Chapter 14 Acids and Bases. Chapter 14 Table of Contents Copyright © Cengage Learning. All rights...

Chapter 14

Acids and Bases

Chapter 14

Table of Contents

Copyright © Cengage Learning. All rights reserved 2

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


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Section 14.2


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Various Ways to Describe Acid Strength

Section 14.2


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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Let’s Think About It… see page 637

• Why aren’t H+ or CN– major species?

Section 14.5


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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


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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


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• What are the major species in solution?

HF, H2O

• Why aren’t H+ and F– major species?

Section 14.5


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• 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


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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


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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


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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


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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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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


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Several Series of Oxyacids and Their Ka Values

Section 14.9


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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

Chapter 14 Acids and Bases. Chapter 14 Table of Contents Copyright © Cengage Learning. All rights...

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Transcript of Chapter 14 Acids and Bases. Chapter 14 Table of Contents Copyright © Cengage Learning. All rights...