Names | |
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Other names | |
Identifiers | |
3D model (JSmol) | |
ChEBI | |
ChemSpider | |
ECHA InfoCard | 100.013.783 |
EC Number |
|
11116 | |
PubChem CID | |
RTECS number |
|
UNII | |
UN number | 3077 |
CompTox Dashboard (EPA) | |
| |
| |
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 |
+1960.0×10−6 cm3/mol | |
Refractive index (nD) | 2.551 |
Structure [1] | |
Corundum | |
R3c (No. 167) | |
a = 495 pm, c = 1358 pm | |
Thermochemistry | |
Std molar entropy (S⦵298) | 81 J·mol−1·K−1 |
Std enthalpy of formation (ΔfH⦵298) | −1128 kJ·mol−1 |
Hazards | |
GHS labelling: | |
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). |
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.
Cr
2O
3 has the corundum 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 to 8.5). [3] It is antiferromagnetic up to 307 K, the Néel temperature. [4] [5] It is not readily attacked by acids.
Cr
2O
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]
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. [6] It is derived from the mineral chromite, (Fe,Mg)Cr
2O
4. The conversion of chromite to chromia proceeds via Na
2Cr
2O
7, which is reduced with sulfur at high temperatures: [7]
The oxide is also formed by the decomposition of chromium salts such as chromium nitrate, or by the exothermic decomposition of ammonium dichromate.
The reaction has a low ignition temperature of less than 200 °C and is frequently used in “volcano” demonstrations. [8]
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]
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.
From 1962, it has been used as an inert marker in livestock intake research. However, its use in animal science research has been diminishing with the increased use of Titanium dioxide due to the latter being consider more food safe, allowing for the public sale of animals used in research trials that use an inert digestion marker. [9]
Chromium(III) oxide is amphoteric. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as [Cr(H
2O)
6]3+
. [10] 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:
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:
Chromates can be formed by the oxidation of chromium(III) oxide and another oxide in a basic environment:
Chromium is a chemical element; it has 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.
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, since rust shares several properties and has a similar composition; however, in chemistry, rust is considered an ill-defined material, described as hydrous ferric oxide.
Chromic acid is jargon for a solution formed by the addition of sulfuric acid to aqueous solutions of dichromate. It consists at least in part of chromium trioxide.
Chromate salts contain the chromate anion, CrO2−
4. Dichromate salts contain the dichromate anion, Cr
2O2−
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.
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.
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.
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 laboratories because it is not deliquescent, in contrast to the more industrially relevant salt sodium dichromate.
Chromium(III) chloride (also called chromic chloride) is an inorganic chemical compound with the chemical formula CrCl3. It forms several hydrates with the formula CrCl3·nH2O, among which are hydrates where n can be 5 (chromium(III) chloride pentahydrate CrCl3·5H2O) or 6 (chromium(III) chloride hexahydrate CrCl3·6H2O). The anhydrous compound with the formula CrCl3 are violet crystals, while the most common form of the chromium(III) chloride are the dark green crystals of hexahydrate, CrCl3·6H2O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.
Chromium trioxide is an 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-purple solid under anhydrous conditions and bright orange when wet. The substance dissolves in water accompanied by hydrolysis. Millions of kilograms are produced annually, mainly for electroplating. Chromium trioxide is a powerful oxidiser, a mutagen, and a carcinogen.
Copper chromite is an inorganic compound with the formula Cu2Cr2O5. It is a black solid that is used to catalyze reactions in organic synthesis.
Chromium compounds are compounds containing the element chromium (Cr). 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.
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.
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.
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.
In chemistry the term chromite has been used in two contexts. Under IUPAC naming conventions, chromate(III) is preferred to chromite.
The Jones oxidation is an organic reaction for the oxidation of primary and secondary alcohols to carboxylic acids and ketones, respectively. It is named after its discoverer, Sir Ewart Jones. The reaction was an early method for the oxidation of alcohols. Its use has subsided because milder, more selective reagents have been developed, e.g. Collins reagent.
Chromyl fluoride is an inorganic compound with the formula CrO2F2. It is a violet-red colored crystalline solid that melts to an orange-red liquid.
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.
Guyanaite (CrOOH) is a chromium oxide mineral that forms as an intergrowth with other chromium oxide minerals known as bracewellite (CrOOH) and grimaldiite (CrOOH) as well as eskolaite (Cr2O3) which in early findings were nearly indistinguishable from one another. These oxides formed so closely as intergrowths with one another that they were initially, and erroneously, identified as a single definite mineral previously known as merumite. Because of its complex history and the previously undiscovered nature of these chromium oxide polymorphs, the relevance of any information found in many early experiments involving the mineral formerly known as merumite in regard to guyanaite is unknown and it is implied that in any further reference of merumite it will have been composed of a mineral assemblage including guyanaite. The rare occurrence and complexity from intergrowth of naturally occurring guyanaite hinders experimental work, leading to laboratory synthesized samples which help to better experiment with the minerals.