Q.56 In acidic solution, permanganate ion is reduced by ferrous ion. The number of electrons involved in the reduction of permanganate ion is _______.
The number of electrons involved in the reduction of permanganate ion by ferrous ion in acidic solution is 5.
This classic redox titration reaction, common in CSIR NET Life Sciences preparation, follows the balanced half-reaction where permanganate (MnO4−) reduces to Mn2+.
Permanganate ion reduction by ferrous ion in acidic solution represents a fundamental redox process in analytical chemistry, where MnO4− acts as a strong oxidizing agent. In this reaction, manganese changes oxidation state from +7 to +2, gaining 5 electrons per MnO4− ion.
Half-Reaction Balancing
Balance the reduction half-reaction step-by-step in acidic medium:
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Start: MnO4−→Mn2+
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Balance Mn: already balanced
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Balance O with H2O: MnO4−→Mn2++4H2O
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Balance H with H+: MnO4−+8H+→Mn2++4H2O
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Balance charge with electrons: left charge +7, right +2, so add 5e− to left
Final: MnO4−+8H++5e−→Mn2++4H2O
Thus, 5 electrons are involved in permanganate ion reduction.
Full Redox Reaction
Ferrous ion oxidation: Fe2+→Fe3++e−
Combine with reduction (multiply Fe by 5): MnO4−+5Fe2++8H+→Mn2++5Fe3++4H2O
Medium-Dependent Reduction
| Medium | Product | Electrons | Half-Reaction Summary |
|---|---|---|---|
| Acidic | Mn2+ | 5 | MnO4−+8H++5e−→Mn2++4H2O |
| Neutral/Alkaline | MnO2 | 3 | MnO4−+2H2O+3e−→MnO2+4OH− |
| Alkaline (initial) | MnO42− | 1 | MnO4−+e−→MnO42− |
The question specifies acidic solution with ferrous ion, confirming Mn2+ product and 5 electrons.
CSIR NET Relevance
This appears in CSIR NET as fill-in-the-blank (Q.56), testing half-reaction balancing and electron count. No options provided; direct answer is 5. Common error: confusing with neutral medium (3 electrons).
