Chapter 21, Problem 77A

Interpretation Introduction

Interpretation: The property of lead (II) sulphate and lead (IV) oxide that makes salt bridges unnecessary in a lead storage battery needs to be explained.

Concept Introduction: Lead-acid battery is a rechargeable battery. The energy density of a lead-acid battery is low as compared to other batteries. When the battery is charged, the chemical energy is stored as the potential difference between pure lead on the negative side and lead oxide on the positive side.

Explanation of Solution

In a lead-acid battery, the reactions at the cathode and anode are as follows:

Anode-Oxidation:

  $\text{Pb}\left(s\right)+{\text{HSO}}_4^{-}\left(aq\right)\to {\text{PbSO}}_{\text{4}}\left(s\right)+{\text{H}}^{+}\left(aq\right)+2e^{-}-\text{0}\text{.36 V}$

Cathode-Reduction:

  ${\text{PbO}}_{\text{2}}\left(s\right)+{\text{HSO}}_4^{-}\left(aq\right)+3{\text{H}}^{+}\left(aq\right)+2e^{-}\to {\text{PbSO}}_4\left(s\right)+{\text{2H}}_{\text{2}}\text{O}\left(l\right)+1.69\text{ V}$

Here, the overall reaction is as follows:

  $\text{Pb}\left(s\right)+{\text{PbO}}_{\text{2}}\left(s\right)+2{\text{H}}_{\text{2}}{\text{SO}}_{\text{4}}\left(aq\right)\to 2{\text{PbSO}}_{\text{4}}\left(s\right)+2{\text{H}}_{\text{2}}\text{O}\left(l\right)$

A lead-acid battery produces energy at the time of discharge. At the time of discharging, both positive and negative plates become lead (II) sulphate and the electrolyte loses sulfuric acid (in dissolved form) to form water.

A salt bridge is needed to separate the two electrolytic solutions of the cell. In lead storage batteries since there is only one electrolyte, a salt bridge is not required.