Hydrolysis Of Salts: Introduction

Hydrolysis Of Salts

Salt is a compound formed by the neutralization reaction between an acid and a base. They generally ionize in water furnishing cations and anions. The cations or anions formed during ionization of salts either exist as hydrated ions in aqueous solutions or interact with water to regenerate the acids and bases. The process of interaction between cations or anions of salts and water is known as hydrolysis of salts. On the basis of hydrolysis, salts are divided into three categories:
  • Acidic salts
  •  Basic salts
  • Neutral salts

Let us discuss hydrolysis of salts of the following types:

  • Salts of strong acid and strong base: Salts formed by the neutralization of strong acid and strong base are neutral in nature as the bonds in the salt solution will not break apart. They generally get hydrated but do not hydrolyse. Therefore, such salts are generally known as neutral salts.

    For example: \(NaCl\)

  •  Salts of weak acid and strong base: Salts formed by the neutralization of weak acid and strong base are basic in nature. For example: \(CH_3COONa\)

    \(CH_3COONa~ (aq)~ →~CH_3COO^-~ (aq)~+~ Na+~ (aq)\)

    Acetate ion formed undergoes hydrolysis to form acetic acid and OH ions.\( CH_3COO^-(aq)~ +~ H_2O~⇋ ~Ch_3COOH (aq)~ +~ OH^- (aq) \)
    As we know acetic acid is a weak acid, it remains unionized in the solution. This results in an increase in concentration of OH  ions which makes the solution alkaline.pH of the solution is greater than7.

Salts of strong acid and weak base: Salts formed by the neutralization of strong acid and weak base are acidic in nature. For example: \( NH_4Cl\)

\( NH_4Cl (aq) ~→ ~Cl^-(aq)~ +~ NH_4^+ (aq)\)

Ammonium ion formed undergoes hydrolysis to form ammonium hydroxide and \(H^+\) ions.

\( NH_4^+ (aq) ~+~ H_2O~ ⇋~ NH_4OH (aq)~ +~ H^+ (aq)\)

As we know ammonium hydroxide is a weak base, it remains unionized in the solution. This results in an increase in concentration of \(H^+\) ions which makes the solution acidic. pH of such solutions is less than 7.
Salts of weak acid and weak base: Salts formed by the neutralization of weak acid and weak base are acidic, basic or neutral depending on the nature of acids and bases involved. For example: \( CH_3COONH_4\).General mechanism for the hydrolysis of ions formed from these salts:

\(CH_3COO^- ~+ ~NH_4^+~ +~ H_2O ~⇋~ CH_3COOH ~+~ NH_4OH\)

Degree of hydrolysis in such cases is independent of the concentration of solution and pH of such solutions is given by:

pH= 7 + \( \frac 12 \) (pKa – pKb)

Hence, we can say that the pH of a solution can be less than 7 or greater than 7 depending on the values of pKa and pKb.

For a detailed discussion on hydrolysis of salts, please visit Byju’s or download the app.


Practise This Question

Calculate the pH of an aqueous solution of 1.0 M ammonium formate assuming complete dissociation. (pKa of formic acid = 3.8 and pKb of ammonia = 4.8)