Chapter 16 Text
Chapter 16 Text
Chapter 16 Text
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<strong>16</strong>.9 Acid-Base Properties of Salt Solutions 641<br />
(b)K& for NH3 is listed in Table <strong>16</strong>.4 and in Appendix D as Kh = 1.8 X 1CT5. Using<br />
Equation <strong>16</strong>.40, we can calculate Ka for the conjugate acid, NH4+:<br />
Kb 1.8 x 1(<br />
PRACTICE EXERCISE<br />
(a) Which of the following anions has the largest base-dissociation constant:<br />
/ PO43 / or N3~? (b) The base quinoline has the following structure:<br />
oio<br />
Its conjugate acid is listed in handbooks as having a pKa of 4.90. What is the base-dissociation<br />
constant for quinoline?<br />
Answers: (a) PO43~(K& - 2.4 x 1(T2}; (b) 7.9 X 10~10<br />
<strong>16</strong>.9 Acid-Base Properties of Salt Solutions<br />
Even before you began this chapter, you were undoubtedly aware of many substances<br />
that are acidic, such as HNO3, HC1, and H2SO4, and others that are basic,<br />
such as NaOH and NHs. However, our recent discussions have indicated that<br />
ions can also exhibit acidic or basic properties. For example, we calculated Ka<br />
for NH4+ and K^ for F~ in Sample Exercise <strong>16</strong>.<strong>16</strong>. Such behavior implies that salt<br />
solutions can be acidic or basic. Before proceeding with further discussions of<br />
acids and bases, let's examine the way dissolved salts can affect pH.<br />
We can assume that when salts dissolve in water, they are completely dissociated;<br />
nearly all salts are strong electrolytes. Consequently, the acid-base properties<br />
of salt solutions are due to the behavior of their constituent cations and<br />
anions. Many ions are able to react with water to generate H+(ag) or OHT(flq).<br />
This type of reaction is often called hydrolysis. The pH of an aqueous salt solution<br />
can be predicted qualitatively by considering the ions of which the salt is<br />
composed. \<br />
Bassam Z. Shakhashiri, "Hydrolysis:<br />
Acidic and Basic Properties of Salts,"<br />
Chemical Demonstrations: A<br />
Handbook for Teachers of Chemistry,<br />
Vol. 3 (The University of Wisconsin<br />
Press, Madison, 1989) pp. 1 03-108.<br />
An Anion's Ability to React with Water<br />
In general, an anion, X~, in solution can be considered the conjugate base of an<br />
acid. For example, Cl~ is the conjugate base of HC1 and C2H3O2^ is the conjugate<br />
base of HC2H3O2. Whether an anion reacts with water to produce hydroxide<br />
depends upon the strength of the acid to which it is conjugate. To identify the acid<br />
and assess its strength, we can simply add a proton to the anion's formula:<br />
X" plus a proton gives HX<br />
H the acid determined in this way is one of the strong acids listed at the beginning<br />
of Section <strong>16</strong>.5, then the anion in question will have a negligible tendency<br />
to abstract protons from water (Section <strong>16</strong>.2) and the following equilibrium will<br />
lie entirely to the left:<br />
H2O(0 [<strong>16</strong>.42]<br />
Consequently, the anion X will not affect the pH of the solution. The presence<br />
°f Cr in an aqueous solution, for example, does not result in the production of<br />
any OH" and does not affect the pH.<br />
/