Redox and examples





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. For example, in the equation:


the oxidation number of zinc has changed from 0 to +2. 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
0.
BATTERY AND FUEL CELLS
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
exchanges.
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) dissociates into two positively
charged hydrogen (2H+) ions and one negatively charged sulfate ion (SO4=).
Note that the sulfate ion has a double negative charge, indicated by the two
minus signs. The copper electrode can start electric current flowing as soon as it
is connected outside the cell to the zinc electrode. It can do this because
copper attracts electrons, which make up the current, more strongly than zinc
does.
The copper electrode cannot attract electrons through the electrolyte,
however, because electrons have a negative electric charge like the sulfate
ions. The negative charges repel each other, and this stops the flow of electrons.
Once the copper electrode starts drawing electrons through an external
connection, a chemical reaction helps to keep the current going. Every zinc
atom that loses electrons to the copper electrode becomes a zinc ion (Zn++)
with a double positive charge. Sulfate ions promptly attract the zinc ions into the
solution where they combine to form dissolved zinc sulfate (Zn++ + SO4 =
ZnSO4).
Before the action starts, the sulfate and hydrogen ions cancel each other
electrically in the solution. Once the hydrogen ions (2H+) are free, they seize
electrons at the copper electrode, become normal hydrogen atoms (H), and
form bubbles of gaseous hydrogen (H2). This allows the copper electrode to
draw more electrons, which keeps the current flowing. In the process, acid and
zinc are consumed. When either is used up, the battery fails.
The simple voltaic cell cannot operate very long because the bubbles of
hydrogen gas that collect at the copper electrode act as an insulator, stopping
further electron flow. This blockage is called polarization. In 1836 John F. Daniell,
an English chemist, produced a cell that was not subject to polarization. In the
Daniell cell, the copper electrode forms the outer shell of