chapter 5 electrochemistry class 12 chemistry textbook solution

    4. Answer the following :

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

    Answer:- 

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

    a) Oxidation at anode (–):
    When the cell provides current, spongy lead undergoes oxidation to form Pb²⁺ ions, resulting in the accumulation of negative charge on lead plates. These Pb²⁺ ions then combine with SO₄²⁻ ions from H₂SO₄ to form insoluble PbSO₄. The overall oxidation process can be expressed as:

    Pb(s) -> Pb(aq)2+ + 2e-       (oxidation)

    Pb(aq)2+ + SO4(aq)2- -> PbSO4(s)   (precipitation)

    _____________________________________________________

    Pb(s) + SO4(aq)2- -> PbSO4(s) + 2e-       .....(1)   (overall oxidation)

    b) Reduction at cathode (+): The electrons generated at the anode travel through the external circuit and re-enter the cell at the cathode. At the cathode, PbO₂ is reduced to Pb²⁺ ions in the presence of H⁺ ions. These Pb²⁺ ions then combine with SO₄²⁻ ions from H₂SO₄ to form insoluble PbSO₄, which coats the electrode. The reduction process can be expressed as:

    Reduction:

    PbO2(s) + 4H+(aq) + 2e- → Pb(aq)2+ + 2H2O(l)

    Precipitation:

    Pb(s) + SO42-(aq) → PbSO4(s)

    Overall reduction:

    PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- → PbSO4(s) + 2H2O(l) (2)

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

    3) Cell reactions during recharging:

    The potential of the lead accumulator is 2 V. It is recharged when the cell potential drops to 1.8 V. To recharge the cell, an external potential slightly greater than 2 V 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 the PbO₂ electrode becoming the anode (+) and the lead electrode becoming the cathode (–).

    a) Oxidation at the anode (+):
    This process is the reverse of the reduction reaction (ii) that occurs during discharge:

    a) Oxidation at the anode (+):

    Oxidation: PbSO4(s) + 2H2O(l) → PbO2(s) + 4H+(aq) + SO42-(aq) + 2e-   (3)

    b) Reduction at the cathode (-):

    Reduction: PbSO4(s) + 2e- → Pb(s) + SO42-(aq)   (4)

    Q 4.8
    Q 4.9
    Q 4.10

    chapter 5 elctrochemistry textbook solution page 118