Salt Formation

Acids and bases may react with each other, forming a salt and (often, but not always) water, in what
is termed a neutralization reaction. For example, a generic acid and a generic base react as follows:

HA  +   BOH →   BA  +   H 2 O

The salt, BA, may precipitate out or remain ionized in solution, depending on its solubility and the
amount produced. Neutralization reactions generally go to completion. The reverse reaction, in
which the salt ions react with water to give back the acid or base, is known as hydrolysis.

Four combinations of strong and weak acids and bases are possible:

The products of a reaction between equal concentrations of a strong acid and a strong base are a
salt and water. The acid and base neutralize each other, so the resulting solution is neutral (pH = 7),
and the ions formed in the reaction do not react with water. The product of a reaction between a
strong acid and a weak base is also a salt but usually no water is formed since weak bases are usually
not hydroxides; however, in this case, the cation of the salt will react with the water solvent,
reforming the weak base. This reaction constitutes hydrolysis. For example:

NH 4 + is the conjugate acid of a weak base (NH 3 ), and is therefore stronger than the conjugate base
(Cl−) of the strong acid HCl. NH 4 + will thus react with OH−, reducing the concentration of OH−. There
will thus be an excess of H+, which will lower the pH of the solution.

1. strong   acid    +   strong  base:   e.g.,   HCl +   NaOH    →   NaCl    +   H 2 O

2. strong   acid    +   weak    base:   e.g.,   HCl +   NH 3    →   NH 4 Cl

3. weak acid    +   strong  base:   e.g.,   HClO    +   NaOH    →   NaClO   +   H 2 O

4. weak acid    +   weak    base:   e.g.,   HClO    +   NH 3    →   NH 4 ClO