Q21 / 150 · MHT CET 2023 (13 May Shift 1) · Chemistry
Identify the overall oxidation reaction that occurs in lead storage cell during discharge.
- A$\mathrm{Pb}_{(\mathrm{aq})}^{++}+\mathrm{SO}_{4(\mathrm{sq})}^{--} \longrightarrow \mathrm{PbSO}_{4(\mathrm{~s})}$
- B$\mathrm{PbSO}_{4(\mathrm{~s})}+2 \mathrm{H}_2 \mathrm{O}_{(0)} \longrightarrow \mathrm{PbO}_{2(\mathrm{~s})}+4\mathrm{H}_{(\mathrm{aq})}^{+}$ $$ +\mathrm{SO}_{4(\mathrm{xq})}^{--}+2 \mathrm{e}^{-} $$
- Correct answerC$\mathrm{Pb}_{(8)}+\mathrm{SO}_{4(\mathrm{aq})}^{--} \longrightarrow \mathrm{PbSO}_{4(\mathrm{~s})}+2 \mathrm{e}^{-}$
- D$\quad \mathrm{PbSO}_{4(\mathrm{~s})}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Pb}_{(\mathrm{s})}+\mathrm{SO}_{4(\mathrm{mp})}^{--}$
Solution
When the lead storage cell provides current (i.e., during discharge), spongy lead ( $\mathrm{Pb}$ ) is oxidised to $\mathrm{Pb}^{2+}$ ions and negative charge accumulates on lead plates. The $\mathrm{Pb}^{2+}$ ions so formed combine with $\mathrm{SO}_4^{2-}$ ions from $\mathrm{H}_2 \mathrm{SO}_4$ to form insoluble $\mathrm{PbSO}_4$. The overall oxidation is the sum of these two processes.
$$
\begin{aligned}
& \mathrm{Pb}_{(\mathrm{s})} \longrightarrow \mathrm{Pb}_{(\text {(4) }}^{2+}+2 \mathrm{e}^{-} \quad \text { (oxidation) } \\
& \mathrm{Pb}_{(\text {(aq) }}^{2+}+\mathrm{SO}_{4(\text { (4) })}^{2-} \longrightarrow \mathrm{PbSO}_{4(\mathrm{~s})} \quad \text { (precipitation) } \\
& \hline \mathrm{Pb}_{(\mathrm{s})}+\mathrm{SO}_{4(\text { (aq) }}^{2-} \longrightarrow \mathrm{PbSO}_{4(\mathrm{~s})}+2 \mathrm{e}^{-}
\end{aligned}
$$
(Overall oxidation)
Hence, the correct answer is option: 'C'.