392 THE TRANSITION ELEMENTS
When iron is refined, the process is essentially one of melting the
iron in presence of materials which will react with the impurities—
for example air (or oxygen) to remove chiefly carbon, and calcium
oxide (added as carbonate) to remove phosphorus. There are a
variety of refining processes, each depending on the composition of
the initial iron and the sort of iron or steel destined as the end product.
Steels have a carbon content of 0.1-1.5%, and addition of other
transition metals imparts certain properties (for example a little
manganese, elasticity and high tensile strength; more manganese,
great hardness; chromium, resistance to chemical attack, as in
stainless steel; nickel, a reduced expansion; tungsten and vanadium,
hardness retained at high temperatures).
Pure iron is prepared by reduction of iron(II) oxide with hydrogen,
or by electrolysis of an iron(II)-containing aqueous solution. It is
a fairly soft metal, existing in different form according to temperature:1041 K n. 1179 K. 1674 K ~.
a-iron ^=± p-iron ;—-± y-iron ~ o-iron
non- face- body-
ferro- magnetic centred centred
magnetic cubic cubic
body-centred —»> no change
cubic lattice of struc-
ture(It should be noted that the magnetic properties of iron are depend-
ent on purity of the iron and the nature of any impurities.)
Iron combines with most non-metals on heating, and forms the
oxides Fe 2 O 3 and (mainly) Fe 3 O 4 when heated in air above 430 K.
Steam above 800 K produces the oxide Fe 3 O 4 and hydrogen. Iron
dissolves in most dilute acids, giving iron(II) solutions, i.e.
Fe + 2H + (aq) -> Fe^2 + (aq) + H 2This follows from the E^ value for the half-reaction
Fe2+(aq) -f 2e" -> Fe(s): £e = - 0.44 V(The impurities in ordinary iron assist dissolution in acid, and are
responsible for the characteristic smell of the hydrogen from this
source.) In dilute nitric acid, ammonium nitrate is formed:
4Fe + 10H+ 4- NO 3 -* 4Fe2+ + NH^ + 3H 2 OConcentrated nitric acid renders the metal "passive; i.e. chemically
unreactive, due to formation of a thin oxide surface film (which can
be removed by scratching or heating in hydrogen).
Iron is a good reducing agent (see the £° value just given): it