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MCQEasyJEE 2025Batteries & Fuel Cells

JEE Chemistry 2025 Question with Solution

On charging the lead storage battery, the oxidation state of lead changes from x1\mathrm{x}_{1} to y1\mathrm{y}_{1} at the anode and from x2\mathrm{x}_{2} to y2\mathrm{y}_{2} at the cathode. The values of x1,y1,x2,y2\mathrm{x}_{1}, \mathrm{y}_{1}, \mathrm{x}_{2}, \mathrm{y}_{2} are respectively:

  • A

    +4,+2,0,+2+4,+2,0,+2

  • B

    +2,0,+2,+4+2,0,+2,+4

  • C

    0,+2,+4,+20,+2,+4,+2

  • D

    +2,0,0,+4+2,0,0,+4

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: The question asks for the change in oxidation state of lead during charging of a lead storage battery at the anode and cathode.

Find: The values of x1,y1,x2,y2\mathrm{x}_{1}, \mathrm{y}_{1}, \mathrm{x}_{2}, \mathrm{y}_{2}.

During charging, the cell reactions are reversed from the discharge process.

At the anode:

PbSO4→Pb\mathrm{PbSO}_{4} \rightarrow \mathrm{Pb}

Lead in PbSO4\mathrm{PbSO}_{4} has oxidation state +2+2, while elemental Pb\mathrm{Pb} has oxidation state 00. So, at the anode:

x1=+2,y1=0\mathrm{x}_{1} = +2, \qquad \mathrm{y}_{1} = 0

At the cathode:

PbSO4→PbO2\mathrm{PbSO}_{4} \rightarrow \mathrm{PbO}_{2}

Lead in PbSO4\mathrm{PbSO}_{4} has oxidation state +2+2 and in PbO2\mathrm{PbO}_{2} it is +4+4. So, at the cathode:

x2=+2,y2=+4\mathrm{x}_{2} = +2, \qquad \mathrm{y}_{2} = +4

Therefore,

x1,y1,x2,y2=+2,0,+2,+4\mathrm{x}_{1}, \mathrm{y}_{1}, \mathrm{x}_{2}, \mathrm{y}_{2} = +2,0,+2,+4

The correct option is B.

Common mistakes

  • Confusing charging with discharging. In charging, the reactions are reversed, so using the discharge direction gives the wrong oxidation-state changes. First identify that the battery is being charged, then reverse the usual discharge reactions.

  • Assigning the oxidation state of lead in PbSO4\mathrm{PbSO}_{4} incorrectly. In PbSO4\mathrm{PbSO}_{4}, the sulfate ion is SO42−\mathrm{SO}_{4}^{2-}, so lead must be +2+2. Do not treat it as elemental lead.

  • Using the electrode names from galvanic-cell discharge without checking the charging process. During charging, the external source drives the reverse reactions, so track the given anode and cathode processes carefully before assigning oxidation states.

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