Match the LIST-I with LIST-II.
| LIST-I Electrolyte | LIST-II Cell/Battery |
| A. Moist paste of NH4Cl and ZnCl2 | I. Fuel cell. |
| B. Paste of KOH and ZnO. | II. Lead storage battery. |
| C. 38% solution of H2SO4. | III. Mercury cell. |
| D. Concentrated aqueous sodium hydroxide solution. | IV. Leclanche cell. |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A-IV, B-III, C-II, D-I
Each commercial cell can be identified by its electrolyte, which is chosen to suit the electrode reactions of that cell.
A. Moist paste of \( \ce{NH4Cl} \) and \( \ce{ZnCl2} \) — This matches IV. Leclanche cell. In this dry cell, the zinc container is the anode and a carbon rod surrounded by powdered \( \ce{MnO2} \) and carbon is the cathode. The space between them is filled with a moist paste of \( \ce{NH4Cl} \) and \( \ce{ZnCl2} \). The cell gives about 1.5 V:
\( \displaystyle \text{Anode: }\ce{Zn(s) -> Zn^{2+} + 2e^{-}} \)
\( \displaystyle \text{Cathode: }\ce{MnO2 + NH4^{+} + e^{-} -> MnO(OH) + NH3} \)
B. Paste of KOH and ZnO. — This matches III. Mercury cell. The anode is a zinc–mercury amalgam and the cathode is a paste of HgO and carbon, with a paste of KOH and ZnO as the electrolyte. The voltage stays constant at about 1.35 V, because no ion in solution takes part in the cell reaction:
\( \displaystyle \text{Anode: }\ce{Zn(Hg) + 2OH^{-} -> ZnO(s) + H2O + 2e^{-}} \)
\( \displaystyle \text{Cathode: }\ce{HgO(s) + H2O + 2e^{-} -> Hg(l) + 2OH^{-}} \)
\( \displaystyle \text{Cell: }\ce{Zn(Hg) + HgO(s) -> ZnO(s) + Hg(l)} \)
C. 38% solution of \( \ce{H2SO4} \). — This matches II. Lead storage battery. Its anode is lead and its cathode is a grid of lead packed with lead dioxide, dipping in a 38% solution of sulphuric acid. On discharge:
\( \displaystyle \text{Anode: }\ce{Pb(s) + SO4^{2-}(aq) -> PbSO4(s) + 2e^{-}} \)
\( \displaystyle \text{Cathode: }\ce{PbO2(s) + SO4^{2-}(aq) + 4H^{+}(aq) + 2e^{-} -> PbSO4(s) + 2H2O(l)} \)
\( \displaystyle \text{Cell: }\ce{Pb(s) + PbO2(s) + 2H2SO4(aq) -> 2PbSO4(s) + 2H2O(l)} \)
Passing current in the opposite direction reverses this reaction, so the battery can be recharged.
D. Concentrated aqueous sodium hydroxide solution. — This matches I. Fuel cell. In the hydrogen–oxygen fuel cell, the two gases are bubbled through porous carbon electrodes into concentrated aqueous sodium hydroxide. It was used in the Apollo space programme:
\( \displaystyle \text{Cathode: }\ce{O2(g) + 2H2O(l) + 4e^{-} -> 4OH^{-}(aq)} \)
\( \displaystyle \text{Anode: }\ce{2H2(g) + 4OH^{-}(aq) -> 4H2O(l) + 4e^{-}} \)
\( \displaystyle \text{Cell: }\ce{2H2(g) + O2(g) -> 2H2O(l)} \)
Shortcut: The pair A–IV appears in Options (1) and (3), and C–II then leaves only Option (1).