ACIDS-PROPERTIES-REACTIONS-ICSE

Acid: An acid is a compound which produces Hydronium ions (H₃O⁺) as the only positively charged ions in its aqueous solution.

Example : Hydrochloric acid, Sulphuric acid, Acetic acid etc.

Hydronium ions are responsible for the acid nature of a solution.

When HCl is dissolved in water it ionizes to form H⁺  and Cl^-. The Hydrogen ion (H⁺ or proton) combines with water molecule and forms Hydronium ion. Thus the resultant solution contains Hydronium ions and Chloride ions. The positively charged ions in this solution are Hydronium ions hence aqueous HCl (Hydrochoric acid) is considered an acid.

Classification of acids:

1.      Based on the origin

Organic acids	Inorganic acids (mineral acids)
Extracted from living or organic matter.	Derived from minerals
e.g. CH3COOH, HCOOH, etc	e.g. HCl, H2SO4, HNO3 etc.

2.      Based on the constituent elements

Hydracids	Oxy-acids
The acids containing Hydrogen atom/s.	The acids containing Hydrogen and Oxygen atoms.
e.g. HCl, HBr, HI etc.	e.g. HNO3, H2SO4 etc.

3.      Based on the strength

Strong acids	Weak acids
1.      Strong acids undergo complete dissociation in their aqueous solutions H2SO4 + 2H2O  --> 2H3O+ + SO42- 2.      give a high concentration of Hydronium ions 3.      their solutions contain ions only. 4.      These are good conductors of electricity(strong electrolytes).	1.      Weak acids undergo partial dissociation in their aqueous solutions. CH3COOH + H2O -->  CH3COO- + H3O+ 2.      give a low concentration of Hydromium ions 3.      their solutions contain ions and molecules. 4.      Bad conductors of electricity (weak electrolytes).

4.      Based on the Basicity of the acids

Basicity of an acid: The number of Hydrogen ions(protons) produced by one molecule of an acid in its aqueous solution.

Monobasic acid	Dibasic acid	Tribasic acid
One molecule of this acid produces one Hydrogen ion in its aqueous solution. e.g. HCl, HNO3, CH3COOH etc. HNO3 + H2O --> H3O+ + NO3-	One molecule of this acid produces two Hydrogen ions in its aqueous solution. e.g. H2SO4, H2SO3 etc. H2SO4 + 2H2O--> 2H3O+ + SO42-	One molecule of this acid produces three Hydrogen ions in its aqueous solution. e.g. H3PO4 H3PO4 + 3H2O-->3H3O+ + PO43-

Preparations of acids:

1.      From non-metals :

H₂ + Cl₂   2HCl

2.      By dissolving  Non-metallic oxides (acidic oxides) in water

CO₂ + H₂O H₂CO₃

SO₂ + H₂O H₂SO₃

3.      By heating salts with non-volatile acids:

NaCl  + Conc. H₂SO₄ HCl + NaHSO₄

            Non-volatile acid

4.      Oxidation of non-metals with strong oxidizing agents:

S +    6HNO_{3 -->} H₂SO₄ + 2H₂O + 6NO₂

                  Oxidizing        Sulphuric

                       agent              acid

Reactions of Acids:

1.      Neutralization : Acids react with bases to give salt and water.

HCl + NaOH -->   NaCl + H₂O

H₂SO₄  + CuO -->   CuSO₄ + H₂O

2.      Dilute acids react with –

i) active metals to produce Hydrogen gas and corresponding salts.

           Zn + 2HCl --> ZnCl₂ + H₂

           H₂SO₄ + Fe --> FeSO₄ + H₂

ii) metal carbonates and bicarbonates to give Carbon dioxide

           2HCl + Na₂CO₃ --> 2NaCl + CO₂ + H₂O

iii) metal sulphites to give Sulphur dioxide gas

           2HCl + Na₂SO_{3 -->}  2NaCl + SO₂ + H₂O

iv) metal sulphides to give Hydrogen sulphide gas.

           2HCl + Na₂S -->  2NaCl + H₂S

3.      Less volatile or non-volatile acids react with salts to give volatile acids.

NaCl  +    Conc. H₂SO_{4 -->} HCl + NaHSO₄

               Non-volatile acid

NaNO₃  +   Conc. H₂SO_{4 -->} HNO₃ + NaHSO₄

                 Non-volatile acid

Acid rain:

                       The gases like NO₂ , NO , SO₂ , SO₃ in the air combine with water vapour and produce acids like Nitrous acid, Nitric acid, Sulphurous acid and Sulphuric acid which results in precipitation or rain and has pH below 5.6

                       SO₂ + H₂O --> H₂SO₃

                       SO₃ + H₂O --> H₂SO₄

                       2NO₂ + H₂O -->  HNO₃ + HNO₂