Chapter 15 149 Chapter 15 Acids and Bases This entire chapter is covered in the AP Chemistry curriculum and will be crucial to success on the AP exam. It is also a large chapter. On the exam, problem 1 in the free response is an equilibrium problem, and most often, it is a K a or K b problem. This chapter combines the acid- base concepts of Chapter 4 and equilibrium concepts from Chapter 14. Basic definitions of acids and bases are reviewed, including the concept that water is a weak electrolyte and can be both an acid and a base. It is acceptable to use either H + or H 3 O + on the exam. K w , the ion product constant, is covered. Students are given the K w value in the AP equations pages for the free response section of the exam, so there is no need to memorize it. Animations are on acid and base ionizations and the media player covers dissociation of weak and strong scids. Major Concepts to Know • In review, the students need to understand why strong acids totally disassociate and weak acids do not. Good visualizations of the process follow that also emphasize solutions are mostly water. The Extent of Disassociation for Weak Acids
Transcript
Chapter 15 149
Chapter 15 Acids and Bases
This entire chapter is covered in the AP Chemistry curriculum and will be crucial to success on
the AP exam. It is also a large chapter. On the exam, problem 1 in the free response is an
equilibrium problem, and most often, it is a Ka or Kb problem. This chapter combines the acid-
base concepts of Chapter 4 and equilibrium concepts from Chapter 14. Basic definitions of acids
and bases are reviewed, including the concept that water is a weak electrolyte and can be both an
acid and a base. It is acceptable to use either H+ or H3O
+ on the exam. Kw, the ion product
constant, is covered. Students are given the Kw value in the AP equations pages for the free
response section of the exam, so there is no need to memorize it. Animations are on acid and
base ionizations and the media player covers dissociation of weak and strong scids.
Major Concepts to Know
• In review, the students need to understand why strong acids totally disassociate and weak
acids do not. Good visualizations of the process follow that also emphasize solutions are
mostly water.
The Extent of Disassociation for Weak Acids
Chapter 15 150
The Extent of Disassociation for Strong Acids
• Students should know the Brønsted-Lowry definition that acids are proton donors and bases
are proton acceptors. Students need to be able to look at equations and identify an acid-base
reaction and also the conjugate acid-base pairs. It will also be important in equations to
recognize that diprotic and triprotic acids only lose one H+ at a time.
Chapter 15 151
Proton Transfer as the Essential Feature of a Brønsted-Lowry
Acid-Based Reaction
• Often, students will not recognize, when asked for the K of the conjugate reaction, that they
can simply use Ka Kb = Kw. Walking them through the algebra of this relationship and
writing out all the equilibrium constant expressions should help to make this clear.
• Students need to understand all solutions have more H2O than anything else, and acid
solutions have more H+ than OH . In neutral solutions, H
+ and OH concentrations are equal,
and basic solutions have more OH than H+.
• A common mistake is for students to indicate an acid when the highest pH is discussed. Yes,
it has a higher H+ concentration, but the pH is lower.
• Important concepts for students include pOH scales, since they will need to be able to
interchange pOH and pH scales using pH + pOH = 14. Although students should indicate an
understanding of logs, it is good to review them. Also, a review of exponents is important as
some students seem to have difficulty recognizing that 105 is smaller than 10
3, for example.
Students should be able to give the –log of H+ concentrations like 1.0 M, 0.1 M, 0.01 M, and
0.001 M without a calculator. This skill will also help them estimate pHs when given the Ka
of an indicator.
• Students often indicate the wrong number of significant figures when converting pH and
pOH to [H+] and [OH
-] or pK to K.values. The first digit before the decimal place in a pH or
pK value sets the power of 10, so it is not counted in the significant figures. Thus, a pH of
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1.23 translates to an [H+] value that should be correctly reported as 5.9 10
2 M.
Conversely, [H+] = 2.5 10
-13 M is a pH of 12.60.
