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6: Ways to Classify Reactions

6: Ways to Classify Reactions

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536 Chapter 16 Acids and Bases

proton to another water molecule to produce a hydronium ion and a hydroxide ion.

H2O(l) ϩ H2O(l) 3

4 H3Oϩ(aq) ϩ OHϪ(aq)

The expression

ϩ



Ϫ



ϩ



Ϫ



Kw ϭ [H3O ][OH ] ϭ [H ][OH ]

is called the ion-product constant. It has been shown

experimentally that at 25 °C,

[Hϩ] ϭ [OHϪ] ϭ 1.0 ϫ 10Ϫ7 M

so Kw ϭ 1.0 ϫ 10Ϫ14.

4. In an acidic solution, [Hϩ] is greater than [OHϪ]. In a

basic solution, [OHϪ] is greater than [Hϩ]. In a neutral

solution, [Hϩ] ϭ [OHϪ].

ϩ



5. To describe [H ] in aqueous solutions, we use the pH

scale.

pH ϭ Ϫlog[Hϩ]

Note that the pH decreases as [Hϩ] (acidity) increases.

6. The pH of strong acid solutions can be calculated directly from the concentration of the acid, because

100% dissociation occurs in aqueous solution.

7. A buffered solution is one that resists a change in its

pH even when a strong acid or base is added to it. A

buffered solution contains a weak acid and its conjugate base.



Active Learning Questions

These questions are designed to be considered by groups of

students in class. Often these questions work well for introducing a particular topic in class.

1. You are asked for the Hϩ concentration in a solution

of NaOH(aq). Because sodium hydroxide is a strong

base, can we say there is no Hϩ, since having Hϩ

would imply that the solution is acidic?

2. Explain why ClϪ does not affect the pH of an aqueous

solution.



8. Can the pH of a solution be negative? Explain.

9. Stanley’s grade-point average (GPA) is 3.28. What is

Stanley’s p(GPA)?

10. A friend asks the following: “Consider a buffered solution made up of the weak acid HA and its salt NaA.

If a strong base like NaOH is added, the HA reacts

with the OHϪ to make AϪ. Thus, the amount of acid

(HA) is decreased, and the amount of base (AϪ) is increased. Analogously, adding HCl to the buffered solution forms more of the acid (HA) by reacting with

the base (AϪ). How can we claim that a buffered solution resists changes in the pH of the solution?” How

would you explain buffering to your friend?

11. Mixing together aqueous solutions of acetic acid and

sodium hydroxide can make a buffered solution.

Explain.

12. Could a buffered solution be made by mixing aqueous solutions of HCl and NaOH? Explain.

13. Consider the equation: HA(aq) ϩ H2O 3

4 H3Oϩ(aq) ϩ

Ϫ

A (aq).

a. If water is a better base than AϪ, which way will

equilibrium lie?

b. If water is a better base than AϪ, does this mean

that HA is a strong or a weak acid?

c. If water is a better base than AϪ, is the value for Ka

greater or less than 1?

14. Choose the answer that best completes the following

statement and defend your answer. When 100.0 mL

of water is added to 100.0 mL of 1.00 M HCl,

a. the pH decreases because the solution is diluted.

b. the pH does not change because water is neutral.

c. the pH is doubled because the volume is now

doubled.

d. the pH increases because the concentration of Hϩ

decreases.

e. the solution is completely neutralized.

15. You mix a solution of a strong acid with a pH of 4 and

an equal volume of a strong acid solution with a pH

of 6. Is the final pH less than 4, between 4 and 5, 5,

between 5 and 6, or greater than 6? Explain.



3. Write the general reaction for an acid acting in water.

What is the base in this case? The conjugate acid? The VP 16. The following figures are molecular-level representaconjugate base?

tions of acid solutions. Label each as a strong acid or

a weak acid.

4. Differentiate among the terms concentrated, dilute,

weak, and strong in describing acids. Use molecularlevel pictures to support your answer.

5. What is meant by “pH”? True or false: A strong acid

always has a lower pH than a weak acid does. Explain.

