5.27.1. Hydrolysis of Salts

Classification of salts by their susceptibility to hydrolysis

Any salt can be represented as the product of a neutralization reaction (from Latin neuter — neither one nor the other) between an acid and a base. Acids and bases can be either strong or weak electrolytes.

The neutralization reaction is an exchange reaction between an acid and a base forming a salt and water:

KOH + HF = KF + H2O.

Acids and bases as electrolytes differ in their strength. For example, ammonium salts can be regarded as formed in a reaction involving a weak base — ammonia hydrate NH3 · H2O. The salt KF is formed by the strong base KОН and the weak acid HF, while ammonium sulfide is formed by a weak base and a weak acid.

Depending on the strength of the base and the acid, four types of salts can be distinguished (Fig. 60.1).

Fig. 60.1. Classification of salts by their susceptibility to hydrolysis

Let us examine the hydrolysis of salts of all four types.

Salts formed by a strong base and a weak acid. An example is sodium acetate CH3COONa. This salt is formed by the strong base NaОН and the weak acid CH3COOH:

NaОН + CH3COOH = CH3COONa + H2O.

In an aqueous solution of sodium acetate, two processes occur:

a) complete dissociation of the strong electrolyte — the salt CH3COONa — into a cation and an anion:

CH3COONa → Na+ + CH3COO–;

b) interaction of acetate ions with water molecules forming the weak electrolyte — acetic acid:

CH3COO– + H2О CH3COOH + ОН–.

An excess of ОН– anions accumulates in the solution and creates a weakly alkaline medium, which indicates that hydrolysis proceeds at the anion CH3COO–.

The equation for the hydrolysis of sodium acetate shows that:

• a) the concentration of hydroxide anions in the solution is higher than in pure water, so the solution of СH3COONa has a weakly alkaline medium (pH > 7);
• b) only the СH3COO– anions participate in the exchange reaction with water and in the formation of the weak acid, which is why it is said that hydrolysis proceeds at the anion.

The hydrolysis equilibrium in this example is strongly shifted to the left — toward the formation of the starting substances — because water is a considerably weaker electrolyte than acetic acid СH3COOH.

Hydrolysis is the reverse of neutralization.

Examples of anions of weak acids whose salts are hydrolyzed by water:

• a) anions of weak monobasic acids: HCOO–, , F–;
• b) anions of weak polybasic acids: S2–, , , , .

Let us examine the hydrolysis of sodium carbonate Na2СO3 — a salt of the strong base NaOH and the weak dibasic acid H2CO3. Hydrolysis proceeds at the anion according to the equation in full ionic form:

2Na+ + + H2O 2Na+ + + OH–.

The equation in reduced ionic form is:

+ H2O + OH–.

In a solution of Na2СO3 an excess of hydroxide anions forms, creating an alkaline medium. A solution of Na2CO3 with a concentration of 0.1 mol/dm3 has a pH of about 11.5.

Salts formed by a strong acid and a weak base. Let us examine the hydrolysis of ammonium chloride NH4Cl. This is a salt of the strong acid — HCl — and the weak base — ammonia hydrate NH3 · H2O.

In an aqueous solution of the salt, two processes occur:

a) complete dissociation of the strong electrolyte — the salt NH4Cl:

NH4Cl → + Cl–;

b) interaction of ammonium ions with water molecules forming the weak electrolyte — ammonia hydrate NH3 · H2O:

+ H2O NH3 · H2O + H+.

This equation shows that:

• a) hydrogen cations Н+ accumulate in the solution and their concentration becomes higher than in pure water, so the solution of NH4Cl has an acidic medium (pH < 7);
• b) only the ammonium cations participate in the exchange reaction with water forming the weak base, which is why it is said that hydrolysis proceeds at the cation.

Multiply charged cations can also enter into reaction with water: the doubly charged Ni2+, Cu2+, Zn2+, Mn2+, Fe2+, Co2+, Pb2+ (except for the cations Mg2+, Ca2+, Sr2+, Ba2+), and the triply charged Fe3+, Al3+, Сr3+.

