Free Study Guide for
Cracolice • Peters
Introductory Chemistry: An Active Learning Approach
Second Edition
www.brookscole.com/chemistry
Chapter 18
Acid–Base
(Proton-Transfer) Reactions
Chapter 18–Assignment A: Acid-Base Theories
Acid-base reactions are everywhere around you. They include small-scale reactions such as
controlling the acidity of your blood and body cells and adding lemon to a cup of tea. They
also include large-scale industrial processes that consume billions of kilograms of chemical
per year. “Acid rain” is a sad example of the importance of acid-base reactions in our lives.
Indeed, acid-base reactions are among the most important classes of reactions; they surely
deserve detailed study.
The main ideas that you should learn from this assignment are:
1)
The hydrogen ion–hydroxide ion concept of acids and bases is based on chemical
properties and is known as the Arrhenius theory of acids and base.
2)
According to the Brønsted-Lowry theory, an acid-base reaction involves a transfer
of a proton from one substance, the acid, to another, the base. An acid is a proton
donor; a base is a proton acceptor. When writing an equation, there must be a
Brønsted-Lowry base whenever there is a Brønsted-Lowry acid.
3)
Acid-base reactions are reversible; they reach a state of equilibrium according to the
general equation HA + B D A– + AB+.
4)
According to the Lewis theory of acids and bases, an acid is an electron-pair
acceptor and a base an electron-pair donor.
5)
Two substances whose formulas differ only by a proton (a hydrogen ion) are a
conjugate acid-base pair. If the acid has the general form HA, its conjugate base
has the general form A– . If the base has the general form B, its conjugate acid has
the general form HB+.
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Chapter 18
Acid–Base (Proton-Transfer) Reactions
Learning Procedures
Study
Sections 18.1–18.4. Focus on Goals 1–7 as you study.
Strategy
This is conceptual material. Learn the definitions and their applications.
Answer
Questions, Exercises, and Problems 1–9. Check your answers with those
at the end of the chapter.
Workbook
If your instructor recommends the Active Learning Workbook, do
Questions, Exercises, and Problems 1–9.
Chapter 18–Assignment B: Relative Acid Strengths, Predicting Acid-Base
Reactions, and Comparing Acid-Base and Redox Reactions
This assignment examines Brønsted-Lowry acid-base reactions in some detail. The
following ideas are included in this short lesson:
1)
Strong acids transfer protons readily; weak acids do not. Strong bases accept
protons readily; weak bases do not.
2)
The relative strengths of Brønsted-Lowry acids and bases may be decided from their
positions in a table.
3)
Proton transfer reactions involve two conjugate acid-base pairs. When a proton
transfer reaction reaches equilibrium, the proton transfer yields the weaker conjugate
acid and the weaker conjugate base.
4)
Acid-base reactions resemble redox reactions in a number of ways.
Learning Procedures
Study
Sections 18.5–18.7. Focus on Goals 8–11 as you study.
Strategy
It is important to be able to use Table 18.1. The Section 18.6 Procedure:
How to Predict the Favored Direction of an Acid-Base Reaction is also
critical to this assignment.
Answer
Questions, Exercises, and Problems 10–17. Check your answers with
those at the end of the chapter.
Workbook
If your instructor recommends the Active Learning Workbook, do
Questions, Exercises, and Problems 10–17.
Chapter 18–Assignment C: The Water Equilibrium and pH Conversions
For this assignment, you may need to review exponents and logarithms. Use Appendix I,
Parts B and C for this review.
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Study Guide for Introductory Chemistry: An Active Learning Approach
Look for the main ideas in this assignment:
1)
Water itself is both a weak acid and a weak base.
2)
Because concentrations of H+(aq) or OH– (aq) are usually small, but can vary over
wide ranges, they are usually expressed in exponential notation and as logarithms.
3)
pH = –log[H+] and pOH = –log[OH– ]. From these equations, you can obtain
[H+] = 10–pH and [OH– ] = 10–pOH. In water solutions at 25°C, pH + pOH = 14.00.
4)
A neutral water solution has pH = 7.00 at 25°C. An acidic water solution has pH
less than 7.00 at 25°C; a basic water solution has pH greater than 7.00 when
measured at 25°C.
Section 18.10, Noninteger pH–[ H+] and pOH–[ OH– ] Conversions, is an optional section.
The new skills needed to complete this optional section are not chemical, but mathematical.
The ideas concerning pH, pOH, [H+], and [OH– ] are the same as in Section 18.9; only the
numbers have been changed.
Learning Procedures
Study
Sections 18.8–18.10. Focus on Goals 12–14 as you study.
Strategy
Most of this material involves calculations. Practice by solving many endof-chapter questions.
Answer
Questions, Exercises, and Problems 18–30. Check your answers with
those at the end of the chapter.
Workbook
If your instructor recommends the Active Learning Workbook, do
Questions, Exercises, and Problems 18–30.
Chapter 18–Assignment D: Summary and Review
Don't be worried by the existence of three different acid-base systems. All three systems
generally agree on which species in a reaction is the acid and which is the base. It's not a
question of one system being right and the others wrong, but rather which system is most
convenient for the reaction being studied. Both good workers and good students know how
to pick the best tool for the job at hand.
Brønsted-Lowry acid-base theory is most convenient for proton-transfer reactions.
Correctly written equations show two acid-base pairs, and the position of equilibrium favors
the formation of the weaker acid and the weaker base. From Table 18.1, you can predict
relative acid and base strength, so you can predict if a proton-transfer reaction will occur.
