Tungsten dioxide

Tungsten oxydichlofide [13520-76-8], WO2CI2, a pale-yeUow crystalline soHd having an mp of 266°C, is soluble in cold water and in alkaline solution, although partly decomposed by hot water. It is prepared by the action of carbon tetrachloride on tungsten dioxide at 250°C in a bomb (13). 

Tungsten dioxide [12036-22-5] WO2, is a brown powder formed by the reduction of WO3 with hydrogen at 575—600°C. Generally, this oxide is obtained as an intermediate in the hydrogen reduction of the trioxide to the metal. On reduction, first a blue oxide, then a brown oxide (WO2), is formed. The composition of the blue oxide was in doubt for a long time. However, it has since been resolved that W2Q03g and W are formed as intermediates, which may also be prepared by the reaction of tungsten with WO3. 

See Chlorine Tungsten dioxide See other METAL OXIDES... 

Tungsten oxytetrabromide [13520-77-9], WOBr black, deliquescent needles having an mp of 277°C and a bp of 327°C, is formed by the action of carbon tetrabromide on tungsten dioxide at 250°C (13). 

Crystals of tungsten dioxide are golden, and when grown by the procedure described above, they are equidimensional with approximately 2-mm. polyhedral faces. The crystallographic symmetry is monoclinic with unit cell parameters a0 — 5.5607 A., b0 = 4.9006 A., c0 = 5.6631 A., and 0 = 120.44°. 

Tungsten dioxide exhibits metallic conductivity (p30o°K 3 X 10 3 A-cm.). The resistivity ratio (p3oo°k/p4.2°k) measured for a typical crystal is about 20. Anal. Weight percent oxygen calcd. for W02 14.81. Found 14.89. 

Molybdenum(IV) oxide (Mo02), is obtained by reducing Mo03 with hydrogen or NH3 below 470°C (above this temperature reduction proceeds to the metal) or by reaction of molybdenum with steam at 800°C. It is a brown-violet solid with a coppery luster, insoluble in nonoxidizing mineral acids but soluble in concentrated nitric acid with oxidation of the molybdenum to Movr. The structure is derived from that of rutile but distorted so that strong Mo—Mo bonds are formed. Tungsten dioxide is similar. Mo—Mo and W—W distances are 2.51 and 2.49 A, respectively. 

Violent reaction with alcohols, N-aryl sulfinamides, dimethyl formamide, polychlorobiphenyl, sodium hydroxide, hydrochloric acid + dinitroanilines. Incandescent reaction when warmed with cesium oxide (above 150°), tellurium, arsenic, tungsten dioxide. Potentially dangerous reaction with hydrocarbons + Lewis acids releases toxic and reactive HCl gas. 

Oxybromides.—Two oxybroniides of tungsten, WOsBrj and WOBrj, are known. When bromine vapour is jiassed over red-hot tungsten dioxide, when a mixture of bromine vapour and air is passed over metallic tungsten, or when the pentabroniide is passed over heated tungsten trioxide, both oxybroniides are formed, and are separated by taking advantage of the fact that the oxybroniide, WOBr, is the more readily volatile. ... 

The density of tungsten dioxide, according to Karsten, is 12T109. It is stable in the air but is readily oxidised. Heated in oxygen, it yields the trioxide at 500° C. in nitrous oxide or nitric oxide, the blue oxide, W2O3, re.sults, while nitrogen peroxide yields the trioxide. ... 

Tungsten dioxide thus acts as a powerful reducer, and will convert mercuric and cui>ric salts to the mercurous and cuprous condition, and precipitate the metal from ammoniacal silver solutions. The amorphous variety is soluble in hydrochloric and sulphuric acids, yielding red solutions which on standing undergo partial oxidation with loss of colour the ciystalline dioxide is unacted upon, even by the hot concentrated acids.Nitric acid has a slow oxidising action. Concentrated alkali solutions dissolve the amorphous oxide, with formation of the tungstate and liberation of hydrogen, but have no action on the crystalline variety. ... 

Tungsten dioxide is reduced to the metal by heating with carbon to about 1020° C. ... 

Synthesis by fusing together a metallic tungstate, either acid or normal, with tungsten dioxide, in absence of air. ... 

Tungsten dioxide (cr) has no melting point since disproportionation occurs to yield W(cr) and before melting starts. 

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