Classification of Acids

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Acids can be classified in various ways, depending on the factors mentioned below:

a) Classification Based on the Strength of the acid.

b) Classification Based on the Concentration of the acid.

c) Classification Based on the presence of Oxygen.

d) Classification Based on the Basicity of the Acid.

Classification Based on the Strength of the acid

The strength of an acid depends on the concentration of the hydronium ions present in a solution. Greater the number of hydronium ions present, greater is the strength of acid. However, some acids do not dissociate to any appreciable extent in water such as carbonic acid. Therefore, these acids will have a low concentration of hydronium ions.

Strong acid

An acid, which dissociates completely or almost completely in water, is classified as a strong acid. It must be noted that in these acids all the hydrogen ions combine with water molecule and exist as hydronium ions.

Examples are: hydrochloric acid, sulphuric acid, nitric acid etc.

Weak acid

An acid that dissociates only partially when dissolved in water, is classified as a weak acid. Most of the molecules remain in solution in molecular form itself in such acid.

Examples are: acetic acid, formic acid, carbonic acid etc.

OR

Classification Based on the Concentration of the Acid

Concentration of an acid solution depends upon the percentage of the acid that is dissolved in water.

Concentrated acid

An acid that has a relatively high percentage of the acid in the aqueous solution is classified as a concentrated acid e.g. concentrated hydrochloric acid, sulphuric acid, nitric acid, acetic acid etc.

Dilute acid

An acid, which has a relatively low percentage of the acid in the aqueous solution, is classified as a dilute acid e.g. dilute hydrochloric acid, sulphuric acid, nitric acid, acetic acid etc.

Classification Based on the Presence of Oxygen

Oxy-acids

Oxy-acids are those, which contain oxygen in their composition.

Examples are:Sulphuric acid (H₂SO₄), Oxalic acid (H₂C₂O₄)

Nitric acid (HNO₃), Acetic acid (CH₃COOH)

Hydracids

Acids, which contain hydrogen together with other elements and not any oxygen in their composition, are called Hydracids.

Examples are: Hydrochloric acid (HCl),Hydroiodic acid (HI),Hydrobromic acid (HBr)

Classification Based on the Basicity of the Acid

The basicity of an acid is equal to the number of replaceable hydrogen ions in an acid. It is also one hydrogen ion or hydronium ion that can react with one hydroxyl ion (OH^-). Stated in another way, basicity is the number of hydroxyl ions, with which one molecule of an acid can combine.

Monobasic acid

A monobasic acid has one hydrogen ion. Hence these acids combine with one hydroxyl group of a base to form salt and water.

Since monobasic acids have only one hydrogen ion, they form only one kind of salt.

Remember :

Monobasic Acids

Hydrochloric acid: HCl

Hydroiodic acid: HI

Nitric acid: HNO₃

Acetic acid: CH₃COOH

Hypochlorous acid: HOCl

Formic acid: HCOOH

Hydrobromic acid: HBr

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Dibasic acid

When an acid can combine with 2 hydroxyl groups it known as a dibasic acid. Such acids dissociate in 2 steps in water.

Since dibasic acid has two hydrogen ions, it can form two kinds of salts, i.e. the normal salt, given above, and a hydrogen salt, as given below:

Remember :

Dibasic acids

Sulphuric acid: H₂SO₄

Sulphurous acid: H₂SO₃

Carbonic acid: H₂CO₃

Oxalic acid: (COOH)₂

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Tribasic acids

The tribasic acids can combine with three hydroxyl groups. These acids must have three replaceable hydrogen ions, and as such they can form three types of salts e.g., phosphoric acid (H₃PO₄).

Phosphoric acid dissociates in water in three steps:

i)

ii)

iii)

Remember :

A normal salt does not contain any hydrogen ions. All the replaceable hydrogen ions are substituted by metal ions.

Examples: NaCl, Na₂SO₄ and Na₃PO₄.

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