6. Consider two separate solutions: one containing a

weak acid, HA, and one containing HCl. Assume that

you start with 10 molecules of each.

a. Draw a molecular-level picture of what each solution looks like.

b. Arrange the following from strongest to weakest

base: ClϪ, H2O, AϪ. Explain.

7. Why is the pH of water at 25 °C equal to 7.00?



H+

B–



H+

A–



17. Answer the following questions concerning buffered

solutions.

a. Explain what a buffered solution does.

b. Describe the substances that make up a buffered

solution.

c. Explain how a buffered solution works.



Chapter Review



Questions and Problems

16.1 Acids and Bases

QUESTIONS

1. What are some physical properties that historically

led chemists to classify various substances as acids

and bases?

2. Write an equation showing how HCl(g) behaves as an

Arrhenius acid when dissolved in water. Write an

equation showing how NaOH(s) behaves as an Arrhenius base when dissolved in water.

3. According to the Brønsted-Lowry model, an acid is

a “proton donor” and a base is a “proton acceptor.”

Explain.

4. How do the components of a conjugate acid–base

pair differ from one another? Give an example of a

conjugate acid–base pair to illustrate your answer.

5. Given the general equation illustrating the reaction

of the acid HA in water,

HA(aq) ϩ H2O(l) S H3Oϩ(aq) ϩ AϪ(aq)

explain why water is considered a base in the BrønstedLowry model.

F 6. The “Chemistry in Focus” segment Gum That Foams



discusses Mad Dawg chewing gum. One of the ingredients in the gum is baking soda, sodium bicarbonate, NaHCO3. Does baking soda behave as an acid or

as a base in the gum?

PROBLEMS

7. Which of the following do not represent a conjugate

acid–base pair? For those pairs that are not conjugate

acid–base pairs, write the correct conjugate acid–base

pair for each species in the pair.

a.

b.

c.

d.



HI, IϪ

HClO, HClO2

H3PO4, PO43Ϫ

H2CO3, CO32Ϫ



8. Which of the following do not represent a conjugate

acid–base pair? For those pairs that are not conjugate

acid–base pairs, write the correct conjugate acid–base

pair for each species in the pair.

a.

b.

c.

d.



HClO4, ClO4Ϫ

NH4ϩ, NH3

NH3, NH2Ϫ

H2O, O2Ϫ



537



10. In each of the following chemical reactions, identify

the conjugate acid–base pairs.

a. NH3(aq) ϩ H2O(l) 3

4 NH4ϩ(aq) ϩ OHϪ(aq)

ϩ

b. NH4 (aq) ϩ H2O(l) 3

4 NH3(aq) ϩ H3Oϩ(aq)

Ϫ

c. NH2 (aq) ϩ H2O(l) S NH3(aq) ϩ OHϪ(aq)

11. Write the conjugate acid for each of the following

bases:

a. PO43Ϫ

b. IO3Ϫ



c. NO3Ϫ

d. NH2Ϫ



12. Write the conjugate acid for each of the following

bases.

a. ClOϪ

b. ClϪ



c. ClO3Ϫ

d. ClO4Ϫ



13. Write the conjugate base for each of the following

acids:

a. H2S

b. HSϪ



c. NH3

d. H2SO3



14. Write the conjugate base for each of the following

acids.

a. HBrO

b. HNO2



c. HSO3Ϫ

d. CH3NH3ϩ



15. Write a chemical equation showing how each of the

following species can behave as indicated when dissolved in water.

a.

b.

c.

d.



HSO3Ϫ as an acid

CO32Ϫ as a base

H2PO4Ϫ as an acid

C2H3O2Ϫ as a base



16. Write a chemical equation showing how each of the

following species can behave as indicated when dissolved in water.

a.

b.

c.

d.



O2Ϫ as a base

NH3 as a base

HSO4Ϫ as an acid

HNO2 as an acid



16.2 Acid Strength

QUESTIONS

17. What does it mean to say that an acid is strong in

aqueous solution? What does this reveal about the

ability of the acid’s anion to attract protons?

