Chapter 16 Acids and Bases. Chapter 16 Table of Contents 2 16.1Acids and Bases 16.2Acid Strength...

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

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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

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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

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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

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

• Forms hydronium ion (H3O+).

Section 16.1

Acids and Bases

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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

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8

Strong Acid

• Completely ionized or completely dissociated

Acid Strength

Section 16.2

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9

Weak Acid

• Most of the acid molecules remain intact.

Acid Strength

Section 16.2

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Behavior of Acids of Different Strengths in Aqueous Solution

Acid Strength

Section 16.2

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11

• A strong acid contains a relatively weak conjugate base.

Acid Strength

Section 16.2

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12

Ways to Describe Acid Strength

Acid Strength

Section 16.2

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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

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14

Common Strong Bases

• Ammonia NH3

• Organic Ammines R-NH2

• Insoluble Hydroxides

Common Weak Bases

• Group I Hydroxides NaOH

Acid Strength

Section 16.2

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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

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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)

Acid Strength

Section 16.2

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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–

Acid Strength

Section 16.2

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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

–

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

–, CN–

Water as an Acid and a Base

Section 16.3

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19


• 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.


Section 16.3

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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.


Section 16.3

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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–]


Section 16.3

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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

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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 16.4

The pH Scale

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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

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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

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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

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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

[H+] = 10–5.85 = 1.4 × 10–6 M

Section 16.4

The pH Scale

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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

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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

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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

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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


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Concept Check

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

pH = 10.82

Section 16.5


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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

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

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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

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• 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

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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

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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

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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

Chapter 16 Acids and Bases. Chapter 16 Table of Contents 2 16.1Acids and Bases 16.2Acid Strength...

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Transcript of Chapter 16 Acids and Bases. Chapter 16 Table of Contents 2 16.1Acids and Bases 16.2Acid Strength...