Question

write a short note on anionic hydrolysis .How to calculate pH in such cases​

Answer 1

Salt hydrolysis: It is defined as a reaction in which cation or anion or both of a salt react with water to produce acidity or alkalinity.

The reaction of an anion or cation with water accompanied by cleavage of O-H bond is called hydrolysis.

A- + H O H HA + OH-

Anionic hydrolysis

B+ + H O H BOH + H+

Cationic hydrolysis

In anionic hydrolysis, the solution becomes slightly basic (p H >7).

In cationic hydrolysis, the solution becomes slightly acidic (p H <7).

Salts of weak acids and strong bases undergo anionic hydrolysis and yield basic solution.

Salts of weak bases and strong acids undergo cationic hydrolysis and yield slightly acidic solution.

Answer 2

Anionic hydrolysis

It is a hydrolysis reaction in which hydrolysis of the salt of a weak acid takes place.This reaction takes place when salt of a strong base and weak acid is dissolved in water.

For example :- Let HB be the weak acid, here hydrolysis of ${B}^{ - } \: takes \: place$

• Since, it is an anion, it Is named as anionic hydrolysis.

The reaction that takes place is :-

${B}^{ - } + H_{2}(O) = > BH \: + O {H}^{ - }$

• We observe that the solution is alkaline.

Here, hydrolysis constant ( $k_{h} ) = \frac{ \: \: k_{w}\: }{ k_{a}} \\ \\ where \: k_{a} = dissociation \: constant \: of \: the \: acid$

Now,

How to calculate pH in anionic hydrolysis ?

Let the concentration of the anion be c , then the pH will be :-

$pH = \frac{1}{2} log(p k_{w} + p k_{a} + logc )$