Salt Hydrolysis MCQ - Practice Questions with Answers

Salt Hydrolysis

When a salt is added in water ions of the salt interact with water to cause acidity or basicity in aqueous solution. This ionic interaction is called salt hydrolysis. Interaction of cation is cationic hydrolysis and interaction of anion is anionic hydrolysis.

• Hydrolysis is reverse of neutralization and an endothermic process.
• If hydrolysis constant is K_h and neutralization constant is Kn. Then,
  Kn = 1/K_h
• A solution of the salt of strong acid and weak base is acidic and for it pH < 7 or [H⁺] > 10^-7. For example, FeCl₃ (weak base + strong acid): Solution is acidic and involves cationic hydrolysis.
• A solution of the salt of strong base and weak acid is basic and for it pH > 7 or [H⁺] < 10^-7. For example, KCN (strong base + weak acid): Solution is basic and involves anionic hydrolysis.
• A solution of the salt of weak acid and weak
  base, then:
  If K_a > K_b, it is acidic
  If K_a < K_b, it is basic
  If K_a = K_b, it is neutral
• CH₃COONH₄ (weak acid + weak base): Solution is neutral and involves both cationic and anionic hydrolysis.
• A solution of the salt of strong acid and strong base is neutral or pH = 7 or [H⁺] = 10^-7 
• A salt of strong acid and strong base is never hydrolyzed however, ions are hydrated. For example, K₂SO₄ (however, strong base + strong acid).

Degree of Hydrolysis: It is defined as the fraction of total salt that has undergone hydrolysis on attainment of equilibrium. It is denoted by h.
Let c be the concentration of salt and h be its degree of hydrolysis.

 
  c
c - ch                      ch           ch

Such salts give alkaline solutions in water. Some of such salts are: CH₃COONa, Na₂CO3, K₂CO₃, KCN, etc. For our discussion, we consider CH₃COONa (sodium acetate) in water. When CH₃COONa is put in water, it completely ionises to give CH₃COO^- (acetate) ions and Na⁺ ions. Now acetate ions (CH₃COO^-) absorb some H⁺ ions from weakly dissociated H₂O molecules to form undissociated CH₃COOH. Na⁺ remains in the ionic state in water. Now for K_w (ionic product) of water to remain constant, H₂O further ionises to produce more H⁺ and OH^- ions. H⁺ ions are taken up by CH₃COO^- ions leaving OH^- ions in excess and hence an alkaline solution is obtained.

Let BA represents such a salt. As it is put in water;

BA dissociates into ions and BOH being strong base also ionises.

Thus, the net reaction is:

Thus, the hydrolysis constant(K_h) is given as:

pH of Solution

pH of a basic solution is given as:

Thus, substituting for K_h, we get:

Such salts give acidic solutions in water. Some of such salts are: NH₄Cl, ZnCl₂, FeCl₃, etc. For the purpose of discussion, we will consider the hydrolysis of NH₄Cl. When NH₄Cl is put in water, it completely ionises in water to give NH₄⁺ and Cl^- ions. NH₄⁺ ions combine with OH^- ions furnished by weakly dissociated water to form NH₄OH (a weak base). Now for keeping K_w constant, water further ionises to give H⁺ and OH^- ions, where OH^- ions are consumed by NH₄⁺ ions leaving behind H⁺ ions in solution to give an acidic solution.
Let BA be one of such salts. When it is put into water, the reaction is as follows.

Thus the net reaction of hydrolysis is as follows:

  
c - ch                          ch                   ch

Considering ionisation of weak base BOH and H₂O.

From expressions for K_h, K_b and K_w, we have :

pH of Solution
Now, pH = - log [H⁺]

Let us consider ammonium acetate (CH₃COONH₄) for our discussion. Both NH₄⁺ ions and CH₃COO^- ions react respectively with OH^- and H⁺ ions furnished by water to form NH₄OH (a weak base) and CH₃COOH (acetic acid).

Let BA represents such a salt.

                
                                                     weak base    weak acid
Initially:      c          c                             0                 0
At equil:   c - ch   c - ch                         ch               ch

Hence, the degree of hydrolysis of such salts is independent of concentration of salt solution.

Now considering the dissociation of both weak base and acid.

Combining K_h, K_b, K_a and K_w, we get:

pH of Solution

Considering K_a for acid, we have:

Since, base and acids are weaker, hence,