18. What does it mean to say that an acid is weak in aqueous solution? What does this reveal about the ability

of the acid’s anion to attract protons?



9. In each of the following chemical equations, identify

the conjugate acid–base pairs.



19. How is the strength of an acid related to the fact that

a competition for protons exists in aqueous solution

between water molecules and the anion of the acid?



4 FϪ(aq) ϩ H3Oϩ(aq)

a. HF(aq) ϩ H2O(l) 3

b. CNϪ(aq) ϩ H2O(l) 3

4 HCN(aq) ϩ OHϪ(aq)

c. HCO3Ϫ(aq) ϩ H2O(l) 3

4 H2CO3(aq) ϩ OHϪ(aq)



20. A strong acid has a weak conjugate base, whereas a

weak acid has a relatively strong conjugate base.

Explain.



All even-numbered Questions and Problems have answers in the back of this book and solutions in the Solutions Guide.



538 Chapter 16 Acids and Bases

21. Write the formula for the hydronium ion. Write an

equation for the formation of the hydronium ion

when an acid is dissolved in water.

22. Name four strong acids. For each of these, write the

equation showing the acid dissociating in water.

23. Organic acids contain the carboxyl group



O

H



Using acetic acid, CH3OCOOH, and propionic acid,

CH3CH2OCOOH, write equations showing how the

carboxyl group enables these substances to behave as

weak acids when dissolved in water.

24. What is an oxyacid? Write the formulas of three acids

that are oxyacids. Write the formulas of three acids

that are not oxyacids.

25. Which of the following acids have relatively strong

conjugate bases?

a.

b.

c.

d.



31. Calculate the [Hϩ] in each of the following solutions,

and indicate whether the solution is acidic or basic.

a.

b.

c.

d.



[OHϪ] ϭ 2.32 ϫ 10Ϫ4 M

[OHϪ] ϭ 8.99 ϫ 10Ϫ10 M

[OHϪ] ϭ 4.34 ϫ 10Ϫ6 M

[OHϪ] ϭ 6.22 ϫ 10Ϫ12 M



32. Calculate the [Hϩ] in each of the following solutions,

and indicate whether the solution is acidic or basic.



C

O



PROBLEMS



HCN

H2S

HBrO4

HNO3



F 26. The “Chemistry in Focus” segment Plants Fight Back



discusses how tobacco plants under attack by disease

produce salicylic acid. Examine the structure of salicylic acid and predict whether it behaves as a monoprotic or a diprotic acid.



16.3 Water as an Acid and a Base

QUESTIONS

27. Water is the most common amphoteric substance,

which means that, depending on the circumstances,

water can behave either as an acid or as a base. Using

HF as an example of an acid and NH3 as an example

of a base, write equations for these substances reacting with water, in which water behaves as a base and

as an acid, respectively.

28. Anions containing hydrogen (for example, HCO3Ϫ

and H2PO42Ϫ) show amphoteric behavior when reacting with other acids or bases. Write equations illustrating the amphoterism of these anions.

29. What is meant by the ion-product constant for water,

Kw? What does this constant signify? Write an equation for the chemical reaction from which the constant is derived.

30. What happens to the hydroxide ion concentration in

aqueous solutions when we increase the hydrogen

ion concentration by adding an acid? What happens

to the hydrogen ion concentration in aqueous solutions when we increase the hydroxide ion concentration by adding a base? Explain.



a.

b.

c.

d.



[OHϪ] ϭ 3.44 ϫ 10Ϫ1 M

[OHϪ] ϭ 9.79 ϫ 10Ϫ11 M

[OHϪ] ϭ 4.89 ϫ 10Ϫ6 M

[OHϪ] ϭ 3.78 ϫ 10Ϫ7 M



33. Calculate the [OHϪ] in each of the following solutions, and indicate whether the solution is acidic or

basic.

a.

b.

c.

d.



