JEE Main 2022 · 29 June · morning · Single correct · +4 / −1
Which one of the following reactions indicates the reducing ability of hydrogen peroxide in basic medium?
- (a) HOCl + H₂O₂ → H₃O⁺ + Cl⁻ + O₂
- (b) PbS + 4H₂O₂ → PbSO₄ + 4H₂O
- (c) 2MnO₄ - + 3H₂O₂ → 2MnO₂ + 3O₂ + 2H₂O + 2OH⁻
- (d) Mn²⁺ + H₂O₂ → Mn⁴⁺ + 2OH⁻
Answer and solution
Answer (c) 2MnO₄ - + 3H₂O₂ → 2MnO₂ + 3O₂ + 2H₂O + 2OH⁻
Solution
Chemistry ma’am · 4 steps
Look for O₂ coming out of H₂O₂, then look for OH⁻ in the equation.
- Reducing agent test. In H₂O₂, oxygen is at -1. When H₂O₂ acts as a reducing agent, it is itself oxidised and its oxygen goes to 0, so O₂ is released. When it acts as an oxidising agent, its oxygen goes to -2 in H₂O or OH⁻, and no O₂ forms.
- Basic medium test. The medium shows up in the equation. OH⁻ among the species means basic medium. H₃O⁺ or H⁺ means acidic medium.
Option What the other species does Role of H₂O₂ Medium (a) Cl goes from +1 to -1 (reduced) reducing, O₂ released acidic (H₃O⁺) (b) S goes from -2 to +6 (oxidised) oxidising neutral (c) Mn goes from +7 to +4 (reduced) reducing, O₂ released basic (OH⁻) (d) Mn goes from +2 to +4 (oxidised) oxidising basic (OH⁻) - Check electrons in (c). Each Mn gains 3 electrons, so 2 Mn gain 6. Each H₂O₂ loses 2 electrons to form O₂, so 3 H₂O₂ lose 6. The transfer balances, and the charge is -2 on both sides.
- So. Only (c) passes both tests. H₂O₂ reduces MnO₄⁻ to MnO₂ and gives off O₂, and the OH⁻ formed marks basic medium.
The idea to keep. Spot the role of H₂O₂ from its products: O₂ out means it is reducing, H₂O or OH⁻ only means it is oxidising. Then read the medium from H⁺ or OH⁻.
Answer from the NTA final answer key. Solution by PrepWiser.