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140 Modern Analytical Chemistry

The equilibrium constant for this reaction is called the solubility product, K sp , and
solubility product
The equilibrium constant for a reaction is given as
in which a solid dissociates into its ions K sp = [Pb ][Cl ] = 1.7 ´10 –5 6.6
– 2
2+
(K sp ).
Note that the precipitate, which is a solid, does not appear in the K sp expression. It
is important to remember, however, that equation 6.6 is valid only if PbCl 2 (s) is
–
2+
present and in equilibrium with the dissolved Pb and Cl . Values for selected solu-
bility products can be found in Appendix 3A.

6D.2 Acid–Base Reactions
A useful definition of acids and bases is that independently introduced by Jo-
hannes Brønsted (1879–1947) and Thomas Lowry (1874–1936) in 1923. In the
acid Brønsted-Lowry definition, acids are proton donors, and bases are proton accep-
A proton donor. tors. Note that these definitions are interrelated. Defining a base as a proton accep-
tor means an acid must be available to provide the proton. For example, in reac-
base tion 6.7 acetic acid, CH 3 COOH, donates a proton to ammonia, NH 3 , which serves
A proton acceptor.
as the base.
+
–
CH 3 COOH(aq)+NH 3 (aq) t CH 3 COO (aq)+NH 4 (aq) 6.7
When an acid and a base react, the products are a new acid and base. For exam-
–
ple, the acetate ion, CH 3 COO , in reaction 6.7 is a base that reacts with the acidic
+
ammonium ion, NH 4 , to produce acetic acid and ammonia. We call the acetate ion
the conjugate base of acetic acid, and the ammonium ion is the conjugate acid of
ammonia.

Strong and Weak Acids The reaction of an acid with its solvent (typically water) is
called an acid dissociation reaction. Acids are divided into two categories based on
the ease with which they can donate protons to the solvent. Strong acids, such as
HCl, almost completely transfer their protons to the solvent molecules.
–
+
HCl(aq)+H 2 O(l) ® H 3 O (aq)+Cl (aq)
+
In this reaction H 2 O serves as the base. The hydronium ion, H 3 O , is the conju-
gate acid of H 2 O, and the chloride ion is the conjugate base of HCl. It is the hy-
dronium ion that is the acidic species in solution, and its concentration deter-
mines the acidity of the resulting solution. We have chosen to use a single arrow
(® ) in place of the double arrows (t) to indicate that we treat HCl as if it were
completely dissociated in aqueous solutions. A solution of 0.10 M HCl is effec-
+
–
tively 0.10 M in H 3 O and 0.10 M in Cl . In aqueous solutions, the common
strong acids are hydrochloric acid (HCl), hydroiodic acid (HI), hydrobromic acid
(HBr), nitric acid (HNO 3 ), perchloric acid (HClO 4 ), and the first proton of sulfu-
ric acid (H 2 SO 4 ).
Weak acids, of which aqueous acetic acid is one example, cannot completely
donate their acidic protons to the solvent. Instead, most of the acid remains undis-
sociated, with only a small fraction present as the conjugate base.
acid dissociation constant + –
The equilibrium constant for a reaction CH 3 COOH(aq)+H 2 O(l) t H 3 O (aq)+CH 3 COO (aq)
in which an acid donates a proton to the The equilibrium constant for this reaction is called an acid dissociation constant,
solvent (K a ).
K a , and is written as

[H O + ][CH COO – ] –5
3
3
K a = = .175 ´10
[CH COOH ]
3