Chapter 18, Problem 34A

Interpretation Introduction

Interpretation: The reason of long lifetime of an alkaline battery than normal dry cell needs to be explained. The overall cell reaction in an alkaline dry cell needs to be determined.

Concept Introduction: In the electrochemical cell, the reactions at cathode and anode occur due to the difference in their reduction electrode potential value.

The EMF of the cell can be calculated with help of electrode reduction potential values.

Answer to Problem 34A

  $\begin{array}{l}{\text{At anode (oxidation) Zn + 2 OH}}^{\text{-}}\stackrel{}{\to }{\text{ZnO + H}}_{\text{2}}{\text{O + 2e}}^{\text{-}}\\ {\text{At cathode (Reduction) 2 MnO}}_{\text{2}}{\text{ + H}}_{\text{2}}{\text{O + 2e}}^{\text{-}}\stackrel{}{\to }{\text{Mn}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+ 2 OH}}^{\text{-}}\\ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\\ {\text{Zn + 2 MnO}}_{\text{2}}\stackrel{}{\to }{\text{ZnO +Mn}}_{\text{2}}{\text{O}}_{\text{3}}\\ \end{array}$

These batteries has almost double energy density, which allows them to provide energy for long time compare to dry cell.

Explanation of Solution

Alkaline batteries last longer due to the chemical substances present in it compare to dry cell.

In an alkaline dry cell battery, there is a zinc anode and a manganese dioxide cathode with a non-acidic electrolyte paste like KOH. It is packed in a steel can.

  ${\text{At cathode (Reduction) 2 MnO}}_{\text{2}}{\text{ + H}}_{\text{2}}{\text{O + 2e}}^{\text{-}}\stackrel{}{\to }{\text{Mn}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+ 2 OH}}^{\text{-}}$

  ${\text{At anode (oxidation) Zn + 2 OH}}^{\text{-}}{}^{\text{-}}\stackrel{}{\to }{\text{ZnO + H}}_{\text{2}}{\text{O + 2e}}^{\text{-}}$

The overall reaction can be written as:

  $\begin{array}{l}{\text{At anode (oxidation) Zn + 2 OH}}^{\text{-}}\stackrel{}{\to }{\text{ZnO + H}}_{\text{2}}{\text{O + 2e}}^{\text{-}}\\ {\text{At cathode (Reduction) 2 MnO}}_{\text{2}}{\text{ + H}}_{\text{2}}{\text{O + 2e}}^{\text{-}}\stackrel{}{\to }{\text{Mn}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+ 2 OH}}^{\text{-}}\\ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\\ {\text{Zn + 2 MnO}}_{\text{2}}\stackrel{}{\to }{\text{ZnO +Mn}}_{\text{2}}{\text{O}}_{\text{3}}\\ \end{array}$

These batteries has almost double energy density, which allows them to provide energy for long time compare to dry cell.

Conclusion

  $\begin{array}{l}{\text{At anode (oxidation) Zn + 2 OH}}^{\text{-}}\stackrel{}{\to }{\text{ZnO + H}}_{\text{2}}{\text{O + 2e}}^{\text{-}}\\ {\text{At cathode (Reduction) 2 MnO}}_{\text{2}}{\text{ + H}}_{\text{2}}{\text{O + 2e}}^{\text{-}}\stackrel{}{\to }{\text{Mn}}_{\text{2}}{\text{O}}_{\text{3}}{\text{+ 2 OH}}^{\text{-}}\\ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\\ {\text{Zn + 2 MnO}}_{\text{2}}\stackrel{}{\to }{\text{ZnO +Mn}}_{\text{2}}{\text{O}}_{\text{3}}\\ \end{array}$