Notes-Part-2-Class-12-Chemistry-Chapter-3-Ionic Equilibria-Maharashtra Board

Ionic Equilibria

Class-12-Chemistry-Chapter-3-Maharashtra Board

Notes-Part-2

Topics to be Learn : Part-1

  • Introduction
  • Types of electrolyte
  • Acids and Bases
  • Ionisation of acids and bases
  • Autoionisation of water

Topics to be Learn : Part-2

  • pH Scale
  • Hydrolysis of salts
  • Buffer solutions
  • Solubility product
  • Common ion effect

pH Scale :

Most of the chemical reactions and industrial processes are carried out in aqueous solutions, hence there is a need to know concentration of H+ and OH ions in the solution.

Sorensen developed a convenient scale to represent the acidic, basic or neutral nature of the solution.

The pH scale is used to express the concentration of H+ and OH along with pH and pOH of the solution.

According to Sorensen,

pH = —log10[H3O+],  pOH = — log1o[OH]

pH + pOH = 14.

Ionic product of water : It is defined as the product of molar concentrations of hydronium ions (or hydrogen ions) and hydroxyl ions at equilibrium in pure water at constant temperature.

It is represented as,

Kw = [H3O+] x [OH]

where Kw is called ionic product of water. At 25 0C

KW = 1 x 1014.

pH : The negative logarithm, to the base 10, of the molar concentration of hydrogen ions, H+ is known as the pH of a solution.

pH = — log10[H+]

pOH : The negative logarithm, to the base 10, of the molar concentration of hydroxyl ions, OH is known as the pOH of a solution.

pOH = —log10[OH]

Relationship between pH and pOH :

Acidity, basicity and neutrality of aqueous solutions :

(i) Neutral solution : For pure water or any aqueous neutral solution at 298 K

[H3O+] = [OH ] = 1.0 × 10-7 M

Hence, pH = -log10[H+] = -log10[1 × 10-7] = 7

(ii) Acidic solution : In acidic solution, there is excess of H3O+ ions, or [H3O+] > [OH] Hence,

[H3O+] > 1 × 10-7 and pH < 7

(iii) Basic solution : In basic solution, the excess of OH ions are present that is [H3O+] < [OH]

or [H3O+] < 1.0 × 10-7 with pH > 7.

Know this :

  • Even if monobasic acid and dibasic acid give same pH, their molar concentrations are different. One mole of monobasic acid like HCL gives 1 mol of H+ while one mole of dibasic acid gives 2 moi H+  in solution. Hence the concentration of dibasic acid will be half of the concentration of monobasic acid. For example, for same pH.  [Monobasic acid] = [Dibasic acid]/2
  • pH of pure water vary with temperature : Ans. Since the increase in temperature, increases the dissociation of water, its pH decreases.
  • pH is crucial for digestion of food and other biochemical reactions in our body.
  • pH of gastric juice is about 2.
  • pH of blood is maintained within range 7.36 to 7.42.
  • Enzymes function effectively only at a certain pH. For example, tiypsin acts best for alkaline pH.

 

Hydrolysis of salts :

Hydrolysis : A reaction in which the cations or anions or both the ions of a salt react with water to produce acidity or basicity or sometimes neutrality is called hydrolysis.

Types of salts :

These are of four types

Types of salts :

Remember...

  • The solutions of salts of strong acids and strong bases are neutral,
  • The solutions of salts of strong acids and weak bases are acidic
  • The solutions of salts of strong bases and weak acids are basic.

Q. An aqueous solution of sodium chloride is neutral.

Answer :

Ionic equation

Na++Cl + H2O ⇌ Na++OH + H++Cl

H2O  ⇌  H+ + OH

Since the solution contains equal number of H+ and OH, it is neutral.

Hence the salt of strong acid and strong base does not undergo hydrolysis.

Hydrolysis of the salt of strong acid and weak base :

Hydrolysis of the salt of weak acid and strong base :

Hydrolysis of the salt of weak acid and weak base :

Buffer solutions :

Buffer solution : It is defined as a solution which resists the change in pH even after the addition of a small amount of a strong acid or a strong base or on dilution or on addition of water.

Types of buffer solutions :

(i) Acidic buffer solution ; It is a solution containing a weak acid e.g. (CH3COOH) and its salt of a strong base. e.g. (CH3COONa).

pH of an acidic buffer is given by following Henderson Hasselbalch equation,

pH = pKa + log10\frac{[Salt]}{[Acid]}

where pKa = — log10 Ka

and Ka is the dissociation constant of weak acid.

(ii) Basic buffer solutions : It is a solution containing a weak base (e. g. NH4OH) and its salt of strong acid, (e.g. NH4Cl).

pOH of a basic buffer is given by Henderson Hassebalch equation,

pOH = pKb + log10 \frac{[Salt]}{[Base]}   

where pKb = — log10 Kb and

Kb is the dissociation constant of a weak base.

Mechanism of action of a acidic buffer :

Mechanism of action of a basic buffer :

Properties (or advantages) of a buffer solution :

  • The pH of a buffer solution is maintained appreciably constant.
  • By addition of a small amount of an acid or a base pH does not change.
  • On dilution with water, pH of the solution doesn’t change.
Applications of buffer solution :

Know this :

Sodium benzoate added to jams and jellies in commercial products maintains the pH constant and acts as a preservative.

Hence jams and jellies are not spoiled for a very long time unlike home made products.

Solubility product :

Solubility : It is defined as the maximum amount of a substance in moles, that can be dissolved at constant temperature to give one litre of its saturated solution.

It is expressed in moles per litre or moles per decimeter cube of a saturated solution at given temperature.

∴ Molar solubility = \frac{\text{Solubility in number of moles of substance}}{\text{Volume of a solution in}dm^{-3}}

                         = \frac{\text{Solubility in gram per}dm^{-3}}{\text{Molecular mass of substance}}

Relationship between solubility and solubility product :

Q. What is the relationship between molar solubility and solubility product for salts given below :

(i) Ag2CrO4 (ii) Ca3(PO4)2 (iii) Cr(OH)3.

Answer :

Ionic product (IP) : It is defined as the product of concentrations in mol dm-3  of ions of an electrolyte in the solution and denoted by IP.

In a saturated solution,

IP = Ksp where Ksp is the solubility product of the electrolyte.

Know this :

The process of dissolution and precipitation of sparingly soluble ionic compounds is important in our everyday life, industry and medicine.

Kidney stone is developed due to the precipitation of insoluble calcium oxalate, CaC2O4.

The process of tooth decay occurs due to dissolution of enamel composed of hydroxyapatite, Ca3(PO4)3OH in acidic medium.

Common ion effect :

Common ion : An ion common to two electrolytes is called common ion.

This is generally applicable to a mixture of a strong and a weak electrolyte.

For example, a solution containing weak electrolyte CH3COOH and strong electrolyte salt CH3COONa.

CH3COONa —> CH3COO- + Na+;

CH3COOH   ⇌  CH3COO-  + H+

Hence CH3COOH and CH3COONa have a common ion CH3COO-

Common ion effect : The suppression of the degree of dissociation of a weak electrolyte by the addition of a strong electrolyte having an ion in common with the weak electrolyte is called common ion effect.

For example, CH3COOH and CH3COONa have common ion CH3COO-

Explanation :

Common ion effect on dissociation of a weak acid :

Common ion effect on dissociation of a weak base :

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