The cathode reaction during the charging of a lead-acid battery leads to the

Options

A. deposition of `Pb`
B. conversion of `PbSO_(4)` to `PbO_(2)`
C. formation of `PbSO_(4)`
D. formation of `PbO_(2)`

Answer

B

Explanation

The lead storage battery used in automobilies is really a number of galvanic cells conncected in series. The glavanic cells are designed so that the cell reaction that produce current can be reversed by application of an external current. The battery is rechargeable and thus long lived.
The chemistry of the automonile battery is based on changes in the oxidation state of lead. In anode half-cell, the oxidation state of lead is changed from `0` to `+2`. In the cathode half-fell. the oxidation state is changed from `+4` to `+2`.
The lead in the `0` oxidation state is in the form of a spongy alloy. The lead in the `+2` oxidation state in the form of `PbSO_(4)`. a white insoluble salt that can sometimes be seen as a coating on dead batteries. Lead in the `+4` oxidation state is in the form of `PbO_(2)`, lead dioxide which is also insoluble.
The chemical system of a charged lead storage battery is not at equilibrium, and an electric current flows when the plates of lead and of lead dioxide are connected. The oxidation process that occurs in the anode is
`Pb(s)+SO_(4)^(2-)(aq.) rarr PbSO_(4)(s)+2e^(-)`
the reduction that occurs when the lead dioxide plate (the cathode) receives the electrons released by the oxidation of lead is
`PbO_(2)+SO_(4)^(2-)(aq.)+4H^(+)(aq.)+2e^(-) rarr PbSO_(4)(s)+2H_(2)O(I)`
Notice that both these process produce `PbSO_(4)`, in which lead is in the `+2` oxidation state.
On charging the battery the reactions are reversed i.e. anode now undergoes reduction while cathode undergoes oxidation. As a result `PbSO_(4)(s)` on anode is converted into `Pb(s)` while `PbSO_(4)(s)` on cathode is converted into `PbSO_(4)(s)` on cathode llis converted into `PbO_(2)(s)`. The overall reaction for the cell is
`Pb(s)+PbO_(2)(s)+2SO_(4)^(2-)(aq.)+4H^(+)(aq.) underset("Charge")overset("Discharge")(hArr) 2PbSO_(4)(s)+2H_(2)O(l)`
As the cell discharges electricity and proceeds towards equilibrium, insoluble lead sulphate is deposited on both plates and sulphuric acid is consumed. The consumption of `H_(2)SO_(4)` reduces the density of the solution in the battery. The extent to which a battery has been discharged can be determine by measurement of the density of the solution with a hydrometer.