22.1. The Nature of Oxidation and Reduction http://www.ck12.org
Thus, we can say that the hydrogen atoms are oxidized because they undergo a partial loss of electrons. Even though
the electrons are not lost completely, which would result in the formation of ions, the hydrogen atoms in water have
less electron density near them than they did in the H 2 molecule. The oxygen is reduced because it undergoes a
partial gain of electrons. The oxygen atom in water has more electron density around it than it did as part of the O 2
molecule.
It is frequently more difficult to identify this type of electron shift than the electron transfer that occurs in an ionic
reaction. In the following lesson, we will introduce the concept of oxidation numbers, which provides a way to keep
track of relative electron density in both ionic and molecular substances.
Corrosion
Many metals that are exposed to the environment eventually degrade. For example, iron turns to rust when it is
exposed for long periods of time to oxygen and water (Figure22.2).
FIGURE 22.2
Rusting is a form of corrosion, a process
in which pure metals are oxidized.Rust is generally a complex mixture of iron oxides, some of which are in the form of hydrates. The equations below
show the steps involved in one of the many processes of rust formation.
2Fe(s) + O 2 (g) + 4H+(aq)→2Fe^2 +(aq) + 2H 2 O(l)
4Fe^2 +(aq) + O 2 (g) + 6H 2 O(l)→2Fe 2 O 3 •H 2 O(s) + 8H+(aq)Iron is first oxidized to iron(II) ions by oxygen. In the second step, the iron(II) ions are further oxidized and combine
with water and oxygen gas to produce a hydrated form of iron(III) oxide known as rust. Rusting is one of many
examples of corrosion. Corrosionis the deterioration of metals by redox processes. Corrosion causes enormous
amounts of damage to buildings, bridges, ships, cars, and other objects. It has been estimated that corrosion costs
the U.S. economy over 100 billion dollars each year. A great amount of time and effort is spent on attempts to limit
or prevent corrosion.