Form 6 Chemistry – PHYSICAL CHEMISTRY 1.2- ACIDS, BASES AND SALTS

                                               SALT HYDROLYSIS 

A salt is a compound which contain metallic or radicle or positive radicle rather than hydrogen (H⁺) and acidic or negative or anion radicle rather than hydroxyl ion (OH‾)

CLASSIFICATION OF SALTS

Salts are categorized into four major classes.

These are:

1.     i. Normal salt (strong salts).

2.     ii. Salts with strong cation and weak anion.

3.     iii. Salts with weak cation and strong anion.

iv. Salts with weak cation and weak Anion.

i) STRONG SALTS
These are salts with strong cation and strong anions.

  ii).SALTS WITH STRONG CATION AN WEAK ANIONS.

These are also known as basic salts mostly are organic salts or salts of carboxylic acid

    iii) SALTS WITH WEAK CATION AND STRONG ANION
These salts are termed as acidic salts

iv.) SALT WITH WEAK CATION AND WEAK ANION

     

Salt hydrolysis is the reaction between salt and water to give acid and base it is a reversible reaction of neutralization reaction.

In salt hydrolysis only weak ions react with water to give the respective products.

HYDROLYSIS OF CLASSES OF SALTS

1.     Hydrolysis of normal salts

The salts having strong cation and strong anion do not undergo Salt hydrolysis process, rather than ionizing in solution to give free ions.

2.     Hydrolysis of basic salts

The salt having strong cation and weak anion undergo the type of hydrolysis termed as Anionic salt hydrolysis

Definition:

Anionic salt hydrolysis is the reaction between water and salt with strong cation and weak anion where by the weak anion react with water to give acid and base.

Example

Initially the salt will ionize

Anion will react with H₂O

Example:

Ionization

Anion will react with H2O

iii) Hydrolysis of salts with weak cation and strong anion. This process is also known as cationic salt hydrolysis
Definition:

Cationic salt hydrolysis is the reaction between water and salt with weak cation and strong anion in which weak cation react with water to give acid and base.

iv) Salt with weak cation and weak anion (weak salt )

This salt undergo both cationic and anionic salt hydrolysis because both weak ions will react with water to give acid and base.

Hydrolysis constant for anionic salts hydrolysis

From hydrolysis equation Kh can be obtained

HYDROLYSIS CONSTANT (Kh)
Definition:

Kh is the ratio of  product of molar concentration of the products to that concentration reactant raised to their power which is equal to the balancing number in a hydrolysis equation.

During hydrolysis the weak acid formed and water molecules will also dissociate.

The weak acid dissociation

where
ka = Dissociation constant of acid (CH₃COOH)
kw = Dissociation constant of water
kh = Hydrolysis constant.

Hydrolysis constant for cationic salt hydrolysis

Consider the hydrolysis of NH₄Br

During hydrolysis the weak base is formed and water molecules will dissociate.

Weak base dissociation

where
kb = Dissociation constant of Base

Example

With examples in each briefly write short note on the following

i)                   Cationic salt hydrolysis

It is the reaction between water and salt with strong anion and weak cation in which the weak cation react with water to give acid and base.

Example

ii)    Anionic salt hydrolysis

It is the reaction between water and salt with strong cation and weak anion in which the weak anion react with water to give acid and base.

Example

The above chemical reaction is Anionic salt hydrolysis

(iii) Hydrolysis constant

Hydrolysis constant is the ratio of the products molar concentration to the reactant concentration raised to their powers which is equal to the balancing number in the hydrolysis equation.

Normally it is denoted by kh

For anionic salt hydrolysis.

(iv) Acidic salt:

Acidic salt is the type of salt which contain a strong anion and weak cation.

Example:

(NH₄)₂SO₄ , NH₄Br and NH₄I.

(V) Basic salt

Basic salt is the type of salt which contain a strong cation and a weak anion.

Example:

CH₃COOK, Na₂C₂O₄ and CH₃CH₂COONa.

(vi) Salt hydrolysis :

Salt hydrolysis is the reaction between salt and water to produce base and acid.

Under this process only weak radicals/ions are associated in the reaction.

Example

C₂O₄^2- + H₂O …….. H₂C₂O₄ + OH^–

NH₄⁺ + H₂O …………………. NH₄OH + H⁺

Example

1.     Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.

Solution

Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion where by the weak cation react with water to produce acid and base while

anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion react with water to produce acid and base.

1.     With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.

Solution

Consider the hydrolysis of Sodium Oxalate.

During hydrolysis acid formed together with water molecules, ionize in a solution.

Acid ionization.

Example

During Hydrolysis the base and water molecule also ionize.

Base ionization

Water ionization

Example 4:

Calculate the Hydrolysis constant for the hydrolysis of 0.05 M NH₄NO₃.if its Kb is 6.67 x10^-6

Solution

DEGREE OF HYDROLYSIS

Definition:

Is the fraction or percentage of the salts that has reacted with water to form acid and base

The degree of Hydrolysis is denoted by h.

