ferric oxide

Iron(III) oxide
Other names Ferric oxide, Hematite, ferric iron, red iron oxide, synthetic maghemite, colcothar, iron sesquioxide, rust
Identifiers
CAS number	1309-37-1 Y
PubChem	518696
ChemSpider	21106565 Y
UNII	1K09F3G675 Y
ChEBI	CHEBI:50819 Y
RTECS number	NO7400000
Jmol-3D images	Image 1
SMILES O1[Fe]2O[Fe]1O2
InChI InChI=1S/2Fe.3O Y Key: JEIPFZHSYJVQDO-UHFFFAOYSA-N Y InChI=1/2Fe.3O/rFe2O3/c3-1-4-2(3)5-1 Key: JEIPFZHSYJVQDO-ZVGCCQCPAC
Properties
Molecular formula	Fe2O3
Molar mass	159.69 g/mol
Appearance	red-brown solid
Odor	odorless
Density	5.242 g/cm3, solid
Melting point	1566 °C (1838 K) decomp.
Solubility in water	insoluble
Structure
Crystal structure	rhombohedral
Thermochemistry
Std enthalpy of formation ΔfHo298	−826 kJ·mol−1[1]
Standard molar entropy So298	90 J·mol−1·K−1[1]
Hazards
EU classification	not listed
Flash point	non-flammable
Related compounds
Other anions	iron(III) fluoride
Other cations	manganese(III) oxide, cobalt(III) oxide
Related compounds	iron(II) oxide, iron(II,III) oxide
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe₂O₃. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare, and iron(II,III) oxide (Fe₃O₄), which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe₂O₃ is the main source of the iron for the steel industry. Fe₂O₃ is ferromagnetic, dark red, and readily attacked by acids. Rust is often called iron(III) oxide, and to some extent this label is useful, because rust shares several properties and has a similar composition. To a chemist, rust is considered an ill-defined material, described as hydrated ferric oxide.

Structure

Fe₂O₃ can be obtained in various polymorphs. In the main ones, α and γ, iron adopts octahedral coordination geometry. That is, each Fe centre is bound to six oxygen ligands.

Alpha phase

α-Fe₂O₃ has the rhombohedral, corundum (α-Al₂O₃) structure and is the most common form. It occurs naturally as the mineral hematite which is mined as the main ore of iron. It is antiferromagnetic below ~260 K (Morin transition temperature), and weak antiferromagnetic between 260 K and 950 K Néel temperature.^[2] It is easy to prepare using both thermal decomposition and precipitation in the liquid phase. Its magnetic properties are dependent on many factors, e.g. pressure, particle size, and magnetic field intensity.

Gamma phase

Cubic, metastable, converts to the alpha phase at high temperatures. Occurs naturally as the mineral maghemite. Ferromagnetic. Ultrafine particles smaller than 10 nanometers are superparamagnetic. Can be prepared by thermal dehydratation of gamma iron(III) oxide-hydroxide, careful oxidation of iron(II,III) oxide. The ultrafine particles can be prepared by thermal decomposition of iron(III) oxalate.^{[citation needed]}

Other phases

Several other phases have been aimed. The beta-phase is cubic face centered, metastable, at temperatures above 500 °C (930 °F) converts to alpha phase. It can be prepared by reduction of hematite by carbon, pyrolysis of iron(III) chloride solution, or thermal bonding of iron(III) sulfate. The epsilon phase is rhombic, shows properties intermediate between alpha and gamma. So far has not been prepared in pure form; it is always mixed with the alpha phase or gamma phases. Material with a high proportion of epsilon phase can be prepared by thermal transformation of the flatchalanta phase. This phase is also metastable, transforming to the alpha phase at between 500 and 750 °C (930 and 1,380 °F). Can also be prepared by oxidation of iron in an electric arc or by sol-gel precipitation from iron(III) nitrate.^{[citation needed]} Additionally at high pressure an amorphous form is claimed.^[3]

Reactions

The most important reaction is its carbothermal reduction, which gives iron used in steel-making:

2 Fe₂O₃ + 3 C → 4 Fe + 3 CO₂

Another redox reaction is the extremely exothermic thermite reaction with aluminium.^[4]

2 Al + Fe₂O₃ → 2 Fe + Al₂O₃

This process is used to weld thick metals such as rails of train tracks by using a ceramic container to funnel the molten iron in between two sections of rail. Thermite is also used in weapons and making small-scale cast-iron sculptures and tools.^{[citation needed]}

Partial reduction with hydrogen at about 400 °C gives magnetite, a black magnetic material that contains both Fe(III) and Fe(II):^[5]

3 Fe₂O₃ + H₂ → 2 Fe₃O₄ + H₂O

Iron(III) oxide is insoluble in water but dissolves readily in strong acid, e.g. hydrochloric and sulfuric acids. It also dissolves well in solutions of the chelating agents such as EDTA and oxalic acid.

Heating iron(III) oxides with other metal oxides or carbonates yields materials known as ferrates:^[5]

ZnO + Fe₂O₃ → Zn(FeO₂)₂

Preparation

Iron (III) oxide is a product of the oxidation of iron. It can be prepared in the laboratory by electrolyzing a solution of sodium bicarbonate, an inert electrolyte, with an iron anode:

4 Fe + 3 O₂ + 2 H₂O → 4 FeO(OH)

The resulting hydrated iron(III) oxide, written here as Fe(O)OH, dehydrates around 200 °C.^[5][6]

2 FeO(OH) → Fe₂O₃ + H₂O

It can also be prepared by the thermal decomposition of Iron (III) hydroxide under temperature above 200 °C.

2 Fe(OH)₃ → Fe₂O₃ + 3H₂O

Uses

Iron industry

The overwhelming application of Iron(III) oxide is as the feedstock of the steel and iron industries, e.g. the production of iron, steel, and many alloys.^[6]

Polishing

A very fine powder of ferric oxide is known as "jeweler's rouge", "red rouge", or simply rouge. It is used to put the final polish on metallic jewelry and lenses, and historically as a cosmetic.

Rouge cuts more slowly than some modern polishes, such as cerium(IV) oxide, but is still used in optics fabrication and by jewelers for the superior finish it can produce. When polishing gold, the rouge slightly stains the gold, which contributes to the appearance of the finished piece. Rouge is sold as a powder, paste, laced on polishing cloths, or solid bar (with a wax or grease binder). Other polishing compounds are also often called "rouge", even when they do not contain iron oxide. Jewelers remove the residual rouge on jewelry by use of ultrasonic cleaning.

Tool Sharpening

Products sold as stropping compound are often applied to a leather strop to assist in getting a razor edge on knives, straight razors, or any other edged tool.

Pigment

Iron(III) oxide is also used as a pigment, under names "Pigment Brown 6", "Pigment Brown 7", and "Pigment Red 101".^[7] Some of them, e.g. Pigment Red 101 and Pigment Brown 6, are Food and Drug Administration (FDA)-approved for use in cosmetics.

Niche uses

Being inexpensive and nontoxic, ferric oxide finds many niche uses. For example, in its granular form (GFO, granular-ferric-oxide), it is used to remove phosphates in aquariums. The crystals are also used, due to their ferromagnetism, in audio tapes, as the level of magnetism records the amplitude of signal to be recorded.^[8]

See also

• Iron oxides

References

External links

Wikimedia Commons has media related to: Iron(III) oxide

• NIOSH Pocket Guide to Chemical Hazards