CHEMICAL REACTION NOTES 

Chemical reactions: -  Chemical reactions are the processes in which new substances with new properties are formed.

• During a chemical reaction, atoms of one element do not change into those of another element. Only a rearrangement of atoms takes place in a chemical reaction

REACTANTS: -   The substances which take part in a chemical reaction are called reactants.

PRODUCTS: - The new substances produced as a result of chemical reaction are called products.

Example:-

(i) Digestion of food

(ii) Respiration

(iii) Rusting of iron

(iv) Burning of magnesium ribbon

(v) formation of curd

(vi) Magnesium + Oxygen Magnesium oxide

(vii) Zinc + Sulphuric acid Zinc sulphate + Hydrogen

Characteristics of Chemical Reactions

(i) Evolution of a gas,

(ii) Formation of a precipitate,

(iii) Change in colour,

(iv) Change in temperature,

(v) Change in state

CHEMICAL EQUATION:-  The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.

BALANCED EQUATIONS: -   A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.

       H₂     +   O₂          →       H₂O

      2H₂     +   O₂          →       2H₂O

     in reactants                    in product

 No. of H atoms: 4         No. of H atoms: 4

 No. of O atoms: 2         No. of O atoms: 2

UNBALANCED CHEMICAL EQUATIONS: -  An unbalanced chemical equation has an unequal number of atoms of one or more elements in the reactants and products.

The chemical equations can be made more informative in three ways

1. By indicating the “physical states” of the reactants and products.

Solid state =(s)     Liquid state = (l)      Gaseous state = (g)     Aqueous state = ( aq) 

2. By indicating the “heat changes” taking place in the reaction. 

“+ Heat”     “+ Heat energy”     “+ Energy”     Example:- C + O₂ → CO₂ + Heat

3. By indicating the “conditions” under which the reaction takes place. 

         sign delta (Δ)                 

       300 atm; 300℃

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          ZnO + CrO₂

            Sunlight

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          Chlorophyll

TYPES OF CHEMICAL REACTIONS

Combination Reactions: - Those reactions in which two or more substances combine to form a single substance, are called combination reactions.

(i) Burning of coal :-

C(s) + O₂(g) → CO₂(g) 

(ii) Formation of water

2H₂(g) + O₂(g) → 2H₂O(l)

(iii) Quick lime + water → Slaked lime

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

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

(i) Burning of Coal

C(s) + O₂ (g) → CO₂(g) + 2H₂O(g) + Heat

(ii) Respiration is also an exothermic reaction

C₂H₁₂O₆(aq) + 6O₂(g) → 6CO₂(aq) + 6H₂O(l) + energy

Decomposition Reactions: - Those reactions in which a compound splits up into two or more simpler substances are known as decomposition reactions.

𝑨 → 𝑩 + 𝑪

 1. Thermal decomposition :- when decomposition is carried out by heating.

(i) 2FeSO₄ (s)           →         Fe₂O₃(S)      +    SO₂(g)   +   SO₃(g)

(Ferrous sulphate)             (Ferric oxide)

Green colour                     Red-brown colour

 

                               Heat       

(ii) CaCO₃(s)            →               Cao(s)      +     CO₂(g)    

  (Lime Stone)                  (Quick lime)

 2. Electrolytic decomposition :- when decomposition is carried Out by passing electricity.

               Electric  

 2H₂O          →       2H₂ + O₂

               current

 3. Photolytic decomposition :- When decomposition is carried Out in presence of sunlight.

                       Sunlight

        2AgCl(s)         →       2Ag(s) + Cl₂(g)

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

Displacement Reaction: - The chemical reaction in which more reactive element from its salt solution.

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Metals	Reactivity
Potassium	More Reactive  React with water
Sodium
Lithium
Barium
Strontium
Calcium
Magnesium	Reacts with acids
Aluminium
Manganese
Zinc
Chromium
Iron
Cobalt
Nickel
Tin
Lead
Hydrogen	Included for Comparison
Copper	Highly unreactive
Mercury
Silver
Gold
Platinum

Double Displacement Reaction : - A reaction in which new compounds are formed by mutual exchange of ions between two compounds.

Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Oxidation: (i) The addition of oxygen to reactant.

(ii) The removal of hydrogen from a reactant.

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

(ii) The removal of oxygen from a reactant.