SECONDARY VOLTAIC CELLS
In secondary voltaic cells,or reversible cells,the original reactants can be regenerated.
This is done by passing a direct current through the cell in the direction opposite of the
discharge current flow. This process is referred to as charging,or recharging, a cell or
battery. The most common example of a secondary voltaic cell is the lead storage battery,
used in most automobiles.THE LEAD STORAGE BATTERY
The lead storage battery is depicted in Figure 21-16. One group of lead plates contains
compressed spongy lead. These alternate with a group of lead plates that contain lead(IV)
oxide, PbO 2. The electrodes are immersed in a solution of about 40% sulfuric acid. When
the cell discharges, the spongy lead is oxidized to lead ions, and the lead plates accumu-
late a negative charge.Pb88nPb^2 2 e (oxidation)The lead ions then react with hydrogen sulfate ions from the sulfuric acid to form insol-
uble lead(II) sulfate. This begins to coat the lead electrode.Pb^2 HSO 4 88nPbSO 4 (s)H (precipitation)Thus, the net process at the anode during dischargeisPbHSO 4 88nPbSO 4 (s)H 2 e (anode during discharge)21-23
888 CHAPTER 21: Electrochemistry
During discharge: H 2 SO 4
solution(b)PbO 2 (s) + 4H+
+ H+(aq) + 2e– → Pb2+(aq) + 2H
(aq)+ H+(aq)2 O
Pb2+(aq) + HSO 4 –HSO 4 –(aq) → PbSO 4 (s)PbO 2 plate (cathode)Pb(s) → Pb2+(aq) + 2e–
Pb2+(aq) + (aq) → PbSO 4 (s)Pb plate (anode)(a)Figure 21-16 (a) A schematic representation of one cell of a lead storage battery. The
reactions shown are those taking place during the dischargeof the cell. Alternate lead grids
are packed with spongy lead and lead(IV) oxide. The grids are immersed in a solution of
sulfuric acid, which serves as the electrolyte. To provide a large reacting surface, each cell
contains several connected grids of each type as in (b); for clarity, only one of each is shown
in (a). Such a cell generates a voltage of about 2 volts. Six of these cells are connected
together in series, so that their voltages add, to make a 12-volt battery.