Match List-I with List-II
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List-I Cell |
List-II Reaction |
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(A) Fuel Cell |
(I) Anode: \( Zn(s) → Zn^{2+} + 2e^- \) Cathode: \( MnO_2 + NH_4^+ + e^- → MnO(OH) + NH_3 \) |
|
(B) A secondary cell |
(II) Cathode: \( O_2 (g) + 2H_2O(l) + 4e^- → 4OH^- (aq) \) Anode: \( 2H_2 (g) + 4OH^-(aq) → 4H_2O(l) + 4e^- \) |
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(C) Mercury cells |
(III) \( Pb(s) + PbO_2 (s) + 2H_2SO_4 (aq) → 2PbSO_4 (s) + 2H_2O(l) \) |
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(D) Leclanche cell (Simplified) |
(IV) \( Zn(Hg) + HgO(s) → ZnO(s) + Hg(l) \) |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A)-(II), (B)-(III), (C)-(IV), (D)-(I)
Each cell is identified by the reactions at its electrodes. A primary cell (dry cell, mercury cell) cannot be recharged once the reaction is over; a secondary cell can be recharged by passing current in the opposite direction; a fuel cell turns the energy of burning a fuel such as hydrogen directly into electricity, with the reactants supplied continuously.
(A) Fuel Cell — This matches (II). In the hydrogen–oxygen fuel cell, with concentrated KOH as electrolyte, hydrogen is oxidised at the anode and oxygen reduced at the cathode; the cell reaction is the combination of hydrogen and oxygen:
\( \displaystyle \ce{2H2(g) + O2(g) -> 2H2O(l)} \)
(B) A secondary cell — This matches (III). The lead storage battery is the common secondary cell. On discharge both electrodes turn into \( \ce{PbSO4} \); on charging this reaction is reversed:
\( \displaystyle \ce{Pb(s) + PbO2(s) + 2H2SO4(aq) -> 2PbSO4(s) + 2H2O(l)} \)
(C) Mercury cells — This matches (IV). The anode is zinc–mercury amalgam and the cathode a paste of HgO and carbon, in KOH:
\( \displaystyle \text{Anode: } \ce{Zn(Hg) + 2OH^- -> ZnO(s) + H2O + 2e^-} \)
\( \displaystyle \text{Cathode: } \ce{HgO(s) + H2O + 2e^- -> Hg(l) + 2OH^-} \)
No ion appears in the cell reaction, so the cell gives a constant voltage (about 1.35 V) throughout its life.
(D) Leclanche cell (Simplified) — This matches (I). In the dry cell the zinc container is the anode and a carbon rod in \( \ce{MnO2} \) and \( \ce{NH4Cl} \) paste is the cathode; at the cathode manganese is reduced from +4 to +3. The ammonia formed combines with \( \ce{Zn^2+} \) to give \( \ce{[Zn(NH3)4]^2+} \).
Shortcut: (A)-(II), the hydrogen–oxygen reactions for the fuel cell, appears only in Option (2).