Match List-I with List-II List-I (Items) List-II (Properties) (A) Pri…

The correct answer is Option (1) → (A)-(IV), (B)-(III), (C)-(I), (D)-(II)

Galvanic cells used as batteries are of two kinds. In a primary battery the reaction occurs only once, and the cell cannot be recharged. A secondary battery can be recharged by passing current through it in the opposite direction. A fuel cell converts the energy of combustion of a fuel such as hydrogen directly into electrical energy. Corrosion is an unwanted electrochemical process on a metal surface.

• (A) Primary batteries — This matches (IV) Mercury cell is used in hearing aids. The mercury cell is a primary cell: zinc-mercury amalgam is the anode and a paste of HgO and carbon the cathode. It gives a constant voltage of about 1.35 V:

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

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

      \( \displaystyle \text{Net: } \ce{Zn(Hg) + HgO(s) -> ZnO(s) + Hg(l)} \)

• (B) Secondary batteries — This matches (III) Large number of charging/discharging and long-lasting. A good secondary cell, such as the lead storage battery, can be discharged and recharged many times; on charging, the discharge reaction is reversed:

      \( \displaystyle \ce{Pb(s) + PbO2(s) + 2H2SO4(aq) <=> 2PbSO4(s) + 2H2O(l)} \)

• (C) Fuel Cells — This matches (I) Pollution free energy converter. In the hydrogen-oxygen fuel cell the only product is water, and the efficiency is about 70%:

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

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

      \( \displaystyle \text{Net: } \ce{2H2(g) + O2(g) -> 2H2O(l)} \)

• (D) Corrosion — This matches (II) Takes place in the presence of air, moisture and \( \ce{H+} \). In rusting, a spot on the iron acts as the anode and oxygen is reduced at another spot in the presence of \( \ce{H+} \) (from \( \ce{H2CO3} \) formed by \( \ce{CO2} \) in water):

      \( \displaystyle \text{Anode: } \ce{2Fe(s) -> 2Fe^2+ + 4e^-} \)

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

      \( \displaystyle \text{Net: } \ce{2Fe(s) + O2(g) + 4H^+(aq) -> 2Fe^2+(aq) + 2H2O(l)} \)

Shortcut: (A)-(IV) appears only in Option (1).