Upon completion of this topic, learners will be able to:

• determine the effects of a salt on pH and pOH.
• predict whether an aqueous solution of a salt will be acidic, basic, or neutral.

A. Laboratory and industrial preparation of salts

Salts can be prepared using several methods. Any particular method chosen depends on whether the salts are soluble or not.

1. Preparation of salts by the action of an acid upon a metal

The solution is filtered, leaving behind insoluble impurities, such as excess zinc and particles of carbon on the filter paper. The ZnSO4 is then crystallized from the solution.

2. Preparation of salts by double decomposition

This method is used to prepare insoluble salts. lt involves the reaction between two soluble compounds to produce one soluble and one insoluble products. Amongst the products, the one of importance is the insoluble salt which precipitates out of the solution.

3. Preparation of salts by neutralization

Recall that neutralization is the reaction between acids and bases to form salts and water only. This method would depend on whether the base is soluble (i.e. alkali) or insoluble in water.

2NaOH(aq) + H₂SO₄(aq) ⟶ Na₂SO₄(aq) + 2H₂O(l)
2NH₃(aq) + H₂SO₄(aq) ⟶ (NH₄)2SO₄(aq)
Pb(OH)₂(s) + 2HNO₃(aq) ⟶ Pb(NO₃)₂(aq) + 2H₂O(l)

B. Chemicals from sodium chloride solution

Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. Deposits of solid salt are also found in several parts of the world.

What is a rock salt? Common salt is an important raw material for various materials of daily use, such as sodium hydroxide, baking soda, washing soda, bleaching powder and many more.

The electrolysis of sodium chloride solution (brine) is an important industrial process because hydrogen, chlorine and sodium hydroxide have many uses. Chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode.

C. Hydrolysis of salts

What does salt hydrolysis mean?

The term salt hydrolysis describes the reaction of an anion or a cation of a salt, or both, with water. Salt hydrolysis usually affects the pH of a solution.

Hydrolysis of salts of strong acids and strong bases

Recall that the conjugate base of a strong acid has virtually no affinity for protons as compared with that of the water molecule. Cations such as K⁺ and Na⁺ from strong bases have no ability to produce H⁺, so they too have no effect on the pH of an aqueous solution. Salts that consist of the cations of strong bases and the anions of strong acids have no effect on [H⁺] when dissolved in water.

Hydrolysis of salts of weak acids and strong bases

The solution of a salt derived from a strong base and a weak acid is basic. For example, the dissociation of sodium acetate (CH₃COO^–Na⁺) in water is given by:

The hydrated Na+ ion has no acidic or basic properties.

Hydrolysis of salts of strong acids and weak bases

When we neutralize a weak base with a strong acid, the product is a salt containing the conjugate acid of the weak base. For example, ammonium chloride, NH₄Cl, is a salt formed by the reaction of the weak base ammonia with the strong acid HCl:

NH₃ (aq) + HCl(aq) ⟶ NH₄Cl(aq)

A solution of this salt contains ammonium ions and chloride ions. The chloride ion has no effect on the acidity of the solution since HCl is a strong acid.

Hydrolysis of salts of weak acids and weak bases

For salts derived from a weak acid and a weak base, both the cation and the anion hydrolyze. However, whether a solution containing such a salt is acidic, basic, or neutral depends on the relative strengths of the weak acid and the weak base.

If K_b for the anion is greater than K_a for the cation, then the solution must be basic because the anion will hydrolyze to a greater extent than the cation. Conversely, if K_b for the anion is smaller than K_a for the cation, the solution will be acidic because cation hydrolysis will be more extensive than anion hydrolysis. If Ka is approximately equal to K_b, the solution will be neutral.