chapter 14 acids and bases. chapter 14 table of contents copyright © cengage learning. all rights...
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Chapter 14
Acids and Bases
Chapter 14
Table of Contents
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14.1The Nature of Acids and Bases
14.2Acid Strength
14.3The pH Scale
14.4Calculating the pH of Strong Acid Solutions
14.5 Calculating the pH of Weak Acid Solutions
14.6Bases
14.7 Polyprotic Acids
14.8 Acid–Base Properties of Salts
14.9 The Effect of Structure on Acid–Base Properties
14.10 Acid–Base Properties of Oxides
14.11 The Lewis Acid–Base Model
Section 14.1
The Nature of Acids and Bases
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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
Section 14.1
The Nature of Acids and Bases
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Acid in Water
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
acid base 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.
Section 14.2
Atomic MassesAcid Strength
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• Strong acid: Ionization equilibrium lies far to the right. Yields a weak conjugate base.
• Weak acid: Ionization equilibrium lies far to the left. Weaker the acid, stronger its conjugate base.
Section 14.2
Atomic MassesAcid Strength
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Section 14.2
Atomic MassesAcid Strength
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Various Ways to Describe Acid Strength
Section 14.2
Atomic MassesAcid Strength
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Concept Check
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
acid base conjugate conjugate acid base
What is the equilibrium constant expression for an acid acting in water?
3H O A
= HA
K
Section 14.2
Atomic MassesAcid Strength
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Concept Check
If the equilibrium lies to the right, the value for Ka is __________.
large (or >1)
If the equilibrium lies to the left, the value for Ka is ___________.
small (or <1)
Section 14.2
Atomic MassesAcid Strength
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Concept Check
HA(aq) + H2O(l) H3O+(aq) + A–(aq)
If water is a better base than A–, do products or reactants dominate at equilibrium?
Does this mean HA is a strong or weak acid?
Is the value for Ka greater or less than 1?
Section 14.2
Atomic MassesAcid Strength
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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)
Section 14.2
Atomic MassesAcid Strength
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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–
Section 14.2
Atomic MassesAcid Strength
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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– CN– C2H3O2–
Section 14.3
The Mole The pH Scale
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• 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.
Section 14.3
The Mole The pH Scale
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Exercise
Calculate the pH for each of the following solutions.
a) 1.0 × 10–4 M H+
pH = 4.00
b) 0.040 M OH–
pH = 12.60
Section 14.3
The Mole The pH Scale
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Exercise
The pH of a solution is 5.85. What is the [H+] for this solution?
[H+] = 1.4 × 10–6 M
Section 14.3
The Mole The pH Scale
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pH and pOH
• Recall:
Kw = [H+][OH–]
–log Kw = –log[H+] – log[OH–]
pKw = pH + pOH
14.00 = pH + pOH
Section 14.3
The Mole The pH Scale
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Exercise
Calculate the pOH for each of the following solutions.
a) 1.0 × 10–4 M H+
pOH = 10.00
b) 0.040 M OH–
pOH = 1.40
Section 14.4
Calculating the pH of Strong Acid Solutions
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Concept Check
Consider an aqueous solution of 2.0 x 10–3 M HCl.
What are the major species in solution?
H+, Cl–, H2O
What is the pH?
pH = 2.70
Section 14.4
Calculating the pH of Strong Acid Solutions
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Concept Check
Calculate the pH of a 1.5 x 10–11 M solution of HCl.
pH = 7.00
Section 14.5
Calculating the pH of Weak Acid Solutions
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Concept Check
Consider a 0.80 M aqueous solution of the weak acid HCN (Ka = 6.2 x 10–10).
What are the major species in solution?
HCN, H2O
Section 14.5
Calculating the pH of Weak Acid Solutions
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Let’s Think About It… see page 637
• Why aren’t H+ or CN– major species?
Section 14.5
Calculating the pH of Weak Acid Solutions
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Consider This
HCN(aq) + H2O(l) H3O+(aq) + CN–(aq)
Ka = 6.2 x 10-10
H2O(l) + H2O(l) H3O+(aq) + OH–(aq)
Kw = 1.0 x 10-14
• Which reaction controls the pH? Explain.
