Chemistry HYDROLYSIS OF SALTS AND PH OF THEIR SOLUTIONS

Salt Solutions

Salts are the reaction product of an acid and a base, other than water. Depending on the nature of an acid or a base there can be four types of salts:

(1) Strong Acid Strong Base Salt (SASB)
(2) Strong Acid Weak Base Salt (SAWB)
(3) Weak Acid Strong Base Salt (WASB)
(4) Weak Acid Weak Base Salt (WAWB)

Salts are strong electrolytes and when dissolved in water, they dissociated almost completely into ions.
The ions of the salt may or may not react with water depending upon their chemical nature. "The phenomenon of the reaction of anion and cation of the salt with `H^+` and `OH^-` ions furnished by water yielding acidic or alkaline solution is known as salt hydrolysis".

Hydrolysis can be of two types -

Anionic Hydrolysis - When the anion of the salt reacts with water to produce an alkaline solution.
`A^(-) + H_2O ⇋ HA + OH^(-)` Alkaline solution (`pH` increases).

Cationic Hydrolysis - When the cation of the salt reacts with water to produce an acidic solution.
`B^(+) + 2H_2O ⇋ B(OH) + H_3O^+` Acidic solution (`pH` lowers down).

Salt Solutions-Strong Acid Strong Base Salt

Ions of such salts do not undergo hydrolysis because both the ions are weak in nature.
For eg., `NaCl` is a salt of `HCl` & `NaOH` which produces `Na^(+)` & `Cl^-` ions in the solution.

`HCl` being a strong acid, `Cl^-` is a weak base. Similarly `NaOH` being a strong base, `Na^+` is a weak acid. Therefore none of the ion will get hydrolyzed and the solution will remain neutral.

Salt Solutions-Strong Acid Weak Base Salt

In the aqueous solution of such salts only the cation undergoes hydrolysis (as it is a strong acid) while the anion doesn't undergo hydrolysis (as it is a weak base).
For eg., `NH_4Cl` is a salt of `HCl` & `NH_4OH` which produces `NH_4^(-)` & `Cl^(-)` ions in the solution.

`HCl` being a strong acid `Cl^(-)` is a weak base. Similarly, `NH_4OH` being a weak base, `NH_4^(-)` is a strong acid. Therefore, `NH_4^(-)` will get hydrolyzed and the solution will become acidic .

`NH_4^+ +H_2O ⇋ NH_4OH +H^(+)`

Salt Solutions-Weak Acid Strong Base Salt

In the aqueous solution of such salts only the anion undergoes hydrolysis (as it is a strong base) while the cation doesn't undergo hydrolysis (as it is a weak acid)
For eg.,` CH_3COONa` is a salt of `CH_3COOH` & `NaOH` which produces `Na^(+)` & `CH_3COO^(-)` ions in the solution.

`CH_3COOH` being a weak acid, `CH_3COO^(-)` is a strong base. Similarly, `NaOH` being a strong base `Na^(+)` is a weak acid.
Therefore, only `CH_3COO^(-)` will get hydrolyzed and the solution will become basic.

`CH_3COO^(-)+H_2O ⇋ CH_3COOH +OH^(-)`

Salt Solutions-Weak Acid Weak Base Salt

In the aqueous solution of such salts both the anion (as it is a strong base) & the cation (as it is a strong acid)
undergo hydrolysis.
For eg., `CH_3COONH_4` is a salt of `CH_3COOH` & `NH_4OH` which produces `NH_4^(+)` & `CH_3COO^-` ions in the solution.

`CH_3COOH` being a weak acid, ` CH_3COO^-` is a strong base. Similarly `NH_4OH` being a weak base `NH_4^(+)` is a strong acid. Therefore, both `CH_3COO^-` & `NH_4^-` will get hydrolyzed.

`CH_3COO^(-) + NH_4^(+)+ H_2O ⇋ NH_4OH + CH_3COOH`