Chapter 16

Acids and Bases

Chapter 16

Table of Contents

2

16.1Acids and Bases

16.2Acid Strength

16.3Water as an Acid and a Base

16.4The pH Scale

16.5 Calculating the pH of Strong Acid Solutions

16.6Buffered Solutions

Section 16.1

Acids and Bases

Return to TOC

3

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 16.1

Acids and Bases

Return to TOC

Copyright © Cengage Learning. All rights reserved 4

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 and is found by removing an H+.

• Conjugate acid is formed when the proton is transferred to the base and is found by adding an H+.

Section 16.1

Acids and Bases

Return to TOC

5

Practice – Conjugate Acids and Bases

• Identify the conjugate acid for each base.

a) F-

b) HCO3-

c) H2O

13-

HF

H2CO3

H3O+

• Identify the conjugate base for each acid.

a) HBr

b) HSO4-

c) H2O

Br-

SO4-2

OH-

Section 16.1

Acids and Bases

Return to TOC

Copyright © Cengage Learning. All rights reserved 6

• Water acts as a base accepting a proton from the acid.

• Forms hydronium ion (H3O+).

Section 16.1

Acids and Bases

Return to TOC

Copyright © Cengage Learning. All rights reserved 7

Concept Check

Which of the following represent conjugate acid–base pairs?

a) HCl, HNO3

b) H3O+, OH–

c) H2SO4, SO42–

d) HCN, CN–

Acid Strength

Section 16.2

Return to TOC

8

Strong Acid

• Completely ionized or completely dissociated

Acid Strength

Section 16.2

Return to TOC

9

Weak Acid

• Most of the acid molecules remain intact.

Acid Strength

Section 16.2

Return to TOC

Copyright © Cengage Learning. All rights reserved 10

Behavior of Acids of Different Strengths in Aqueous Solution

Acid Strength

Section 16.2

Return to TOC

11

• A strong acid contains a relatively weak conjugate base.

Acid Strength

Section 16.2

Return to TOC

12

Ways to Describe Acid Strength

Acid Strength

Section 16.2

Return to TOC

Copyright © Cengage Learning. All rights reserved 13

Common Strong Acids

• Sulfuric acid, H2SO4

• Hydrochloric acid, HCl

• Nitric acid, HNO3

• Perchloric acid, HClO4

Common Weak Acids

• Hydroflouric Acid HF

• Phosphoric Acid H3PO4

• Organic Acids

Acid Strength

Section 16.2

Return to TOC

14

Common Strong Bases

• Ammonia NH3

• Organic Ammines R-NH2

• Insoluble Hydroxides

Common Weak Bases

• Group I Hydroxides NaOH

Acid Strength

Section 16.2

Return to TOC

15

• Oxyacid – acidic proton is attached to an oxygen atom• Organic acid – have a carbon atom backbone and

commonly contain the carboxyl group:

Typically a weak acid

Acid Strength

Section 16.2

Return to TOC

Copyright © Cengage Learning. All rights reserved 16

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)

Acid Strength

Section 16.2

Return to TOC

Copyright © Cengage Learning. All rights reserved 17

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–

Acid Strength

Section 16.2

Return to TOC

18

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

–

The bases from weakest to strongest are: Cl–, H2O, C2H3O2

–, CN–

Water as an Acid and a Base

Section 16.3

Return to TOC

19

Water as an Acid and a Base

• Water is amphoteric: H2O(l) + H2O(l) H3O+(aq) + OH–(aq)

Behaves either as an acid or as a base.• At 25°C:

Kw = [H+][OH–] = 1.0 × 10–14

[X] = molarity of X or moles X per Liter

• No matter what the solution contains, the product of [H+] and [OH–] must always equal 1.0 × 10–14 at 25°C.

Water as an Acid and a Base

Section 16.3

Return to TOC

20

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.

Water as an Acid and a Base

Section 16.3

Return to TOC

Copyright © Cengage Learning. All rights reserved 21

Concept Check

In an acidic aqueous solution, which statement below is correct?

a) [H+] < 1.0 × 10–7 Mb) [H+] > 1.0 × 10–7 Mc) [OH–] > 1.0 × 10–7 Md) [H+] < [OH–]

Water as an Acid and a Base

Section 16.3

Return to TOC

22

Exercise

In an aqueous solution in which [OH–] = 2.0 x 10–10 M, the [H+] = ________ M, and the solution is ________.

a) 2.0 × 10–10 M ; basicb) 1.0 × 10–14 M ; acidicc) 5.0 × 10–5 M ; acidicd) 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.

Section 16.4

The pH Scale

Return to TOC

23

• 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 16.4

The pH Scale

Return to TOC

Copyright © Cengage Learning. All rights reserved 24

pH Range

• pH = 7; neutral• pH > 7; basic

Higher the pH, more basic.• pH < 7; acidic

Lower the pH, more acidic.

Section 16.4

The pH Scale

Return to TOC

25

The pH Scale and pH Values of Some Common Substances

Blood cells can exist only over a narrow pH range.

Section 16.4

The pH Scale

Return to TOC

Copyright © Cengage Learning. All rights reserved 26

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)

[H+] = 2.5 × 10–13 M H+

pH = –log[H+] = –log(2.5 × 10–13 M) = 12.60

Section 16.4

The pH Scale

Return to TOC

Copyright © Cengage Learning. All rights reserved 27

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

Section 16.4

The pH Scale

Return to TOC

28

pH and pOH

• Recall:

Kw = [H+][OH–]

–log Kw = –log[H+] – log[OH–]

pKw = pH + pOH

14.00 = pH + pOH

Section 16.4

The pH Scale

Return to TOC

Copyright © Cengage Learning. All rights reserved 29

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

Section 16.4

The pH Scale

Return to TOC

30

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

Section 16.5

Calculating the pH of Strong Acid Solutions

Return to TOC

Copyright © Cengage Learning. All rights reserved 31

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

Section 16.5

Calculating the pH of Strong Acid Solutions

Return to TOC

Copyright © Cengage Learning. All rights reserved 32

Concept Check

Calculate the pH of a 1.5 × 10–11 M solution of HCl.

pH = 10.82

Section 16.5

Calculating the pH of Strong Acid Solutions

Return to TOC

33

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

Section 16.4

The pH Scale

Return to TOC

Practice Calculating ph, pOH, [H+] and [OH-]

34

pH [H+] [OH-] pOH

_____ ______ 6.8x10-7 ______

9.2 ______ ______ ______

6.177.83

4.8

1.5x10-8

6 x10-10 2x10-5

Section 16.6

Buffered Solutions

Return to TOC

35

• 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.

Section 16.6

Buffered Solutions

Return to TOC

Copyright © Cengage Learning. All rights reserved 36

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)

Section 16.6

Buffered Solutions

Return to TOC

37

How Buffers Really Work

HA H+ + A-

A- + H2O HA + OH-

Consider the Equilibrium and La Chatelier’s Principle.

As you add H+ to a buffered system containing equal amounts of weak acid and its conjugate base, it will go through this equilibrium treadmill and keep the H+ concentration in check.

Section 16.6

Buffered Solutions

Return to TOC

Copyright © Cengage Learning. All rights reserved 38

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) NaOH + NH3 → NaNH4Ob) NaOH + NH4

+ → Na+ + NH3 + H2Oc) NaOH + Cl– → NaCl + OH–

d) NaOH + H2O → NaH3O2