Unit 7: Redox Reactions — Class XI Chemistry

Oxidation and reduction occur simultaneously — the basis of energy, metallurgy, and life

7.1 Classical Idea of Redox Reactions

OIL RIG: Oxidation Is Loss — Reduction Is Gain (of electrons)
Oxidation: Increase in oxidation number | Reduction: Decrease in oxidation number
Oxidising agent: Gets reduced (accepts electrons) | Reducing agent: Gets oxidised (donates electrons)

7.2 Redox as Electron Transfer

Redox reactions can be understood as electron transfer processes. For example, in the formation of NaCl:

2Na(s) → 2Na⁺(g) + 2e⁻ (oxidation — loss of electrons)
Cl₂(g) + 2e⁻ → 2Cl⁻(g) (reduction — gain of electrons)

Each step is called a half reaction. The sum of half reactions gives the overall reaction.

Competitive Electron Transfer

When zinc is placed in Cu(NO₃)₂ solution, copper is displaced: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Zinc releases electrons to copper, showing Zn > Cu in electron-releasing tendency.

7.3 Oxidation Number

Oxidation number is a book-keeping tool — assumes complete electron transfer even in covalent compounds. The sum of oxidation numbers in a neutral compound is zero; in a polyatomic ion, it equals the ion charge.

7.4 Types of Redox Reactions

Combination

A + B → C. Both or one in elemental form. All combustion reactions are redox.

Decomposition

C → A + B. Opposite of combination. Not all are redox (e.g., CaCO₃ decomposition).

Displacement

X + YZ → XZ + Y. Reactive element displaces less reactive. Metal or non-metal.

Disproportionation

Same element is both oxidised and reduced. E.g., Cu⁺ → Cu²⁺ + Cu⁰.

7.5 Balancing Redox Reactions

Method 1 — Oxidation Number: Equalise total increase and decrease in O.N. by multiplying appropriate coefficients.
Method 2 — Half-Reaction: Split into oxidation and reduction halves, balance each separately (atoms, then charge with e⁻), equalise electrons, then add.

7.6 Applications

Summary

Redox reactions involve simultaneous oxidation and reduction. Oxidation is loss of electrons (increase in O.N.), reduction is gain (decrease in O.N.). Oxidation number rules allow identification of oxidised and reduced species. Redox reactions are classified as combination, decomposition, displacement, and disproportionation. They are balanced using oxidation number or half-reaction methods. Redox reactions are fundamental to batteries, metallurgy, corrosion, and biological processes like photosynthesis and respiration.