Chromium(III) oxide

Last updated
Chromium(III) oxide
Cr2o3 gruener farbstoff.jpg
Corundum struct.png
Names
Other names
Chromium sesquioxide
Chromia
Chrome green
Eskolaite
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.013.783 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 215-160-9
11116
PubChem CID
RTECS number
  • GB6475000
UNII
UN number 3077
  • InChI=1S/2Cr.3O Yes check.svgY
    Key: QDOXWKRWXJOMAK-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/2Cr.3O/rCr2O3/c3-1-5-2-4
    Key: QDOXWKRWXJOMAK-LUXALHLMAR
  • O=[Cr]O[Cr]=O
Properties
Cr2O3
Molar mass 151.9904 g/mol
Appearancelight 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
+1960.0×10−6 cm3/mol
2.551
Structure [1]
Corundum
R3c (No. 167)
a = 495 pm, c = 1358 pm
Thermochemistry
Std molar
entropy
(S298)
81 J·mol−1·K−1
−1128 kJ·mol−1
Hazards
GHS labelling:
GHS-pictogram-exclam.svg GHS-pictogram-silhouette.svg
Danger
H302, H317, H319, H360
P201, P202, P261, P264, P270, P272, P280, P281, P301+P312, P302+P352, P305+P351+P338, P308+P313, P321, P330, P333+P313, P337+P313, P363, P405, P501
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 1 mg/m3 [2]
REL (Recommended)
TWA 0.5 mg/m3 [2]
IDLH (Immediate danger)
250 mg/m3 [2]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Chromium(III) oxide (or chromia) is an inorganic compound with 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.

Contents

Structure and properties

Cr
2
O
3
has the corundum structure, consisting of a hexagonal close packed array of oxide anions with 23 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). [3] It is antiferromagnetic up to 307 K, the Néel temperature. [4] [5] It is not readily attacked by acids.

Occurrence

Eskolaite mineral Eskolaite.jpg
Eskolaite mineral

Cr
2
O
3
occurs naturally as the 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. [3]

Production

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. [6] 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: [7]

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. [8]

Applications

Because of its considerable stability, chromia is a commonly used pigment. It was originally called viridian. It is used in paints, inks, and glasses. It is the colorant in "chrome green" and "institutional green." Chromium(III) oxide is a precursor to the magnetic pigment chromium dioxide, by the following reaction: [7]

Cr
2
O
3
+ 3 CrO
3
→ 5 CrO
2
+ O
2

Along with many other oxides, it is used as a compound 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. It is available in powder or wax form, and in this context it is known as "green compound".

It is used as a component of refractories due to its high melting point.

Reactions

Chromium(III) oxide is amphoteric. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as [Cr(H
2
O)
6
]3+
. [9] It is also attacked by concentrated alkali to yield salts of [Cr(OH)
6
]3−
.

When heated with finely divided carbon or aluminium, it is reduced to chromium metal:

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

Related Research Articles

<span class="mw-page-title-main">Chromium</span> Chemical element, symbol Cr and atomic number 24

Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.

In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively charged sodium ions and negatively charged chloride ions.

<span class="mw-page-title-main">Iron(III) oxide</span> Chemical compound

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 iron for the steel industry. Fe2O3 is readily attacked by acids. Iron(III) oxide is often called rust, and to some extent this label is useful, because rust shares several properties and has a similar composition; however, in chemistry, rust is considered an ill-defined material, described as Hydrous ferric oxide.

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 molecular species, H2CrO4 of which the trioxide is the anhydride. Chromic acid features chromium in an oxidation state of +6 (or VI). It is a strong and corrosive oxidising agent.

<span class="mw-page-title-main">Chromate and dichromate</span> Chromium(VI) anions

Chromate salts contain the chromate anion, CrO2−
4
. Dichromate salts contain the dichromate anion, Cr
2
O2−
7
. They are oxyanions of chromium in the +6 oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible.

<span class="mw-page-title-main">Chromite</span> Crystalline mineral

Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite (MgCr2O4). A substitution of the element aluminium can also occur, leading to hercynite (FeAl2O4). Chromite today is mined particularly to make stainless steel through the production of ferrochrome (FeCr), which is an iron-chromium alloy.

<span class="mw-page-title-main">Copper(II) oxide</span> Chemical compound – an oxide of copper with formula CuO

Copper(II) oxide or cupric oxide is an inorganic compound with the formula CuO. A black solid, it is one of the two stable oxides of copper, the other being Cu2O or copper(I) oxide (cuprous oxide). As a mineral, it is known as tenorite. It is a product of copper mining and the precursor to many other copper-containing products and chemical compounds.

<span class="mw-page-title-main">Potassium dichromate</span> Chemical compound

Potassium dichromate, K2Cr2O7, is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health. It is a crystalline ionic solid with a very bright, red-orange color. The salt is popular in the laboratory because it is not deliquescent, in contrast to the more industrially relevant salt sodium dichromate.

