Iron in its free state has a valence of zero and is very reactive since its common valence
state is +2 or +3. It loses two electrons to become the ferrous ion. The valence has
gone from 0 to +2, thus iron has been oxidized. It can undergo further oxidation to the
+3 valence state:
-
Fe+2 -le
---> Fe+3
Here the ferrous ion has lost another electron to become a ferric ion.
c. Reduction. In inorganic chemistry, reduction is defined as the gain of
electrons or a decrease in the valence of an element. Consider the reduction of
elemental oxygen:
O2 + 4e - -----> 2 O -2
Observe that oxygen is a diatomic molecule in its free elemental form and has a valence
of zero. Since the most common valence state of oxygen is -2, oxygen accepts
electrons readily to become the oxygen anion. The valence of each oxygen atom has
gone from 0 to -2, thus oxygen had been reduced. If the valence is made smaller
(reduced), reduction has occurred.
d. Oxidizing and Reducing Agents. For all practical purposes, it is impossible
to simply add or subtract electrons from an element except in an electrolytic cell. In
fact, the oxidation of one element and the reduction of another always occur
simultaneously. One element loses the electrons; the other element gains the electrons
that are lost by the first. Consider these two reactions when they are combined:
2Fe - 4e - -----> 2Fe+2
O2 + 4e - ------> 2O-2
2Fe + O2 ------> 2FeO
This is an oxidation-reduction reaction that is very common in our industrialized society.
The oxidation of iron by atmospheric oxygen gives us iron oxide, commonly known as
rust. In this reaction, oxygen was reduced, going from a zero to a -2 state by receiving
electrons from iron. Because it accepted the electrons from iron and allowed the iron to
oxidize, oxygen is called an oxidizing agent. Iron, which gave up electrons, is called the
reducing agent. General characteristics of reducing and oxidizing are shown in the
following table.
MD0803
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