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Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO 3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. [6] This compound is a dark-purple solid under anhydrous conditions and bright orange when wet. The substance dissolves in water ...
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+. [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: Cr 2 O 3 + 2 Al → 2 Cr ...
Molecular chromic acid could in principle be made by adding chromium trioxide to water (cf. manufacture of sulfuric acid). CrO 3 + H 2 O ⇌ H 2 CrO 4. In practice, the reverse reaction occurs: molecular chromic acid dehydrates. Some insights can be gleaned from observations on the reaction of dichromate solutions with sulfuric acid.
Relative to the potassium salt, the main advantage of sodium dichromate is its greater solubility in water and polar solvents like acetic acid. For hexavalent chrome plating, chromate is converted to the so-called chromic acid (essentially chromium trioxide) by sulfuric acid. Sodium dichromate can be used in fluorene to fluorenone conversion.
Engelbrecht and von Grosse's synthesis of CrO 2 F 2, and most successive syntheses, involve treating chromium trioxide with a fluorinating agent: [5] CrO 3 + 2 HF → CrO 2 F 2 + H 2 O. The reaction is reversible, as water will readily hydrolyze CrO 2 F 2 back to CrO 3.
Further condensation reactions can occur in strongly acidic solution with the formation of trichromates, Cr 3 O 2− 10, and tetrachromates, Cr 4 O 2− 13. [2] All polyoxyanions of chromium(VI) have structures made up of tetrahedral CrO 4 units sharing corners. [3] The hydrogen chromate ion, HCrO 4 −, is a weak acid: HCrO − 4 ⇌ CrO 2−
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The Cornforth reagent is prepared by slow addition of a concentrated aqueous solution of chromium trioxide to pyridine. The reaction may cause explosion, which is avoided by thoroughly dissolving the trioxide in water and cooling the solution by ice. The product is filtered, washed with acetone and dried, yielding an orange powder.