Q16 / 150 · MHT CET 2025 (20 Apr Shift 2) · Chemistry
Which of the following change takes place at positive electrode during working of dry cell?
- A$\mathrm{Zn}_{(\mathrm{s})}$ is oxidised to $\mathrm{Zn}^{+2}{ }_{(\mathrm{aq})}$
- B$\mathrm{Zn}^{+2}{ }_{(\mathrm{aq})}$ is reduced to $\mathrm{Zn}_{(\mathrm{s})}$
- Correct answerC$\mathrm{MnO}_{2(\mathrm{~s})}$ is reduced to $\mathrm{Mn}_2 \mathrm{O}_{3(\mathrm{~s})}$
- D$\mathrm{Mn}_2 \mathrm{O}_{3(\mathrm{~s})}$ is oxidised to $\mathrm{MnO}_{2(\mathrm{~s})}$
Solution
Dry Cell (Leclanché Cell) Reactions
A dry cell uses zinc as the anode and a carbon rod surrounded by
$\mathrm{MnO_2}$, carbon powder, and $\mathrm{NH_4Cl}$ as the cathode.
At the anode (negative electrode), zinc undergoes oxidation:
$\mathrm{Zn}_{(s)} \rightarrow \mathrm{Zn}^{2+}_{(aq)} + 2\mathrm{e}^-$
At the cathode (positive electrode), manganese dioxide is reduced:
$2\mathrm{MnO}_{2(s)} + 2\mathrm{NH}^{+}_{4(aq)} + 2\mathrm{e}^- \rightarrow \mathrm{Mn}_2\mathrm{O}_{3(s)} + 2\mathrm{NH}_{3(aq)} + \mathrm{H}_2\mathrm{O}_{(l)}$
Manganese in $\mathrm{MnO_2}$ has an oxidation state of +4, while in $\mathrm{Mn_2O_3}$ it is +3.
The decrease in oxidation number confirms reduction is occurring at the positive electrode.
The correct change is: $\mathrm{MnO}_{2(s)}$ reduced to $\mathrm{Mn}_2\mathrm{O}_{3(s)}$.
Hence, the correct answer is option: 'C'.