Cliffs AP Chemistry, 3rd Edition

In hydrolysis, a salt reacts with water. The ions that hydrolyze do so because a weak acid or a
weak base is formed. The process of hydrolysis removes ions from the solution and is the dri-
ving force for the reaction. The reaction may produce a solution that is acidic, basic or neutral
according to the following chart:

Strong base + strong acid No hydrolysis — neutral

Salt formed — NaCl

HOH HO+-+ + 2

NaOH + HCl K

HOH K

11

w

==+-

77 AA

Methyl Red (end point = pH 5)

Bromthymol Blue (end point = pH 7)

Phenolphthalein (end point = pH 9)

Strong base + weak acid Basic — only anion hydrolyzes

Salt formed — NaAc

NaOH + HAc

Ac--++H O 2 +HAc OH

K

Ac

HAc OH

H

H

K

K

b==- # HAcw

+

7

6 7

7

7

A

@ A

A

A

Phenolphthalein (end point = pH 9)

Weak base + strong acid Acidic — only cation hydrolyzes

Salt formed — NH 4 Cl

NH 42 +++H O+NH 33 H O+

NH 3 + HCl

4

K

NH

NH H O

OH

OH

K

K

a NHw

33

3

==# -

+ -

7 +

6 7

7

7

A

@ A

A

A

Methyl Red (end point = pH 5) or

Methyl Orange (end point = pH 4)

Weak base + weak acid Variable pH— both ions hydrolyze.

pH depends upon extent of hydrolysis

of each ion involved.

Salt formed — NH 4 CN

NH 42 +++H O+NH 33 H O+

NH 3 + HCN

CN--++H O 2 +HCN OH

K

NH CN

NH HCN

HOH

HOH

KK

K

a NH w HCN

4

3

3

==-+-# #

+-

7 + 7

66

77

77

A A

@@

AA

AA

pH of NH 4 CN (aq) is greater than 7 because CN–(Kb= 2.0 × 10 –5) is

more basic than NH 4 +(Ka= 5.6 × 10 –10) is acidic.

If solutions undergo only very small changes in pH after small amounts of strong acids or bases
are added, the solution is called a buffer. Buffers may be prepared by either combining a weak
acid and a salt of the acid or by adding a weak base to a salt of the base. Where both ions of a
salt can hydrolyze, that salt may also act as a buffer (for example, NH 4 AC, pH=7.0). The solu-
tions that result may have a common ion and resist changes in pH.

Laboratory Experiments