Chapter 20.2, Problem 15SSC

Interpretation Introduction

Interpretation: The component that gets reduced and oxidized in a zinc-carbon dry cell battery is to be identified and the advantage of alkaline dry cell battery over earlier used dry cell battery is to be explained.

Concept introduction: Electrochemical cell is a device used to convert chemical energy produced in redox reaction into electrical energy. A battery is a series of electrochemical cells connected in a series that produces an electric current.

There are majorly three types of commercial cells which are as follows:

1. Primary cells
2. Secondary cells
3. Fuel cells

Primary cells are those in which redox reaction occurs once and then they cannot be used again, become dead. Example: Dry cell.

Secondary cells are those in which can be recharged again and again and can be re-used. An example is the lead storage battery.

Fuel cells are those in which electrical energy is produced by the combustion of fuels.

Answer to Problem 15SSC

In zinc-carbon dry cell battery, zinc gets oxidized to ${\text{Zn}}^{\text{2+}}$ ion and $\text{Mn}$ is reduced from oxidation state +4 to +3.

The advantage of an alkaline dry cell over earlier used dry cell is that they do not need a graphite rod as a cathode so, can be made smaller for use and also in an earlier dry cell at cathode ammonia is released on reduction which makes the battery no longer useful.

Explanation of Solution

A dry cell is an electrochemical cell mostly used in clocks and transistors. It consists of a cylindrical zinc container which acts as an anode. A graphite rod in mid of it, acts as a cathode, without touching base. The gap between the anode and cathode is packed with ${\text{NH}}_{\text{4}}\text{Cl}$ and ${\text{ZnCl}}_2$ and graphite rod is covered on the periphery by powdered ${\text{MnO}}_{\text{2}}$ and carbon.

The reactions that occur at the anode is as follows:

  $\text{Zn}\left(\text{s}\right)\to {\text{Zn}}^{\text{2+}}\left(\text{aq}\right)+2{\text{e}}^{-}$

The reactions that occur at the cathode is as follows:

  ${\text{2MnO}}_2\left(\text{s}\right)+2{\text{NH}}_4^{+}\left(\text{aq}\right)+2{\text{e}}^{-}\to {\text{Mn}}_2{\text{O}}_3\left(\text{s}\right)+2{\text{NH}}_3\left(\text{g}\right)+{\text{H}}_{\text{2}}\text{O}$

Here $\text{Mn}$ is reduced from oxidation state +4 to +3.

In an alkaline dry cell, the powdered $\text{Zn}$ that is present provides a larger surface area for reaction to occur. $\text{Zn}$ is mixed with $\text{KOH}$ paste and is caged in a steel case. In this cell, the cathode is ${\text{MnO}}_{\text{2}}$ , also mixed with $\text{KOH}$ .

The reactions that occur at the anode is as follows:

  $\text{Zn}\left(\text{s}\right)+2{\text{OH}}^{\text{-}}\left(\text{aq}\right)\to \text{ZnO}\left(\text{s}\right)+{\text{H}}_{\text{2}}\text{O}\left(\text{l}\right)+2{\text{e}}^{-}$

The reactions that occur at the cathode is as follows:

  ${\text{MnO}}_2\left(\text{s}\right)+2{\text{H}}_{\text{2}}\text{O}\left(\text{s}\right)+2{\text{e}}^{-}\to \text{Mn}{\left(\text{OH}\right)}_2\left(\text{s}\right)+2{\text{OH}}^{-}\left(\text{aq}\right)$

The advantage of an alkaline dry cell over earlier used dry cell is that they do not need a graphite rod as a cathode so it can be made smaller for use, and also in an earlier dry cell at cathode, ammonia is released on reduction which makes the battery no longer useful.

Conclusion

In zinc-carbon dry cell battery, zinc gets oxidized to ${\text{Zn}}^{\text{2+}}$ ion and $\text{Mn}$ is reduced from oxidation state +4 to +3.

The advantage of an alkaline dry cell over earlier used dry cell is that they do not need a graphite rod as a cathode so it can be made smaller for use, and also in an earlier dry cell at cathode, ammonia is released on reduction which makes the battery no longer useful