Question

Write anode and cathode reactions that occur in dry cells. How does a dry cell differ from a mercury cell?

Answer 1

Hint: Oxidation reaction occurs at anode. Zinc metal loses two electrons to form zinc cations. Reduction reaction occurs at cathode. In the presence of ammonium ions, manganese dioxide gains electrons. In mercury cells, mercuric oxide and zinc react in presence of an alkali.

Complete step by step answer:
In dry cells, a chemical reaction generates electricity.
In the dry cell, oxidation reaction occurs at zinc anode to form zinc cations. Zinc atom loses two electrons to form zinc cations. Write the oxidation half reaction.
$$\text{Zn }\to \text{ Z}{\text{n}}^{2+}\text{ + 2 }{\text{e}}^{-}$$
Reduction reaction occurs at graphite cathode. Write the reduction half reaction.
$${\text{2 NH}}_4^{+}\text{ + 2 Mn}{\text{O}}_2\text{ + 2 }{\text{e}}^{-}\to \text{ M}{\text{n}}_{\text{2}}{\text{O}}_{\text{3}}\text{ + 2 N}{\text{H}}_3\text{ + }{\text{H}}_2\text{O}$$
Write the overall cell reaction:
$$\text{Zn(s) + 2Mn}{\text{O}}_{\text{2}}{\text{(s) + 2NH}}_4^{+}\text{(aq)}\to \text{M}{\text{n}}_{\text{2}}{\text{O}}_{\text{3}}\text{(s) + }{\text{H}}_{\text{2}}\text{O(l) + Z}{\text{n}}^{2+}\text{ + 2N}{\text{H}}_{\text{3}}\text{(g)}$$
The voltage obtained from the dry cell is 1.5 volts.
List the differences between Dry cell and mercury cell, in the following table.

Dry cell	Mercury cell
Zinc is converted to zinc chloride. Due to this, zinc casing becomes porous. Due to this, the life span of dry cells is very short.	The concentration of ions does not change during its lifetime. Hence, mercury cells have relatively longer life span.
The potential given decreases with its usage.	The potential remains constant throughout its life.
The anode used is zinc.	The anode used is zinc amalgam.
In the dry cell, a paste is formed by absorbing the electrolyte on a solid. This prevents spilling and increases the ease of handling.	In mercury cells an aqueous alkaline (potassium hydroxide) electrolyte is used. So spilling is possible. This reduces the ease of handling.

Additional Information: Electrolyte in the dry cell is in the form of a paste so as to avoid spillage. Electrolyte is an acid dissolved in water, which generates positive ions (cations) and negative ions (anions). Ions move towards oppositely charged electrodes. Electrodes are metal rods immersed in electrolyte.

Note: In dry cells, paste of ammonium chloride electrolyte is used, so that amount of water can be minimized. Due to this dry cell can be used in any orientation without any spillage. This increases the ease of handling and convenience of its usage. Hence, dry cells can be used into portable equipment.