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Redox Reactions – Class 11 Chemistry notes, important questions and MCQs

NCERT Class 11 Chemistry, chapter 7. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Redox reactions involve transfer of electrons. Oxidation numbers help identify oxidation and reduction, and balance redox equations.

Concepts

Oxidation: loss of electrons / increase in oxidation number; reduction: gain of electrons / decrease.
Oxidising agent gets reduced; reducing agent gets oxidised.
Oxidation number rules: free element 0; O usually −2 (−1 in peroxides); H +1 (−1 in metal hydrides); sum in a neutral compound is 0.

Types and balancing

Combination, decomposition, displacement and disproportionation reactions.
Balance by the oxidation-number method or the half-reaction (ion-electron) method.
Redox titrations, e.g. KMnO₄ (Mn +7 → +2 in acid) acts as a self-indicator.

Key terms

Disproportionation: same element is both oxidised and reduced
Oxidation number: charge an atom would have if all bonds were ionic
Standard electrode potential: potential of a half-cell measured against the standard hydrogen electrode under standard conditions

Important questions with answers

Why is fluorine always in the −1 oxidation state in its compounds?2 marks

Fluorine is the most electronegative element, so it always attracts the shared electrons and never shows a positive oxidation state.

Identify the oxidant and reductant in 2Al + Fe₂O₃ → Al₂O₃ + 2Fe, and state where this reaction is used.3 marks

Al goes from 0 to +3 (oxidised), so it is the reductant; Fe goes from +3 to 0 (reduced), so Fe₂O₃ is the oxidant. This is the thermite reaction, used to weld railway tracks.

Balance the following by the ion-electron method in acidic medium: Cr₂O₇²⁻ + SO₂ → Cr³⁺ + SO₄²⁻.5 marks

Reduction: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. Oxidation: SO₂ + 2H₂O → SO₄²⁻ + 4H⁺ + 2e⁻; multiply by 3: 3SO₂ + 6H₂O → 3SO₄²⁻ + 12H⁺ + 6e⁻. Adding and cancelling: Cr₂O₇²⁻ + 3SO₂ + 2H⁺ → 2Cr³⁺ + 3SO₄²⁻ + H₂O. Check: atoms and charge balance (left −2 + 2 = 0; right +6 − 6 = 0).

Quick revision

OIL RIG: oxidation is loss, reduction is gain.
Oxidising agent is reduced.
KMnO₄ is a self-indicator.

Exam tips

In H₂O₂ oxygen is −1; in OF₂ it is +2.
In the ion-electron method, balance O with H₂O, H with H⁺ (acidic medium), then charge with electrons, and finally equalise electrons in both half-reactions.

Practice MCQs

Oxidation is
1. loss of electrons
2. gain of electrons
3. gain of hydrogen
4. gain of protons
Show answer
Answer: (a) loss of electrons. OIL.
Oxidation number of an element in its free state is
1. 0
2. +1
3. −1
4. variable
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Answer: (a) 0. Zero.
Oxidation number of Mn in KMnO₄ is
1. +2
2. +6
3. +4
4. +7
Show answer
Answer: (d) +7. 1 + x − 8 = 0.
In Zn + CuSO₄ → ZnSO₄ + Cu, the reducing agent is
1. Zn
2. Cu²⁺
3. SO₄²⁻
4. Cu
Show answer
Answer: (a) Zn. Zn is oxidised.
Oxidation number of S in H₂SO₄ is
1. +6
2. +4
3. −2
4. +2
Show answer
Answer: (a) +6. 2 + x − 8 = 0.
Oxidation number of Cr in K₂Cr₂O₇ is
1. +3
2. +6
3. +7
4. +12
Show answer
Answer: (b) +6. 2 + 2x − 14 = 0.
2H₂O₂ → 2H₂O + O₂ is an example of
1. disproportionation
2. combination
3. displacement
4. neutralisation
Show answer
Answer: (a) disproportionation. O goes from −1 to −2 and 0.
In the balanced reaction MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O, the number of electrons gained per MnO₄⁻ is
1. 5
2. 3
3. 7
4. 2
Show answer
Answer: (a) 5. Mn from +7 to +2.

All chapters of Class 11 Chemistry

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.