MCQ Imp. Questions Worksheet NCERT Solutions

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Important Points

1. Chemical reaction: The reaction in which the original state of the particles changes and it cannot be reversed by simple physical means, is known as a chemical reaction.
   Examples: fermentation of grapes, burning of wood, etc. Burning of wood produces charcoal and we cannot get back wood from charcoal on reversing the conditions.
2. Change in state: a chemical reaction is accompanied by change in state, colour, evolution of gas or change in temperature. The chemical reaction is represented as

Reactants → Products

3. Example of a chemical reaction is burning of magnesium ribbon with a dazzling white flame to form a white powder (magnesium oxide).

2Mg + O2 → 2MgO

4. Chemical equation: Representation of a chemical reaction in terms of chemical symbols and formulae of the reactants and products is known as chemical equation. A chemical equation represents the reactants, products and their physical states symbolically. For example,

Magnesium + Oxygen → Magnesium oxide

2Mg(s) + O2(g) → 2MgO(s)

The substances that undergo chemical change in the reaction, i.e., magnesium and oxygen, are the Reactants. The new substance, magnesium oxide, formed during the reaction is the Product.

5. Writing a chemical reaction in terms of chemical equation: A chemical equation represents a chemical reaction. While writing a chemical equation the reactants are written on the left-hand side of the equation while products on the right hand side.
   Examples:

2H₂ + O₂  ⇌  2H₂O

N₂ + 3H₂ ⇌ 2NH₃

CH₄ + 2O₂ ⇌ CO₂ + 2H₂O

6. Balanced chemical equation: The chemical equation in which the number of atoms of different elements is same on both sides of the arrow is called a balanced chemical equation.

“This is in accordance to the law of conservation of mass.”

Let us try to balance the following chemical equation:

Fe + H₂O → Fe₃O₄ + H₂

Number of atoms of different elements present in the unbalanced equation,

Element	Number of atoms in reactants (LHS)	Number of atoms in products (RHS)
Fe	1	3
H	2	2
O	1	4

Now select the element which has the maximum number of atoms Fe₃O₄. There are four oxygen atoms on the RHS and only one on the LHS. To balance the oxygen atoms,

Atoms of oxygen	In reactants	In products
(i) Initial	1 (in H2O)	4 (in Fe3O4)
(ii) To balance	1 x 4	4

So, multiplying H₂O molecules by four, we get,

Fe + 4H₂O → Fe₃O₄ + H₂

To balance H atoms, make the number of molecules of hydrogen as four on the RHS.

Fe + 4H₂O → Fe₃O₄ + 4H₂

To equalise Fe, we multiply Fe atoms by three on the LHS. Hence,

3Fe + 4H₂O → Fe₃O₄ + 4H₂ (Balanced equation)

To make chemical equation more informative gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s) respectively. Therefore,

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

Types of chemical reactions: 

i) Combination reaction: The reactions in which two or more substances combine to form a new substance is called combination reaction.
For example,

2Mg(s) + O2 (Fe)→ 2MgO(s)

CaO(s) + H2O(l) → Ca(OH)2(aq)

ii) Decomposition reaction: The reaction in which a single compound breaks up into two or more simpler substances is called decomposition reaction. For example,

2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)

The decomposition of a substance by passing electric current through it is known as Electrolysis. Fr example on passing electric current through water the following reaction occurs,

2H₂O → 2H₂ + O₂

The decomposition of a substance on heating is known as Thermal-Decomposition. For example

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

CaCO₃(s) → CaO (s) + CO₂

The decomposition of a substance by absorbing light energy is called Photochemical Decomposition. For example,

2AgCl (s) → 2Ag(s) + Cl₂(g)

2AgBr (s) → 2Ag(s) + Br₂(g)

The above two reactions are used in black and white photography. Decomposition reactions are opposite of combination reactions.

iii) Displacement reaction: The chemical reaction in which a more reactive element displaces a less reactive element from its salt solution is known as displacement reaction. For example,

Zn(s) + CuSO₄(aq) →  ZnSO₄(aq) + Cu(s)

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂ (aq) + 2Ag(s)

iv) Double displacement reaction: In this reaction two different atoms or group of atoms are mutually exchanged. E.g.

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

A white, insoluble substance, i.e., BaS04 is formed which is called precipitate.

v) Precipitation Reaction: Any reaction that produces a precipitate is called a precipitation reaction. E.g.

Pb(NO₃)₂ (aq)  + 2KI (aq)   →  PbI₂ (s) + 2KNO₃(aq)

vi) Oxidation: Oxidation is the gain of oxygen or loss of hydrogen. E.g.

2Cu  +  O₂   →   2CuO

vii) Reduction: Reduction is the loss of oxygen or gain of hydrogen.E.g.

CuO +H₂  →  Cu  +H₂O

viii) Redox reaction: The reaction in which one reactant gets oxidised while other gets reduced. E.g.

 

ix) Exothermic reactions: Reaction in which heat is released along with the formation of products. E.g.

• Respiration (Oxidation of Glucose),

C₆H₁₂O₆  + 6O₂  ⟶   6CO₂  + 6H₂O + E

• Burning of Methane (decomposition of vegetable matter into compost),

CH₄(g) + 2O₂(g) ⟶ CO₂(g) + 2H₂O(g) + E

Respiration and decomposition of vegetable matter into compost are exothermic reactions.

x) Endothermic reactions: The reactions which require energy in form of heat, light or electricity are called endothermic reactions. E.g.

2Ba(OH)₂ + 2NH₄Cl ⟶ 2BaCl₂ + 2NH₄OH

xi) Corrosion: The process of slow deterioration of some metals like iron, copper and silver into their compounds due to their reaction with oxygen, water, acids, gases, etc. present in the atmosphere is called corrosion.

xii) Rusting: The process in which iron reacts with oxygen and moisture present in the air to form a reddish brown coating called rust on its surface.

xiii) Rancidity: The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time. This is called rancidity. It is caused due to oxidation of fat and oil present in food material. It can be prevented by using various methods such as by adding antioxidants to the food materials, storing food in air tight containers and by flushing out air with nitrogen.

Important Terms

• Electrolysis: The decomposition of a substance by passing electric current through it is known as Electrolysis
• Combination reaction: is a reaction in which two or more substances combine to form a new substance.
• Decomposition reaction: is a reaction in which a single compound breaks up into two or more simpler substances.
• Displacement reaction: is a reaction in which a more reactive element displaces a less reactive element from its salt solution.
• Redox reaction: is the reaction in which oxidation and reduction takes place simultaneously.
• Chemical reaction: is a process in which old bond breaks up and new bonds are formed.
• Chemical equation: is the representation of a chemical reaction in terms of chemical symbols and formulae.