NEET Chemistry Notes Chemical Equilibrium-Salt Hydrolysis

NEET Chemistry Notes Chemical Equilibrium-Salt Hydrolysis

Salt Hydrolysis

Salt Hydrolysis

The process of salt hydrolysis is actually the reverse process of neutralisation. The reaction of an anion or cation with water accompanied by cleavage of O—H bond, is called hydrolysis.
Salt hydrolysis affects the pH of the solution.

Neutral Salts

Salts of strong acids and strong bases (i.e. neutral salts) do not undergo hydrolysis
e.g. NaCI, CaS0₄ etc.
If such salt is dissolved in water, pH of the solution remains 7

Acidic Salts

Salt of a strong acid and weak base, e.g. NH₄Cl are called acidic salts. Such salts undergo cationic hydrolysis.
pH of acidic salt solution will be less than 7.
For such salts

Basic Salts

Salt of strong base and weak acid, e.g. NaN0₂, NaCN, CH₃COONa  are termed as basic salts.
Such salts undergo anionic hydrolysis.
pH of basic salt solution will be more than 7.
For basic salts,

Salts of Weak Acid and Weak Base

The salts other than halides, sulphates, nitrates of metals fall into this category, e.g. CH₃COONH₄ etc.

There is no effect of dilution on the hydrolysis of salts of weak acid and weak base because , pH and K_h are all independent of concentration, C.

Solubility Product

It is defined as the product of molar concentration of its ions in a concentrated solution, each concentration terms raised to the power equal to the number of ions produced on dissociation of one mtileetile of electrolyte.

Ionic Product (Q)
(At Any Stage of Reaction)

It is the product of ions and give the direction of reaction.
If Q > K_sp, precipitate will be formed.
If Q < K_sp,precipitate does not form.
If Q = K_sp, reaction is at equilibrium.

Buffer Solution

The solution, which maintains its pH constant or reserve acidic or basic nature even upon addition of small amounts of acid or base, is called buffer solution. The ability of buffer solution to resist changes in pH on addition of acid or base is called buffer action.

Characteristics of a Buffer Solution

A buffer solution should exhibit following characteristics:

• Buffer solutions possess a definite pH value.
•  Their pH value remains constant on keeping long or dilution.
•  The pH value is not changed on the addition of a strong acid in acidic buffer and a strong base in basic buffer.

Acidic Buffer

A buffer solution pH of which is less than 7 is called acidic buffer.
Weak acid + salt of the acid with strong base
e.g. CH₃COOH + CH₃COONa; HCN + NaCN

Basic Buffer

A buffer solution having pH more than 7 is called basic buffer.
Weak base + salt of the base with strong acid

Henderson’s Equation for Buffer Solution

For acidic buffer

Buffer Capacity

Buffer capacity is quantitatively defined as the number of moles of acid or base added in 1L of solution to change the pH by unity.

Buffer capacity is maximum when
[salt] = [acid], pH = pK_a for acidic buffer
[salt] = [base], pH =pK_h for basic buffer
Greater the buffer capacity, larger is its capacity to resist the change in pH value.

Acid-Base Indicators

An acid-base indicator is a substance which possess one colour in acid solution and altogether different colour in alkaline medium, i.e. its colour changes with pH. The selection of indicator for a given titration depends upon its range. Every indicator works only under specific pH range.

Universal Indicator

It is a mixture of certain indicators in required proportion which show colour changes oyer a considerable pH range.

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