Department – Science and Humanities

Prepared by – Mamata Mohanta

( Lect. In Chemistry)

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1. Properties of Acids & Bases�

Acid & Base Theory :-

• Three different theories have been put forth in order to define acids and bases. These theories include the Arrhenius theory, the Bronsted-Lowry theory, and the Lewis theory of acids and bases. A brief description of each of these theories is provided in this subsection.  Acids and bases can be defined via three different theories.
• The Arrhenius theory of acids and bases states that “an acid generates H⁺ ions in a solution whereas a base produces an OH^– ion in its solution”.
• The Bronsted-Lowry theory defines “an acid as a proton donor and a base as a proton acceptor”.
• Finally, the Lewis definition of acids and bases describes “acids as electron-pair acceptors and bases as electron-pair donors”.

Limitation of Arrhenius Concept :-� 1. It fails to explain the acidic nature of substances like CO₂, SO₂,SO₃ etc. which do not have hydrogen and similarly it also fails to explain the basic nature of substances like NH₃, CaO, MgO etc which do not have -OH group.� 2. It fails to explain the acidic and basic behaviour of substances in solvents other than water such as ammonia,alcohol.etc�

The limitations of the Lewis theory :-

1. Lewis theory does not fit in the concepts that involve acid-base reaction
2. Most of the Lewis acid does not obey the catalytic property
3. Lewis’s theory does not explain the relative strengths of acids and bases.
4. Lewis’s theory provides a general idea, which involves all the coordination compounds and coordination reactions, which is not always true.
5. The concept does not explain the well known protonic acids behaviour, such as Hcl, H2So4, etc, which does not form the coordination bonds with bases. Hence, according to Lewis, those are not considered as acids.

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What is Salt ?�

• Salt is an ionic compound that has a cation other than H⁺ and an anion other than OH^– and is obtained along with water in the neutrlization reaction between acids and bases.

Eg:- NaCl, CuCl₂ etc.

Acid + Base → Salt + water

Types of Salt : -

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• Normal Salt –

A normal salt is a salt formesd by the complete replacement of hydrogen ion from an acid.

KCl, NaCl, FeCl2, Na2SO4 are some example of Normal Salt .

• Acidic Salt :-

A normal salt which is formed by neutralization of strong acid and weak base is called Acidic salt .

Ex :- Sodium carbonate (NaHCO3), Sodium bisulfate (NaHSO4), Potassium bisulfite (KHSO4),

Ammonium chloride (NH4Cl) .

• Basic Salt :-

A normal salt which is formed by neutralization of weak acid and Strong base is called Basic salt .

Ex - potassium cyanide (KCN), sodium hydroxide (NaOH, )Magnesium oxychloride (Mg(OH)Cl) .

• Double Salt :-

A Double Salt is a mixture of two salt which on dissolution of water gives two types of metal ion .

Ex :- Mohr’s salt ((NH4)2Fe(SO4)2.6H2O), Potash alum, (K2SO4Al2(SO4)3.24H2O)

• Complex Salt :-

A complex salt is a salt which contains a complex ion or complex neutral molecule in which there is a central

metal ion surrounded by a number of neutral molecules or negative ions.

Ex – Potassium ferrocyanide K4 [Fe (CN6}, Tetra amino cupric sulphate [Cu (NH3)4]SO4

• Mixed Salts : –

The salt that consists of a fixed proportion of two salts, often sharing either a common cation or common anion is known as mixed salt.

Ex. :- Bleaching powder(Ca(ClO)2), Sodium potassium sulfate (NaKSO4)

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