Question

Explain the dry cell with diagram?

Answer 1

Dry Cell : It is a primary cell based on Leclanche cell invented by G. Leclanche in 1868. In a primary cell, the electrode reactions cannot be reversed by an external source of electrical energy. In this cell, the cell reaction takes place only once i.e., this cell is not rechargeable. It is.generally used intorched, transistors, radios, calculators, tape recorders etc. It consists of a hollow zinc cylinder which is filled with a paste of $N{H}_{4}Cl$ nad a little $ZnC{l}_2$. This paste is made with the help of water. The zinc cylinder acts as anode while cathode is a graphite rod (Carbon). The carbon rod is surrounded by a black paste of $Mn{O}_2$ and carbon powder. The zinc cylinder has an outer insulation of cardboard case.
Dry cells are sealed with wax or other material to protect the moisture from evaporation. When the electrodes are connected, the cell operates.
The electrode reactions are complex. Metallic $Zn$ is oxidized to $Z{n}^{2+}$ and the electrons liberated are left on the container. The reactions which take place at electrodes can be represented as :
At anode : $Z{n}_{\left(s\right)}\to Z{n}_{\left(aq\right)}^{2+}+2e^{-}$ (Oxidation)
At cathode : $\underset{–}{2N{H}_{4\left(aq\right)}^{+}+2Mn{O}_{2\left(s\right)}+3e-\to M{n}_2{O}_{3\left(s\right)}+H_2{O}_{\left(I\right)}+2N{H}_{3\left(g\right)}\left(Reduction\right)}$
Overall reaction :
$Z{n}_{\left(s\right)}+2N{H}_{4\left(aq\right)}^{+}+2Mn{O}_{2\left(s\right)}Z{n}_{\left(aq\right)}^{2+}+M{n}_2{O}_{3\left(s\right)}+H_2{O}_{\left(I\right)}+2N{H}_{3\left(g\right)}$
The ammoia gas produced at the cathode, combines with $Zn2+$ ions to form a soluble compound containing ion $\left[Zn\right(N{H}_3{)}_4{]}^{2+}$.
$Z{n}_{\left(aq\right)}^{2+}+4N{H}_{3\left(g\right)}\to \left[Zn\right(N{H}_3{)}_4{]}_{\left(aq\right)}^{2+}$
This reaction prevents polarization due to formation of ammonia. It also prevents the substantial increase of concentration of $Z{n}^{2+}$ ions which would decrease the cell potential. This potential of dry cell is approximately 1.5V.
Defect : Due to acidic nature of $N{H}_{4}Cl$ zinc container corrodes due to which holes develop through which the chemicals come out.
Now a day, the cells are made leakage resistant. In it $KOH$ is used in place of $N{H}_{4}Cl$ by which zinc does not corrode.