Chemical reactions and equations CBSE class 10th Notes Chemistry

Chemical Reactions and Equations

Chemical Reaction

The process in which new substances with new properties are formed from one or more substances is called a Chemical Reaction.

Examples:

1. Rusting of Iron
2. Burning of Wood
3. Formation of Curd
4. Respiration, etc.

We use the following symbols for physical states in a chemical equation:

1. (g): Gas
2. (l): Liquid
3. (s): Solid
4. (aq): Aqueous

Chemical Equation

A Chemical Equation is the symbolic representation of a chemical reaction.

Example:
Magnesium is burnt in air to form magnesium oxide, represented as:
Mg + O₂ → MgO

Physical States

1. Solid: (s)
2. Liquid: (l)
3. Gas: (g)
4. Aqueous Solution: (aq)

Heat Changes

1. Reactant(s) → Product(s) + Heat
2. Reactant(s) + Heat → Product(s)

Conditions to Yield Products

Adding conditions to a chemical equation makes it more informative.

Classification of Chemical Reactions Based on Energy Change

Exothermic Reactions

Reactions in which heat is released along with the formation of products.

Example:
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

Endothermic Reactions

Reactions that require energy in the form of heat, light, or electricity to break reactants.

Example:
2Pb(NO₃)₂ + Heat → 2PbO + ...

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Chemical Reactions and Equations

Characteristics of a Chemical Reaction

• Change in Colour
• Change in Temperature
• Change in State
• Evolution of Gas
• Formation of Precipitate
• Endothermic Reaction
• Exothermic Reaction

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Balanced Chemical Equation

A balanced chemical equation ensures that:

• The number of atoms of each element in reactants = The number of atoms of each element in products

Law of Conservation of Mass

Mass of Reactants = Mass of Products

“Mass is neither created nor destroyed in a chemical reaction.”

Example of Balancing

Balancing Questions:

Example: H₂ + O₂ → H₂O

• In this equation, the number of H atoms in the reactant side is 2, and on the product side, it is also 2.
• The number of O atoms in the reactant side is 2, but on the product side, it is 1.

To balance this equation, we use the hit-and-trial method:

2H₂ + O₂ → 2H₂O

Now, the number of atoms on the reactant side is equal to that on the product side. Thus, the equation is balanced.

Steps to Balance Chemical Equations

Chemical reactions and equations notes include the means to balance equations by following methods:

• Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction and thus remains conserved.
• Ensure that the number of elements on the product side equals the reactant side.

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Types of Chemical Reactions

Based on Chemical Change

There are 8 types of chemical reactions:

1. Combination Reaction: The reaction in which two or more substances combine to form a new single substance.
   Example: C + O₂ → CO₂
2. Decomposition Reaction: The reaction in which a single substance decomposes to give two or more substances.
   Example: NaCl → Na + Cl
   Types of Decomposition Reactions:
   • Thermal Decomposition: Decomposition by heating.
     Example: CaCO₃ → CaO + CO₂
   • Electrolytic Decomposition: Decomposition by electricity.
     Example: 2H₂O + Electric Current → 2H₂ + O₂
   • Photolytic Decomposition: Decomposition in sunlight.
     Example: 2AgCl + Sunlight → 2Ag + Cl₂
3. Displacement Reaction: A reaction in which an element displaces another element from its solution.
   Example: Fe + CuSO₄ → FeSO₄ + Cu
4. Double Displacement Reaction: A reaction in which two different atoms or groups of atoms are mutually exchanged.
   Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
5. Precipitation Reaction: A reaction involving the formation of an insoluble substance (precipitate).
   Example: Pb(NO₃)₂ + 2KI → 2KNO₃ + PbI₂
6. Oxidation Reaction: A reaction where a substance gains oxygen or loses hydrogen.
   Example: 2Cu + O₂ + Heat → 2CuO
7. Reduction Reaction: A reaction where a substance gains hydrogen or loses oxygen.
   Example: C + 2H₂ → CH₄
8. Redox Reaction: A reaction in which one reactant gets oxidized, and the other gets reduced. Both oxidation and reduction occur simultaneously.
   Examples:
   ZnO + C → Zn + CO
   MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂

Effects of Oxidation in Daily Life

1) Corrosion:

When metals are exposed to moisture or acids for some time, a layer of hydrated oxide forms, weakening the metal.
Examples:

• Rusting of Iron
• Black coating on Silver
• Green coating on Copper

Prevention Methods: Galvanization, electroplating, or painting.

2) Rancidity:

Oxidation of fats and oils when exposed to air leads to bad smell and taste of food.

Prevention Methods:

• Adding antioxidants
• Storing food in airtight containers
• Replacing air with nitrogen
• Refrigeration

Prepared by: Pratap Sanjay Sir