In a primary dry cell, choose the incorrect statement.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Mn is oxidized from +4 to +6 oxidation state.
In the dry cell (Leclanché cell) zinc is oxidised at the anode and manganese dioxide is reduced at the cathode:
\( \displaystyle \text{Anode:} \quad \ce{Zn(s) -> Zn^2+ + 2e^-} \)
\( \displaystyle \text{Cathode:} \quad \ce{MnO2 + NH4^+ + e^- -> MnO(OH) + NH3} \)
The oxidation state of Mn changes as follows:
\( \displaystyle \ce{MnO2}: \; x + 2(-2) = 0 \Rightarrow x = +4 \qquad \ce{MnO(OH)}: \; x + (-2) + (-1) = 0 \Rightarrow x = +3 \)
At the cathode manganese gains an electron and is reduced from +4 to +3; it is not oxidised, and it never reaches +6.
Option (1) Mn is oxidized from +4 to +6 oxidation state. — Incorrect about the dry cell. Manganese is reduced from +4 to +3; it is not oxidised and never reaches +6.
Option (2) Zinc container acts as anode. — Correct about the dry cell. Zinc is oxidised to \( \ce{Zn^2+} \), and oxidation takes place at the anode.
Option (3) Mn is reduced from +4 to +3 oxidation state. — Correct about the dry cell. \( \ce{MnO2} \) (+4) gains one electron and becomes \( \ce{MnO(OH)} \) (+3).
Option (4) Ammonia forms a complex with Zn. — Correct about the dry cell. The ammonia formed at the cathode combines with the zinc ions:
\( \displaystyle \ce{Zn^2+ + 4NH3 -> [Zn(NH3)4]^2+} \)
NCERT (2026-27 edition):
Primary Batteries
