CHAPTER 15



chapter 15

Acids and Bases

Chapter Terms and Definitions

Numbers in parentheses after definitions give the text sections in which the terms are explained. Starred terms are italicized in the text. Where a term does not fall directly under a text section heading, additional information is given for you to locate it.

oxygen*  means “acid former” in Greek (15.1, introductory section)

Arrhenius concept*  acid–base theory that defines an acid as a substance that, when dissolved in water, increases the concentration of hydronium ion, H3O+(aq), and a base as a substance that, when dissolved in water, increases the concentration of hydroxide ion, OH((aq) (15.1)

strong acid*  according to Arrhenius, a substance that completely ionizes in aqueous solution to give a hydronium ion, H3O+(aq), and an anion (15.1)

strong base*  according to Arrhenius, a substance that completely ionizes in aqueous solution to give a hydroxide ion, OH–(aq), and a cation (15.1)

weak*  describes acids and bases that are not ionized completely in solution and exist in equilibrium with the corresponding ions (15.1)

acid (Brønsted–Lowry)  species that donates the proton in a proton-transfer reaction (15.2)

base (Brønsted–Lowry)  species that accepts the proton in a proton-transfer reaction (15.2)

conjugate acid–base pair  in an acid–base equilibrium, two species that differ by the loss or gain of a proton (15.2)

conjugate acid*  in a conjugate acid–base pair, the species that can donate a proton (15.2)

conjugate base*  in a conjugate acid–base pair, the species that can accept a proton (15.2)

amphiprotic species  species that can function as either a Brønsted–Lowry acid or base, that is, can either lose or gain a proton (15.2)

amphoteric*  describes a species, such as aluminum oxide, that can act as an acid or a base (15.2, marginal note)

Lewis acid  species that can form a covalent bond by accepting an electron pair from another species (15.3)

Lewis base  species that can form a covalent bond by donating an electron pair to another species (15.3)

complex ion(s)*  species produced when a metal ion forms a covalent bond by accepting an electron pair from a molecule or another ion (15.3)

amines*  nitrogen-containing bases that are considered to be derivatives of ammonia, in which one or more hydrogen atoms have been substituted by carbon-containing groups

leveling effect*  phenomenon whereby, in water, the strong acids appear to be the same, although they show differing degrees of ionization in other solvents (15.4)

self-ionization (autoionization)  reaction in which two like molecules react to give ions (15.6)

ion-product constant for water (Kw)  equilibrium value of the ion product [H3O+][OH(], which equals 1.00 ( 10(14 at 25(C (15.6)

pH  –log[H3O+] (15.8)

Chapter Diagnostic Test

1. What is the Arrhenius concept of an acid?

2. Give the conjugate base for each of the following acids.

a. HBr

b. NH4+

c. HCO3(

d. HC2H3O2

e. H2SO4

3. Give the conjugate acid for each of the following bases.

a. CN(

b. H2O

c. HCO3(

d. SO42(

e. H2PO4(

4. Using text Table 15.2, predict the direction for each of the following acid–base reactions.

a. H3O+ + CO32( [pic] HCO3( + H2O

b. NO3( + HF [pic] HNO3 + F(

c. HS( + NH4+ [pic] H2S + NH3

d. H2S + CN( [pic] HS( + HCN

5. For each of the following reactions, what type of acid is present? Identify which species are the acids and which are the bases.

a. Cu2+ + 4NH3 [pic] Cu(NH3)42+

b. COCl2 + AlCl3 [pic] COCl+ + AlCl4(

6. A solution of NaOH is determined to be 7.6 ( 10(3 M NaOH. What are the hydroxide and hydronium ion concentrations of this solution?

7. The hydroxide ion concentration in skim milk was measured to be 4.2 ( 10–8 M. What is the pH of this milk?

8. Will the following reaction occur as written?

HClO(aq) + CO32((aq) → ClO–(aq) + HCO3–(aq)

Answers to Chapter Diagnostic Test

If you missed an answer, study the text section and problem-solving skill (PS Sk.) given in parentheses after the answer.

1. According to the Arrhenius concept, an acid is any hydrogen-containing substance that yields a hydronium ion in a water solution. (15.1)

2.

a. Br(

b. NH3

c. CO32(

d. C2H3O2(

e. HSO4( (15.2, PS Sk. 1)

3.

a. HCN

b. H3O+

c. H2CO3

d. HSO4(

e. H3PO4 (15.2, PS Sk. 1)

4.

a. To the right

b. To the left

c. To the left

d. To the right (15.4, PS Sk. 3)

5. Lewis acids

a. Cu2+, acid; NH3, base

b. COCl2, base; AlCl3, acid (15.3, PS Sk. 2)

6. [OH–] = 7.6 ( 10–3 M; [H3O+] = 1.3 ( 10–12 M (15.7, PS Sk. 4)

7. pH = 6.62 (15.8, PS Sk. 5)

8. Yes (15.4, PS Sk. 3)

Summary of Chapter Topics

15.1 Arrhenius Concept of Acids and Bases

Learning Objective

• Define acid and base according to the Arrhenius concept.

15.2 Brønsted–Lowry Concept of Acids and Bases

Learning Objectives

• Define acid and base according to the Brønsted–Lowry concept.

• Define the term conjugate acid–base pair.

• Identify acid and base species. (Example 15.1)

• Define amphiprotic species.

