Class 9 Chemistry Chapter 7 Redox Reactions: Solved Previous Year Questions (2020–2025)

Redox reactions involve simultaneous oxidation and reduction. According to NCERT Class 9 Chemistry Chapter 7, oxidation is loss of electrons or gain of oxygen, while reduction is gain of electrons or loss of oxygen. Redox reactions occur in combustion, rusting, respiration, and photosynthesis. Understanding oxidation numbers (OS) is key: they help identify which element is oxidized and which is reduced. The chapter emphasizes that in any redox reaction, electrons lost equal electrons gained, maintaining charge balance throughout the process.

NCERT Chapter 7 teaches oxidation number rules systematically. Elements in their free state have OS = 0. Monatomic ions have OS equal to charge. Oxygen is usually −2 (except in peroxides where it's −1). Hydrogen is +1 with nonmetals, −1 with metals. Alkali metals are +1, alkaline earth metals are +2. For compounds, sum of OS = 0; for polyatomic ions, sum = charge. Example: In KMnO₄, K is +1, O is −2, so Mn is +7. Mastering this method is essential for identifying oxidation and reduction in any reaction.

Common PYQ: Identify oxidized and reduced species in: Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag. Solution: Cu goes from 0 to +2 (oxidized, loses 2e⁻). Ag⁺ goes from +1 to 0 (reduced, gains 1e⁻). Cu is the reducing agent; Ag⁺ is the oxidizing agent. This pattern appears frequently in board exams. Students must write half-reactions clearly: oxidation: Cu → Cu²⁺ + 2e⁻; reduction: Ag⁺ + e⁻ → Ag. Balance electrons by multiplying the reduction half-reaction by 2.

The half-reaction method, taught in NCERT Chapter 7, separates oxidation and reduction into two parts. Step 1: Write oxidation and reduction half-reactions separately. Step 2: Balance atoms other than O and H. Step 3: Balance O using H₂O; balance H using H⁺ (acidic) or OH⁻ (basic). Step 4: Balance charge by adding electrons. Step 5: Equalize electrons in both half-reactions by multiplying. Step 6: Add half-reactions and cancel species appearing on both sides. This systematic approach ensures accuracy and is preferred in CBSE examinations over inspection method.

NCERT Chapter 7 connects redox chemistry to practical scenarios: rusting of iron (combination of oxidation and corrosion), burning of fuels (combustion—rapid oxidation releasing energy), respiration in cells (glucose oxidation providing ATP), photosynthesis (reduction of CO₂ to glucose), electroplating (galvanic cells), and battery operation. Understanding these applications helps students see why redox reactions matter beyond the classroom. Board exams often include application-based questions where students explain real phenomena using redox concepts, making conceptual clarity essential.

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PYQ 2: Balance MnO₄⁻ + C₂O₄²⁻ + H⁺ → Mn²⁺ + CO₂ + H₂O. Solution: Mn(+7→+2, gains 5e⁻), C(+3→+4, loses 1e⁻ per C, 2e⁻ total). Multiply Mn by 2, C by 5. PYQ 3: Balance Fe²⁺ + MnO₄⁻ + H⁺ → Fe³⁺ + Mn²⁺ + H₂O. Solution: Fe(+2→+3, 1e⁻), Mn(+7→+2, 5e⁻). Multiply Fe by 5. These equations appear in CBSE exams regularly. Students who practice half-reaction balancing systematically rarely make errors in timed examinations.

Misconception 1: Redox reactions always involve metals. Reality: Many occur between nonmetals (e.g., H₂ + Cl₂ → 2HCl). Misconception 2: Oxidation number = formal charge. Reality: OS are assigned by convention; formal charge is calculated differently. Misconception 3: Reduction always involves gaining oxygen. Reality: Loss of oxygen is also reduction in older definitions, but electron transfer is the modern, universal definition NCERT emphasizes. Misconception 4: All reactions involving O₂ are redox. Reality: Combustion is redox, but not all O₂ reactions are (e.g., drying agents absorb, not react redox).

CBSE board papers feature 1-mark, 2-mark, 3-mark, and 5-mark questions on redox reactions. 1-mark: Identify OS of an atom in a compound. 2-mark: Distinguish between oxidation and reduction; identify oxidizing/reducing agent. 3-mark: Balance a simple redox equation; write half-reactions; explain applications. 5-mark: Balance complex equations (acidic/basic medium); write balanced half-reactions; state oxidation state changes; identify oxidizing/reducing agents; explain why reaction occurs. Practicing all question types builds exam readiness. Board toppers solve PYQs systematically, timing themselves to manage 3 hours effectively.

Tip 1: Always assign oxidation numbers before balancing; write them above each atom. Tip 2: Clearly state OS before and after; identify what oxidized/reduced. Tip 3: Use half-reaction method for complex equations; show all intermediate steps. Tip 4: In acidic medium, use H⁺ and H₂O; in basic medium, use OH⁻ (NCERT Chapter 7 emphasizes this distinction). Tip 5: Double-check charge and mass balance before finalizing. Tip 6: If asked for oxidizing/reducing agent, name the entire species, not just the atom (e.g., KMnO₄, not Mn). Practice with timed mock tests to internalize these habits.