TYPES OF CHEMICAL REACTIONS - 12th class Science

 TYPES OF CHEMICAL REACTIONS - 12th class Science

(1)COMBINATION REACTION: The reaction in which two or more reactant combine to form a single product. 

 e.g. (1) Burning of coal

C(s) + O₂(g) → CO₂(g)

 (2) Formation of water

2H₂ (g) + O₂(g) → 2H₂0 (1)

(3) CaO(s) + H₂0 (1)→Ca(OH)₂(aq)

    Quick lime                Slaked lime

Exothermic Reacions: Reaction in which heat is released along with formation of products.

 e.g., (i) Burning of natural gas.

CH₄(g) + O₂ (g)→ CO₂ (g) + 2H₂O(g) + Heat 

(ü) Respiration is also an exothermic reaction.

C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(aq) + 6H₂O (1) + energy 

(2). DECOMPOSITION REACTION: The reaction in which a compound splits into two or more simple substances  is called decomposition reaction.

                                                            A→ B+C 

• Thermal decomposition : When decomposition is carried out by heating. 

             Eg: (1) 2FeSO₄ (s) →Heat > Fe₂O₃(s) + SO₂(g) + SO₃ (g)

                    (Ferrous sulphate)          (Ferric oxide)

                    Green colour                Red-brown colour 

          (2) CaCO₃ (s) →(Heat)CaO (s) + CO₂ (g)

              (Lime stone)     (Quick lime)

•  Electrolytic Decomposition: When decomposition is carried out by passing electricity.

            Eg: (1)2H₂0 (electric current) → 2H₂ + O₂

•  Photolytic Decomposition: When decomposition is carried out in presence of sunlight. 

              e.g., (i) 2AgCl (s) (sunlight) → 2Ag (s) + Cl₂(g)

                       2AgBr (s) (Sunlight) → 2Ag (s) + Br₂ (g)

 Above reaction is used in black and white photography.

• Endothermic Reaction: The reactions which require energy in the form of heat, light or electricity to  break reactants are called endothermic reactions. 

(3)DISPLACEMENT REACTION: The chemical reactions in which more reactive element displaces less reactive element from its salt solution. 

Fe (s)+ CuSO₄(aq) → FeSO₄ (aq) + Cu (s)

The iron nail becomes brownish in colour by deposition of Cu and blue colour of CuSO₄ changes dirty green clour due to formation of FeSO₄. 

Zn + CuSO₄ + ZnSO₄ + Cu

Zn more reactive than copper.

 (4) DOUBLE DISPLACEMENT REACTION: A reaction in which new compounds are formed by

mutual exchange of ions between two compounds.

Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq)

                                (Sodium sulphate)  +  (Barium chloride) → (Barium sulphate) + (Sodium chloride) 

      White precipitate of BaSO₄ is formed, so it is also called precipitation reaction. 

(5)OXIDATION AND REDUCTION: 

Oxidation : It is a process of gaining oxygen during a reaction.

 2Cu + O₂ (Heat) → 2CuO

 CuO + H2(Heat) → Cu + H₂O 

Reduction : Reduction is just reverse of oxidation. It is a process of losing oxygen during a reaction.

                         CuO + H₂ → Cu + H₂O 

In this reaction CuO is reduced to Cu and H₂ is oxidized H₂O. In other words, one reactant gets oxidised while the other gets reduced. Such reactions are called oxidation-reduction reactions or redox reactions.