iron(II) chloride A green-yellow
deliquescent compound, FeCl 2 ;
hexagonal; r.d. 3.16; m.p. 670°C. It
also exists in hydrated forms:
FeCl 2 .2H 2 O (green monoclinic; r.d.
2.36) and FeCl 2 .4H 2 O (blue-green
monoclinic deliquescent; r.d. 1.93).
Anhydrous iron(II) chloride can be
made by passing a stream of dry hy-
drogen chloride over the heated
metal; the hydrated forms can be
made using dilute hydrochloric acid
or by recrystallizing with water. It is
converted into iron(III) chloride by
the action of chlorine.
iron(III) chloride A black-brown
solid, FeCl 3 ; hexagonal; r.d. 2.9; m.p.
306 °C; decomposes at 315°C. It also
exists as the hexahydrate FeCl 3 .6H 2 O,
a brown-yellow deliquescent crys-
talline substance (m.p. 37°C; b.p.
280–285°C). Iron(III) chloride is pre-
pared by passing dry chlorine over
iron wire or steel wool. The reaction
proceeds with incandescence when
started and iron(III) chloride sublimes
as almost black iridescent scales. The
compound is rapidly hydrolysed in
moist air. In solution it is partly
hydrolysed; hydrolysis can be sup-
pressed by the addition of hydro-
chloric acid. The compound dissolves
in many organic solvents, forming
solutions of low electrical conductiv-
ity: in ethanol, ethoxyethane, and
pyridine the molecular weight cor-
responds to FeCl 3 but is higher in
other solvents corresponding to
Fe 2 Cl 6. The vapour is also dimerized.
In many ways the compound resem-
bles aluminium chloride, which it
may replace in Friedel–Crafts reac-
tions.
iron(II) oxide A black solid, FeO;
cubic; r.d. 5.7; m.p. 1420°C. It can be
obtained by heating iron(II) oxalate;
the carbon monoxide formed pro-
duces a reducing atmosphere thus
preventing oxidation to iron(III)
oxide. The compound has the sodium
chloride structure, indicating its
ionic nature, but the crystal lattice is
deÜcient in iron(II) ions and it is non-
stoichiometric. Iron(II) oxide dis-
solves readily in dilute acids.iron(III) oxide A red-brown to
black insoluble solid, Fe 2 O 3 ; trigonal;
r.d. 5.24; m.p. 1565°C. There is also a
hydrated form, Fe 2 O 3 .xH 2 O, which is
a red-brown powder; r.d. 2.44–3.60.
(See rusting.)
Iron(III) oxide occurs naturally as
*haematite and can be prepared by
heating iron(III) hydroxide or iron(II)
sulphate. It is readily reduced on
heating in a stream of carbon
monoxide or hydrogen.iron pyrites See pyrite.iron(II) sulphate An off-white
solid, FeSO 4 .H 2 O; monoclinic; r.d.
2.970. There is also a heptahydrate,
FeSO 4 .7H 2 O; blue-green monoclinic;
r.d. 1.898; m.p. 64°C. The heptahy-
drate is the best known iron(II) salt
and is sometimes called green vitriol
or copperas. It is obtained by the ac-
tion of dilute sulphuric acid on iron
in a reducing atmosphere. The anhy-
drous compound is very hygroscopic.
It decomposes at red heat to give
iron(III) oxide, sulphur trioxide, and
sulphur dioxide. A solution of iron(II)
sulphate is gradually oxidized on ex-
posure to air, a basic iron(III) sul-
phate being deposited.iron(III) sulphate A yellow hygro-
scopic compound, Fe 2 (SO 4 ) 3 ; rhom-
bic; r.d. 3.097; decomposes above
480 °C. It is obtained by heating an
aqueous acidiÜed solution of iron(II)
sulphate with hydrogen peroxide:
2FeSO 4 + H 2 SO 4 + H 2 O 2 →
Fe 2 (SO 4 ) 3 + 2H 2 O
On crystallizing, the hydrate
Fe 2 (SO 4 ) 3 .9H 2 O is formed. The acid
sulphate Fe 2 (SO 4 ) 3 .H 2 SO 4 .8H 2 O is de-293 iron(III) sulphate
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