• The strong acids and bases essentially completely ionize. The acids are HI, HBr, HCl, HNO3,
HClO4, and H2SO4;; the bases are NaOH, KOH, LiOH, CsOH, RbOH, and Ba(OH)2. They
are also considered strong electrolytes. In these cases, the formal molar concentrations of the
acids or bases thus equal the concentrations of H+ or OH , respectively. For example, a 0.1 M
solution of NaOH has a pH of 13.0 and [Na+] and [OH ] = 0.1 M. Students often miss the
first ionization equation for the strong acid H2SO4. It is H2SO4(aq) H+
(aq) + HSO4 (aq).
• Weak acids and bases differ because they do not ionize completely. This means weak acids
and bases are weak electrolytes, and Ka and Kb must be used to solve for concentrations of
species and, hence, pHs. When doing problems, students frequently fail to recognize the
validity of balanced equations. If given the acid HA, when it ionizes (dissociates), the
concentrations of the ions are equal. If HA H+ + A , then [H
+] = [A ].
• HF is the most common acid students incorrectly indicate as strong. Students do not need to
know any Ka or Kb values, but they do need to be able to solve Ka and Kb problems. Using the
ICE chart previously mentioned in solving Kc and Kp problems will help the students
throughout the acid-base problems and later in buffer problems.
• Most problems on the AP exam, as with Kc and Kp, have had less than five percent reaction
allowing for the dropping of –x values. So students can escape from solving quadratic
Chapter 15 153
equations, although they should be able to solve the quadratic and understand when it is
required. However, they also need practice in estimation, so they must know when x or + x
can be neglected. Students need to do a variety of problems and show the calculation of
percent ionization.
• When doing calculations of pH with diprotic and triprotic acids, the majority of the H+ comes
from the first ionization. Students should be able to explain by looking at the Ka values. For
instance, with 0.10 M phosphoric acid, the first Ka value of
7.5 10 3
(7.5 103 = x
2/0.01 x) gives an [H
+] value of 8.7 10
3 M, while the second Ka is
6.2 10 8 and the third is 4.8 10
13, which contribute very little to the concentration. H
+
values are so small in relationship to the first one that they do not change the value from the
first step when using correct significant figures. Later in Chapter 16, when buffers are
discussed, they will need to look at Ka values and indicate which will be the best choice to
make certain buffers.
• The next section covers acid strength. A common question on the AP exam is explaining why
HF is weak and HCl is strong. Students need to understand the concept of the hydrogen bond
to explain this.
• When comparing acids like HClO3 and HClO2, it is important that students do not just
memorize “more O’s means stronger.” They must be able to draw the molecules correctly
and indicate the location of electron density. They should also be able to explain why
reducing electron density on the central Cl atom (by adding more electron-withdrawing
oxygen) makes it easier to remove the H+ from the OH group since the O-H bond becomes
more polar, making the acid stronger.
Chapter 15 154
Lewis Structures of the Oxoacids of Chlorine
• Many students will erroneously assume that the way the formula is often written indicates
the Hs are attached to atoms like Br or Cl. It is important to note that this means an
argument cannot be used to predict comparison of strengths between HCl and the oxyacids.
Important Functional Groups in Organic Compounds
Chapter 15 155
• Students need to be able to identify which salts are acidic, basic, or neutral and be able to
explain why. They should be able to write hydrolysis equations and then solve problems to
calculate the pH of solutions of those salts.
• The vocabulary of percent ionization, percent dissociation, and percent hydrolysis often
confuse students. Hydrolysis problems are best tackled by assuming the salt ionizes
completely and then “back reacts” with water using the ICE formalism, where I is the
completely dissociated salt ion concentration.
• Oxides that form acids and bases are covered briefly here. In the equation section of the free
response, acidic oxide and basic oxide reactions in water are often given.
Chapter 15 156
Oxides of the Representative Elements
• The chapter concludes with a review of Lewis acids and bases. This concept is not difficult
but is unlikely to be on recent AP exams. Addition reactions of the type NH3 + BF3 are
expected to be known as descriptive chemistry items, as are the formation of complex ions
such as Ag(NH3)2+, where NH3 is a Lewis base and Ag
+ is a Lewis acid. Lewis acids are
electron acceptors and bases are electron donors. This broadens the concept of acids and
