COMEDK 2025 Evening Solved Paper

Concept:Identify the incorrect half‑cell reactions in the given electrochemical cells.Explanation:The cathode reaction shown for the Lead‑Storage Cell (A) is wrong.In a lead‑acid storage cell, the correct cathode reaction on discharge is:$\mathrm{PbO}_2 + 4\mathrm{H}^+ + \mathrm{SO}_4^{2-} + 2e^- \rightarrow \mathrm{PbSO}_4 + 2\mathrm{H}_2\mathrm{O}$.The reaction given in the table ($\mathrm{Pb}+2\mathrm{SO}_4^{2-}+4\mathrm{H}^++2e^-\rightarrow \mathrm{PbSO}_4+2\mathrm{H}_2\mathrm{O}+\mathrm{SO}_2$) is incorrect because it involves Pb instead of PbO₂ and produces SO₂.The cathode reaction shown for the Leclanché Cell (B) is also incorrect.In a Leclanché (dry) cell, the correct reduction is:$\mathrm{MnO}_2 + \mathrm{NH}_4^+ + e^- \rightarrow \mathrm{MnOOH} + \mathrm{NH}_3$.The given reaction ($\mathrm{MnO}_2+\mathrm{NH}_4^++2e^-\rightarrow \mathrm{MnO}+\mathrm{OH}+\mathrm{NH}_3$) is wrong because it uses 2 electrons and produces MnO and OH instead of MnOOH.The anode reaction of the Mercury Cell (C) and the cathode reaction of the Fuel Cell (D) are correctly written as per standard electrochemical reactions.Answer:A & B (Option D).