Several chemical compounds consisting of two simple elements - Cr and O - belong to the class of inorganic compounds - oxides. Their common name is chromium oxide, then in brackets it is customary to indicate the valency of the metal in Roman numerals. Other names and chemical formulas:
- chromium (II) oxide - chromium oxide, CrO;
- chromium (III) oxide - chrome greens, chromium sesquioxide, Cr2O3;
- chromium (IV) oxide - chromium oxide, CrO2;
- chromium (VI) oxide - chromic anhydride, chromium trioxide, CrO3.
A compound in which the metal is hexavalent is higher chromium oxide. This odorless solid, in appearance it is dark red crystals (in the air they are diffused due to strong hygroscopicity). Molar mass is 99.99 g / mol. The density at 20 ° C is 2.70 g / cm³. Melting point - 197 ° C, boiling point - 251 ° C. At 0 ° C, 61.7 g / 100 dissolves in water, 63 g / 100 ml at 25 ° C, and 67.45 g / 100 ml at 100 ° C. The oxide also dissolves in sulfuric acid (it is a chromic mixture that is used in laboratory practice for washing chemical dishes) and nitric acid, ethyl alcohol, ethyl ether, acetic acid, acetone. At 450 ° C it decomposes to Cr2O3.
Chromium (VI) oxide is used in the electrolysis process (to obtain pure chromium), for the chromation of galvanized products, in electrolytic chromium plating, as a strong oxidizing agent (for the production of indigo and isatin). Higher chromium oxide is used to detect alcohol in expired air. The interaction proceeds according to the scheme: 4CrO3 + 6H2SO4 + 3C2H5OH → 2Cr2 (SO4) 3 + 3CH3COOH + 9H2O. The presence of alcohol is indicated by a color change in the solution (acquires a green color).
Chromium (VI) oxide, like all hexavalent Cr compounds, is a strong poison (lethal dose - 0.1 g). Due to its high activity, CrO3 causes a fire (with explosions) of organic substances in contact with them. Despite its low volatility, higher chromium oxide is dangerous if inhaled, as it causes lung cancer. Upon contact with the skin (even when it is removed soon) it causes irritation, dermatitis, eczema, and provokes the development of cancer.
In appearance, the tetravalent chromium oxide CrO2 is a solid in the form of black tetrahedral ferromagnetic crystals. Chromium oxide 4 has a molar mass of 83.9949 g / mol, density 4.89 g / cm³. The substance melts while decomposing at a temperature of 375 ° C. It does not dissolve in water. Used in magnetic recording media as a working substance. With the growing popularity of CDs and DVDs, the use of chromium (IV) oxide has declined. It was first synthesized in 1956 by Norman L. Cox, a chemist at EI DuPont, by decomposing chromium trioxide in the presence of water at a temperature of 640 ° C and a pressure of 200 MPa. DuPont is licensed by Sony in Japan and BASF in Germany.
Chromium oxide 3 Cr2O3 is a solid crystalline substance from light to dark green in color. The molar mass is 151.99 g / mol. Density - 5.22 g / cm³. Melting point - 2435 ° , boiling point - 4000 ° . The refractive index of the pure substance is 2.511. This oxide is not soluble in water, in alcohol, acetone, acid. Since its density approaches the density of corundum, it is introduced into the composition of polishing agents (for example, GOI paste). This is one of the main chromium oxides that is used as a pigment. For the first time using secret technology, it was obtained in 1838 in the form of a transparent hydrated form. It is found in nature in the form of chrome iron ore FeO • Cr2O3.
Divalent chromium oxide is a solid substance of black or red color with a melting point of 1550 ° . Melts with decomposition. Molar mass is 67.996 g / mol. Chromium (II) oxide of red color is not pyrophoric, and the same substance of black color is pyrophoric. The powder spontaneously ignites in air, so it can only be stored under a layer in water, since it does not interact with it. It is very difficult to obtain pure black chromium oxide in its pure form.
For chromium oxides with a lower valency, the basic properties are characteristic, and for an oxide with a higher valency, acidic.