Metal Chromium III Oxides

After the mixture is cooled, the solid is washed with water to remove the excess double chloride and the alkali salts formed. After filtration the divalent metal chromium(III) oxide is dried by heating in air at 200°. 

TABLE III Analyses and X-ny Data for Divalent Metal ChromiumfUI) Oxides 

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Owing to the slow rates of diffusion of the cations, the direct solid-state reaction of the oxides Cr2 03 and MO at an elevated temperature is not a good preparation of divalent metal chromium(III) oxides. They can be prepared by more elaborate methods, such as controlled reduction of dichromates MCr207,1 reaction of dichromium tungsten oxide Cr2W06 with a molten divalent metal fluoride2 at 1400°, pyrolysis of complexes,3 and pulverization of slurries containing Cr2 03 and a divalent metal salt.4... 

Metal chromium(III) oxides are often used as catalysts, especially dehydrogenation catalysts hence it is important to prepare them as powders.5 The synthesis described here involves the double-decomposition reaction between lithium chromium(III) oxide and molten divalent metal chlorides, and produces finely divided powders. 

Tlie purity of metal(Il) chromium(III) oxides was checked by chemical analysis. Chromium and divalent metal contents agree with the calculated values within less than 1%. Divalent metal chromium(III) oxides crystallize in the cubic spinel system. Chemical analysis data, cubic lattice constants and powder diffraction lines are given in Table III. 

Divalent metal chromium(III) oxides produced by a double-decomposition reaction between LiCr02 and a molten metal(II) salt are fine powders. Electron microscopic examination shows that these powders are constituted of grains, the repartition size of which is heterogeneous (500-3000 A), but each grain is made of smaller crystallites. The average diameter of the crystallites, determined by a radiocrystallographical method (measurement of the widening of X-rays 220 and 335), is about 250 A. 

A detailed study of the dehydrogenation of 10.1 l-dihydro-5//-benz[6,/]azcpinc (47) over metal oxides at 550 C revealed that cobalt(II) oxide, iron(III) oxide and manganese(III) oxide are effective catalysts (yields 30-40%), but formation of 5//-dibenz[7),/]azepinc (48) is accompanied by ring contraction of the dihydro compound to 9-methylacridine and acridine in 3-20 % yield.111 In contrast, tin(IV) oxide, zinc(II) oxide. chromium(III) oxide, cerium(IV) oxide and magnesium oxide arc less-effective catalysts (7-14% yield) but provide pure 5H-dibenz[b,/]azepine. On the basis of these results, optimum conditions (83 88% selectivity 94-98 % yield) for the formation of the dibenzazepine are proposed which employ a K2CO,/ Mn203/Sn02/Mg0 catalyst (1 7 3 10) at 550 C. 

C04-0102. Write the balanced redox reactions for the formation of each of the following oxides from the reaction of molecular oxygen with pure metal (a) strontium oxide (b) chromium(III) oxide (c) tin(IV) oxide. 

Pure chromium metal is made by a two-step reduction sequence. First, sodium dichromate is reduced to chromium(III) oxide by heating in the presence of charcoal ... 

Chromium (III) oxide gave rise to a very violent reaction with lithium heated to 180 C. The temperature exceeds 1000 C. It is a typical thermite reaction during which a more electropositive metai violentiy reduces an oxide by forming a metal, which is chromium in this particular case. However, the mixture needs to reach a high temperature to be able to react. 

Cuprous oxide is also reduced violently by electropositive metals as discovered in an accident which occurred with aluminium. With chromium (III) oxide the reaction enables one to make copper chromite, which is a very common catalyst. The activity of copper chromite is such that it frequently combusts at the end of the reaction. 

Lithium is used to reduce metallic oxides in metallurgical operations, and the reactions, after initiation at moderate temperatures, are violently exothermic and rapid. Chromium(III) oxide reacts at 185°C, reaching 965° similarly molybdenum trioxide (180 to 1400°), niobium pentoxide (320 to 490°), titanium dioxide (200-400 to 1400°), tungsten trioxide (200 to 1030°), vanadium pentoxide (394 to 768°) also iron(II) sulfide (260 to 945°), and manganese tclluridc (230 to 600°C)... 

Chromium metal is produced hy thermal reduction of chromium(III) oxide, Cr203 by aluminum, silicon or carbon. The starting material in all these thermal reduction processes are Cr203 which is obtained from the natural ore chromite after the removal of iron oxide and other impurities. In the aluminum reduction process, the oxide is mixed with A1 powder and ignited in a refractory-lined vessel. The heat of reaction is sufficient to sustain the reaction at the required high temperature. Chromium obtained is about 98% pure, containing traces of carbon, sulfur and nitrogen. 

The use of chromium(III) oxide as a pigment for toys, cosmetics, and in plastics and paints that come in contact with food is permitted in national and international regulations [3.62] -[3.68]. Maximum limits for heavy metals or their soluble fractions are usually a prerequisite. Because pure starting materials are used, these limits are satisfied by most types of chromium oxide. 

What mass of aluminum is needed to reduce 10.0 kg of chromium(III) oxide to produce chromium metal The chemical equation for the reaction is... 

In addition to its use in making steels, chromium is widely used to electroplate metallic objects with an attractive, protective coating (Section 18.12). Chromium is hard and lustrous, takes a high polish, and resists corrosion because an invisible, microscopic film of chromium(III) oxide shields the surface from further oxidation. 

Metal Oxides. Reduction of metal oxides such as chromium(III) oxide, molybdenum trioxide, niobium pentoxide, tungsten trioxide, and vanadium pentoxide is violently exothermic and rapid.9... 

Carbon tetrafluoride, 1 34 3 178 Carbon tetraiodide, 3 37 Carbonyl azide, formation of, by carbohydrazide, 4 35 Carbonyl fluoride, 6 155 Carbonyls, metal, 2 229 metal, nomenclature of, 2 264 structure of, 2 232 Catalysts, beryllium chloride, 5 25 boron fluoride, 1 23 chromium(III) oxide gel, 2 190... 

Chromium(III) oxide, CrgOg, is a product of the oxidation of the metal and of the thermal decomposition of ammonium dichromate ... 

Chromium metal is used in metal alloys and as a surface plating on other metals to minimize corrosion. It can be obtained by reducing the chromium(III) in chromium(III) oxide, Ct20j, to the uncharged metal with finely divided aluminum. The Ct203 is found in an ore called chromite. What is the maximum mass, in kilograms, of chromium that can be made from... 

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