Online Notes on Chapter=1 ( Chemical Reactions and Equations) Part 2

This is Online Notes on Chapter=1 ( Chemical Reactions and Equations))  Part 2 for preparation of CBSE BOARD Examination, NTSE etc.




=> TYPES OF CHEMICAL REACTIONS:

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

e.g. (i) Burning of coal

C (s) + O₂ (g) → CO₂ (g)

(ii) Formation of water

2H₂ (g) + O₂ (g) → 2H₂O (l)

(iii) CaO (s) + H2O (l) → Ca(OH)₂ (aq)

Quick lime Slaked lime

=> Exothermic Reactions : 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

(ii) Respiration is also an exothermic reaction.

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

II. 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.

e.g., (i)  2FeSO₄ (s)    ==HEAT==>    Fe₂O₃ (s)      + SO₂ (g) + SO₃ (g)

           (Ferrous sulphate)                  (Ferric oxide)

Green colour Red-brown colour

(ii)   CaCO3 (s)    ==Heat==>        CaO (s) + CO2 (g)
        (Lime stone)                     (Quick lime)

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

e.g., 2H₂O   ==Electric current==>    2H₂ + O₂

e.g., 2AgCl (s)   ==Sunlight==>    2Ag (s) + Cl₂ (g)

2AgBr (s)    ==Sunlight==> 2Ag (s) + Br₂ (g)

* Above reaction is used in black & white photography.

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

III. DISPLACEMENT REACTION : The chemical reaction 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 colour due to formation of FeSO₄.

Zn + CuSO₄ → ZnSO₄ + Cu

Zn is more reactive than copper.

IV. 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          (Barium           (Barium         (Sodium

sulphate)          chloride)         sulphate)       chloride)

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

V. OXIDATION AND REDUCTION :

Oxidation :
i) The addition of oxygen to substance.

(ii) The removal of hydrogen from a substance.

C + O₂ → CO₂

2Cu + O₂ ==Heat==>  2CuO

CuO + H₂ ==Heat==> Cu + H₂O

Reduction :
(i) The addition of hydrogen to substance.

(ii) The removal of oxygen from a substance.

In this reaction CuO is reduced to Cu and H₂ is oxidized to H₂O. So, oxidation and  reduction taking place together is redox reaction.

=> Effects of Oxidation in Daily Life:

1) Corrosion :
• When a metal is exposed to substances such as moisture, acid etc. for some
time, a layer of hydrated oxide is formed which weakens the metal and hence
metal is said to be corrode

• Rusting of iron, black coating on silver and green coating on copper are
examples of corrosion.

• Corrosion can be prevented by galvanization, electroplating or painting.

2) Rancidity : The oxidation of fats and oils when exposed to air is known as
rancidity. It leads to bad smell and bad taste of food.

=> Methods to Prevent Rancidity :

(i) By adding antioxidants

(ii) Keeping food in air tight containers

(iii) Replacing air by nitrogen


(iv) Refrigeration.