Section 14.5
Calculating the pH of Weak Acid Solutions
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Exercise
Calculate the pH of a 0.50 M aqueous solution of the weak acid HF.
(Ka = 7.2 x 10–4)
Section 14.5
Calculating the pH of Weak Acid Solutions
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Let’s Think About It…
• What are the major species in solution?
HF, H2O
• Why aren’t H+ and F– major species?
Section 14.5
Calculating the pH of Weak Acid Solutions
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Let’s Think About It…
• What are the possibilities for the dominant reaction?
HF(aq) + H2O(l) H3O+(aq) + F–(aq) Ka=7.2 x 10-4
H2O(l) + H2O(l) H3O+(aq) + OH–(aq) Kw=1.0 x 10-14
• Which reaction controls the pH? Why?
Section 14.5
Calculating the pH of Weak Acid Solutions
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Steps Toward Solving for pH
Ka = 7.2 x 10–4
pH = 1.72
HF(aq) + H2O H3O+(aq) + F–(aq)
Initial 0.50 M ~ 0 ~ 0
Change –x +x +x
Equilibrium 0.50–x x x
Section 14.5
Calculating the pH of Weak Acid Solutions
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Percent Dissociation (Ionization)
• For a given weak acid, the percent dissociation increases as the acid becomes more dilute.
amount dissociated (mol/L)Percent dissociation = 100%
initial concentration (mol/L)
Section 14.5
Calculating the pH of Weak Acid Solutions
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Example 14.11 at page 642
A solution of 0.100 M lactic acid (HC3HO2) is 3.7% ionized in water. Calculate the Ka value for formic acid.
Section 14.5
Calculating the pH of Weak Acid Solutions
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Concept Check
The value of Ka for a 4.00 M formic acid solution should be:
higher than lower than the same as
the value of Ka of an 8.00 M formic acid solution.
Section 14.6
Bases
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• Arrhenius: bases produce OH– ions.• Brønsted–Lowry: bases are proton acceptors.• In a basic solution at 25°C, pH > 7.• Ionic compounds containing OH– are generally
considered strong bases. LiOH, NaOH, KOH, Ca(OH)2
• pOH = –log[OH–] • pH = 14.00 – pOH
Section 14.6
Bases
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• Equilibrium expression for weak bases uses Kb.
CN–(aq) + H2O(l) HCN(aq) + OH–(aq)
b
HCN OH =
CN
K
Section 14.6
Bases
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• pH calculations for solutions of weak bases are very similar to those for weak acids.
• Kw = [H+][OH–] = 1.0 × 10–14
• pOH = –log[OH–] • pH = 14.00 – pOH
Section 14.6
Bases
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Concept Check
Calculate the pH of a 2.0 M solution of ammonia (NH3).
(Kb = 1.8 × 10–5)
pH = 11.78
Section 14.7
Polyprotic Acids
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• Acids that can furnish more than one proton.• Always dissociates in a stepwise manner, one
proton at a time.• The conjugate base of the first dissociation
equilibrium becomes the acid in the second step.
• For a typical weak polyprotic acid:
Ka1 > Ka2 > Ka3
• For a typical polyprotic acid in water, only the first dissociation step is important to pH.
Section 14.7
Polyprotic Acids
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Exercise
Calculate the pH of a 1.00 M solution of H3PO4.
Ka1 = 7.5 x 10-3
Ka2 = 6.2 x 10-8
Ka3 = 4.8 x 10-13
pH = 1.08
Section 14.7
Polyprotic Acids
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Concept Check
Calculate the equilibrium concentration of PO43-
in a 1.00 M solution of H3PO4.
Ka1 = 7.5 x 10-3
Ka2 = 6.2 x 10-8
Ka3 = 4.8 x 10-13
[PO43-] = 3.6 x 10-19 M
Section 14.7
Polyprotic Acids
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Amino acids are polyprotic
Amino acids from which proteins are built have an acidic carboxylic acid group, a basic amino group, and a variable substituent designated R:
The resulting structure, with positive and negative sites, is called a zwitterion.
