CBSE Class 10 Science Chemical Reactions And Chemical Equations Notes Set 02

Chemical Reaction: In a chemical reaction, a new substance is formed which is completely different in properties from the original substance, so in a chemical reaction, a chemical change takes place.

Example: The burning of magnesium in the air to form magnesium oxide is an example of a chemical reaction.
2Mg(s) + O₂(g) △→ 2MgO(s)

Before burning in air, the magnesium ribbon is cleaned by rubbing with sandpaper.
This is done to remove the protective layer of basic magnesium carbonate from the surface of the magnesium ribbon.

Characteristics of Chemical Reactions:
(i) Evolution of gas
(ii) Change in Colour
(iii) Change in state of substance
(iv) Change in temperature
(v) Formation of precipitate

Balanced Chemical Equation: A balanced chemical equation has the number of atoms of each element equal on both sides.
Example: Zn + H₂SO₄ → ZnSO₄ + H₂

Types of Chemical Reactions:
Chemical reactions can be classified in following types:
(i) Combination Reaction: Reactions in which two or more reactants combine to form one product are called Combination Reactions.

Example:
When magnesium is burnt in the air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.

Mg(s) + O₂(g) → 2MgO(s)
Magnesium + Oxygen → Magnesium Oxide

(ii) Decomposition Reaction: Reactions in which one compound decomposes in two or more compounds or elements are known as Decomposition Reaction. A decomposition reaction is just the opposite of combination reaction.

Example:
When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.
CaCO₃(s) heat−→− CaO(s) + CO₂(g)
Calcium carbonate → Calcium oxide + Carbon dioxide

Thermal Decomposition: The decomposition of a substance on heating is known as Thermal Decomposition.
Example: 2Pb (NO₃)₂(s) heat−→− 2PbO(s) + 4NO₂ (g) + O₂(g)

Electrolytic Decomposition: Reactions in which compounds decompose into simpler compounds because of passing of electricity, are known as Electrolytic Decomposition. This is also known as Electrolysis.

Example: When electricity is passed in water, it decomposes into hydrogen and oxygen.
2H₂O (l) → 2H₂ (g) + O₂(g)

Photolysis or Photo Decomposition Reaction: Reactions in which a compound decomposes because of sunlight are known as Photolysis or Photo Decomposition Reaction.

Example: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.
2AgCl(s) (white) Sunlight−→−−−−− 2Ag(s) (grey) + Cl₂ (g)

(iii) Displacement Reaction: The chemical reaction in which a more reactive element displaces a less reactive element from a compound is known as Displacement Reactions. Displacement reactions are also known as Substitution Reaction or Single Displacement/ replacement reactions.

Example:
When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.
Zn(s) + 2HCl (aq) → ZnCl₂ (aq) + H₂(g)

(iv) Double Displacement Reaction: Reactions in which ions are exchanged between two reactants forming new compounds are called Double Displacement Reactions.

Example:
When the solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.
BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄(s) (Precipitate) + 2NaCl (aq)

Precipitation Reaction: The reaction in which precipitate is formed by the mixing of the aqueous solution of two salts is called Precipitation Reaction.

(v) Oxidation and Reduction Reactions:

Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as Oxidation.
Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized.

Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called Reduction.
The compound or element which goes under reduction in called to be reduced.
Oxidation and Reduction take place together.

Oxidizing agent:
* The substance which gives oxygen for oxidation is called an Oxidizing agent.
* The substance which removes hydrogen is also called an Oxidizing agent.

Reducing agent:
* The substance which gives hydrogen for reduction is called a Reducing agent.
* The substance which removes oxygen is also called a Reducing agent.

REDOX REACTIONS: The reaction in which oxidation and reduction both take place simultaneously is called Redox reaction.
When copper oxide is heated with hydrogen, then copper metal and hydrogen are formed.
CuO + H₂ → Cu + H₂O

(i) In this reaction, CuO is changing into Cu. Oxygen is being removed from copper oxide. Removal of oxygen from a substance is called Reduction, so copper oxide is being reduced to copper.
ii) In this reaction, H2 is changing to H2O. Oxygen is being added to hydrogen. Addition of oxygen to a substance is called Oxidation, so hydrogen is being oxidized to water.

* The substance which gets oxidized is the reducing agent.
* The substance which gets reduced is the oxidizing agent

(vi) Exothermic and Endothermic Reactions:
Exothermic Reaction: Reaction which produces energy is called Exothermic Reaction. Most of the decomposition reactions are exothermic.

Example:
Respiration is a decomposition reaction in which energy is released.

Effects of Oxidation Reactions in Everyday life: Corrosion and Rancidity.

Corrosion: The process of slow conversion of metals into their undesirable compounds due to their reaction with oxygen, water, acids, gases etc. present in the atmosphere is called Corrosion.

Rusting: Iron when reacts with oxygen and moisture forms red substance which is called Rusting.

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 the oxidation of fat and oil present in food materials.

Methods to prevent rancidity:
* By adding anti-oxidant.
* Vacuum packing.
* Replacing air by nitrogen.
Refrigeration of foodstuff.