Chemical Reactions

• A change in which new substances with different properties are formed is called a chemical reaction.
• These changes cannot be reversed to the original state.

Examples:
(i) Rusting of iron
(ii) Digestion of food

Reactants: Substances that take part in a chemical reaction are called reactants.
Products: Substances that are formed during a chemical reaction are called products.

Example:
CH_4​  +  2O₂​  →  CO₂  ​+  2H_2​O

Reactants:

• CH₄ (Methane)
• O₂ (Oxygen)

Products:

• CO₂ (Carbon dioxide)
• H₂O (Water)

Chemical Equation

A chemical equation is a brief representation that expresses chemical reactions using the chemical formulas of reactants and products.

Writing Physical States in a Chemical Equation:
(s) = Solid
(l) = Liquid
(g) = Gas
(aq) = Aqueous (dissolved in water)

For example: Mg(s) + O2(g) → MgO(s)

Types of Chemical Reactions:

I. Combination Reaction
II. Decomposition Reaction
III. Displacement Reaction
IV. Double Displacement Reaction
V. Oxidation and Reduction

I. Combination Reaction:

In this reaction, two or more reactants combine to form a single product.
A + B → AB

Examples:
(i) Combustion of coal
C(s) + O₂(g) → CO₂(g)
(ii) Formation of water
2H₂(g) + O₂(g) → 2H₂O(l)
(iii) Reaction of quicklime with water
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
(Quicklime) (Slaked lime)

Exothermic Reactions: Reactions that release heat along with the formation of products.

Examples:
(i) Combustion of natural gas
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Energy

(ii) Respiration is an exothermic Reactions : The body requires energy, which we obtain from food. During digestion, carbohydrates present in food such as rice, potatoes, and bread break down into glucose (C₆H₁₂O₆). This glucose, when combined with oxygen (O₂) in cells, provides energy.

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

II. Decomposition Reaction:

In this reaction, a single reactant breaks down into two or more products.

AB → A + B

Example:
Decomposition of limestone (CaCO₃):
CaCO₃ → CaO + CO₂

Endothermic Reactions: Reactions in which energy (usually heat) is absorbed.

Types of Decomposition Reactions:

(i) Thermal Decomposition: Decomposition by heat.
Example:
Ferrous sulfate (green color) decomposes to ferric oxide (brown-red color).
(ii) Electrolytic Decomposition: Decomposition by electric current.
(iii) Photolytic Decomposition: Decomposition in the presence of sunlight.

Example:
2AgCl(s) → 2Ag(s) + Cl₂(g)

This reaction is used in black-and-white photography.

III. Displacement Reaction:

In these reactions, a more reactive element displaces a less reactive element from its compound.

Examples:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Here, zinc (Zn) and iron (Fe), being more reactive than copper (Cu), displace copper from copper sulfate (CuSO₄) to form ferrous sulfate (FeSO₄) and zinc sulfate (ZnSO₄).

Reactivity Series:

• Lithium (Li)
• Potassium (K)
• Sodium (Na)
• Calcium (Ca)
• Magnesium (Mg)
• Aluminum (Al)
• Zinc (Zn)
• Iron (Fe)
• Tin (Sn)
• Lead (Pb)
• Hydrogen (H)
• Copper (Cu)
• Silver (Ag)
• Platinum (Pt)
• Gold (Au)

Short trick : introduce a simple mnemonic. For example:

“Please Stop Calling Me A Zebra, I Like Her Call Smart Goat.”

IV. Double Displacement Reaction:

In this reaction, ions are exchanged between two compounds to form products.

Example:
Na₂SO₄ (aq)     +      BaCl₂ (aq)     →     BaSO₄ (s)     +      2NaCl (aq)
(Sodium sulfate) (Barium chloride) (Barium sulfate) (Sodium chloride)

A white precipitate of barium sulfate (BaSO₄) is formed. Hence, this reaction is also known as a precipitation reaction.

V. Oxidation and Reduction:

Oxidation:
(i) Increase in oxygen in a substance.
(ii) Decrease in hydrogen in a substance.

Reduction:
(i) Decrease in oxygen in a substance.
(ii) Increase in hydrogen in a substance.

Example:
CuO + H2 → Cu + H2O

In this reaction, copper oxide (CuO) is reduced to copper (Cu), and hydrogen (H₂) is oxidized to water (H₂O). This reaction involves both oxidation and reduction, hence it is called a redox reaction.

Effects of Oxidation Reactions in Daily Life:

1. Corrosion:

When a metal comes in contact with oxygen, moisture (humidity), acid, or other chemicals in the air, its surface slowly deteriorates or weakens. This is called corrosion.

Examples:

• Rusting of iron

• Copper turning green

• Silver turning black

Prevention of Corrosion:

• Galvanization: Coating of zinc (Zn) on metal.

• Painting and varnishing to protect metal from atmospheric contact.

• Use of stainless steel.

2. Rancidity:

Fats and oils, when exposed to air, get oxidized and their taste and smell change. This is called rancidity.

Examples:

• Spoiling of oily chips

• Rancid smell in ghee or oil

Prevention of Rancidity:

• Use of antioxidants

• Storing food in airtight containers

• Replacing air with nitrogen

• Keeping food in cold places.