<span class="mw-page-title-main">Chromium(III) chloride</span> Chemical compound

Chromium(III) chloride (also called chromic chloride) describes any of several chemical compounds with the formula CrCl3 · xH2O, where x can be 0, 5, and 6. The anhydrous compound with the formula CrCl3 is a violet solid. The most common form of the trichloride is the dark green hexahydrate, CrCl3 · 6 H2O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.

<span class="mw-page-title-main">Copper chromite</span> Chemical compound

Copper chromite is an inorganic compound with the formula Cu2Cr2O5. It is a black solid that is used to catalyze reactions in organic synthesis.

<span class="mw-page-title-main">Chromium(III) fluoride</span> Chemical compound

Chromium(III) fluoride is the name for the inorganic compounds with the chemical formula CrF3 as well as several related hydrates. The compound CrF3 is a green crystalline solid that is insoluble in common solvents, but the coloured hydrates [Cr(H2O)6]F3 and [Cr(H2O)6]F3•3H2O are soluble in water. The trihydrate is green, and the hexahydrate is violet. The anhydrous form sublimes at 1100–1200 °C.

<span class="mw-page-title-main">Sodium dichromate</span> Inorganic compound

Sodium dichromate is the inorganic compound with the formula Na2Cr2O7. However, the salt is usually handled as its dihydrate Na2Cr2O7·2H2O. Virtually all chromium ore is processed via conversion to sodium dichromate and virtually all compounds and materials based on chromium are prepared from this salt. In terms of reactivity and appearance, sodium dichromate and potassium dichromate are very similar. The sodium salt is, however, around twenty times more soluble in water than the potassium salt (49 g/L at 0 °C) and its equivalent weight is also lower, which is often desirable.

<span class="mw-page-title-main">Chromyl chloride</span> Chemical compound

Chromyl chloride is the inorganic compound with the formula CrO2Cl2. It is a reddish brown compound that is a volatile liquid at room temperature, which is unusual for transition metal complexes.

<span class="mw-page-title-main">Chromium compounds</span> Chemical compounds containing chromium

Chromium is a member of group 6, of the transition metals. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.

<span class="mw-page-title-main">Ammonium dichromate</span> Chemical compound

Ammonium dichromate is an inorganic compound with the formula (NH4)2Cr2O7. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state, commonly known as hexavalent chromium. It is a salt consisting of ammonium ions and dichromate ions.

<span class="mw-page-title-main">Chromium(III) sulfate</span> Chemical compound

Chromium(III) sulfate usually refers to the inorganic compounds with the formula Cr2(SO4)3.x(H2O), where x can range from 0 to 18. Additionally, ill-defined but commercially important "basic chromium sulfates" are known. These salts are usually either violet or green solids that are soluble in water. It is commonly used in tanning leather.

<span class="mw-page-title-main">Sodium chromate</span> Chemical compound

Sodium chromate is the inorganic compound with the formula Na2CrO4. It exists as a yellow hygroscopic solid, which can form tetra-, hexa-, and decahydrates. It is an intermediate in the extraction of chromium from its ores.

<span class="mw-page-title-main">Chromite (compound)</span>

In chemistry the term chromite has been used in two contexts. Under IUPAC naming conventions, chromate(III) is preferred to chromite.

  1. For compounds containing an oxyanion of chromium in oxidation state 3
  2. For other compounds of chromium(III) as a means of distinguishing a chemical species such as hexacyanochromite(III). [Cr(CN)6]3− from an analogous compound in which chromium is a different oxidation state.

Chromium(III) hydroxide is a gelatinous green inorganic compound with the chemical formula Cr(OH)3. It is a polymer with an undefined structure and low solubility. It is amphoteric, dissolving in both strong alkalis and strong acids.

References

  1. Abdullah, M. M.; Rajab, Fahd M.; Al-Abbas, Saleh M. (2014). "Structural and optical characterization of Cr2O3 nanostructures: Evaluation of its dielectric properties". AIP Advances. AIP Publishing. 4 (2): 027121. Bibcode:2014AIPA....4b7121A. doi: 10.1063/1.4867012 . ISSN   2158-3226.
  2. 1 2 3 NIOSH Pocket Guide to Chemical Hazards. "#0141". National Institute for Occupational Safety and Health (NIOSH).
  3. 1 2 "Eskolaite". Webminerals. Retrieved 2009-06-06.
  4. J.E Greedan, (1994), Magnetic oxides in Encyclopedia of Inorganic chemistry R. Bruce King, Ed. John Wiley & Sons. ISBN   0-471-93620-0
  5. A. F. Holleman and E. Wiberg "Inorganic Chemistry" Academic Press, 2001, New York. ISBN   0-12-352651-5.
  6. Eastaugh, Nicholas; Chaplin, Tracey; Siddall, Ruth (2004). The pigment compendium: a dictionary of historical pigments . Butterworth-Heinemann. p.  391. ISBN   0-7506-5749-9.
  7. 1 2 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
  8. "Ammonium dichromate volcano". www.rsc.org. Retrieved 2019-02-26.
  9. R. Scholder "Sodium Hexahydroxochromate(III)" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 2, 1688ff.