Section 14.7
Polyprotic Acids
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Section 14.7
Polyprotic Acids
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Section 14.7
Polyprotic Acids
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Finding the pH 1) Titration of polyprotic acids
(ex) H3PO4 (H3A)
][HA]A[H 22
]A[H2
]A[HA][H 23
AH3
pH
NAOH ofV
2pKapH
2
pKapKapH 21
1pKapH
50% 50%
100%
50% 50%
100%
Section 14.7
Polyprotic Acids
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Finding the pH
2) solutions containing Amphoteric Anions (H2A-) as the only A-B major species
-23
a
a
2
2
23
(aq)2
(aq)3(aq)2(aq)2
HAAH
K
K
AH
HA AHK
HAAHAHAH
1
2
Section 14.7
Polyprotic Acids
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Finding the pH
① .
② .
AH
AH
K
H
AH
AH HK
AH
AH HK
AHHAH from
K
K
AH
AHK
2
3
a3
-2
a
3
-2
a
2(aq)3
a
a
2
2
23
1
1
1
1
2
Section 14.7
Polyprotic Acids
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Finding the pH
&
2
pKpKpH
KKH
K
H
K
K
21
21
11
2
aa
aa
2
aa
a
Section 14.7
Polyprotic Acids
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Which is the principle species ?
HAA pKpH
HAA pKpH
1HA
A 4.20pH at
HA
AlogpKapH
a
a
Section 14.7
Polyprotic Acids
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For H2A
HA
ApKpH
AH
HApKpH
2
a
2
a
2
1
22
21
AAH
)pK(pK2
1pH
Which is the principle species ?
Section 14.7
Polyprotic Acids
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• 3) For H3A
Section 14.7
Polyprotic Acids
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What is the principal form of arginine at pH 10.0? Approximately what fraction is in this form? What is the second most abundant form at this pH?
Section 14.7
Polyprotic Acids
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Proteins are polyprotic acids and bases
myoglobin
Section 14.7
Polyprotic Acids
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Isoelectric pH (isoionic pH)
The pH at which the average charge of the polyprotic acid is zero
Section 14.7
Polyprotic Acids
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(ex)
21
2
pKpK2
1pH where
AAH &HA as formmost :pH cisoelectri
H3+N CHCOOH
CH3
H3+N C
H
CH3
COO- + H+
H3+N CHCO2
-
CH3
H3+N CHCO2
-
CH3
+ H+
alanine cation H2A+
A- alanine antion
HA
HA
pK1=2.35
pK2=9.87
Section 14.7
Polyprotic Acids
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Isoelectric focusing : A technique of protein separation
pH gradient designProtein stop migrating in an electric field at isoelectric pH
Section 14.8
Acid–Base Properties of Salts
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Qualitative Prediction of pH of Salt Solutions (from Weak Parents)
Section 14.8
Acid–Base Properties of Salts
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Concept Check
Consider a 0.30 M solution of NaF. The Ka for HF is 7.2 x 10-4.
What are the major species?
Na+, F-, H2O
Section 14.9
The Effect of Structure on Acid–Base Properties
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Models of Acids and Bases
Bond Polarity (high is good)
Bond Strength (low is good)
Section 14.9
The Effect of Structure on Acid–Base Properties
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Several Series of Oxyacids and Their Ka Values
Section 14.9
The Effect of Structure on Acid–Base Properties
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Comparison of Electronegativity of X and Ka Value
Section 14.10
Acid–Base Properties of Oxides
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Oxides
• Acidic Oxides (Acid Anhydrides): O – X bond is strong and covalent.
SO2, NO2, CO2
• Basic Oxides (Basic Anhydrides): O – X bond is ionic.
K2O, CaO
Section 14.11
The Lewis Acid–Base Model
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Lewis Acids and Bases
• Lewis acid: electron pair acceptor• Lewis base: electron pair donor
Lewis acid Lewis base
OH
HO
H
HAlAl3+ + 6
6
3+
Section 14.11
The Lewis Acid–Base Model
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Three Models for Acids and Bases