Question

What happens when lead storage battery is discharged?

• A. $S{O}_2$ is evolved
• B. Lead sulphate is consumed
• C. Lead is formed
• D. $H_{2}S{O}_4$ is consumed

Answer 1

The correct option is D

$H_{2}S{O}_4$ is consumed

In lead storage battery, lead grids filled with spongy lead will act as anode and lead grids filled with $Pb{O}_2$ will act as cathode. 38% solution of sulphuric acid will act as the electrolyte for the cell.
When the battery discharge,
At anode: $Pb\left(s\right)+S{O}_4^{2-}\left(aq\right)\to PbS{O}_4\left(s\right)+2e^{-}$
Most of the lead sulphate precipitates formed gets sticks to the lead rod. Thus, it is not consumed during discharging.

At cathode: $Pb{O}_2\left(s\right)+S{O}_4^{2-}\left(aq\right)+4H^{+}\left(aq\right)+2e^{-}\to PbS{O}_4\left(s\right)+2H_{2}O\left(l\right)$
Here also formed lead sulphate precipitate gets sticks
to cathode rod.

Overall reaction during discharge:
$Pb\left(s\right)+Pb{O}_2\left(s\right)+2H_{2}S{O}_4\left(aq\right)\to 2PbS{O}_4\left(s\right)+2H_{2}O\left(l\right)$
$E_{cell}=2.05V$

$H_{2}S{O}_4$ is consumed in the cell reaction during discharge. So, its concentration in the solution decreases and results into decrease in the density of the solution.