Question
Describe about the working principle of Leclanche cell.
✓
Answer
1. Leclanche cell:
Anode: Zinc container
Cathode: Graphite rod in contact with $MnO _2$
Electrolyte: Ammonium chloride and Zinc chloride in water.
emf of the cell $=1.5 V$
2. Cell reaction:
Oxidation at anode
$Zn _{( s )} \rightarrow Zn ^{2+}{ }_{( aq )}+2 e _{-}$
Reduction at cathode
$
2 NH ^{+}{ }_{4( aq )}+2 e ^{-} \rightarrow 2 NH _{3( aq )}+ H _{2( aq )}
$
3. The hydrogen gas is oxidised to water by $MnO _2$
$
H _{2( g )}+2 MnO _{2( s )} \rightarrow Mn _2 O _{3( s )}+ H _2 O _{(1)} \ldots \ldots . \text { (3) }
$
Adding equations 1,2,3 the overall redox reaction
$
Zn _{( s )}+2 NH ^{+}{ }_{4( aq )}+2 MnO _{2( s )} \rightarrow Zn ^{2+}{ }_{( aq )}+ Mn _2 O _{3( s )}+ H _2 O _{(1)}+2 NH _3
$
4. The ammonia produced at the cathode combines with $Zn ^{2+}$ to form a complex ion [ $Zn$ $\left.\left( NH _3\right)_4\right]_{2+}$ (aq). As the reaction proceeds, the concentration of $NH _4{ }^{+}$will decrease and the aqueous $NH _3$ will increase which lead to the decrease in the emf of the cell.
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