write down the 4 features of lead accumulators
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Some electrochemical cells are rechargeable – the electrode reactions are reversible and the process can be repeated many times. Such cells can be used to store electricity. The most common type of heavy duty rechargeable cell is the familiar lead-acid accumulator (‘car battery’) found in most combustion-engined vehicles.
A simple lead-acid cell consisting of strips of lead and an electrolyte of dilute sulfuric acid is constructed and charged for different lengths of time. The cell is then discharged through a light bulb and the time the bulb remains lit measured. Plotting a graph of this time against the charging time shows the relationship between the electrical energy put into the cell and the energy released.
During charging (electrode signs as in charging circuit)
(+) electrode: PbSO4(s) + 2H2O(l) → PbO2(s) + 4H+(aq) + SO42-(aq) + 2e-
(-) electrode: PbSO4(s) + 2e- → Pb(s) + SO42-(aq)
Discharging (electrode signs as for cell)
(+) electrode: PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- → PbSO4(s) + 2H2O(l)
(-) electrode: Pb(s) + SO42-(aq) → PbSO4(s) + 2e-
The overall, reversible cell reaction is therefore:
PbO2(s) + 4H+(aq) + 2SO42-(aq) + Pb(s) ⇌ 2PbSO4(s) + 2H2O(l)
Thus during charging the sulfuric acid concentration rises, and during discharge it falls. A side reaction which may result from over-charging is the liberation of hydrogen gas at the (-) electrode, resulting from the reduction of H+(aq) ions. This has caused explosions in the past when the electrolyte level in batteries has been investigated with the aid of a lighted match!
The advantages of this cell reaction for use in a commercial battery could be discussed, eg the formation of insoluble lead or lead compounds on the electrodes during charge and discharge, the only changes in the electrolyte being a change in concentration. Commercial cells need to be robust, cheap to construct and, for certain applications, able to sustain large currents. The lead-acid accumulator fulfils all these criteria, but has the disadvantage of being very heavy.
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