Salt hydrolysis.

It is the reaction of the cation or the anion or both the ions of the salt with water to produce either acidic or basic solution. Hydrolysis is the reverse of neutralization.

(1)     Hydrolysis constant : The general equation for the hydrolysis of a salt (BA),

BA + H₂O ⇌ HA + BOH

Applying the law of chemical equilibrium, we get

, where K is the equilibrium constant.

Since water is present in very large excess in the aqueous solution, its concentration [H₂O] may be regarded as constant so,

,  where K­_h is called the hydrolysis constant.

 

(2)     Degree of hydrolysis : It is defined as the fraction (or percentage) of the total salt which is hydrolysed at equilibrium. For example, if 90% of a salt solution is hydrolysed, its degree of hydrolysis is 0.90 or as 90%. It is generally represented by ‘h’.

(3)     Salts of strong acids and strong bases do not undergo hydrolysis (they undergo only ionization) hence the resulting aqueous solution is neutral. Halides, nitrates and sulphates of Na and K, SrCl₂ and BaCl₂ are of this type. As an illustration, let us discuss the hydrolysis of NaCl. We may write,

NaCl + H₂O → NaOH + HCl  or  Na⁺ + Cl^– + H₂O → Na⁺ + OH^– + H⁺ + Cl^–  or  H₂O → H⁺ + OH^–

Further in the resulting solution, [H⁺] = [OH^–].

 

(4)     Salts of weak acids and strong bases undergo anionic hydrolysis to produce basic solutions Na and K salts other than halides, nitrates and sulphates are of this type, e.g., NaOOCCH₃, Na₂CO₃, NaCN, Na₃PO₄, Na₂HPO₄, Na₂B₄O₇. 10H₂O, etc.

A(aq) + H₂O(l) ⇌ HA(aq) + OH^–(aq)   

At start               C                0       0

At equil               C(1–h)       Ch    Ch

Where C is the molarity of the salt solution and h is the degree of hydrolysis.

K_h (Hydrolysis constant) =

f h<<1,, then,   K_h = Ch² or 

Relation between hydrolysis constant (K_h), K_w and K_a is as follows,

For calculation of hydrogen ion concentration ;

  or 

 

(5)     Salts of strong acids and weak bases undergo cationic hydrolysis to produce acidic solutions. Halides, sulphates and nitrates of all metals except alkali metals, alkaline earth metals are of this type, e.g.,

FeCl₃, NH₄Cl, CuSO₄, AlCl₃ etc

B⁺(aq) + H₂O(l) ⇌ BOH(aq) + H⁺ (aq)

At start               C                          0                 0

At equil               C(1–h)                Ch             Ch

         

If h<<1, then ,   K_h = Ch²  or 

Relation between K_h, K_w and K_b is as follows,

For calculation of H⁺ ion concentration,

  or 

 

(6)     Salts of weak acids and weak bases undergo anionic as well as cationic hydrolysis to produce an aqueous solution which may be neutral or acidic or basic depending upon the relative strengths of weak acid and weak base. Such a salt cannot be Na or K salt or it cannot be halide, nitrate or sulphate, it could be

NH₄OOCCH₃, (NH₃)₃PO₄, Ca₃(PO₄)₂, aniline acetate,

(NH₄)₂CO₃, AlPO₄ etc.

A^–(aq) + H₂O(l) ⇌ HA(aq) + OH^–(aq)

B⁺(aq) + H₂O(l) ⇌ BOH(aq) + H⁺(aq)

B⁺(aq) + A^–(aq) + H₂O(l) ⇌ HA(aq) + BOH(aq)

At start               C                C                0                 0

At equil               C(1–h)       C(1–h)       Ch              Ch

If [OH^–] = [H⁺], possible when K_a = K_b, there is no net hydrolysis, the resulting aqueous solution is neutral e.g., NH₄OOCCH₃.

If [OH^–] > [H⁺], possible when K_a < K_b, there is net anionic hydrolysis, the resulting aqueous solution is basic, e.g., NH₄CN.

If [OH^–] < [H⁺], possible when K_a > K_b there is net cationic hydrolysis, the resulting aqueous solution is acidic.

                            

If [OH^–] = [H⁺] and if h<<1, then, K_h = h² or h = √K_h i.e., the degree of hydrolysis of such a salt is independent of the concentration of salt solution.

Relation between K_h, K_w, K_a and K_b is as follows,

For Calculation of H⁺ ion concentration,