Chapter 18, Problem 33A

Interpretation Introduction

Interpretation: The overall chemical equation in lead storage automobile battery with oxidized and reduced species during charging and recharging process 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 33A

  ${\text{Pb}}_{\text{(s)}}{\text{ + SO}}^{\text{2-}}{}_{\text{4(aq)}}\stackrel{}{\to }{\text{ PbSO}}_{\text{4(s)}}{\text{ + 2e}}^{\text{-}}\text{ at anode}$

  ${\text{PbO}}_{\text{2}}{}_{\text{(s)}}{\text{ + SO}}^{\text{2-}}{}_{\text{4(aq)}}{\text{ + 4 H}}^{\text{+}}{}_{\text{(aq)}}\text{ + 2e- }\stackrel{}{\to }{\text{ PbSO}}_{\text{4(s)}}{\text{ + 2H}}_{\text{2}}\text{O at cathode}$

Oxidized species = Solid lead

Reduced species = Lead ions in PbO₂

These batteries can recharge because passing electricity can reverse the reaction to form reactants again.

Explanation of Solution

In a lead storage battery, lead is anode and a grid of lead is cathode with lead dioxide (PbO₂). The battery contains 38% of sulphuric acid as an electrolyte solution.

As battery remains in use the concentration of sulphuric acid changes due to given reactions:

  ${\text{Pb}}_{\text{(s)}}{\text{ + SO}}^{\text{2-}}{}_{\text{4(aq)}}\stackrel{}{\to }{\text{ PbSO}}_{\text{4(s)}}{\text{ + 2e}}^{\text{-}}\text{ at anode}$

  ${\text{PbO}}_{\text{2}}{}_{\text{(s)}}{\text{ + SO}}^{\text{2-}}{}_{\text{4(aq)}}{\text{ + 4 H}}^{\text{+}}{}_{\text{(aq)}}\text{ + 2e- }\stackrel{}{\to }{\text{ PbSO}}_{\text{4(s)}}{\text{ + 2H}}_{\text{2}}\text{O at cathode}$

When battery recharge, the reverse reaction occurs which converts lead sulphate to ${\text{PbO}}_{\text{2}}$ and regenerate sulphuric acid. These batteries can recharge because passing electricity can reverse the reaction to form reactants again. When battery recharge, the reverse reaction occurs which converts lead sulphate to PbO2 and regenerate sulphuric acid.

Conclusion

When battery recharge, the reverse reaction occurs which converts lead sulphate to ${\text{PbO}}_{\text{2}}$ and regenerate sulphuric acid.