Match the LIST-I with LIST-II.LIST-ICell/Electrode reactionLIST-IIRea…

The correct answer is Option (2) → A-III, B-I, C-IV, D-II

Electrochemical devices are grouped by how their reaction is used:

• Primary battery: the reaction occurs only once and cannot be reversed; once the reactants are used up, the cell is dead (dry cell, mercury cell).

• Secondary battery: the reaction can be reversed by passing current in the opposite direction, so the battery can be recharged (lead storage battery, nickel–cadmium cell).

• Fuel cell: converts the energy of combustion of a fuel such as H₂ directly into electricity; the reactants are supplied continuously.

• Reference electrode: its potential is fixed and all other electrodes are measured against it; the standard hydrogen electrode is given the value 0 V.

• A. Zn(Hg) + HgO(s) → ZnO(s) + Hg(l) — This matches III. Primary battery. This is the cell reaction of the mercury cell, used in watches and hearing aids. It gives a steady 1.35 V and cannot be recharged.

      \( \displaystyle \text{Anode: }\ce{Zn(Hg) + 2OH^- -> ZnO(s) + H2O + 2e^-} \)

      \( \displaystyle \text{Cathode: }\ce{HgO(s) + H2O + 2e^- -> Hg(l) + 2OH^-} \)

• B. 2H₂(g) + O₂(g) → 2H₂O(l) — This matches I. Fuel cell. In the H₂–O₂ fuel cell, hydrogen and oxygen are bubbled through porous carbon electrodes into concentrated NaOH solution, and the only product is water.

      \( \displaystyle \text{Anode: }\ce{2H2(g) + 4OH^-(aq) -> 4H2O(l) + 4e^-} \)

      \( \displaystyle \text{Cathode: }\ce{O2(g) + 2H2O(l) + 4e^- -> 4OH^-(aq)} \)

• C. Pb(s) + PbO₂(s) + 2H₂SO₄(aq) → 2PbSO₄(s) + 2H₂O(l) — This matches IV. Secondary battery. This is the discharge reaction of the lead storage battery. On charging, the reaction runs backwards and \( \ce{PbSO4} \) is converted back into Pb and \( \ce{PbO2} \). On discharge:

      \( \displaystyle \text{Anode: }\ce{Pb(s) + SO4^2-(aq) -> PbSO4(s) + 2e^-} \)

      \( \displaystyle \text{Cathode: }\ce{PbO2(s) + SO4^2-(aq) + 4H^+(aq) + 2e^- -> PbSO4(s) + 2H2O(l)} \)

• D. H⁺(aq) + e⁻ → ½H₂(g) — This matches II. Reference electrode. This is the half reaction of the standard hydrogen electrode, whose E° is taken as 0 V at all temperatures.

Shortcut: The pair C–IV appears only in Option (2).