The Foundations of Chemistry

SOLVOLYSIS

Solvolysisis the reaction of a substance with the solvent in which it is dissolved. The

solvolysis reactions that we will consider in this chapter occur in aqueous solutions so they

are called hydrolysisreactions. Hydrolysisis the reaction of a substance with water. Some

hydrolysis reactions involve reaction with H 3 O
or OHions. One common kind of

hydrolysis involves reaction of the anion of a weak acidwith water to form nonionized acid

molecules and OHions. This upsets the H 3 O
/OHbalance in water and produces

basic solutions. This reaction is usually represented as

A 
H 2 O 34 HA 
OH (excess OH, so solution is basic)

anion of weak acid

weak acid

Recall that in

neutral solutions [H 3 O
][OH]1.0 10 ^7 M

basic solutions [H 3 O
][OH] or [OH]1.0 10 ^7 M

acidic solutions [H 3 O
][OH] or [H 3 O
]1.0 10 ^7 M

In Brønsted–Lowry terminology anions of strong acids are extremely weak bases,

whereas anions of weak acids are stronger bases (Section 10-4). To refresh your memory,

consider the following examples.

Nitric acid, a common strong acid, is essentially completely ionized in dilute aqueous

solution. Diluteaqueous solutions of HNO 3 contain equal concentrations of H 3 O
and

NO 3 ions. In dilute aqueous solution nitrate ions show almost no tendency to react with

H 3 O
ions to form nonionized HNO 3 ; thus, NO 3 is a very weak base.

100%

HNO 3 
H 2 O8888nH 3 O

NO 3 

On the other hand, acetic acid (a weak acid) is only slightly ionized in dilute aqueous

solution. Acetate ions have a strong tendency to react with H 3 O
to form CH 3 COOH

molecules. Acetic acid ionizes only slightly.

18-6

We often omit (aq) from molecules
and ions in aqueous solutions to save
space and time.

TABLE 18-8 Comparison of 0.10 M

Solutions of Two Polyprotic

Acids (Examples 18-16, 18-17)

0.10 M 0.10 M

H 3 PO 4 H 2 SO 4

Ka1 7.5 10 ^3 
very large

Ka2 6.2 10 ^8 
1.2 10 ^2

Ka3 3.6 10 ^13 
—

[H 3 O
] 2.4 10 ^2 M 
0.11 M

pH 1.62 
0.96

[nonionized molecules] 7.6 10 ^2 M 
 0 M

Examples of conjugate acid–base pairs.

Acid Conjugate Base
strong (HCl) 88n very weak (Cl)
weak (HCN) 88n stronger, but still
weak (CN)

Base Conjugate Acid
strong (OH) 88n very weak (H 2 O)
weak (NH 3 ) 88n stronger, but still
weak (NH 4
)