[Hϩ] ϭ 4.01 ϫ 10Ϫ4 M

[Hϩ] ϭ 7.22 ϫ 10Ϫ6 M

[Hϩ] ϭ 8.05 ϫ 10Ϫ7 M

[Hϩ] ϭ 5.43 ϫ 10Ϫ9 M



34. Calculate the [OHϪ] in each of the following solutions, and indicate whether the solution is acidic or

basic.

a.

b.

c.

d.



[Hϩ] ϭ 1.02 ϫ 10Ϫ7 M

[Hϩ] ϭ 9.77 ϫ 10Ϫ8 M

[Hϩ] ϭ 3.41 ϫ 10Ϫ3 M

[Hϩ] ϭ 4.79 ϫ 10Ϫ11 M



35. For each pair of concentrations, tell which represents

the more acidic solution.

a. [Hϩ] ϭ 1.2 ϫ 10Ϫ3 M or [Hϩ] ϭ 4.5 ϫ 10Ϫ4 M

b. [Hϩ] ϭ 2.6 ϫ 10Ϫ6 M or [Hϩ] ϭ 4.3 ϫ 10Ϫ8 M

c. [Hϩ] ϭ 0.000010 M or [Hϩ] ϭ 0.0000010 M

36. For each pair of concentrations, tell which represents

the more basic solution.

a. [Hϩ] ϭ 3.99 ϫ 10Ϫ6 M or [OHϪ] ϭ 6.03 ϫ 10Ϫ4 M

b. [Hϩ] ϭ 1.79 ϫ 10Ϫ5 M or [OHϪ] ϭ 4.21 ϫ 10Ϫ6 M

c. [Hϩ] ϭ 7.81 ϫ 10Ϫ3 M or [OHϪ] ϭ 8.04 ϫ 10Ϫ4 M



16.4 The pH Scale

QUESTIONS

37. Why do scientists tend to express the acidity of a solution in terms of its pH, rather than in terms of the

molarity of hydrogen ion present? How is pH defined

mathematically?

38. Using Figure 16.3, list the approximate pH value of

five “everyday” solutions. How do the familiar properties (such as the sour taste for acids) of these solutions correspond to their indicated pH?

39. For a hydrogen ion concentration of 2.33 ϫ 10Ϫ6 M,

how many decimal places should we give when expressing the pH of the solution?



All even-numbered Questions and Problems have answers in the back of this book and solutions in the Solutions Guide.



Chapter Review

F 40. The “Chemistry in Focus” segment Garden-Variety



Acid–Base Indicators discusses acid–base indicators

found in nature. What colors are exhibited by red

cabbage juice under acid conditions? Under basic

conditions?

PROBLEMS

41. Calculate the pH corresponding to each of the hydrogen ion concentrations given below, and indicate

whether each solution is acidic or basic.

a.

b.

c.

d.



ϩ



Ϫ3



[H ] ϭ 4.02 ϫ 10 M

[Hϩ] ϭ 8.99 ϫ 10Ϫ7 M

[Hϩ] ϭ 2.39 ϫ 10Ϫ6 M

[Hϩ] ϭ 1.89 ϫ 10Ϫ10 M



a.

b.

c.

d.



[Hϩ] ϭ 9.35 ϫ 10Ϫ2 M

[Hϩ] ϭ 3.75 ϫ 10Ϫ4 M

[Hϩ] ϭ 8.36 ϫ 10Ϫ6 M

[Hϩ] ϭ 5.42 ϫ 10Ϫ8 M



[OHϪ] ϭ 4.73 ϫ 10Ϫ4 M

[OHϪ] ϭ 5.99 ϫ 10Ϫ1 M

[OHϪ] ϭ 2.87 ϫ 10Ϫ8 M

[OHϪ] ϭ 6.39 ϫ 10Ϫ3 M



44. Calculate the pH corresponding to each of the hydroxide ion concentrations given below, and indicate

whether each solution is acidic or basic.

a.

b.

c.

d.



