Inorganic and Applied Chemistry

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Inorganic and Applied Chemistry

Example 6- H:

Lead battery

The lead battery is used primarily in cars where they deliver current to the start engine. The reason for this

widely use through many years is that lead batteries work well with good performance at typical outdoor

temperatures. The anode in the lead battery is the lead electrode while the cathode typically consists of a

lead electrode covered with lead oxide. Both electrodes are placed in an electrolytic solution of sulphuric

acid. The following half cell reaction takes place at the anode:

Pb(s) + HSO 4 - (aq)  PbSO 4 (s) + H+(aq) + 2 e-

while the following half cell reaction takes place at the cathode:

PbO 2 (s) + HSO 4 - (aq) + 3 H+(aq) + 2 e- PbSO 4 (s) + 2 H 2 O(l)

Hereby the total overall cell reaction is:

Pb(s) +3 H+(aq) + PbO 2 (s) + 2 HSO 4 - (aq)  2 PbSO 4 (s) + 2 H 2 O(l)

A typical lead battery for the use in cars has six cells in series. As each cell each gives 2 volt the total

voltage becomes 12 volt.

Please note from the overall cell reaction that sulphuric acid is used in the reaction and the battery is

hereby slowly discharged. Hereby the density of the electrolytic solution changes and one of the ways to

determine the condition of the battery is to measure the density of the electrolytic solution. The solid lead

sulphate that is formed during the cell reaction is set on the surfaces of the electrodes.

The lead battery may be recharged by passing a current of electrons in the opposite direction of the cell

reaction above. This happens continuously while the car is driving as the energy for the recharge is

generated by the combustion reaction in the engine of the car. The figure below shows the principle in the

set-up of the lead battery.

Electrochemistry