Chapter 21, Problem 33A

Interpretation Introduction

Interpretation: The structure of a dry cell needs to be described. The substance that is oxidized or reduced needs to be determined.

Concept Introduction: A dry cell is a type of voltaic cell in which electrolysis is in the form of a paste. Here, oxidation takes place at the anode and reduction occurs at the cathode. Dry cells are not rechargeable because the cathode reaction is not reversible in this case.

Explanation of Solution

In a type of dry cell, a zinc container is filled with a paste (electrolyte) of manganese (IV) oxide or ${\text{MnO}}_{\text{2}}$ , ammonium chloride or ${\text{NH}}_{\text{4}}\text{Cl}$ , zinc chloride or ${\text{ZnCl}}_{\text{2}}$ , and water or ${\text{H}}_{\text{2}}\text{O}$ . Here, a graphite rod is placed in the paste. The zinc container acts as an anode and the graphite rod as the cathode. The mixing of the content of the cell is prevented by the thick paste; thus, a salt bridge is not required in a dry cell. In a dry cell, the graphite rod acts as a conductor and does not undergo reduction even though it is a cathode. Here, the reduction of manganese (IV) oxide occurs at the cathode.

The oxidation and reduction half-cell reactions are represented as follows:

Oxidation reaction (anode): $\text{Zn}\left(s\right)\to {\text{Zn}}^{2+}\left(aq\right)+2e$

Reduction reaction (cathode): ${\text{2MnO}}_{\text{2}}\left(s\right)+{\text{2NH}}_4^{+}\left(aq\right)+2e\to {\text{Mn}}_{\text{2}}{\text{O}}_{\text{3}}\left(s\right)+2{\text{NH}}_{\text{3}}+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

Therefore, the substances oxidized and reduced are zinc (solid) and manganese oxide (solid) respectively.