Chapter 16 Acids and Bases€¦ · Chapter 16 Acids and Bases Copyright © Cengage Learning. All...

Chapter 16

Acids and Bases

Copyright © Cengage Learning. All rights reserved 1

Acids and Bases

Section 16.1


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

Acids and Bases

Section 16.1


Acid in Water

HA(aq) + H2O(l) H3O+(aq) + A–(aq)

acid base conjugate conjugateacid 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.

Acids and Bases

Section 16.1


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

• Forms hydronium ion (H3O+).

Acids and Bases

Section 16.1


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

Strong Acid

• Completely ionized or completely dissociated


Acid Strength

Section 16.2

Weak Acid

• Most of the acid molecules remain intact.


Acid Strength

Section 16.2

Behavior of Acids of Different Strengths in Aqueous Solution


Acid Strength

Section 16.2

• A strong acid contains a relatively weak conjugate base.


Acid Strength

Section 16.2

Ways to Describe Acid Strength


Acid Strength

Section 16.2

Common Strong Acids

• Sulfuric acid, H2SO4

• Hydrochloric acid, HCl

• Nitric acid, HNO3

• Perchloric acid, HClO4


And HI and HBr

Acid Strength

Section 16.2

• 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


Acid Strength

Section 16.2

Concept Check

Consider a 1.0 M solution of HCl.

Order the following from strongest to weakestbase and explain:

H2O(l)

A–(aq) (from weak acid HA)

Cl–(aq)


Acid Strength

Section 16.2

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

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 strongestand explain your answer:

H2O Cl– CN– C2H3O2–


Acid Strength

Section 16.2

Let’s Think About It…

• H2O(l) + H2O(l) H3O+(aq) + OH–(aq)

acid base conjugate conjugateacid base

• At 25C, Kw = 1.0 x 10–14

The bases from weakest to strongest are:

Cl–, H2O, C2H3O2–, CN–


Water as an Acid and a Base

Section 16.3


• 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.



Section 16.3

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

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

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.


The pH Scale

Section 16.4

• 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.


pOH = -log[OH-]

The pH Scale

Section 16.4

pH Range

• pH = 7; neutral

• pH > 7; basic

Higher the pH, more basic.

• pH < 7; acidic

Lower the pH, more acidic.


The pH Scale

Section 16.4

The pH Scale and pH Values of Some Common Substances


The pH Scale

Section 16.4

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


The pH Scale

Section 16.4

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


The pH Scale

Section 16.4

pH and pOH

• Recall:

Kw = [H+][OH–]

–log Kw = –log[H+] – log[OH–]

pKw = pH + pOH

14.00 = pH + pOH


The pH Scale

Section 16.4

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


The pH Scale

Section 16.4

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


Calculating the pH of Strong Acid Solutions

Section 16.5

• Determine the [H+].

2.0 M HCl → 2.0 M H+ and 2.0 M Cl–

• pH = log[H+]



Section 16.5

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

Concept Check

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

pH = 7.00



Section 16.5

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


Buffered Solutions

Section 16.6

• 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.


You are NOT responsible for Buffers…

Buffered Solutions

Section 16.6

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)


Buffered Solutions

Section 16.6

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 Acids and Bases Copyright © Cengage Learning. All...

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Transcript of Chapter 16 Acids and Bases€¦ · Chapter 16 Acids and Bases Copyright © Cengage Learning. All...