HYDROLYSIS OF IONS

Contents for this page	Related topics
The pH of salt solutions The reaction of ions with water Additional questions	The common ion effect The strength of acids The pH scale	Data Glossary
Learning Outcomes
After studying this section, you will (a) understand how salts react with water to produce an excess of hydrogen or hydroxyl ions and (b) be able to recognize those salts which give rise to acidic solutions, and those salts which give rise to alkaline solutions.

The pH of salt solutions:

When pure sodium chloride, NaCl, is dissolved in pure water, the pH of the resulting solution is 7.0. This is not the case for all salts:

0.1 mol.dm^-3 sodium ethanoate, CH₃COO^-Na⁺ has pH = 9, iin other words, it is an alkaline solution.

0.1 mol.dm^-3 ammonium chloride, H₄⁺Cl^-, has pH = 5, meaning that it is an acidic solution.

Neutralization of an acid by a base to form a salt does not necessarily produce a solution with a neutral pH!

In general . . .

Salts of strong acids (HCl, HNO₃, H₂SO₄) and strong bases (NaOH, KOH) will produce neutral aqueous solutions (pH = 7).

Salts of weak acids (CH₃COOH, H₂CO₃) and strong bases will produce alkaline aqueous solutions (pH > 7).

Salts of strong acids and weak bases (NH₃) will produce acidic aqueous solutions (pH < 7).

The reaction of ions with water:

Remember that water undergoes an acid-base equilibrium of its own:

It is important to bear in mind that whenever an aqueous solution has [H₃O⁺] = [OH^-], that solution will be neutral and have a pH = 7. An excess of [H₃O⁺] over [OH^-] will result in an acidic solution, while an excess of [OH^-] over [H₃O⁺] will result in a basic, or alkaline solution.

It turns out that various ions can react with water and so disturb the delicate balance between [H₃O⁺] and [OH^-], which is a characteristic property of pure water. We call this process the HYDROLYSIS of the ion.

Let us look at the case of sodium ethanoate, typical of the salt of a weak acid and a strong base:

Equal amounts of H₃O⁺ and OH (left hand side of the above reaction) represent pure water. The CH₃COO^- ion reacts as a base towards the acid H₃O⁺. This results in an excess of OH^- ions over the H₃O⁺ ions, with consequent loss of the neutrality of the solution. In other words, [OH^-] > [H₃O⁺], and the solution becomes basic.

Salts of weak acids and strong bases have anions which act as bases in aqueous solution, and so will give rise to aqueous solutions which have pH values above 7.

Ammonium chloride behaves differently in water:

As before, equal amounts of H₃O⁺ ions and OH^- ions (left hand side of the above reaction) represent pure water. Here the NH₄⁺ ion behaves as an acid towards the base OH^- ion. This results in an excess of H₃O⁺ ions over the OH^- ions, resulting in the loss of the neutrality of the solution. In other words, [H₃O⁺] > [OH^-], and the solution becomes acidic.

Salts of weak bases and strong acids have cations which behave as acids in aqueous solution, and so will give rise to solutions with pH values less than 7.

The ions from which the salts of strong acids and strong bases are constituted do not react with water, and so do not disturb the neutrality of their aqueous solutions.

Additional questions