[HKAL 2006 II Q4a]
The overall reaction for the discharging process of a lead-acid cell can be represented by the following equation:
PbO2(s) + Pb(s) + 4H+(aq) + 2SO4 → 2-(aq) 2PbSO4(s) + 2H2O(l)
(i) Write the half equation of the cathodic reaction and that of the anodic reaction during discharge for a lead-acid cell.
(ii) Using the IUPAC conventions, write the cell diagram of a lead-acid cell.
(iii) Based on the above information, explain why a lead-acid cell is rechargeable.
(iv) A lead-acid accumulator used in automobiles consists of six lead-acid cells connected in series.
Suggest why
(I) the state of charge of a lead-acid accumulator can be estimated by measuring the density of the acid in the accumulator, and
(II) an excessively high voltage should not be used to charge a lead-acid accumulator.
