# BHARDWAJ CLASSES

## Class 10 CBSE Chemical Reactions and Equations - Notes

In this chapter we are going to study about the changes around us. There are two types of changes:

Physical change: In physical change, no new substance is formed, only physical states are changed.

Chemical change: In chemical change, new substance with new properties is formed. Whenever a chemical change occurs, a chemical reaction takes place.

Chemical Reaction: It takes place by Bond breaking between atoms and formation of new bonds between different atoms. New substances with different properties are formed. For example: Iron reacts with oxygen in the presence of moisture to form reddish brown iron oxide.

Whenever a chemical reaction takes place one or more of the following changes take place :
•  Change in state
•  Change in colour
•  Evolution of gas
•  Change in temperature

Chemical Equation: It is the symbolic representation of chemical reaction, with the help of symbols of elements and formulae of compounds.

Reactants: The elements and compounds taking part in the reaction are called reactants. They must be written on the left hand side of the chemical equation.

Products: The substance formed in a chemical reaction are called products. They are written on the right hand side separated from the reactants by an arrowhead .

Balanced Chemical Equation: The equation in which number of atoms of each element on the reactants and products side are equal, so as to follow the law of conservation of mass.

Law of conservation of mass: The total mass of the reactants must be equal to the total mass of products that's why we balance every chemical equation.

Types of chemical reactions:

1. Combination reaction: Those reactions in which two or more elements or compounds combine to form one compound are called combination reactions. Ex.

2. Decomposition Reaction: Those reactions in which a compound breaks into simpler substances are called decomposition reactions. Ex
$2Pb{\left(N{O}_{3}\right)}_{2}\text{\hspace{0.17em}}\to \text{\hspace{0.17em}}2PbO+4N{O}_{2}+{O}_{2}$

(i) Thermal decomposition: Those decomposition reactions in which compound breaks down into simpler substances by heat. Ex.
$CaC{O}_{3}\left(s\right)\text{\hspace{0.17em}}\stackrel{\text{Heat}}{\to }\text{\hspace{0.17em}}CaO\left(s\right)+C{O}_{2}\left(g\right)$
(ii) Photo decomposition: Those decomposition reactions in which sunlight is used for decomposition of a compound. Ex.
$\underset{\text{white}}{AgCl\left(s\right)}\text{\hspace{0.17em}}\stackrel{\text{sunlight}}{\to }\underset{\text{grey}}{2Ag\left(s\right)}+C{l}_{2}\left(g\right)$
(iii) Electrolysis( decomposition by electricity): Those reactions in which a compound is broken down into simpler substances by passing electric current through it, in aqueous solution or molten state. Ex.
$2{H}_{2}O\left(l\right)\text{\hspace{0.17em}}\stackrel{\text{Electricity}}{\to }2{H}_{2}\left(g\right)+{O}_{2}\left(g\right)$

3. Displacement Reactions: Those reactions in which a more reactive element can displace a less reactive metal from its salt solution are called displacement reactions. Ex.
$Fe(s)+CuS O 4 (aq) → FeS O 4 (aq)+Cu(s)$

4. Double Displacement Reactions: Those reactions in which two compounds exchange their ions to form two new compounds are called double displacement reactions. Ex.
$NaOH\left(aq\right)+HCl\left(aq\right)\text{\hspace{0.17em}}\to \text{\hspace{0.17em}}NaCl\left(aq\right)+{H}_{2}O\left(l\right)$

5. Precipitation Reactions: Those reactions in which one of the product formed is a solid (insoluble in water) are called precipitation reactions. Ex.

6. Exothermic Reactions: Those reactions in which heat is evolved are called exothermic reactions. Ex. Respiration is an exothermic reaction.
${C}_{6}{H}_{12}{O}_{6}\left(s\right)+6{O}_{2}\left(g\right)\text{\hspace{0.17em}}\to \text{\hspace{0.17em}}6C{O}_{2}\left(g\right)+6{H}_{2}O\left(l\right)+Heat$

7. Endothermic Reactions: Those reactions in which heat is absorbed are called endothermic reactions. Ex.
$2KCl{O}_{3}\left(s\right)\stackrel{\text{Heat}}{\to }2KCl\left(s\right)+3{O}_{2}\left(g\right)$

8. Oxidation Reactions: Those reactions in which oxygen is added or hydrogen is removed are called Oxidation reactions. Ex.

9. Reduction Reactions: Those reactions in which hydrogen is added or oxygen is removed are called reduction reactions. Ex.
$2Na\left(s\right)+{H}_{2}\left(g\right)\text{\hspace{0.17em}}\to \text{\hspace{0.17em}}2NaH\left(s\right)$

10. Redox Reactions: Those reaction in which oxidation as well as reduction takes place simultaneously are called redox reactions. Ex.

Oxidizing agent: Those substances which can add oxygen or remove hydrogen are called oxidizing agents.

Reducing agent: Those substances which can add hydrogen or remove oxygen are called reducing agents.

Corrosion: The process of slowly eating up of metal due to attack of atmospheric gases such as oxygen, carbon dioxide, hydrogen sulphide, water vapour on the surface of metal so as to convert the metal into oxide, carbonates, sulphide is known as corrosion. Ex-

•  When an iron article exposed to moist air, its surface is covered with a brown substance called rust which is hydrated ferric oxide.

• Copper objects lose their Shine after some time because this object acquires a green coating of basic copper carbonate due to attack of oxygen, carbon dioxide and water vapour present in air.

• Silver metal loses its Shine due to a coating of black silver sulphide by the action of hydrogen sulphide gas present in the air.

$Ag\left(s\right)+{H}_{2}S\left(g\right)\text{\hspace{0.17em}}\to \text{\hspace{0.17em}}\underset{\text{Black}}{A{g}_{2}S\left(s\right)}+{H}_{2}\left(g\right)$

Disadvantages of corrosion: Corrosion causes damage to bodies of vehicles, bridges, iron railings, ship etc and all the objects that are made up of metals.

Prevention of corrosion: oiling, greasing, painting, coating with zinc ( galvanization) are methods to prevent corrosion of iron.

Rancidity: The process in which food get spoilt due to oxidation is called rancidity. It causes to change in taste, smell and colour.

Prevention from Rancidity:

•  Antioxidants are added to prevent oils and fats from oxidation.
•  Chips and other snacks are packed in Nitrogen gas which is least reactive and prevent food by oxidation.
•  Some snacks like peanuts, Walnuts are vacuum packed.
•  Refrigeration of food is also the method to prevent rancidity.