Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/553

Rh TUNGSTEN GROUP.] CHEMISTRY 541 the formation of the arsenic compounds than in the forma tion of the corresponding phosphorus compounds : Reaction. Units of heat developed. Remarks. As,, 0,.. 154,580 As 2, 5 219.380 Formation of the solid As, 4, H 3 215,630 compounds. As 2, 5 A( . 225,380 ) Formation of the com- 4s, Go 4.0 147,030 &amp;gt; pounds in aqueous solu- As,0 4, H 3 , Aq 215,230 64,800 ) tion. Oxidation of the trioxide As,OqAo, Oo. ... 78,350 when dry, and in solution. As/) 5 , 3H. 2 O 6,800 AsnO *, Ao 6,000 Dissolution of the trioxide As,0o, An 7,550 and pentoxide and of As0 4 H 3) Af[ - 400 arsenic acid in water. The oxides of antimony are white or yellowish ; the trioxide is fusible and volatile, and is partly converted into the tetroxide when heated in air ; the tetroxide is infusible, and unalterable by heat ; the pentoxide is con verted into the tetroxide at a red heat. The vapour density of the trioxide has not been determined, but as it may be obtained in crystals isomorphous with those of arsenic trioxide its formula is probably Sb 4 . The oxides of antimony all exhibit feeble acid properties, dissolving in alkalies, but the salts mostly have little stability ; the most stable salts apparently are those which correspond to the potassium salt of the formula K 2 Sb 4 O n . The oxides of antimony are soluble in hydrochloric and concentrated sulphuric acids ; the trioxide may thus be converted into the sulphate, Sb. 2 (SO 4 ) 3, which is decomposed by water into an acid and a basic salt. The bismuth oxides are yellow or brown, and are devoid of acid properties, but tkey dissolve in acids ; the pentoxide is decomposed and converted into the tetroxide when heated to about 220 C., and the latter is reduced to the trioxide at a somewhat higher temperature. Bismuth sulphate, Bi. 2 (S0 4 ) 3, like antimony sulphate, is decomposed by water into an acid and a basic salt. On boiling a solution of acid potassium tartrate, HKC 4 H 4 0, ;, with antimony trioxide the so-called tartar emetic (SbO)KC 4 H 4 O,3, is formed, the oxygenated radicle SbO displacing an atom of hydrogen in the potassium tartrate ; corresponding arsenic and bismuth compounds are formed in a similar manner. Niobium and tantalum pentoxides are white and infusible ; they do not volatilize ; after ignition they are insoluble in all acids, but dissolve on fusion with alkalies. They exhibit marked acid properties, combining with basic oxides in various proportions. The hydrated oxides obtained on decomposing the chlorides by water are acid to litmus, and are soluble in hydrochloric and hydrofluoric acids. The fluorides of niobium and tantalum, and niobium oxyfluoride, readily form double salts with other fluorides, which in many cases are isomorphous with certain titanium, zirconium, tin, and tungsten compounds which do not exactly correspond to them in composition, but in which apparently fluorine and oxygen displace one another isomorphously ; thus, the following pairs of compounds are isomorphous : (NH 4 ) 3 ZrF 7 (XHJ 3 XbOF 6 Arsenic and antimony, like phosphorus and nitrogen, form gaseous trihydrides, AsH 3 and SbH 3, and arsenic is said to furnish also a solid dihydride, As H 4 . Arsine and stibine, AsH 3 and SbH 3 , are formed whenever hydrogen is evolved in presence of an arsenic or antimony compound ; they are extremely unstable compounds, especially the latter, and are entirely devoid of basic properties. The K 2 NbF 7 HK 3 SnF s (XH 4 ).,XbOF 3 K 2 TaF 7 HK 3 NbOF 7 (NHJ 2 WO,F 4 corresponding compounds of arsenic and antimony with positive hydrocarbon radicles, however, like the analogous nitrogen and phosphorus compounds, are far more stable and combine readily with other elements. Thus, triethyl- amine, N(C 2 H 5 ) 3, and triethylphosphine, P(C 2 H 5 ) 3 , com bine with acids, forming salts corresponding to the ammo nium salts such as the chloride PH(0 2 H 5 ) 3 C1 ; the triethylphosphonium salts, moreover, are stable in presence of water, which at once decomposes phosphonium iodide. Triethylstibine, Sb(C 2 H 5 ) 3, decomposes hydrochloric acid with evolution of hydrogen, forming Sb(C 2 H 5 ) 3 Cl 2 ; trimethylarsine, however, does not react with acids, but with chlorine forms the compound As(CH 3 ) 3 Cl 2 ,the existence of which furnishes a proof that arsenic may function as a pentad element. The phosphorus, antimony, and arsenic compounds absorb oxygen with great avidity, forming powerfully basic oxides such as PO(C 2 H 5 ) 3. They also furnish hydroxides such as N(C 2 H 5 ) 4 .OH, P(C. 2 H 5 ) 4 .OH, &c. (p. 575), which exhibit the closest resemblance to the alkalies ; Thomsen, indeed, has shown that the heat of neutralization of the hydroxide N(CH 3 ) 4 .OH is equal to that of the alkalies. The existence of these hydroxides, and their behaviour, afford the strongest reason for believing that a solution of ammonia at least partially consists of ammonium hydroxide, NH 4 .OH (p. 510). MOLYBDENUM: TUNGSTEN URANIUM. Name. Symbol. At wt Sp.gr. At. vol. Molybdenum Mo 95-8 8-6 11-1 Tungsten W 184 18-2 10-1 Uranium u (?) 180 18-4 9-8 Molybdenum and tungsten are closely related to chromium. They may be obtained by reducing their oxides at a high temperature in a stream of hydrogen. Molybdenum, after fusion, is white, and has a silvery lustre ; the powder obtained by reducing tungsten trioxide has a grey colour, and strong lustre. Both are difficult of fusion, and insoluble in most acids except nitric acid ; when heated in air or oxygen they are finally converted into the trioxides Mo0 3 and W0 3. They form the follow ing chlorides and oxides : Mo a Cl 4 W a Cl 4 Mo 2 Cl 6 MoCl 5 WC1 S Mo a 3 WC1 S Mo0 3 wo, chloride MoCL WC1 4 Mo 2 4 Wo,O 4 Molybdenum pentachloride is the highest obtained by heating molybdenum in chlorine gas; it is a black, crystalline substance, which forms a colourless solution when dissolved in a considerable quantity of water ; when heated in air it is converted into a colourless oxychloride, MoO 2 Cl 2 ; according to Debray, its vapour density corresponds with the formula MoCl 5. The remaining chlorides are not distinctly crystalline ; the tetrachloride is brown, the trichloride red, and the dichloride yellow ; the molecular formulae of these chlorides are undetermined. In an atmosphere of carbon dioxide the dichloride bears a bright red heat without melting or volatilizing, the trichloride is resolved into di- and tetra chloride, and the latter when again heated, splits up into pentachloride which sublimes, and trichloride which remains ; the di- and bd-chlorides are insoluble, but the tetrachloride is readily soluble in water. Tungsten is converted by chlorine at a moderate heat into a hexachloride, WC1 G, which forms black -violet crystals ; it melts at 275 C., and boils at 341 C. ; from the determination of its vapour density it appears that it undergoes decomposition when heated. When pure it is insoluble in water below 60 C., but if the slightest trace