Page 158 - Modern Analytical Chemistry

1400-CH06 9/9/99 7:40 AM Page 141

Chapter 6 Equilibrium Chemistry 141

Note that the concentration of H 2 O is omitted from the K a expression because its
value is so large that it is unaffected by the dissociation reaction.* The magnitude
of K a provides information about the relative strength of a weak acid, with a
smaller K a corresponding to a weaker acid. The ammonium ion, for example,
with a K a of 5.70 ´10 –10 , is a weaker acid than acetic acid.
Monoprotic weak acids, such as acetic acid, have only a single acidic proton
and a single acid dissociation constant. Some acids, such as phosphoric acid, can
donate more than one proton and are called polyprotic weak acids. Polyprotic acids
are described by a series of acid dissociation steps, each characterized by it own acid
dissociation constant. Phosphoric acid, for example, has three acid dissociation re-
actions and acid dissociation constants.
–
+
HPO 4 () + H O( ) l t H O () + H PO ()
aq
aq
aq
3
2
3
2
4
–
+
[ HPO ][ H O ]
2
3
4
.
1 = = 711 ´ 10 3 –
K a
[ HPO 4 ]
3
+
–
aq
aq
aq
H PO () + H O( ) l t H O () + HPO 4 2 - ()
2
2
3
4
+
[ HPO 4 2 - ][ H O ] –8
3
K a 2 = = . 632 ´ 10
–
–
[ HPO ]
2
4
l
HPO 2 - () +H O t H 3 O + () +PO 3 - ()
( )
aq
aq
aq
4 2 4
[PO 3 - ][H O + ] –13
3
4
.
3 a K = 2 - =45 ´ 10
[HPO 4 ]
The decrease in the acid dissociation constant from K a1 to K a3 tells us that each suc-
cessive proton is harder to remove. Consequently, H 3 PO 4 is a stronger acid than
2–
–
–
H 2 PO 4 , and H 2 PO 4 is a stronger acid than HPO 4 .
Strong and Weak Bases Just as the acidity of an aqueous solution is a measure of
+
the concentration of the hydronium ion, H 3 O , the basicity of an aqueous solution
–
is a measure of the concentration of the hydroxide ion, OH . The most common
example of a strong base is an alkali metal hydroxide, such as sodium hydroxide,
which completely dissociates to produce the hydroxide ion.
–
+
NaOH(aq) ® Na (aq)+OH (aq)
Weak bases only partially accept protons from the solvent and are characterized by
a base dissociation constant, K b . For example, the base dissociation reaction and
base dissociation constant
base dissociation constant for the acetate ion are
The equilibrium constant for a reaction
–
–
l
aq
aq
CH COO () + H O( ) t OH () + CH COOH() in which a base accepts a proton from
aq
3
2
3
the solvent (K b ).
-
]
[
[ CH COOH OH ] – 10
3
K b = = 571 ´ 10
.
–
[ CH COO ]
3
Polyprotic bases, like polyprotic acids, also have more than one base dissociation re-
action and base dissociation constant.
Amphiprotic Species Some species can behave as either an acid or a base. For ex-
ample, the following two reactions show the chemical reactivity of the bicarbonate
–
ion, HCO 3 , in water.
*The concentration of pure water is approximately 55.5 M