Question

During discharging of a lead storage battery, which of the following is ${\text{/}}$ are true
A.${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ is produced
B.${{\text{H}}_{\text{2}}}{\text{O}}$ is consumed
C.${\text{PbS}}{{\text{O}}_{\text{4}}}$ is formed at both electrodes
D.Density of electrolytic solution decreases

Answer 1

Lead storage battery is a secondary cell which consists of lead anode and a grid of lead packed with lead dioxide $\left( {{\text{Pb}}{{\text{O}}_{\text{2}}}} \right)$ as cathode. A solution of 38% Sulphuric acid is used as an electrolyte.

Complete step by step answer:
A battery that is used as a source of electrical energy is a galvanic cell where the chemical energy produced during the redox reaction is converted in electrical energy.
Batteries are of two types
$\left( {\text{i}} \right)$ Primary battery: Here the reaction occurs only once and after a certain period of time the battery is dead and cannot be reused.
$\left( {{\text{ii}}} \right)$ Secondary batteries: They are rechargeable after a use by passing current in the opposite direction.
Lead storage batteries are an important class of secondary batteries where lead acts as anode and a grid of lead packed with lead oxide as cathode. The electrolyte used is 38% solution of Sulphuric acid
The cell reactions are
Anode: $Pb_{(s)} +SO_4^{2-}\to PbSO_4+2e-$
Cathode: {\text{Pb}}{{\text{O}}_{\text{2}}}_{\left( {\text{s}} \right)}{\text{ + S}}{{\text{O}}_{\text{4}}}{^{{\text{2 - }}}_{\left( {{\text{aq}}} \right)}}{\text{ + 4}}{{\text{H}}^{\text{ + }}}_{\left( {{\text{aq}}} \right)}{\text{ + 2}}{{\text{e}}^{\text{\\_}}} \to {\text{PbS}}{{\text{O}}_{{\text{4}}\left( {\text{s}} \right)}}{\text{ + 2}}{{\text{H}}_{\text{2}}}{{\text{O}}_{\left( {\text{l}} \right)}}
The overall cell reaction consists of both the cathode and anode reactions
${\text{P}}{{\text{b}}_{\left( {\text{s}} \right)}}{\text{ + Pb}}{{\text{O}}_{\text{2}}}_{\left( {\text{s}} \right)}{\text{ + 2}}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{{\text{4}}\left( {{\text{aq}}} \right)}} \to {\text{2PbS}}{{\text{O}}_{\text{4}}}_{\left( {\text{s}} \right)}{\text{ + 2}}{{\text{H}}_{\text{2}}}{{\text{O}}_{\left( {\text{l}} \right)}}$
In the above reaction ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ is consumed and ${{\text{H}}_{\text{2}}}{\text{O}}$ is released.
Density is $\dfrac{{\text{m}}}{{\text{V}}}$
Since there is a consumption of Sulphuric acid the density of electrolytic solution decreases
Thus the correct answer is C and D.

Additional information: The above reaction is reversed on charging the battery where ${\text{PbS}}{{\text{O}}_{\text{4}}}_{\left( {\text{s}} \right)}$ of anode is converted into ${\text{Pb}}$ and ${\text{PbS}}{{\text{O}}_{\text{4}}}$ of cathode is converted into ${\text{Pb}}{{\text{O}}_{\text{2}}}$
The net reaction is represented as
${\text{2PbS}}{{\text{O}}_{\text{4}}}{\text{ + 2}}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{Pb + Pb}}{{\text{O}}_{\text{2}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$
During the charging of the lead storage battery the Sulphuric acid added as an electrolyte is regenerated. Thus the lead storage batteries can be reused by regenerating sulphuric acid.

Note:
The role of Lead storage batteries is to store energy for particular period time.The battery manufactured should be reasonably light, compact and its voltage should not vary during the use. Lead storage batteries are mature technology and reliable, they have high discharge rates.