MHT CET · Chemistry · Electrochemistry

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 \(\mathrm{PbSO}_{4(\mathrm{~s})}+2 \mathrm{H}_2 \mathrm{O}_{(0)} \longrightarrow \mathrm{PbO}_{2(\mathrm{~s})}+4\mathrm{H}_{(\mathrm{aq})}^{+}\) \(
+\mathrm{SO}_{4(\mathrm{xq})}^{--}+2 \mathrm{e}^{-}
\)
C \(\mathrm{Pb}_{(8)}+\mathrm{SO}_{4(\mathrm{aq})}^{--} \longrightarrow \mathrm{PbSO}_{4(\mathrm{~s})}+2 \mathrm{e}^{-}\)
D \(\quad \mathrm{PbSO}_{4(\mathrm{~s})}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Pb}_{(\mathrm{s})}+\mathrm{SO}_{4(\mathrm{mp})}^{--}\)

Verified Solution

Answer & Solution

Correct Answer

(C) \(\mathrm{Pb}_{(8)}+\mathrm{SO}_{4(\mathrm{aq})}^{--} \longrightarrow \mathrm{PbSO}_{4(\mathrm{~s})}+2 \mathrm{e}^{-}\)

Step-by-step Solution

Detailed explanation

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.
\(
\begin{aligned}
& \mathrm{Pb}_{(\mathrm{s})} \longrightarrow \mathrm{Pb}_{(\text {(4) }}^{2+}+2 \mathrm{e}^{-} \text { (oxidation) } \\
& \mathrm{Pb}_{(\text {(aq) }}^{2+}+\mathrm{SO}_{4(\text { (4) })}^{2-} \longrightarrow \mathrm{PbSO}_{4(\mathrm{~s})}\text {(precipitation) } \\
& \hline \mathrm{Pb}_{(\mathrm{s})}+\mathrm{SO}_{4(\text { (aq) }}^{2-} \longrightarrow \mathrm{PbSO}_{4(\mathrm{~s})}+2 \mathrm{e}^{-}
\end{aligned}
\)
(Overall oxidation)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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