Chromium(III) oxide

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Chromium(III) oxide
Chromium(III) oxide.jpg
OTC Cr2O3 sold as pigment. It is impure and indurated.
Names
IUPAC name
Chromium(III) oxide
Other names
Chrome green
Chromia
Chromium sesquioxide
Dichromium trioxide
Eskolaite
Properties
Cr2O3
Molar mass 151.9904 g/mol
Appearance Green solid
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)
3·10-5 g/100 ml (20 °C)
Solubility Reacts with strong acids, molten alkali
Insoluble in alcohol, ethers, ketones
Vapor pressure ~0 mmHg
Thermochemistry
81 J·mol−1·K−1
−1128 kJ/mol
Hazards
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Related compounds
Related compounds
Chromium trioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Chromium(III) oxide, also known as chrome green, chromia, or chromium sesquioxide is the inorganic compound with chemical formula Cr2O3.

Properties

Chemical

Chromium(III) oxide reacts with acids to form chromium(III) salts, though samples that have been heavily annealed at high temperature, such as commercial varieties, are not readily attacked by acids or bases. Molten alkalis react with chromium(III) oxide to form chromates or chromites. Molten potassium nitrate reacts with it to form potassium dichromate, though indurated chromia reacts very slowly. Chromium(III) oxide can be used to produce a low-energy thermite with aluminum or magnesium powder, which is difficult to ignite.

Physical

Chromium(III) oxide is a dark green powder and is slightly hygroscopic. It turns brown when heated, but reverts to its dark green color when cooled.

Availability

Chrome green is available at pottery shops as a green pigment, sometimes with traces of calcium carbonate. This material, however, is generally too unreactive to produce other chromium compounds.

Purer chromium(III) oxide can be purchased from chemical suppliers.

Preparation

Chromium(III) oxide can be prepared by reducing potassium dichromate with sulfur:

K2Cr2O7 + S → K2SO4 + Cr2O3

It can also be prepared from the thermal decomposition of ammonium dichromate:

(NH4)2Cr2O7 → Cr2O3 + N2 + 4 H2O

or by reacting chromium trioxide with ethanol.

It is the most common byproduct of the reduction of chromium(VI) compounds, especially in neutral solution.

Projects

Handling

Safety

Chromium(III) oxide is not extremely reactive. If ingested it may cause nausea, vomiting and diarrhea. As it stains, gloves and protective clothing should be worn.

Storage

Chromia doesn't require any special storage.

Disposal

While not as toxic as chromium(VI) compounds, it's best to avoid dumping it in the environment.

References

Relevant Sciencemadness threads