|The Chemistry of Iron
Iron is one of the most widely used metals. It is the fourth most abundant
element in the Earth's crust and appears as several different minearals.
These include hematite (Fe2O3), magnetite (Fe3O4),
siderite (FeCO3), and iron pyrite (FeS2). Note that
hematite is a compound of Fe(III) whereas magnetite contains both Fe(III)
and Fe(III) and may better be formulated as [Fe(II)(Fe(III)2]O3.
Iron pyrite is gold in color and has a distinctive lustor. For this reason
it is often referred to as fool's gold. Since sulfur is in the same group
as oxygen it is tempting to think of this compound as containing Fe(IV).
However, this is not a stable oxidation state for iron. In pyrite, the
sulfur is present as the disulfide ion (S22-); thereofre
the iron is present as Fe(II). While an abundant mineral, pyrite is not
used as as source of iron as the sulfur is not easily removed.
Iron is fairly reactive and will react with acids. An exposed iron surface quickly develops a coating of rust, which is a hydrated form of iron(III) oxide. This process is acceleretd in a moist environment. Finely divided iron will burst into flame when heated in an atmosphere of pure oxygen.
Iron is a ferromagnetic material. In a ferromagnetic substance, the magnetic fields of individual atoms are aligned and reinforce each other to produce a large magnetic field. The phenomenon is most pronounced for iron but also occurs in cobalt and nickel to a lesser extent. Iron and alloys of iron are used to make such things as magnets and transfomer cores.
Steel is an important alloy of iron. All steel contains 0.50 - 1.50
percent carbon. Generally, the the higher the carbon content, the harder
and more brittle the steel. Other metals are sometimes added depending
upon the desired properties. For example, magnets are often made of steel
containing nickel and cobalt. Stainless steel contains chromim and nickel
and is resistant to corrosion. The addition of manganese produces a steel
which is very hard and durable.
Iron in the +2 oxidation state is known as the ferrous ion. This
ion is pale green in color and is very easily oxidized to the ferric ion;
even traces of dissolved oxygen in solution will accomplish this oxidation.
For this reason, solutions of the ferrous ion are sometimes used as reducing
Iron in the +3 oxidation state is known as the ferric ion. The
color of the hydrated ferous ion [Fe(H2O)6]2+
is actually violet. However, most solutions contining the ferric ion are
usually yellow or yellow-brown due to the formtion of [Fe(H2O)5(OH)]2+.
This ion is formed due to the high charge density of Fe3+ ion.
Water molecules are coordinated so strongly that one one of the O-H bonds
weakens, resulting in the formation of H3O+. As a
result solutions of ferric ion are often acidic.