Chemistry, Third edition

16 · ACID–BASE EQUILIBRIA

3.a salt of a strong acid and a weak base (SA–WB);

4.a salt of a weak acid and a weak base (WA–WB).

Sodium chloride solution (made with pure water), is neutral with a pH of 7

at 25 °C. This may lead us to suppose that solutions of all ionic salts are neutral. In

fact,only solutions of salts made from strong acids and strong bases are always neutral.

Solutions of other salts are usually either acidic or basic. The reasons for this be-

haviour are that:

1.These salts react with the water producing a weak acid or a weak base.

2.Since the weak acid or weak base is only partially ionized in water, formation of

these molecules ties up hydroxide or hydronium ions.

3.This produces unequal concentrations of hydronium and hydroxide ions, and

the resulting solution of salt is then acidic or basic.

Reactions with water are called hydrolysis, and the reactions of a salt with water are

referred to as salt hydrolysis. We now look at one example of this effect.

Hydrolysis of a salt of a weak acid and a strong base

Using sodium ethanoate (CH 3 COO,Na) as an example, the ions present in an

aqueous solution of this salt are

H 2 O

CH 3 COO,Na(s) —CH 3 COO(aq)Na(aq) (from the salt)

2H 2 O(l)\===\H 3 O(aq)OH(aq) (from the water)

The potential products of any reaction between the ions in solution are NaOH (from

the reaction of Na(aq) and OH(aq)) and CH 3 COOH (from the reaction of

CH 3 COO(aq) and H 3 O(aq)). The NaOH is fully ionized in solution, but the

ethanoic acid is only partially ionized. This means that [OH(aq)][H 3 O(aq)]

and the solution is basic.

Generalizing,

Solutions of salts of weak acids and strong bases are basic.

Consideration of the possible reactions in solutions of the other classes of salts

leads to the conclusions given in Table 16.3.

Table 16.3The pH of solutions of the four classes of salt

Class of salt Example Formula Acidic Basic Neutral

SA–SB sodium chloride NaCl 

WA–SB sodium ethanoate CH 3 COONa 

SA–WB ammonium chloride NH 4 Cl 

WA–WB ammonium cyanide NH 4 CN *

* If Ka(WA)Kb(WB) the solution is acidic. If Ka(WA)Kb(WB) the solution is basic. If Ka(WA)

Kb(WB) the solution is neutral.

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