[OHϪ] ϭ 8.63 ϫ 10Ϫ3 M

[OHϪ] ϭ 7.44 ϫ 10Ϫ6 M

[OHϪ] ϭ 9.35 ϫ 10Ϫ9 M

[OHϪ] ϭ 1.21 ϫ 10Ϫ11 M



45. Calculate the pH corresponding to each of the pOH

values listed, and indicate whether each solution is

acidic, basic, or neutral.

a. pOH ϭ 4.32

b. pOH ϭ 8.90



c. pOH ϭ 1.81

d. pOH ϭ 13.1



46. Calculate the pOH value corresponding to each of

the pH values listed, and tell whether each solution is

acidic or basic.

a. pH ϭ 9.78

b. pH ϭ 4.01



c. pH ϭ 2.79

d. pH ϭ 11.21



47. For each hydrogen ion concentration listed, calculate

the pH of the solution as well as the concentration of

hydroxide ion in the solution. Indicate whether each

solution is acidic or basic.

a.

b.

c.

d.



ϩ



Ϫ8



[H ] ϭ 4.76 ϫ 10 M

[Hϩ] ϭ 8.92 ϫ 10Ϫ3 M

[Hϩ] ϭ 7.00 ϫ 10Ϫ5 M

[Hϩ] ϭ 1.25 ϫ 10Ϫ12 M



a.

b.

c.

d.



[Hϩ] ϭ 1.91 ϫ 10Ϫ2 M

[Hϩ] ϭ 4.83 ϫ 10Ϫ7 M

[Hϩ] ϭ 8.92 ϫ 10Ϫ11 M

[Hϩ] ϭ 6.14 ϫ 10Ϫ5 M



49. Calculate the hydrogen ion concentration, in moles

per liter, for solutions with each of the following pH

values.

c. pH ϭ 1.02

d. pH ϭ 7.00



50. Calculate the hydrogen ion concentration, in moles

per liter, for solutions with each of the following pH

values.

a. pH ϭ 11.21

b. pH ϭ 4.39



43. Calculate the pH corresponding to each of the hydroxide ion concentrations given below, and indicate

whether each solution is acidic or basic.

a.

b.

c.

d.



48. For each hydrogen ion concentration listed, calculate

the pH of the solution as well as the concentration of

hydroxide ion in the solution. Indicate whether each

solution is acidic or basic.



a. pH ϭ 9.01

b. pH ϭ 6.89



42. Calculate the pH corresponding to each of the hydrogen ion concentrations given below, and indicate

whether each solution is acidic or basic.



539



c. pH ϭ 7.44

d. pH ϭ 1.38



51. Calculate the hydrogen ion concentration, in moles

per liter, for solutions with each of the following pOH

values.

a.

b.

c.

d.



pOH ϭ 4.95

pOH ϭ 7.00

pOH ϭ 12.94

pOH ϭ 1.02



52. Calculate the hydrogen ion concentration, in moles

per liter, for solutions with each of the following pH

or pOH values.

a.

b.

c.

d.



pOH ϭ 4.99

pH ϭ 7.74

pOH ϭ 10.74

pH ϭ 2.25



53. Calculate the pH of each of the following solutions

from the information given.

a.

b.

c.

d.



[Hϩ] ϭ 4.78 ϫ 10Ϫ2 M

pOH ϭ 4.56

[OHϪ] ϭ 9.74 ϫ 10Ϫ3 M

[Hϩ] ϭ 1.24 ϫ 10Ϫ8 M



54. Calculate the pH of each of the following solutions

from the information given.

a.

b.

c.

d.



[Hϩ] ϭ 4.39 ϫ 10Ϫ6 M

pOH ϭ 10.36

[OHϪ] ϭ 9.37 ϫ 10Ϫ9 M

[Hϩ] ϭ 3.31 ϫ 10Ϫ1 M



16.5 Calculating the pH of Strong Acid Solutions

QUESTIONS

55. When 1 mole of gaseous hydrogen chloride is dissolved in enough water to make 1 L of solution, approximately how many HCl molecules remain in the

solution? Explain.



All even-numbered Questions and Problems have answers in the back of this book and solutions in the Solutions Guide.



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