Lead-Acid Storage Battery
The negative
terminal, the
anode, is made of
a soft, spongy,
Lead (symbol Pb,
shown in black).
The positive
terminal, the
cathode, is Lead
Oxide (symbol
PbO2, shown
grey). 35%
Sulfuric Acid in
water acts as the
electrolyte
solution and it
+
dissociates into a Hydrogen ions, H , and a sulfate ion, HSO4-. In the
diagram a single atom of Lead is shown at left and a single Lead Oxide
molecule at right. We’ll see what happens to them!.
When the switch is closed and the battery is supplying current, 2 electrons
are pulled away from the Lead atom: they’re attracted to the positive side
of the battery.
At the same
time, the Lead
ion (Pb+2)
reacts with the
Sulfate part of
the Hydrogen
Sulfate ion
(HSO4-),
leaving a
Hydrogen ion
behind.
When the lead
loses the
electrons we
say it is
“oxidized”: its valance has increased from 0 to +2. So, at the ANODE we get
OXIDATION of Lead. The oxidation equation for this half-cell reaction is:
Pb(s) + HSO4-(aq) ------> PbSO4 (s) + H+(aq) + 2e-
(+0.36 volts)
illumination!
When the
Lead and
Sulfate
combine they
form Lead
Sulfate which
precipitates
onto the
electrode
surface.
Meanwhile,
the electrons
pass through
the light bulb,
heat the
filament, and
provide
After passing through the light bulb, the electrons approach the Lead
Oxide molecule. An important feature of the molecule is the oxidation state
of each atom. Oxygen atoms can be doubly ionized, giving O2 an “oxidation
number” of
(-4). To
balance the
molecule,
which must
be neutral,
Lead (Pb)
must have
an
“oxidation
number” of
+4.
Remember
that the
concept of
oxidation
number is
artificial: it
can’t be determined by experiment, unlike the charge on an ion. It’s really
reflective of how the electrons are shared by the individual atoms in a
molecule.
The electrons weaken the Lead Oxide (PbO2) bond and the Lead ion, now
Pb2+, attracts the Hydrogen Sulfate ion (HSO4-), as it did at the other
electrode.
This reaction
frees up two
Oxygen ions
(O2-). Notice
the Lead has
gained two
electrons here
and has
undergone
what is called
reduction: its
valency has
decreased
from +4 to +2.
The Lead (Pb2+)
reacts with the
Hydrogen
Sulfate ion
(HSO4-),
releasing a
Hydrogen ion,
and the aqueous
Oxygen ions
react with the
Hydrogen ions
in solution.
Notice that
since the electrons have passed through the bulb, no further light is
supplied. In an actual reaction, there are many, many millions of electrons so
the battery would supply light for much longer.
The Oxygen ions
and Hydrogen
ions combine
(what do they
form?), and the
Lead and
Sulfate ions also
combine. See
what they
become in the
next drawing!
Oxygen and
Hydrogen
combine to form
Water (H2O)
and the Lead
ion and the
Sulfate ion
form Lead
Sulfate - PbSO4
(again!). Recall
from above that
at the
CATHODE we
get
REDUCTION of
the lead. The reduction equation for this half-cell reaction is:
PbO2(s) + HSO4-(aq) + 3H+(aq) + 2e- -----> PbSO4(s) + 2H2O
(+1.68 volts)
Now we combine the Oxidation-Reduction equations and add them to find
the overall reaction. Like terms on opposite sides cancel:
Pb(s) + HSO4-(aq) ----> PbSO4 (s) + H+(aq) + 2ePbO2(s) + HSO4-(aq) + 3H+(aq) + 2e- ----> PbSO4(s) + 2H2O
+0.36 volts
+1.68 volts
Pb(s) + PbO2(s) + 2HSO4-(aq) + 3H+(aq) + 2e- ----> 2PbSO4 (s) + H+(aq) + 2e- + 2H2O
+2.04 volts
Pb(s) + PbO2(s) + 2HSO4-(aq) + 2H+(aq) ----> 2PbSO4 (s) + 2H2O +2.04 volts
As the battery continues to operate, the acid in the electrolyte is slowly
replaced by water. When it’s all water the battery is dead!
Fortunately this reaction is reversible. If we pass a direct current through
the cell in the opposite direction, the Lead Sulfate dissociates back into
Lead and Sulfate ions and the Water dissociates back into Oxygen and
Hydrogen. Lead Oxide is restored on the positive electrode and pure Lead
on the negative side…and we can use it all over again!
The above discussion represents only a single cell of the lead-acid battery
you would find in your car (unless it’s an electric car!). It produces 2.04
volts. By placing SIX of these cells together in a “series” connection you get
6 x 2.04 = 12.24 volts, the amount of voltage found in your typical car
battery.