3. Different types of Hydrolysis

Individual possible cases of hydrolysis of salts are compared below (from the point of view of the acidity or alkalinity of the solution and the nature of the product obtained). The basis for the comparison is, on one hand, the strength of the corresponding acids and bases, and, on the other hand, the type of salt with regard to valency of the cation and the anion.

I. Strong base, weak acid. The solution is alkaline. The following cases are possible, depending on the valency of the cation or the anion.

a) Cation and anion monovalent. The hydrolysis products are free acid and free base, e.g.:

NaCN + HOH ↔ NaOH + HCN

or, in net ionic form, CN^-+ HOH ↔ HCN + OH^- (pH>7).

b) Cation monovalent, anion multivalent. This is the most typical case. Hydrolysis gives acid salts and free alkali, e.g.:

Na₃PO₄ + HOH ↔ Na₂HPO₄ + NaOH

or, in net ionic form, PO₄^3-+ HOH ↔ HPO₄^2- + OH^- (pH>7).

In a large volume of water hydrolysis proceeds further:

Na₂HPO₄ + HOH ↔ NaH₂PO₄+ NaOH

or, in net ionic form, HPO₄^2-+ HOH ↔ H₂PO₄^-+ OH^-.

However, it does not reach the stage of formation of free weak acid owing to the accumulation of free alkali (OH^- ions) in the solution.

c) Cation multivalent, anion monovalent. This case is rather rare. Hydrolysis results in basic salt and free acid. But the solution is alkaline since it contains more OH^- ions from the basic salt (formed by the strong base) than H⁺ ions from the weak acid, e.g.:

Ba(CN)₂+ HOH ↔ BaOHCN+ HCN

or, in ionic form, CN^-+ HOH ↔ HCN + OH^-.

d) Cation and anion multivalent. This case is never encountered in practice, as the compounds of this group are all insoluble in water.

II. Weak base, strong acid. The solution is acid. The following cases are possible, depending on the valency of the cation or the anion.

a) Cation and anion monovalent. The hydrolysis gives free acid and free base, e.g.:

NH₄NO₃ + HOH ↔ NH₄OH + HNO₃

or, in ionic form, NH₄⁺ + HOH ↔ NH₄OH + H⁺.

b) Cation multivalent, anion monovalent. This is the most typical case. Hydrolysis results in basic salts and free acid, e.g.:

AlCl₃ + HOH ↔ Al(OH)Cl₂ + HCl

or, in net ionic form, Al³⁺+ HOH ↔ Al(OH)²⁺ + H⁺ (pH<7).

In a large volume of water hydrolysis proceeds further:

Al(OH)Cl₂ + HOH ↔ Al(OH)₂Cl + HCl

or, in ionic form, Al(OH)²⁺+ HOH ↔ Al(OH)₂⁺ + H⁺.

However, it does not reach the stage of formation of free weak base owing to the accumulation of free acid (H⁺ ions) in the solution.

c) Cation monovalent, anion multivalent. This is very rare case. The result of hydrolysis is an acid salt and free base. In this case, the solution contains more H⁺ ions from the acid salt (formed by the strong acid) than OH^- ions from the weak base, e.g.:

(NH₄)₂SO₄ + HOH ↔ NH₄OH + NH₄HSO₄

or, in net ionic form, NH₄⁺ + HOH ↔ NH₄OH + H⁺ (pH<7).

d) Cation and anion multivalent. Hydrolysis results in basic salt and free acid, e.g.:

Fe₂(SO₄)₃ + 2 HOH ↔ 2 Fe(OH)SO₄ +H₂SO₄

or in net ionic form, Fe³⁺ + HOH ↔ Fe(OH)²⁺ + H⁺(pH<7).

III. Weak base, weak acid. In this case the alkalinity or acidity of the solution depends on the relative strengths of the acid and the base. The nature of the hydrolysis products depends on the strengths of the acid and the base as well as on the valences of the cation and the anion. For example, Aluminum Acetate hydrolyses bring to forming of basic salts, according to the equations:

Al(CH₃COO)₃ + HOH ↔ Al(OH)(CH₃COO)₂ + CH₃COOH

Al(OH)(CH₃COO)₂ + HOH ↔ Al(OH)₂(CH₃COO) + CH₃COOH (pH≈7).

While Aluminium Sulfide, a salt of a much weaker acid, is hydrolyzed to free base and the free acid:

Al₂S₃ + 6HOH ↔ 2 Al(OH)₃ + 3H₂S.

The last case concerns to the salts marked as symbol - (dash) in Table of salts and bases Solubility in water (Appendix 5).

IV. Strong base, strong acid. Solutions of such salts have neutral medium, since they are practically unhydrolyzed. This group includes NaCl, Na₂SO₄, BaCl₂ etc.

PRACTICE PROBLEMS

1. What is the value of pH in solutions in which the concentration of Hydrogen ions is: 1·10^-7; 1·10^-5; 1·10^-10?

2. What is pH value of 0,1 M Nitric acid solution if its dissociation is complete?

3. Haw will the concentration of hydroxide ions change if pH of solution decreases from 10 to 4?

4. What aqueous solution of salt will have acidic medium? Write the corresponding equations:

a) Aluminium Nitrate, Potassium Sulfide, Potassium Chloride;

b) Potassium Carbonate, Iron (II) Sulfate, Lithium Chloride;

c) Copper (II) Chloride, Sodium Chloride, Lithium Carbonate.

5. What salt aqueous solution will have alkaline medium? Write the corresponding equations:

a) Calcium Chloride, Aluminium Chloride, Sodium Silicate;

b) Lithium Nitrate, Potassium Carbonate, Zinc Nitrate;

c) Potassium Chloride, Aluminium Chloride, Sodium Sulfide.

6. Give examples of salt hydrolysis in cation ( singly-, doubly- and triple-charged).

7. Give examples of salt hydrolysis in anion (singly- and doubly-charged).