What are anode and cathode for Leclanche’ dry cell ? Write electr…

Question

What are anode and cathode for Leclanche’ dry cell ? Write electrode reactions and overall cell reaction when it generates electricity.

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Answer

A dry cell has zinc vessel as anode and graphite rod as cathode and moist paste of $ZnCl _2, MnO _2, NH _4 Cl$ as electrolytes.
At anode:
$Zn _{( s )} \rightarrow Zn _{( aq )}^{2+}+2 e ^{-}$(Oxidation half reaction)
At graphite (c) cathode:
$2 NH _{4( e )}^{+}+2 e ^{-} \rightarrow 2 NH _{3( aq )}+ H _{2( g )}$ (Reduction half reaction)
$2 MnO _{2( s )}+ H _2 \rightarrow Mn _2 O _{3( s )}+ H _2 O _{(1)}$
There is a side reaction inside the cell, between $Zn ^{2+}$ ions and aqueous $NH _3$.
$
Zn _{( aq )}^{2+}+4 NH _{3( aq )} \longrightarrow\left[ Zn \left( NH _3\right)_4\right]_{( aq )}^{2+}
$

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