Chromium(III) oxide is the
inorganic compoundTraditionally, inorganic compounds are considered to be of a mineral, not biological, origin. Complementarily, most organic compounds are traditionally viewed as being of biological origin...
of the
formulaA chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
Cr
2O
3. It is one of principal oxides of
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, 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 odourless, tasteless, and malleable...
and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.
Cr
2O
3 adopts the
corundumCorundum 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...
structure, consisting of a hexagonal close packed array of oxide anions with 2/3 of the octahedral holes occupied by chromium. Similar to corundum, Cr
2O
3 is a hard, brittle material (Mohs hardness 8-8.5).
It is
antiferromagneticIn 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.
Chromium(III) oxide is the
inorganic compoundTraditionally, inorganic compounds are considered to be of a mineral, not biological, origin. Complementarily, most organic compounds are traditionally viewed as being of biological origin...
of the
formulaA chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
Cr
2O
3. It is one of principal oxides of
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, 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 odourless, tasteless, and malleable...
and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.
Structure and properties
Cr
2O
3 adopts the
corundumCorundum 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...
structure, consisting of a hexagonal close packed array of oxide anions with 2/3 of the octahedral holes occupied by chromium. Similar to corundum, Cr
2O
3 is a hard, brittle material (Mohs hardness 8-8.5).
It is
antiferromagneticIn 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 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
chromiteChromite is iron magnesium chromium oxide: Cr
2O
4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts; also, aluminium and ferric iron commonly substitute for chromium....
).
Occurrence
Cr
2O
3 occurs naturally in mineral
Eskolaite, which is found in chromium-rich termolite skarns, metaquartzites, and chlorite veins. Eskolaite is also a rare component of chondrite meteorites. The mineral is named after Finnish geologist
Pentti EskolaPentti Eelis Eskola was a Finnish geologist who developed the concept of metamorphic facies. He won the Wollaston Medal in 1958, and was given a state funeral upon his death....
.
Production
The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr
2O
3 in 1838 via a secret process, sold as a pigment. It is derived from the mineral
chromiteChromite is iron magnesium chromium oxide: Cr
2O
4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts; also, aluminium and ferric iron commonly substitute for chromium....
, (Fe,Mg)Cr
2O
4. The conversion of chromite to chromia proceeds via
Na2Cr2O7Sodium 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...
, 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.
2Cr
2O
7 → Cr
2O
3 + N
2 + 4 H
2O
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
thermiteThermite is a pyrotechnic composition of a metal powder and a metal oxide, which produces an aluminothermic reaction known as a thermite reaction. It is not explosive, but can create short bursts of extremely high temperatures focused on a very small area for a short period of time.Thermites can...
-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 commonly used pigment and was originally called
viridianViridian 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 dioxideChromium 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
Reactions
Chromium(III) oxide is amphoteric. Although insoluble in water, it dissolves in acid to produce hydrated chromium ions, [Cr(H
2O)
6]
3+. It dissolves in concentrated alkali to yield chromite ions. When heated 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) chlorideChromium chloride is a violet coloured solid with the formula CrCl
3. The most common form of CrCl
3 sold commercially is a dark green hexahydrate with the formula [CrCl
24]Cl
.2H
2O...
:
- Cr2O3 + 3 Cl2 + 3 C → 2 CrCl3 + 3 CO
See also
- 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....
- 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...
- Chromic acid
Chromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H2CrO4 and...