Match List-I with List-II. List-I List-II A Cd(s) + 2Ni (OH) _ 3...

(a)$Cd\left(s\right) + 2Ni{\left(OH\right)}_{3}\left(s\right) \to CdO\left(s\right) + 2Ni{\left(OH\right)}_{2}\left(s\right) + {H}_{2}O\left(l\right)$
Discharge of secondary Battery

(b)$Zn\left(Hg\right) + HgO\left(s\right) \to ZnO\left(s\right) + Hg\left(l\right)$
(Primary Battery Mercury cell)

(c) $2{PbSO}_{4}\left(s\right) + 2{H}_{2}O\left(l\right) \to Pb\left(s\right) + {PbO}_{2}\left(s\right) + 2{H}_{2}{SO}_{4}\left(aq\right)$ Charging of secondary Battery

(d) $2{H}_{2}\left(g\right) + {O}_{2}\left(g\right) \to 2{H}_{2}O\left(l\right) -$ Fuel cell

Mercury cell, suitable for low current devices like hearing aids, watches, etc. consists of zinc - mercury amalgam as anode and a paste of $HgO$ and carbon as the cathode. The electrolyte is a paste of $KOH$ and $ZnO$. The electrode reactions for the cell are given below:

Anode: $Zn\left(Hg\right) + 2{OH}^{-} \to ZnO\left(s\right) + {H}_{2}O + 2{e}^{-}$

Cathode: $HgO + {H}_{2}O + 2{e}^{-} \to Hg\left(l\right) + 2{OH}^{-}$

Another important secondary cell is the nickel-cadmium cell which has longer life than the lead storage cell but more expensive to manufacture.

The overall reaction during discharge is:

$Cd\left(s\right) + 2Ni{\left(OH\right)}_{3}\left(s\right) \to CdO\left(s\right) + 2Ni{\left(OH\right)}_{2}\left(s\right) + {H}_{2}O\left(l\right)$

Fuel cell using ${H}_{2}$ and ${O}_{2}$ produces electricity.

Cathode: ${O}_{2}\left(g\right) + 2{H}_{2}O\left(l\right) + 4{e}^{-} \to 4{OH}^{-}\left(aq\right)$

Anode: $2{H}_{2}\left(g\right) + 4{OH}^{-}\left(aq\right) \to 4{H}_{2}O\left(l\right) + 4{e}^{-}$

Overall reaction being:

$2{H}_{2}\left(g\right) + {O}_{2}\left(g\right) \to 2{H}_{2}O\left(l\right)$

(ii) Cell reaction of lead storage battery during recharge -

The reverse of oxidation at anode during discharge is the reduction reaction at cathode during recharging.

${Pb}_{\left(aq\right)}^{2 +} + 2{e}^{-} \to {Pb}_{\left(s\right)}$

${PbSO}_{4\left(s\right)} \to {Pb}_{\left(aq\right)}^{2 +} + {SO}_{4\left(aq\right)}^{2 -}$

Final Reaction: ${PbSO}_{4\left(s\right)} + 2{e}^{-} \to {Pb}_{\left(s\right)} + {SO}_{4\left(aq\right)}^{2 -}$