Problem-Solving Skill

1. Identifying acid and base species. Given a proton-transfer reaction, label the acids and bases, and name the conjugate acid–base pairs (Example 15.1).

Memorize the Brønsted–Lowry definition that an acid is a proton donor. This will help you to remember that a base is a proton acceptor.

Exercise 15.1

For the reaction

H2CO3(aq) + CN((aq) [pic] HCN(aq) + HCO3((aq)

label each species as an acid or a base. For the base on the left, what is the conjugate acid?

Known: A Brønsted–Lowry acid is a proton donor; a conjugate acid is the base without its proton.

Solution: Write labels under the appropriate species.

H2CO3(aq) + CN((aq) [pic] HCN(aq) + HCO3((aq)

|acid |base | acid | base |

The base on the left is CN(. Its conjugate acid is HCN.

15.3 Lewis Concept of Acids and Bases

Learning Objectives

• Define Lewis acid and Lewis base.

• Identify Lewis acid and Lewis base species. (Example 15.2)

Problem-Solving Skill

2. Identifying Lewis acid and base species. Given a reaction involving the donation of an electron pair, identify the Lewis acid and the Lewis base (Example 15.2).

The Lewis acid is an electron-pair acceptor. Memorizing this definition will help you to remember that a Lewis base is an electron-pair donor.

Exercise 15.2

Identify the Lewis acid and the Lewis base in each of the following reactions. Write the chemical equations using electron-dot formulas.

a. BF3 + CH3OH ( [pic]

b. O2( + CO2 ( CO32(

Solution:

a. The equation, using labeled electron-dot formulas, is

|[pic] |+ |[pic] |( |[pic] |

| Lewis acid | |Lewis base | | |

b. The equation, using labeled electron-dot formulas, is

|[pic] |+ |[pic] |( |[pic] |

|Lewis base | |Lewis acid | | |

15.4 Relative Strengths of Acids and Bases

Learning Objectives

• Understand the relationship between the strength of an acid and that of its conjugate base.

• Decide whether reactants or products are favored in an acid–base reaction. (Example 15.3)

Problem-Solving Skill

3. Deciding whether reactants or products are favored in an acid–base reaction. Given an acid–base reaction and the relative strengths of acids (or bases), decide whether reactants or products are favored (Example 15.3).

Exercise 15.3

Determine the direction of the following reaction from the relative strengths of acids and bases.

H2S(aq) + C2H3O2((aq) [pic] HC2H3O2(aq) + HS((aq)

Known: Equilibrium favors the acid of lesser strength.

Solution: Text Table 15.2 shows that acetic acid is stronger than H2S. Thus the reaction goes from right to left.

15.5 Molecular Structure and Acid Strength

Learning Objectives

• Note the two factors that determine relative acid strengths.

• Understand the periodic trends in the strengths of the binary acids HX.

• Understand the rules for determining the relative strengths of oxoacids.

• Understand the relative acid strengths of a polyprotic acid and its anions.

Exercise 15.4

Which member of each of the following pairs is the stronger acid?

a. NH3, PH3

b. HI, H2Te

c. HSO3(, H2SO3

d. H3AsO4, H3AsO3

e. HSO4(, HSeO4(

a. Known: N and P are in the same periodic table group; acidity increases with size and thus down the group.

Solution: PH3 is the stronger acid.

b. Known: I and Te are in the same period; acidity increases with electronegativity and thus to the right.

Solution: HI is the stronger acid.

c. Known: HSO3( is an anion of H2SO3; the anion with its negative charge holds the hydrogen more tightly.

Solution: H2SO3 is the stronger acid.

d. Known: Both species are oxoacids of As; bond polarity and thus acid strength increase with the oxidation state of As.

Solution: H3AsO4, with the higher oxidation state of As, is the stronger acid.

e. Known: Both species are anions of oxoacids of elements in the same periodic group with the same oxidation number; acidity increases with increasing electronegativity of the central atom.

Solution: HSO4( is the stronger acid because S is more electronegative than Se.

15.6 Self-Ionization of Water

Learning Objectives

• Define self-ionization (or autoionization).

• Define the ion-product constant for water.

Memorize the expression for and value of the ion-product constant for water Kw(=([H3O+][OH(] = 1.0 ( 10(14 at 25(C. You won’t be able to solve acid–base problems without it. Although we do not report units for an equilibrium constant, in using Kw in calculations to find [H3O+] or [OH(], we attribute to Kw the units of M2.

15.7 Solutions of a Strong Acid or Base

Learning Objective

• Calculate the concentrations of H3O+ and OH- in solutions of a strong acid or base. (Example 15.4)

Problem-Solving Skill

4. Calculating concentrations of H3O+ and OH( in solutions of a strong acid or base. Given the concentration of a strong acid or base, calculate the hydronium ion and hydroxide ion concentrations (Example 15.4).

Memorize the information relating hydronium ion concentration and acidity: In an acidic solution, [H3O+] is greater than (>) 1.00 ( 10–7 M; in a neutral solution, [H3O+](=(1.00(((10–7 M; and in a basic solution, [H3O+] is less than ( 1.00 ( 10–7 M; in a neutral solution,

[H3O+] = 1.00 ( 10–7 M; in a basic solution, [H3O+] < 1.00 ( 10–7 M;

Kw = [H3O+][OH–] = 1.00 ( 10–14

Solution: Find [H3O+].

[H3O+] = [pic]= [pic] = 1.0 ( 10–9 M

Since [H3O+] is less than ( ................
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