Chromium(III) oxide
Encyclopedia
Chromium oxide is the inorganic compound
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 of the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 Cr
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

2O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

3. It is one of principal oxides of chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

 and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.

Structure and properties

Cr2O3 adopts the corundum
Corundum
Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

 structure, consisting of a hexagonal close packed array of oxide anions with 2/3 of the octahedral holes occupied by chromium. Similar to corundum, Cr2O3 is a hard, brittle material (Mohs hardness 8-8.5).
It is antiferromagnetic
Antiferromagnetism
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usuallyrelated to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism...

 up to 307 K, the Neel temperature. It is not readily attacked by acids or bases, although molten alkali gives chromites (salts with the CrO anion, not to be confused with the related mineral chromite
Chromite
Chromite is an iron chromium oxide: FeCr2O4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite ; substitution of aluminium occurs leading to hercynite .-Occurrence:Chromite is found in...

).
It turns brown when heated, but reverts to its dark green color when cooled.
It is also hygroscopic.

Occurrence

Cr2O3 occurs naturally in mineral eskolaite, which is found in chromium-rich tremolite skarns, metaquartzites, and chlorite veins. Eskolaite is also a rare component of chondrite meteorites. The mineral is named after Finnish geologist Pentti Eskola.

Production

The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr2O3 in 1838 via a secret process, sold as a pigment. It is derived from the mineral chromite
Chromite
Chromite is an iron chromium oxide: FeCr2O4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite ; substitution of aluminium occurs leading to hercynite .-Occurrence:Chromite is found in...

, (Fe,Mg)Cr2O4. The conversion of chromite to chromia proceeds via Na2Cr2O7
Sodium dichromate
Sodium dichromate is the chemical compound with the formula Na2Cr2O7. Usually, however, the salt is handled as its dihydrate Na2Cr2O7·2H2O. Virtually all chromium ore is processed via conversion to sodium dichromate. In this way, many millions of kilograms of sodium dichromate are produced...

, which is reduced with sulfur at high temperatures:
Na2Cr2O7 + S → Na2SO4 + Cr2O3


The oxide is also formed by the decomposition of chromium salts such as chromium nitrate or by the exothermic decomposition of ammonium dichromate.
2Cr2O7 → Cr2O3 + N2 + 4 H2O

The reaction has a low ignition temperature of less than 200 C and is frequently used in "volcano" demonstrations.

Chromium oxide can be converted into elemental chromium metal through a thermite
Thermite
Thermite is a pyrotechnic composition of a metal powder and a metal oxide that produces an exothermic oxidation-reduction reaction known as a thermite reaction. If aluminium is the reducing agent it is called an aluminothermic reaction...

-like reaction: unlike iron oxide thermites, chromium oxide thermites creates few or no sparks, smoke or sound, but glow brightly. Because of the very high melting point of chromium, chromium thermite casting is impractical.

Applications

Because of its considerable stability, chromia is a commonly used pigment and was originally called viridian
Viridian
Viridian is a blue-green pigment, a hydrated chromium oxide, of medium saturation and relatively dark in value. It is composed more of green than blue. Specifically, it is a dark shade of spring green, the color between green and cyan on the color wheel...

. It is used in paints, inks, and glasses. It is the colourant in "chrome green" and "institutional green." Chromium(III) oxide is the precursor to the magnetic pigment chromium dioxide
Chromium(IV) oxide
Chromium dioxide or chromium oxide is a synthetic magnetic substance once widely used in magnetic tape emulsion. With the increasing popularity of CDs and DVDs, the use of chromium oxide has declined. However, it is still used in data tape applications for enterprise-class storage systems...

, according to the following reaction:
Cr2O3 + 3 CrO3 → 5 CrO2 + O2


It is one of the materials that are used when polishing the edges of knives on a piece of leather (also called stropping). In this context it is often known as green compound.

Reactions

Chromium(III) oxide is amphoteric. Although insoluble in water, it dissolves in acid to produce hydrated chromium ions, [Cr(H2O)6]3+. It dissolves in concentrated alkali to yield chromite ions. When heated with finely divided aluminium or carbon, it is reduced to chromium metal and aluminum oxide:
Cr2O3 + 2 Al → 2 Cr + Al2O3


Heating with chlorine and carbon yields chromium(III) chloride
Chromium(III) chloride
Chromium chloride is a violet coloured solid with the formula CrCl3. The most common form of CrCl3 sold commercially is a dark green hexahydrate with the formula [CrCl24]Cl.2H2O. Two other hydrates are known, pale green [CrCl5]Cl2.H2O and violet [Cr6]Cl3...

:
Cr2O3 + 3 Cl2 + 3 C → 2 CrCl3 + 3 CO


Chromates can be formed by the oxidation of chromium(III) oxide and another oxide in a basic environment:
2 Cr2O3 + 4 MO + 3 O2 → 4 MCrO4

See also

  • Chromium(II) oxide
    Chromium(II) oxide
    Chromium oxide is an inorganic compound composed of chromium and oxygen. It is a black powder that crystallises in the rock salt structure.Hypophosphites may reduce chromium oxide to chromium oxide:It is readily oxidized by the atmosphere....

  • Chromium(IV) oxide
    Chromium(IV) oxide
    Chromium dioxide or chromium oxide is a synthetic magnetic substance once widely used in magnetic tape emulsion. With the increasing popularity of CDs and DVDs, the use of chromium oxide has declined. However, it is still used in data tape applications for enterprise-class storage systems...

  • Chromium trioxide
    Chromium trioxide
    Chromium trioxide is the inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.This compound is a dark-red/orange brown solid, which dissolves in water concomitant with hydrolysis...

  • Chromic acid
    Chromic acid
    The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the...

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