JEE Main 2022 — Chemistry Physical Chemistry

(a)$\mathrm{Cd}(s)+2\mathrm{Ni}{(\mathrm{OH})}_{3}(s)\rightarrow \mathrm{CdO}(s)+2\mathrm{Ni}{(\mathrm{OH})}_{2}(s)+{H}_{2}O(l)$Cd(s)+2Ni(OH)3​(s)→CdO(s)+2Ni(OH)2​(s)+H2​O(l)

Discharge of secondary Battery

(b)$\mathrm{Zn}(\mathrm{Hg})+\mathrm{HgO}(s)\rightarrow \mathrm{ZnO}(s)+\mathrm{Hg}(l)$Zn(Hg)+HgO(s)→ZnO(s)+Hg(l)

(Primary Battery Mercury cell)

(c) $2{\mathrm{PbSO}}_{4}(s)+2{H}_{2}O(l)\rightarrow \mathrm{Pb}(s)+{\mathrm{PbO}}_{2}(s)+2{H}_{2}{\mathrm{SO}}_{4}(\mathrm{aq})$2PbSO4​(s)+2H2​O(l)→Pb(s)+PbO2​(s)+2H2​SO4​(aq) Charging of secondary Battery

(d) $2{H}_{2}(g)+{O}_{2}(g)\rightarrow 2{H}_{2}O(l)-$2H2​(g)+O2​(g)→2H2​O(l)− 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 $\mathrm{HgO}$HgO and carbon as the cathode. The electrolyte is a paste of $\mathrm{KOH}$KOH and $\mathrm{ZnO}$ZnO. The electrode reactions for the cell are given below:

Anode: $\mathrm{Zn}(\mathrm{Hg})+2{\mathrm{OH}}^{-}\rightarrow \mathrm{ZnO}(s)+{H}_{2}O+2{e}^{-}$Zn(Hg)+2OH−→ZnO(s)+H2​O+2e−

Cathode: $\mathrm{HgO}+{H}_{2}O+2{e}^{-}\rightarrow \mathrm{Hg}(l)+2{\mathrm{OH}}^{-}$HgO+H2​O+2e−→Hg(l)+2OH−

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:

$$\mathrm{Cd}(s)+2\mathrm{Ni}{(\mathrm{OH})}_{3}(s)\rightarrow \mathrm{CdO}(s)+2\mathrm{Ni}{(\mathrm{OH})}_{2}(s)+{H}_{2}O(l)$$Cd(s)+2Ni(OH)3​(s)→CdO(s)+2Ni(OH)2​(s)+H2​O(l)

Fuel cell using ${H}_{2}$H2​ and ${O}_{2}$O2​ produces electricity.

Cathode: ${O}_{2}(g)+2{H}_{2}O(l)+4{e}^{-}\rightarrow 4{\mathrm{OH}}^{-}(\mathrm{aq})$O2​(g)+2H2​O(l)+4e−→4OH−(aq)

Anode: $2{H}_{2}(g)+4{\mathrm{OH}}^{-}(\mathrm{aq})\rightarrow 4{H}_{2}O(l)+4{e}^{-}$2H2​(g)+4OH−(aq)→4H2​O(l)+4e−

Overall reaction being:

$$2{H}_{2}(g)+{O}_{2}(g)\rightarrow 2{H}_{2}O(l)$$2H2​(g)+O2​(g)→2H2​O(l)

(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.

$${\mathrm{Pb}}_{(\mathrm{aq})}^{2+}+2{e}^{-}\rightarrow {\mathrm{Pb}}_{(s)}$$Pb(aq)2+​+2e−→Pb(s)​

$${\mathrm{PbSO}}_{4(s)}\rightarrow {\mathrm{Pb}}_{(\mathrm{aq})}^{2+}+{\mathrm{SO}}_{4(\mathrm{aq})}^{2-}$$PbSO4(s)​→Pb(aq)2+​+SO4(aq)2−​

Final Reaction: ${\mathrm{PbSO}}_{4(s)}+2{e}^{-}\rightarrow {\mathrm{Pb}}_{(s)}+{\mathrm{SO}}_{4(\mathrm{aq})}^{2-}$PbSO4(s)​+2e−→Pb(s)​+SO4(aq)2−​