23.1. Electrochemical Cells http://www.ck12.org
redox reaction. Eventually, both metal poles are converted to PbSO 4 , and the battery can no longer produce a flow
of electrons. However, the battery can be recharged by forcing the reaction to run in the opposite direction by using
an external source of energy (such as the energy given off by the combustion of gasoline).
The description given above describes the process that occurs in one cell of a lead-acid battery. Because cars require
a battery with a higher voltage than can be obtained with a single cell, car batteries generally consist of several
cells connected together to produce the desired voltage output. A typical car battery will generate twelve volts of
electricity.
Dry Cell Batteries
The car battery is an example of a “wet cell” battery, because there is a liquid electrolyte (sulfuric acid) present in
the system. These batteries must operate in an upright position so the liquid material does not spill out. In contrast,
dry cell batteriescontain a paste that serves as the positive electrode. The composition and voltage depends on the
specific battery, but typical voltage outputs are in the 1.0-1.5 volt range.
One common material used for the positive electrode is a mixture of ammonium chloride and manganese dioxide.
The half-reaction at this pole is the following:
2 NH 4 ++ 2 MnO 2 + 2e−→Mn 2 O 3 + 2 NH 3 + H 2 OSimplifying this to just the atom being oxidized, we get the following:
Mn^4 ++ e−→Mn^3 +The electrons for this half-reaction are provided by oxidation of metallic zinc:
Zn→Zn+^2 + 2e−Also included in this setup are ZnCl 2 (part of the paste) and an inert carbon rod that acts as a conductor. Over time,
the zinc container will be partially consumed and weakened by the oxidation half-reaction. These batteries cannot
be recharged. Running the reaction in reverse would regenerate solid zinc, but not in a form that would have the
same structural properties as the original container.
“Alkaline” batteries use either NaOH or KOH in place of the ammonium chloride. The half-reactions in an alkaline
battery are shown below:
Zn + 2 OH−→ZnO + H 2 O + 2e−
2 MnO 2 + 2e−+ H 2 O→Mn 2 O 3 + 2 OH−This type of battery lasts longer than the ammonium chloride version, because zinc corrodes more slowly in a basic
medium than it does in an acidic medium.
Figure23.4 illustrates a typical setup for a dry cell battery. The zinc container provides the source of electrons, and
the manganese-containing paste provides a place for those electrons to be used. Because there are insulating layers
in between these two components, very little electron flow occurs when the battery is not hooked up to anything.
However, a low resistance pathway is provided if the two terminals are connected with a wire. Starting at the zinc
container, electrons can flow freely through the negative terminal, the wire, the positive terminal, and the conductive
carbon rod, finally reaching the electron-accepting paste, in which manganese is reduced. If the wire runs through
a light or mechanical device, the movement of the electrons through the wire will provide the power necessary to
operate the device.
Rechargeable dry cell batteries are used in a wide variety of applications, from small devices like digital cameras and
cell phones to cars, including both hybrids that run on a combination of gas and electricity and all-electric vehicles
that must be recharged after each short trip. These batteries are generally of the nickel-metal hydride (NiMH) type.