Question

Explain the construction and working of dry cell.

Answer 1

1. A dry cell is an electrochemical cell in which electrolyte is in form of paste.
2. Construction: It consists of a brass cap, airtight seal, electrolyte, carbon rod which is a positive electrode, and zinc container which is a negative electrode as shown in the figure below

1. A zinc cylindrical container consists of a chemical in form of paste. The electrolyte is a moist paste of ammonium chloride $\left({\mathrm{NH}}_4\mathrm{CI}\right)$, and zinc chloride ${\mathrm{ZnCl}}_2$ with magnesium dioxide ${\mathrm{MnO}}_2$ and powdered carbon present in it. A carbon rod is placed at the center of the cell surrounded by the chemicals. A zinc container is a negative electrode on which negative charges are accumulated and the carbon rod at the center is the positive electrode where positive charges are accumulated. A brass cap is present above the carbon rod to improve the electrical conductivity and the top of the cylindrical container is sealed and made airtight.
2. Working: When a dry cell is externally connected to a wire following reaction takes place

Zinc atoms from the container lose electrons to external wire and produce ion $\underset{\left(\mathrm{Zinc}\right)}{\mathrm{Zn}}\to \underset{(\mathrm{Zinc}\mathrm{ions})}{{\mathrm{Zn}}^{2+}}+\underset{\left(\mathrm{electrons}\right)}{2{\mathrm{e}}^{-}}$

This electron flows through the wire to a cell which causes an electric current. These electrons combine with ammonium ions

$$\begin{gathered} 2{{\mathrm{NH}}_4}^{+}+2{\mathrm{e}}^{-}\to 2{\mathrm{NH}}_3+{\mathrm{H}}_2 \\ (\mathrm{Ammonium}\mathrm{ions})\left(\mathrm{Electrons}\right)\left(\mathrm{Ammomnia}\right)(\mathrm{Hydrogen}\mathrm{gas}) \end{gathered}$$

${\mathrm{NH}}_3$ and ${\mathrm{H}}_2$ are in form of ions. This hydrogen gas reacts with $\mathrm{Mn}{\mathrm{O}}_3$ to form magnesium oxide ${\mathrm{Mn}}_2{\mathrm{O}}_3$ and water

$$\begin{gathered} 2\mathrm{Mn}{\mathrm{O}}_2+{\mathrm{H}}_2\to {\mathrm{Mn}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O} \\ (\mathrm{Magnesium}\mathrm{dioxide})(\mathrm{Hydrogen}\mathrm{gas})(\mathrm{Magnesium}\mathrm{oxide})\left(\mathrm{Water}\right) \end{gathered}$$

The zinc ions formed in the electrolyte react with chlorine ions originating from ammonium chloride to form zinc chloride.

$$\begin{gathered} {\mathrm{Zn}}^{2+}+2{\mathrm{Cl}}^{-}\to {\mathrm{ZnCl}}_2 \\ (\mathrm{Zinc}\mathrm{ions})(\mathrm{Chlorine}\mathrm{ions})(\mathrm{Zinc}\mathrm{chloride}) \end{gathered}$$

The powdered carbon present in the electrolyte enhances the flow of charge.