 16.4
The pH Scale
pH Range
• pH = 7; neutral
• pH > 7; basic
 Higher the pH, more basic.
• pH < 7; acidic
 Lower the pH, more acidic.
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Section 16.4
The pH Scale
The pH Scale and pH Values
of Some Common
Substances
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Section 16.4
The pH Scale
Exercise
Calculate the pH for each of the following
solutions.
a) 1.0 × 10–4 M H+
pH = –log[H+] = –log(1.0 × 10–4 M) = 4.00
b) 0.040 M OH–
Kw = [H+][OH–] = 1.00 × 10–14 = [H+](0.040 M) = 2.5 × 10–13 M H+
pH = –log[H+] = –log(2.5 × 10–13 M) = 12.60
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Section 16.4
The pH Scale
Exercise
The pH of a solution is 5.85. What is the [H+]
for this solution?
[H+] = 1.4 × 10–6 M
[H+] = 10^–5.85 = 1.4 × 10–6 M
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Section 16.4
The pH Scale
pH and pOH
• Recall:
Kw = [H+][OH–]
–log Kw = –log[H+] – log[OH–]
pKw = pH + pOH
14.00 = pH + pOH
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Section 16.4
The pH Scale
Exercise
Calculate the pOH for each of the following
solutions.
a) 1.0 × 10–4 M H+
pH = –log[H+] = –log(1.0 × 10–4 M) = 4.00;
So, 14.00 = 4.00 + pOH; pOH = 10.00
b) 0.040 M OH–
pOH = –log[OH–] = –log(0.040 M) = 1.40
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Section 16.4
The pH Scale
Exercise
The pH of a solution is 5.85. What is the
[OH–] for this solution?
[OH–] = 7.1 × 10–9 M
14.00 = 5.85 + pOH; pOH = 8.15
[OH–] = 10^–8.15 = 7.1 × 10–9 M
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Section 16.5
Calculating the pH of Strong Acid Solutions
• Determine the [H+].
2.0 M HCl → 2.0 M H+ and 2.0 M Cl–
• pH = −log[H+]
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Section 16.5
Calculating the pH of Strong Acid Solutions
Concept Check
Consider an aqueous solution of 2.0 × 10–3
M HCl. What is the pH?
pH = 2.70
2.0 × 10–3 M HCl → 2.0 × 10–3 M H+ and 2.0 × 10–3 M Cl–
pH = –log[H+]
pH = –log(2.0 × 10–3 M)
pH = 2.70
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Section 16.5
Calculating the pH of Strong Acid Solutions
Concept Check
Calculate the pH of a 1.5 × 10–11 M solution
of HCl.
pH = 7.00
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Section 16.5
Calculating the pH of Strong Acid Solutions
Concept Check
Calculate the pH of a 1.5 × 10–2 M solution of
HNO3.
pH = 1.82
pH = –log[H+]
pH = –log(1.5 × 10–2 M)
pH = 1.82
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Section 16.6
Buffered Solutions
• Buffered solution – resists a change in its pH
when either an acid or a base has been added.
 Presence of a weak acid and its conjugate
base buffers the solution.
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Section 16.6
Buffered Solutions
The Characteristics of a Buffer
1. The solution contains a weak acid HA and its
conjugate base A–.
2. The buffer resists changes in pH by reacting
with any added H+ or OH– so that these ions do
not accumulate.
3. Any added H+ reacts with the base A–.
H+(aq) + A–(aq) → HA(aq)
4. Any added OH– reacts with the weak acid HA.
OH–(aq) + HA(aq) → H2O(l) + A–(aq)
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Section 16.6
Buffered Solutions
Concept Check
If a solution is buffered with NH3 to which has been
added NH4Cl, what reaction will occur if a strong base
such as NaOH is added?
a)
b)
c)
d)
NaOH + NH3 → NaNH4O
NaOH + NH4+ → Na+ + NH3 + H2O
NaOH + Cl– → NaCl + OH–
NaOH + H2O → NaH3O2
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