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| Classification of Acids |
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| Acids can be classified in various ways, depending on the factors mentioned below: |
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| a) Classification Based on the Strength of the acid. |
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| b) Classification Based on the Concentration of the acid. |
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| c) Classification Based on the presence of Oxygen. |
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| d) Classification Based on the Basicity of the Acid. |
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| 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. |
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| Strong acid |
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| 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. |
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| Examples are: hydrochloric acid, sulphuric acid, nitric acid etc. |
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| Weak acid |
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| 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. |
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| Examples are: acetic acid, formic acid, carbonic acid etc. |
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| Concentration of an acid solution depends upon the percentage of the acid that is dissolved in water. |
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| Concentrated acid |
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| 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. |
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| Dilute acid |
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| 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. |
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| Oxy-acids |
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| Oxy-acids are those, which contain oxygen in their composition. |
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| Examples are:Sulphuric acid (H2SO4), Oxalic acid (H2C2O4) |
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| Nitric acid (HNO3), Acetic acid (CH3COOH) |
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| Hydracids |
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| Acids, which contain hydrogen together with other elements and not any oxygen in their composition, are called Hydracids. |
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| Examples are: Hydrochloric acid (HCl),Hydroiodic acid (HI),Hydrobromic acid (HBr) |
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| 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. |
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| Monobasic acid |
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| A monobasic acid has one hydrogen ion. Hence these acids combine with one hydroxyl group of a base to form salt and water. |
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| Since monobasic acids have only one hydrogen ion, they form only one kind of salt. |
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| Monobasic Acids |
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| Hydrochloric acid: HCl |
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| Hydroiodic acid: HI |
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| Nitric acid: HNO3 |
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| Acetic acid: CH3COOH |
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| Hypochlorous acid: HOCl |
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| Formic acid: HCOOH |
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| Hydrobromic acid: HBr |
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| Dibasic acid |
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| When an acid can combine with 2 hydroxyl groups it known as a dibasic acid. Such acids dissociate in 2 steps in water. |
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| 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: |
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| Dibasic acids |
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| Sulphuric acid: H2SO4 |
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| Sulphurous acid: H2SO3 |
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| Carbonic acid: H2CO3 |
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| Oxalic acid: (COOH)2 |
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| Tribasic acids |
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| 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). |
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| Phosphoric acid dissociates in water in three steps: |
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i)  |
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ii)  |
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iii)  |
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| A normal salt does not contain any hydrogen ions. All the replaceable hydrogen ions are substituted by metal ions. |
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| Examples: NaCl, Na2SO4 and Na3PO4. |
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