Chromium(III) oxide

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Chromium(III) oxide
Cr2o3 gruener farbstoff.jpg
Corundum struct.png
Names
Other names
Chromium sesquioxide
Chromia
Chrome green
Eskolaite
Identifiers
1308-38-9 YesY
3D model (Jmol) Interactive image
ChEBI CHEBI:48242 YesY
ChemSpider 451305 YesY
ECHA InfoCard 100.013.783
PubChem 517277
RTECS number GB6475000
UNII X5Z09SU859 YesY
Properties
Cr2O3
Molar mass 151.9904 g/mol
Appearance light to dark green, fine crystals
Density 5.22 g/cm3
Melting point 2,435 °C (4,415 °F; 2,708 K)
Boiling point 4,000 °C (7,230 °F; 4,270 K)
insoluble
Solubility in alcohol insoluble in alcohol, acetone, acids
2.551
Structure
hexagonal
Thermochemistry
81 J·mol−1·K−1
−1128 kJ·mol−1
Hazards
US health exposure limits (NIOSH):
PEL (Permissible)
TWA 1 mg/m3[1]
REL (Recommended)
TWA 0.5 mg/m3[1]
IDLH (Immediate danger)
250 mg/m3[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY verify (what is YesYN ?)
Infobox references

Chromium(III) oxide is the inorganic compound of the formula Cr
2
O
3
. It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.

Structure and properties[edit]

Cr
2
O
3
adopts the corundum structure, consisting of a hexagonal close packed array of oxide anions with ⅔ of the octahedral holes occupied by chromium. Similar to corundum, Cr
2
O
3
is a hard, brittle material (Mohs hardness 8 to 8.5).[2] It is antiferromagnetic up to 307 K, the Néel temperature.[3][4] It is not readily attacked by acids or bases, although molten alkali gives chromate salts with the CrO
4
anion. It turns brown when heated, but reverts to its dark green color when cooled. It is also hygroscopic.[citation needed]

Occurrence[edit]

Eskolaite mineral

Cr
2
O
3
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.[2]

Production[edit]

The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr
2
O
3
in 1838 via a secret process, sold as a pigment.[5] It is derived from the mineral chromite, (Fe,Mg)Cr
2
O
4
. The conversion of chromite to chromia proceeds via Na
2
Cr
2
O
7
, which is reduced with sulfur at high temperatures:[6]

Na
2
Cr
2
O
7
+ S → Na
2
SO
4
+ Cr
2
O
3

The oxide is also formed by the decomposition of chromium salts such as chromium nitrate or by the exothermic decomposition of ammonium dichromate.

(NH
4
)
2
Cr
2
O
7
Cr
2
O
3
+ N
2
+ 4 H
2
O

The reaction has a low ignition temperature of less than 200 °C and is frequently used in “volcano” demonstrations.[7]

Applications[edit]

Because of its considerable stability, chromia is a commonly used pigment and was originally called viridian. It is used in paints, inks, and glasses. It is the colourant in "chrome green" and "institutional green." Chromium(III) oxide is a precursor to the magnetic pigment chromium dioxide, according to the following reaction:[6]

Cr
2
O
3
+ 3 CrO
3
→ 5 CrO
2
+ O
2

It is one of the materials that are used when polishing (also called stropping) the edges of knives, razors, surfaces of optical devices etc. on a piece of leather, balsa, cloth or other material. In this context it is alternatively known as "green compound".

Reactions[edit]

Chromium(III) oxide is amphoteric. Although insoluble in water, it dissolves in acid to produce hydrated chromium ions, [Cr(H
2
O)
6
]3+
which react with base to give salts of [Cr(OH)
6
]3−
.[8] It dissolves in concentrated alkali to yield chromite ions.

When heated with finely divided carbon it can be reduced to chromium metal with release of carbon dioxide.[citation needed] When heated with finely divided aluminium it is reduced to chromium metal and aluminum oxide:

Cr
2
O
3
+ 2 Al → 2 Cr + Al
2
O
3

Unlike the classic thermite reaction involving iron oxides, the chromium oxide thermite creates few or no sparks, smoke or sound, but glows brightly. Because of the very high melting point of chromium, chromium thermite casting is impractical.

Heating with chlorine and carbon yields chromium(III) chloride and carbon monoxide:

Cr
2
O
3
+ 3 Cl
2
+ 3 C → 2 CrCl
3
+ 3 CO

Chromates can be formed by the oxidation of chromium(III) oxide and another oxide in a basic environment:

2 Cr
2
O
3
+ 4 MO + 3 O
2
→ 4 MCrO
4

See also[edit]

References[edit]

  1. ^ a b c "NIOSH Pocket Guide to Chemical Hazards #0141". National Institute for Occupational Safety and Health (NIOSH). 
  2. ^ a b "Eskolaite". Webminerals. Retrieved 2009-06-06. 
  3. ^ J.E Greedan, (1994), Magnetic oxides in Encyclopedia of Inorganic chemistry R. Bruce King, Ed. John Wiley & Sons. ISBN 0-471-93620-0
  4. ^ A. F. Holleman and E. Wiberg "Inorganic Chemistry" Academic Press, 2001, New York. ISBN 0-12-352651-5.
  5. ^ Eastaugh, Nicholas; Chaplin, Tracey; Siddall, Ruth (2004). The pigment compendium: a dictionary of historical pigments. Butterworth-Heinemann. p. 391. ISBN 0-7506-5749-9. 
  6. ^ a b Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. doi:10.1002/14356007.a07_067
  7. ^ Ammonium dichromate volcano Retrieved 2009-06-06.
  8. ^ R. Scholder "Sodium Hexahydroxochromate(III)" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 2, 1688ff.