Consider the anionic salt hydrolysis below:

Hydrolysis Reaction:

But

(1 – h) 1

  But c = 1/v.

But

Then

This is for anionic salt hydrolysis

Where
h is the hydrolysis constant

Kw is the water dissociation constant

Ka is the acid dissociation constant

            THE HYDROLYSIS OF SALT WITH WEAK CATION AND ANION

Consider the salt of weak cation and weak anion as AB. During hydrolysis, the salt will ionize.

During the hydrolysis process there is formation of

(i) Weak acid

(ii) Weak base

Ionization of weak acid

Ionization of weak base

Ionization of water

For salt that undergo both cationic and anionic salt hydrolysis (weak salt)

Example

Consider the hydrolysis of ammonium acetate, then derive the relationship between Kh,Kw,Ka and Kb for such hydrolysis.

Solution

During the hydrolysis, weak acid and weak base is formed ionization of weak acid

PH EQUATIONS FOR SALT HYDROLYSIS

a) FOR ANIONIC HYDROLYSIS

Consider the hydrolysis of CH₃COONa

But, from water dissociation

note
pH = -LOG[H⁺]

This is the equation for anionic salt hydrolysis

b) FOR CATIONIC HYDROLYSIS

This is for the salt undergoing cationic salt hydrolysis

OR

Example

a)     What do you understand by

                             I.         Salt hydrolysis

         Is the reaction between water and saltl to produce acid and base.

                            II.       Cationic hydrolysis

         Is the reaction between water and salt with strong anion and weak cation at which the weak cation react with water to give acid and base.

                            III.       Anionic hydrolysis

         Is the reaction between water and salt with strong cation and weak anion in which the weak anion react with water to produce acid and base.

                           IV.       Conjugate pair

        Is the pair of conjugate acid or conjugate base or acid respectively.

b. For cationic hydrolysis show that

Solution. Consider the hydrolysis of NH₄CL

NH₄⁺ H₂0 NH₄ OH + H⁺

1 – h h h

At equilibrium (1 – h)c hc hc

But

Assume that 1 – h 1 (h is very small)

calculate the conc. of H₃O⁺ present in 0.0001M HCOOK ka. HCOOK = 2.4 X 10^-4

Kw = 10^-14

Solution

HCOOK HCOO⁻ + K⁺

Example 2:

a.     Define

p^H is the negative logarithm of hydrogen molar concentration present in the solution.

Acidic salt is the type of salt that consist of strong anion and weak cation.

Basic salt is the type of salt that consist of strong cation and weak anion.

b.     Normal salt do not undergo anionic or cationic salt hydrolysis. Briefly account for this statement.

Soln.

Normal salt do not undergo neither anionic nor cationic salt hydrolysis because they have strong acidic and basic radicle and the hydrolysis take place to the weak radicles only. Hence they just ionize in the water only.

c.      Calculate the p^H of the solution with 0.0152M. CH₃COOK if KaCH₃COOH =2.18 X 10^-8 at a particular temperature.

Soln.

From p^H equation.

Alternatively

d.     Calculate the p^H of the solution with 0.001M of NH₄NO₃ if K_b NH₄OH is 1.8 x 10^-4.

Soln.

Example 3:

Calculate the hydrolysis constant and the p^H of 0.625M solution of CH₃COONa. K_aof CH₃C-OOH= 1.754 X 10^-5 K_w =1 x 10^-14.

Soln.

–

–

—

E-xample 4.

a.     -What is the p^H of 0.2M solution of NaCN? K_a for HCN =4 X 10 ^-10 ionic product of -water =10^-14.

Soln.-

b.     -Calculate the p^H of 0.2M NH₄Cl. K_b = 1.8×10^-5.

S-oln.

–

Also

Amount dissociated = 0.0248

% hydrolysed = 0.0248 × 100%

= 2.48%

The percentage hydrolysis is 2.48%

B. calculate the hydrolysis constant and degree of hydrolysis of NH₄Cl in 0.001m solution k_b=1.8 × 10^-5

Solution.

The hydrolysis constant is 5.55 × 10^-10

The hydrolysis constant is 7.45 × 10^-4

C. calculate the degree of hydrolysis of ammonium acetate if the dissociation constant of ammonium hydroxide is 1.8 × 10^-5 that of acetic acid is 1.8 × 10^-4 and the ionic product of H₂ois 1 × 10^-14 (0.55 × 10^-2 ).

Solution

Data

Ka = 1.8 × 10^-5

Kn = 1.8 × 10^-5

Kw = 1 × 10^-14

From hydrolysis equation

Example 6.

What is hydrolysis constant of salt?

Answer

Is the ratio of products concentration to reactant concentration in moles/dm³ raised to their powers which is equal to the balancing number in a hydrolysis equation

Why aqueous solution of sodium carbonate is alkaline derive an expression for the hydrolysis constant and p^H of this solution.