Question:
What happens in the oxidation number of Mn during working of dry cell.
What happens in the oxidation number of Mn during working of dry cell.
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In the Leclanche dry cell manganese dioxide is reduced to Mn2O3.
Updated On: Oct 1, 2026
- Increases by 1
- decreases by 1
- Increases by 2
- decreases by 2
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The Correct Option is B
Solution and Explanation
Step 1: Understanding the Concept:
In a dry (Leclanche) cell, zinc is the anode and is oxidised. Manganese dioxide mixed with carbon at the cathode is reduced.
Step 2: Key Formula or Approach:
Cathode reaction: \(\text{MnO}_2 + \text{NH}_4^+ + e^- \to \text{MnO(OH)} + \text{NH}_3\). The overall form can also be written as \(2\text{MnO}_2 \to \text{Mn}_2\text{O}_3\).
Step 3: Detailed Explanation:
Oxidation number of Mn in \(\text{MnO}_2\): \(x + 2(-2) = 0\), so \(x = +4\).
Oxidation number of Mn in \(\text{Mn}_2\text{O}_3\): \(2x + 3(-2) = 0\), so \(x = +3\).
\[ +4 \to +3 \]
So the oxidation number of Mn decreases by \(1\). It is reduced by one electron per Mn atom. An increase would mean oxidation, but Mn is at the cathode and gains electrons.
Final Answer:
Mn goes from \(+4\) to \(+3\), a decrease of \(1\), option (B). \[ \boxed{\text{Decreases by } 1} \]
In a dry (Leclanche) cell, zinc is the anode and is oxidised. Manganese dioxide mixed with carbon at the cathode is reduced.
Step 2: Key Formula or Approach:
Cathode reaction: \(\text{MnO}_2 + \text{NH}_4^+ + e^- \to \text{MnO(OH)} + \text{NH}_3\). The overall form can also be written as \(2\text{MnO}_2 \to \text{Mn}_2\text{O}_3\).
Step 3: Detailed Explanation:
Oxidation number of Mn in \(\text{MnO}_2\): \(x + 2(-2) = 0\), so \(x = +4\).
Oxidation number of Mn in \(\text{Mn}_2\text{O}_3\): \(2x + 3(-2) = 0\), so \(x = +3\).
\[ +4 \to +3 \]
So the oxidation number of Mn decreases by \(1\). It is reduced by one electron per Mn atom. An increase would mean oxidation, but Mn is at the cathode and gains electrons.
Final Answer:
Mn goes from \(+4\) to \(+3\), a decrease of \(1\), option (B). \[ \boxed{\text{Decreases by } 1} \]
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