Let us examine the hydrolysis of copper(II) nitrate Сu(NO3)2. This is a salt of the strong acid — HNO3 — and the weak base — Cu(OH)2.

In this case it is customary to say that hydrolysis proceeds at the cation Cu2+. The hydrolysis equation in full ionic form:

Cu2+ + + H2O Cu(OH)+ + + H+.

The hydrolysis equation in reduced ionic form:

Cu2+ + H2O Cu(OH)+ + H+.

The products of hydrolysis are the basic salt Cu(OH)NO3 and nitric acid HNO3.

The medium of an aqueous solution of copper(II) nitrate is acidic (pH ≈ 4.5), since the solution contains an excess of Н+ cations.

Salts formed by a weak base and a weak acid. Such salts undergo hydrolysis at both the cation and the anion. In this case, the Н+ ions that appear from hydrolysis at the cation are bound by the ОН– ions that form from hydrolysis at the anion, which intensifies the hydrolysis. This reaction is often irreversible. For example, aluminum sulfide Al2S3 undergoes irreversible hydrolysis in water, forming insoluble aluminum hydroxide and gaseous hydrogen sulfide:

Al2S3 + 6H2O = 2Al(OH)3↓ + 3H2S↑.

Therefore aluminum sulfide Al2S3 cannot be obtained by an exchange reaction between aqueous solutions of two salts, for example, aluminum nitrate Al(NO3)3 and potassium sulfide K2S.

Other cases of irreversible hydrolysis are also possible. They are not hard to predict, since for a process to be irreversible it is necessary that at least one of the hydrolysis products leave the reaction sphere. Let us give an example of the combined irreversible hydrolysis of Al3+ cations and anions:

2Al(NO3)3 + 3Na2CO3 + 3H2O = 2Al(OH)3↓ + 6NaNO3 + 3CO2↑.

For salts that undergo irreversible hydrolysis, you will find a note in the table "Solubility of Acids, Bases and Salts in Water": "does not exist in aqueous solution."

Let us summarize what you have learned about hydrolysis at both the cation and the anion:

• a) if a salt is hydrolyzed at the cation and the anion simultaneously, the equilibrium of this reaction is shifted further to the right than for the hydrolysis of these ions separately;
• b) the position of the hydrolysis equilibrium at the cation and anion does not depend on the salt concentration (prove this for yourself);
• c) the reaction of the medium in this type of hydrolysis can be neutral, weakly acidic, or weakly alkaline, depending on the strength of the resulting base and acid (a weakly acidic medium indicates that the base is a weaker electrolyte than the acid, and a weakly alkaline medium indicates the opposite);
• d) salts can be irreversibly hydrolyzed at the cation and anion provided that at least one of the hydrolysis products leaves the reaction sphere.

Let us summarize information about the hydrolysis of various cations and anions in the composition of normal salts, by cation and anion, in Table 19.1.

Table 19.1. Hydrolysis of cations and anions

Salt composition	Ions for which hydrolysis does not occur	Ions for which hydrolysis occurs, in order of increasing strength	Ions for which strong hydrolysis occurs
Cations	K+, Na+, Li+, Ba2+, Sr2+, Ca2+, Mg2+, Ag+	Mn2+, Co2+, Zn2+, Ni2+, Fe2+, Pb2+, Cu2+, Be2+	Al3+, Cr3+, Sn2+, Fe3+
Anions	Cl–, Br–, I–, , , ,	F–, , HCOO–, CH3COO–, , ClO–	, , , S2–

Not hydrolyzed: soluble halides (except fluorides), nitrates, perchlorates, sulfates and permanganates of alkali and alkaline-earth metals.

Note that reversible hydrolysis reactions obey Le Chatelier's principle, so the hydrolysis of a salt can be either intensified or weakened.