Question

Answer the following:

Write the electrode reactions when lead storage cell generates electricity. What are the anode and cathode and write the electrode reactions during its recharging?

Answer 1

1) Cell reactions when lead storage cell generates electricity (discharging):

a) Oxidation at anode (–):

When the cell provides current, spongy lead is oxidised to Pb²⁺ ions and negative charge accumulates on lead plates. The Pb²⁺ ions so formed combine with `"SO"_4^(2-)` ions from H₂SO₄ to form insoluble PbSO₄. The net oxidation is the sum of these two processes.

`"Pb"_(("s")) -> "Pb"_(("aq"))^(2+) + 2"e"^-`       (oxidation)

`"Pb"_(("aq"))^(2+) + "SO"_(4("aq"))^(2-) -> "PbSO"_(4("s"))`  (precipitation)

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`"Pb"_(("s")) + "SO"_(4("aq"))^(2-) -> "PbSO"_(4("s")) + 2"e"^-`     .....(1)  (overall oxidation)

b) Reduction at cathode (+):

The electrons produced at the anode travel through external circuit and re-enter the cell at the cathode. At cathode, PbO₂ is reduced to Pb²⁺ ions in presence of H⁺ ions. Pb²⁺ ions formed combine with `"SO"_4^(2-)` ions from H₂SO₄ to form insoluble PbSO₄ that gets coated on the electrode.

`"PbO"_(2("s")) + 4"H"_(("aq"))^+ + 2"e"^(-) -> "Pb"_(("aq"))^(2+) + 2"H"_2"O"_(("l"))`  (reduction)

`"Pb"_(("s")) + "SO"_(4("aq"))^(2-) -> "PbSO"_(4("s"))`  (precipitation)

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`"PbO"_(2("s")) + 4"H"_(("aq"))^+ + "SO"_(4("aq"))^(2-) + 2"e"^(-) -> "PbSO"_(4("s")) + 2"H"_2"O"_(("l"))`     .....(2)    (overall reduction)

2) During recharging, the anode and cathode are interchanged with PbO2 electrode being anode (+) and lead electrode cathode (–).

3) Cell reactions during recharging:

The potential of the lead accumulator is 2 V. It is recharged when cell potential drops to 1.8 V. To recharge the cell external potential slightly greater than 2 V it is applied across the electrodes. During recharging, the cell functions as an electrolytic cell, and electrolytes are regenerated. The anode and cathode are interchanged with PbO₂ electrode being anode (+) and lead electrode cathode (–).

a) Oxidation at anode (+):

It is reverse of reduction reaction (ii) at cathode that occurs during discharge.

`"PbSO"_(4("s")) + 2"H"_2"O"_(("l")) -> "PbO"_(2("s")) + 4"H"_(("aq"))^+ + "SO"_(4("aq"))^(2-) + 2"e"^(-)`    .....(3)

b) Reduction at cathode (–):

It is reverse of oxidation reaction (i) at anode that occurs during discharge.

`"PbSO"_(4("s")) + 2"e"^(-) -> "Pb"_(("s")) + "SO"_(4("aq"))^(2-)`      ...(4)