⚗️ Chemical Reactions and Equations: Complete NCERT Notes

📑 In This Chapter Guide (Table of Contents)

1. 1. What is a Chemical Reaction & How to Identify It?
2. 2. Step-by-Step Guide to Balancing Chemical Equations
3. 3. Major Types of Chemical Reactions Explained
4. 4. Everyday Redox: Corrosion and Rancidity
5. 💡 Frequently Asked Questions (FAQ)

1. What is a Chemical Reaction & How to Identify It?

A chemical reaction occurs when one or more substances (reactants) undergo a transformation to form new substances (products) with entirely different chemical properties. During this process, chemical bonds are broken and new bonds are formed.

Key Observations Confirming a Chemical Change:

• Change in State: E.g., Reaction of Hydrogen gas with Oxygen gas producing liquid water: 2H₂(g) + O₂(g) → 2H₂O(l).
• Change in Color: E.g., Rusting of iron or heating blue Copper Sulphate crystals turning white anhydrous salt.
• Evolution of Gas: E.g., Reaction of Zinc granules with dilute Sulphuric acid releasing effervescent Hydrogen gas: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑.
• Change in Temperature: Exothermic reactions (heat released, e.g., quicklime with water) vs. Endothermic reactions (heat absorbed, e.g., photosynthesis).
• Formation of a Precipitate: Insoluble solid formed during double displacement reactions.

2. Step-by-Step Guide to Balancing Chemical Equations

According to the Law of Conservation of Mass, matter can neither be created nor destroyed in a chemical reaction. Therefore, the total number of atoms of each element must remain equal on both reactant and product sides.

Hit and Trial Method Example: Balance the reaction of Iron with Steam: Fe + H₂O → Fe₃O₄ + H₂

1. Step 1: Count atoms on both sides: Reactants (Fe=1, H=2, O=1) vs Products (Fe=3, H=2, O=4).
2. Step 2: Balance Oxygen first by putting coefficient 4 before H₂O: Fe + 4H₂O → Fe₃O₄ + H₂.
3. Step 3: Hydrogen is now 8 on left, so put coefficient 4 before H₂: Fe + 4H₂O → Fe₃O₄ + 4H₂.
4. Step 4: Balance Iron by putting coefficient 3 before Fe: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
5. Final Balanced Equation: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g).

3. Major Types of Chemical Reactions Explained

Understanding the fundamental reaction categories is crucial for solving classification questions in board exams:

• Combination Reaction: Two or more simple substances combine to form a single compound.
  CaO(s) [Quicklime] + H₂O(l) → Ca(OH)₂(aq) [Slaked Lime] + Heat. (Highly exothermic).
• Decomposition Reaction: A single compound splits into two or more simpler substances. Requires energy input:
  • Thermal: CaCO₃(s) --[Heat]--> CaO(s) + CO₂(g) (Manufacture of cement).
  • Electrolytic: 2H₂O(l) --[Electric Current]--> 2H₂(g) + O₂(g) (Volume ratio H₂:O₂ is 2:1).
  • Photolytic: 2AgBr(s) --[Sunlight]--> 2Ag(s) + Br₂(g) (Used in photographic film development).
• Displacement Reaction: A more reactive metal displaces a less reactive metal from its salt solution:
  Zn(s) + CuSO₄(aq) [Blue] → ZnSO₄(aq) [Colorless] + Cu(s) [Reddish-brown].
• Double Displacement (Precipitation Reaction): Exchange of ions between two aqueous compounds:
  Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) [Bright Yellow ppt] + 2KNO₃(aq).
• Redox Reactions: Oxidation is gain of oxygen or loss of hydrogen/electrons; Reduction is loss of oxygen or gain of hydrogen/electrons.
  MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ (MnO₂ is reduced to MnCl₂; HCl is oxidized to Cl₂).

4. Everyday Redox: Corrosion and Rancidity

Corrosion: Slow degradation of metals caused by exposure to atmospheric moisture, oxygen, and acids.

• Rusting of Iron: Forms hydrated ferric oxide: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O (Reddish-brown flaky layer).
• Corrosion of Copper: Forms basic copper carbonate CuCO₃·Cu(OH)₂ (Green coating).
• Tarnishing of Silver: Forms silver sulphide Ag₂S (Black layer upon reaction with atmospheric H₂S).
• Prevention: Galvanization (zinc coating), electroplating, anodizing, painting, and greasing.

Rancidity: Aerial oxidation of fats and oils leading to foul smell and unpleasant taste in food. Prevented by flushing packets with unreactive nitrogen gas, vacuum packaging, and adding synthetic antioxidants like BHA and BHT.

💡 Frequently Asked Questions (FAQ)

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