Introduction
Salt is a compound formed by the neutralisation reaction between an acid and a base. They generally ionise in water furnishing cations and anions. The cations or anions formed during the ionisation 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:
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Salts of strong acid and strong base: Salts formed by the neutralisation 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 neutralisation of weak acid and strong base are basic in nature. For example: CH3COONa
\(\begin{array}{l}CH_3COONa~ (aq)~ →~CH_3COO^-~ (aq)~+~ Na^+~ (aq)\end{array} \)
Acetate ion formed undergoes hydrolysis to form acetic acid and OH– ions.
\(\begin{array}{l} CH_3COO^-(aq)~ +~ H_2O~⇋ ~CH_3COOH (aq)~ +~ OH^- (aq) \end{array} \)
As we know acetic acid is a weak acid, it remains unionised in the solution. This results in an increase in the concentration of OH
– ions which makes the solution alkaline. The
pH of the solution is greater than 7.
Salts of strong acid and weak base: Salts formed by the neutralisation of strong acid and weak base are acidic in nature. For example: \(\begin{array}{l} NH_4Cl\end{array} \)
\(\begin{array}{l} NH_4Cl (aq) ~→ ~Cl^-(aq)~ +~ NH_4^+ (aq)\end{array} \)
Ammonium ion formed undergoes hydrolysis to form ammonium hydroxide and H+ ions.
\(\begin{array}{l} NH_4^+ (aq) ~+~ H_2O~ ⇋~ NH_4OH (aq)~ +~ H^+ (aq)\end{array} \)
As we know ammonium hydroxide is a weak base, it remains unionised in the solution. This results in an increase in the concentration of H+ ions which make the solution acidic. the pH of such solutions is less than 7.
Salts of a weak acid and weak base: Salts formed by the neutralisation of weak acid and weak base are acidic, basic or neutral, depending on the nature of acids and bases involved. For example: CH3COONH4. A general mechanism for the hydrolysis of ions formed from these salts:
\(\begin{array}{l}CH_3COO^- ~+ ~NH_4^+~ +~ H_2O ~⇋~ CH_3COOH ~+~ NH_4OH\end{array} \)
The degree of hydrolysis in such cases is independent of the concentration of the solution and the pH of such solutions is given by:
\(\begin{array}{l} pH= 7 + \frac 12 \end{array} \)
(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.
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