Because the weaker acid and base in a reaction are less highly ionized than the stronger acid
and base, this is another example of an ion-combination reaction forming a “more”
molecular product.
134
Copyright © 2004 Brooks/Cole, a division of Thomson Learning, Inc.
No part of this work may be reproduced without the written permission of the publisher.
Chapter 18
Acid–Base (Proton-Transfer) Reactions
A memory aid for Lewis acids and bases is: Lewis acid = electron acceptor. Most cations
are Lewis acids, and anions tend to be Lewis bases. All Lewis bases have unshared electron
pairs. All Brønsted-Lowry bases are also Lewis bases, but H+(aq) is only one of many
Lewis acids.
Be sure you can do the pH loop given in Figure 18.3. Some words concerning calculations
are in order. If you understand what you are doing mathematically when you push buttons
on a calculator, that little box can be your best friend. If you don't understand what's going
on when you push the LOG or the INV button, the calculator can be your worst enemy.
Why? Most errors committed when using calculators are typing errors; data are entered
incorrectly on the keyboard. The calculator then gives an incorrect answer. If you have a feel
for the size of the answer, you can tell if you've punched in the wrong data. If you don't have
a feel for the size of the answer, the calculator quickly leads you down the primrose path to
numeric oblivion.
Learning Procedures
Review
your lecture and textbook notes.
the Chapter in Review and the Key Terms and Concepts, and read the Study
Hints and Pitfalls to Avoid.
Answer
Concept-Linking Exercises 1–11. Check your answers with those at the end
of the chapter.
Questions, Exercises, and Problems 31–35. Include Questions 36–38 if
assigned by your instructor. Check your answers with those at the end of the
chapter.
Workbook
If your instructor recommends the Active Learning Workbook, do
Questions, Exercises, and Problems 31–33. Include Questions 34–36 if
assigned by your instructor.
Take
the chapter summary test that follows. Check your answers with those at the
end of this assignment.
Chapter 18 Sample Test
Instructions: You may refer to Table 18.1.
1)
Which of these properties is not related to the H+(aq) ion?
a) turns litmus indicator red
b) slippery feeling on the skin
c) sour taste
d) reacts with and neutralizes a base
2)
According to the Brønsted-Lowry acid-base theory, an acid is a(n)
a) electron donor
b) electron acceptor
c) proton donor
d) proton acceptor
3)
According to the Lewis acid-base theory, a base in a(n)
a) electron donor
b) electron acceptor
c) proton donor
d) proton acceptor
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Study Guide for Introductory Chemistry: An Active Learning Approach
4)
Identify the Lewis acid among the following:
a) NH3
b) Cl–
c) O2
d) K+
5)
For the reaction HX– + HY– D H2 X + Y2 –, pick the conjugate acid-base pair among
the following:
a) HX– , HY–
b) HX, Y2 –
c) HY– , H2 X
2–
–
–
2–
d) Y , HY
e) HX ,Y
6)
The stronger of two Brønsted-Lowry bases tends to:
a) lose protons more easily
b) lose protons less easily
c) gain protons more easily
d) gain protons less easily
7)
Using Table 18.1, identify the strongest acid among the following:
a) HF
b) SO3 2 –
c) NH4 +
d) H2 PO4 –
8)
At 25°C, the numeric value of Kw is:
a) 1.0 ¥ 10– 7
b) 1.0 ¥ 10– 1 4
9)
A solution with a pH of 7.6 would be classified as:
a) strongly acidic
b) about neutral
c) strongly basic
10)
In a solution in which [OH– ] = 10– 5, pH and pOH are, respectively,
a) 10– 1 4 and 10– 9
b) 5 and 9
c) 5 and 14
d) 9 and 5
e) 14 and 5
11)
Which of the following solutions is most basic?
a) [OH– ] = 10– 5
b) pH = 7
c) pOH = 9
c) 1.0 ¥ 101 4
d) 14.00
d) [H+] = 10– 2
Questions 12 and 13: For each pair of reactants below, complete the equation for a single
proton transfer reaction; state if the reaction would be favored in the forward or reverse
direction. You may use Table 18.1.
12)
HCN(aq) + SO3 2 –(aq) D
13)
HSO3 – (aq) + HPO4 2 –(aq) D
14)
A solution has a pH of 2.87. Calculate its pOH, [H+], and [OH– ].
136
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No part of this work may be reproduced without the written permission of the publisher.
Chapter 18
Acid–Base (Proton-Transfer) Reactions
Answers to Chapter 18 Sample Test
1) b
2) c
3) a
4) d
5) d
6) c
7) a
8) b
9) b
10) d 11) a
12)
HCN(aq) + SO3 2 –(aq) D CN– (aq) + HSO3 – (aq)
13)
HSO3 – (aq) + HPO4 2 –(aq) D SO3 2 –(aq) + H2 PO4 – (aq) Reverse
HSO3 – (aq) + HPO4 2 –(aq) D H2 SO3 (aq) + PO4 3 –(aq) Reverse
14)
pH = 2.87; pOH = 14.00 – pH = 14.00 – 2.87 = 11.13;
[H+] = 10–pH = 10–2.87 = 1.3 ¥ 10– 3; [OH– ] = 10–pOH = 10–11.13 = 7.4 ¥ 10– 1 2
Reverse
137
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No part of this work may be reproduced without the written permission of the publisher.