Chemical Reactions and Equations

Why This Chapter Matters

Chapter 1 of Class 10 Science — tested every year with 5-7 marks. Balancing equations, identifying reaction types, and explaining oxidation/reduction. Foundation for Class 11-12 Chemistry.

Prerequisites

Core Concepts

1. Chemical Equation

Represents a chemical reaction using symbols and formulas.

Word equation: Magnesium + Oxygen -> Magnesium oxide

Chemical equation: 2Mg + O2 -> 2MgO

Balanced equation = same atoms of each element on both sides.

2. Balancing (Hit and Trial)

Example: Balance Fe + H2O -> Fe3O4 + H2

Step 1: Fe: 1 vs 3 -> add 3Fe: 3Fe + H2O -> Fe3O4 + H2

Step 2: O: 1 vs 4 -> 3Fe + 4H2O -> Fe3O4 + H2

Step 3: H: 8 vs 2 -> 3Fe + 4H2O -> Fe3O4 + 4H2 (balanced!)

3. Types of Chemical Reactions

Combination: A + B -> AB

2H2 + O2 -> 2H2O | C + O2 -> CO2 | CaO + H2O -> Ca(OH)2

Decomposition: AB -> A + B (opposite of combination)

2H2O -> 2H2 + O2 (electrolytic) | CaCO3 -> CaO + CO2 (thermal) | 2AgCl -> 2Ag + Cl2 (photolytic)

Displacement: A + BC -> AC + B (A is more reactive than B)

Zn + CuSO4 -> ZnSO4 + Cu | Fe + CuSO4 -> FeSO4 + Cu

Reactivity: K > Na > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au

Double Displacement: AB + CD -> AD + CB

Na2SO4 + BaCl2 -> BaSO4(ppt) + 2NaCl | AgNO3 + NaCl -> AgCl(ppt) + NaNO3

Oxidation/Reduction (Redox):

Oxidation = gain of O / loss of H / loss of electrons

Reduction = loss of O / gain of H / gain of electrons

CuO + H2 -> Cu + H2O (CuO reduced, H2 oxidised)

4. Effects of Oxidation

Corrosion: Fe + O2 + H2O -> Fe2O3.xH2O (rust)

Prevention: painting, galvanising, alloying, electroplating

Rancidity: Oxidation of fats/oils -> bad smell

Prevention: antioxidants (BHA), refrigeration, nitrogen flushing, airtight containers

Solved Examples

Q1: Balance: KClO3 -> KCl + O2

2KClO3 -> 2KCl + 3O2 (K:2=2, Cl:2=2, O:6=6) Balanced!

Q2: BaCl2 + H2SO4 -> BaSO4 + 2HCl. Identify type.

Two compounds exchanging ions -> Double Displacement + Precipitation

Q3: CuO + H2 -> Cu + H2O. What is oxidised/reduced?

CuO loses oxygen = REDUCED | H2 gains oxygen = OXIDISED

PYQs

2023: Why does colour of CuSO4 change when Fe nail is dipped in it?

Fe + CuSO4 -> FeSO4 + Cu. Blue fades as Cu deposits on nail.

2022: Write balanced equation for thermal decomposition of calcium carbonate.

CaCO3 --(heat)--> CaO + CO2

2021: What is a precipitation reaction? Example.

When two solutions mix and form insoluble precipitate.

Na2SO4 + BaCl2 -> BaSO4(ppt) + 2NaCl

2020: Define oxidation and reduction (in terms of oxygen).

Oxidation = gain of oxygen | Reduction = loss of oxygen

Example: 2CuO + C -> 2Cu + CO2 (C oxidised, CuO reduced)

MCQ Practice

Q1. 2AgCl -> 2Ag + Cl2 is:

(A) Combination (B) Decomposition (C) Displacement (D) Double displacement

Answer: B

Q2. Which gas evolved when acid reacts with metal?

(A) O2 (B) CO2 (C) H2 (D) N2

Answer: C

Q3 (Hard). In Zn + CuSO4 -> ZnSO4 + Cu, which is oxidised?

Answer: Zn (goes from 0 to +2 oxidation state, loses electrons)

Revision Notes

TYPES: Combination | Decomposition | Displacement | Double Displacement | Redox

OXIDATION vs REDUCTION:
Gains O / Loses H / Loses e- = Oxidised
Loses O / Gains H / Gains e- = Reduced

REACTIVITY: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au

IMPORTANT EQUATIONS:
2H2 + O2 -> 2H2O
2H2O -> 2H2 + O2 (electrolysis)
CaCO3 -> CaO + CO2 (limestone decomposition)
Fe + CuSO4 -> FeSO4 + Cu (displacement)

Common mistakes: Not balancing equations. Confusing oxidising agent WITH oxidised substance (they are opposite!).

Related Topics

Chapter 2 - Acids Bases Salts | Chapter 3 - Metals Non-metals