Chapter 20.2, Problem 16SSC

Interpretation Introduction

Interpretation:

The process that occurs when battery is recharged 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 series which produce 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. Example is lead storage battery.

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

Answer to Problem 16SSC

When battery is recharged, cell operates like electrolytic cell that is electrical energy is supplied by external source and reactions at electrode reverse.

Explanation of Solution

Secondary cells like lead storage battery can be recharged and reused. A lead storage battery consists of lead as anode and lead grid packed with lead dioxide as cathode. These electrodes are placed alternately separated by glass fibre sheets, suspended in sulfuric acid as electrolyte.

To increase their output, cells are connected in parallel.

The reaction at anode and cathode occur as follows:

$\begin{array}{l}\text{Pb}\left(\text{s}\right)+{\text{SO}}_{\text{4}}^{\text{2}-}\left(\text{aq}\right)\to {\text{PbSO}}_4\left(\text{s}\right)+2{\text{e}}^{-}\left(\text{at anode}\right)\\ {\text{PbO}}_2\left(\text{s}\right)+{\text{SO}}_{\text{4}}^{\text{2}-}\left(\text{aq}\right)+4{\text{H}}^{+}\left(\text{aq}\right)+2{\text{e}}^{-}\to {\text{PbSO}}_4\left(\text{s}\right)+2{\text{H}}_2\text{O}\left(\text{at cathode}\right)\end{array}$

When battery is recharged, cell operates like electrolytic cell that is electrical energy is supplied by external source and reactions at electrode reverse.

During recharging reaction at anode and cathode occur as follows:

$\begin{array}{l}{\text{PbSO}}_4\left(\text{s}\right)+2{\text{e}}^{-}\to \text{Pb}\left(\text{s}\right)+{\text{SO}}_{\text{4}}^{\text{2}-}\left(\text{aq}\right)\left(\text{at anode}\right)\\ {\text{PbSO}}_4\left(\text{s}\right)+2{\text{H}}_2\text{O}\to {\text{PbO}}_2\left(\text{s}\right)+{\text{SO}}_{\text{4}}^{\text{2}-}\left(\text{aq}\right)+4{\text{H}}^{+}\left(\text{aq}\right)+2{\text{e}}^{-}\left(\text{at cathode}\right)\end{array}$

Conclusion

When battery is recharged, cell operates like electrolytic cell that is electrical energy is supplied by external source and reactions at electrode reverse.