Releasing Energy through Reactions in Batteries
Reactions that involve a change in oxidation number are called
oxidation-reduction reactions. An element is oxidized if the oxidation number
has become more positive in value. The term reduction
describes the opposite process, in which the oxidation number becomes more
negative in value. In the same equation, for example, the hydrogen is reduced.
The oxidation number has changed from +1 to 0. If everything is counted
through the entire equation, oxidation and reduction are equal and balance to
When electric energy is needed, batteries and fuel cells are one way to
provide it. A battery chemically stores and then releases energy. A fuel cell
converts energy produced by a chemical reaction directly into usable power.
Batteries range in size from single-cell models smaller than coins to
multi-cell units that fill large rooms. Portable radios, pocket calculators, watches,
and hearing aids are typical devices powered by batteries. Very large battery
installations supply standby energy for equipment such as that in telephone
Alessandro Volta, an Italian professor, devised the first battery in 1800 to
provide steady electric current for study and practical use. Before that time,
only static electricity--a novelty with no practical value--could be produced.
Batteries are either primary or secondary. A primary battery produces its
energy by consuming one of the chemicals it contains. When the chemical is
gone, the battery no longer produces energy and must be replaced. The
carbon-zinc batteries used in flashlights and tape recorders are primary.
Secondary batteries, or storage batteries, obtain energy by transforming certain
kinds of chemicals to other kinds. When the change is complete, the battery no
longer produces energy. It can be renewed, or recharged, however, by sending
current from another source through it to restore the chemicals to their original
state. An automobile battery, called a lead-acid battery, is secondary.
The simplest arrangement of parts that will produce current is called a
cell. A battery combines two or more cells to produce higher voltage or more
current. Connecting the cells in series increases the voltage. Connecting them in
parallel raises the current, or amperage.
A very simple form of cell is one called a voltaic cell, in honor of Volta. It uses a
strip or rod of copper, another of zinc, and sulfuric acid mixed with water. The
pieces of metal are called electrodes. The solution is called the electrolyte. The
copper electrode is the cathode, or positive electrode, because it has a positive
electric charge. The zinc electrode is the anode, or negative electrode,
because it has a negative electric charge.
When the cell is not in use, the molecules of the acid in the electrolyte
separate into electrically charged portions called ions. In chemical symbols, this
means the sulfuric acid electrolyte (H2SO4)...