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Acids

In any chemistry laboratory, we find acids such as hydrochloric acid, sulphuric acid and nitric acid. These acids are called mineral acids because they can be prepared from naturally occurring compounds called minerals. Mineral acids are generally stronger than household acids (used for cleaning toilets etc.).
 
  • Oxides of non-metals are acidic oxides, e.g., sulphur dioxide, carbon dioxide.
    Acidic oxides dissolve in water to form acids.
  • Oxides of metals are basic oxides, e.g. sodium oxide, calcium oxide.
    Basic oxides dissolve in water to form bases.

 

Properties of Acids
  1. They are sour in taste.
  2. They react with alkali metals to liberate hydrogen.
  3. They react with bases to form salt and water.
  4. They turn blue litmus paper red.
  5. They react with carbonates and bicarbonates to liberate carbon dioxide.
  6. They conduct electricity.

Different Concepts of Acids

  1. Arrhenius concept: According to Arrhenius, acids are substances which dissociate to produce hydrogen ions (H+) when dissolved in water.
     
    HCl (aq) Description: 32730.jpg H(aq) + Cl (aq)
  2. Bronsted–Lowry concept: According to this concept, an acid is a proton (H+) donor.
     
    H2O + HCl Description: 32730.jpg H3O+ + Cl
     
    In this example, HCl is an acid which donates a proton to water, leading to the formation of hydronium ions (H3O+).
  3. Lewis concept: According to this concept developed by Gilbert N. Lewis, an acid is an electron pair acceptor.
     

Classification of Acids

Acids are classified into different types as shown in Table.
 

Classification of Acids

 

Parameters

Types

Source

(a) Organic acids: Derived from plants, e.g. oxalic acid, citric acid
 
(b) Inorganic acids: Derived from minerals, e.g. hydrochloric acid, sulphuric acid

Molecular composition

(a) Hydracids: Contain hydrogen and non-metallic element other than oxygen, e.g. hydrochloric acid, hydroiodic acid
 
(b) Oxyacids: Contain hydrogen, another element and oxygen, e.g. nitric acid, sulphuric acid

Strength (concentration of the hydronium ion)

(a) Strong acid: Complete dissociation in aqueous solution, thereby producing a high concentration of hydrogen or hydronium ions, e.g. hydrochloric acid, sulphuric acid
 
(contains only ions)
 
(b) Weak acid: Partial dissociation in aqueous solution, thereby producing a low concentration of hydrogen or hydronium ions, e.g. acetic acid, carbonic acid
 
(contains molecules and ions)

Concentration

(a) Concentrated acid: Has a relatively high percentage of acid in its aqueous solution
 
(b) Dilute acid: Has a relatively low percentage of acid in its aqueous solution

Basicity (number of hydrogen ions that can be produced per molecule of the acid in aqueous solution)

(a) Monobasic acid: Ionises in aqueous solution to produce one replaceable hydrogen ion per molecule of the acid, e.g. HCl, HNO3, CH3COOH
 
 
(b) Dibasic acid: Ionises in aqueous solution to produce two hydrogen ions per molecule of the acid, e.g. H2SO4
 
(c) Tribasic acid: Ionises in aqueous solution to produce three replaceable hydrogen ions per molecule of the acid, e.g. H3PO4

Preparation of Acids

  1. From non-metals: Hydrogen combines with non-metals to form acids.
     
    H2 + Cl2 Description: 32737.jpg 2HCl (hydrochloric acid)
  2. From acidic oxides: Acidic oxides dissolve in water to give an acid.
     
    CO2 + H2O Description: 32737.jpg H2CO3 (carbonic acid)
  3. From salts: Normal salts of more volatile acids are displaced by less or non-volatile liquid.
     
  4.  By oxidation of non-metals: Sulphur is oxidised by concentrated nitric acid to form sulphuric acid.
     
    S + 6HNO3 Description: 32737.jpg H2SO4 + 2H2O + 6NO2

Sulphuric acid is also known as oil of vitriol.

Chemical Properties of Acids

  1. Neutralisation reactions: Acids neutralise bases to form salt and water.
     
    HCl + NaOH Description: 32743.jpg NaCl + H2O
  2. Reaction with active metals: Acids react with active metals to liberate hydrogen.
     
    Zn + 2HCl Description: 32743.jpg ZnCl2 + H2
  3. Reaction with salt: Acids on heating with salt undergo double decomposition reaction wherein less volatile acid displaces the more volatile acid.
     

Uses of Acids

  1. Sulphuric acid is used in the manufacture of other acids such as hydrochloric and nitric acid.
  2. Sulphuric acid is used to remove the surface oxide layers on metals before the metals are coated with paint that prevents rusting.
  3. Nitric acid is used in the manufacture of fertilisers, plastics, photographic films and dyes.
  4. Nitric acid is also used in the preparation of explosives such as dynamite and TNT.
  5. Boric acid is used as an eye wash.
  6. Citric acid is used as a food preservative.
  7. Oxalic acid is used to remove ink stain.
  8. Carbonic acid is used to flavour soft drinks.
  9. Tartaric acid is used in baking powder.
  10. Hydrochloric acid is used in pickling (to remove impurities, stains etc.) of steel.
  11. Acetic acid is used to preserve pickles.
  12. Sulphuric acid is used in car batteries.





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