Reactions of the Elements

Yellow forms of arsenic and antimony (the latter very unstable) have been described. These are presumably the nonmetallic modifications of these elements, analogous to white phosphorus, and also consisting of discrete molecules (tetrahedral quartets) in the solid state. The grey or metallic forms of arsenic and antimony are the most stable. They are far denser than the yellow forms, are insoluble in organic solvents, and have appreciable electrical conductivities. Black amorphous forms of arsenic and antimony are also known, and an additional allotrope of antimony, explosive (but always impure), has been described. 

The most important sources of phosphorus are phosphate rocks containing either apatite, (a mixed fluoride-phosphate of calcium, Ca2FP04 Ca3(P04)2), or calcium phosphate itself. These yield elemental phosphorus when heated with a mixture of carbon and silica the latter forms a fusible slag with the CaO formed during the reaction, and the phosphorus formed from reduction by the carbon is distilled away from the mixture. 

The heavier congeners of phosphorus are often found as their sulfides which may be reduced with iron metal directly or roasted in air to form oxides (in turn reducible by carbon). Bismuth is often found in the elemental state. 

Reactions of the Group Vb elements with the halogens are relatively straightforward. Reactions with oxygen and with sulfur are less so. 

Oxidation of phosphorus by oxygen, for example, may yield three distinct oxides (P4O, P4O10, and PjO ), whereas oxidations by sulfur may yield one or more of three possible sulfides (P4S, P4S7, and P Sio). (The compound or compounds formed in a reaction between phosphorus and oxygen or sulfur depend not only on the atomic ratios of the components used, but also on reaction conditions.) Certain of the more important binary halides, oxides, and sulfides of this group are described in later sections of this chapter, but the list is only partial. 

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REACTIONS OF THE ELEMENTS WITH ALKALIS (SODIUM HYDROXIDE)... 

Galhum triiodide [13450-914], Gal, is obtained by direct reaction of the elements or by reaction of iodine solution in carbon disulfide on galhum. 

When prepared by direct reaction of the elements, magnesium hydride is stable in air and only mildly reactive with water. 

Reactions of Hydrogen and Other Elements. Hydrogen forms compounds with almost every other element. Direct reaction of the elements is possible in many cases. Hydrogen combines direcdy with the halogens, X2, to form the corresponding hydrogen halide. 

Synthesis from Hydrogen and Chlorine. Less than 10% of the U.S. production capacity of HCl is made by the direct reaction of the elements. 

Manufacture. Phosphoms sulfides are manufactured commercially by direct reaction of the elements. Elemental phosphoms and sulfur are measured into a reaction vessel containing a heel of molten phosphoms sulfide. The reaction can be batch or continuous. The ratio of phosphoms to sulfur in the feed determines which phosphoms sulfur compound (Table 5) is formed. The reaction temperature can be the boiling point or lower. For the boiling reactor (27,28), the phosphoms sulfide product is first purified by distillation and then condensed to a Hquid. Alternatively, the Hquid product can be formed directly in a nondistiUed process (29—31), which may involve a subsequent distillation step (30), and in which the phosphoms is often cleaned up prior to use (30—32). For either process, the Hquid phosphoms sulfide product is soHdified, and usually sized to form a commercial material. 

Both antimony tribromide and antimony ttiiodide are prepared by reaction of the elements. Their chemistry is similar to that of SbCl in that they readily hydroly2e, form complex haUde ions, and form a wide variety of adducts with ethers, aldehydes, mercaptans, etc. They are soluble in carbon disulfide, acetone, and chloroform. There has been considerable interest in the compounds antimony bromide sulfide [14794-85-5] antimony iodide sulfide [13868-38-1] ISSb, and antimony iodide selenide [15513-79-8] with respect to their soHd-state properties, ferroelectricity, pyroelectricity, photoconduction, and dielectric polarization. 

On the industrial (multitonne) scale it can also be prepared by direct high-pressure reaction of the elements or preferably via the intermediate formation of the Na analogue. 

The III-V semiconductors can all be made by direct reaction of the elements at high temperature and under high pressure when necessary. Some properties of the Al compounds are in Table 7.11 from which it is clear that there are trends to lower mp and energy band-gap Eg with increasing atomic number. 

The most important nitride of Si is Si3N4 this is formed by direct reaction of the elements above 1300 or more economically by heating Si02 and coke in a stream of N2/H2 at 1500". The compound is of considerable interest as an engineering material since it is almost completely inert chemically, and retains its strength, shape and resistance to corrosion and wear even above 1000°. ° Its great hardness (Mohs 9), high... 

H28 is readily prepared in the laboratory by treating Fe8 with dilute HCl in a Kipp apparatus. Purer samples can be made by hydrolysing Ca8, Ba8 or AI283, and the purest gas is prepared by direct reaction of the elements at 600°C. 

Sulfur iodides are a topic of considerable current interest, although compounds containing S-I bonds were, in fact, unknown until fairly recently. The failure to prepare sulfur iodides by direct reaction of the elements probably reflects the comparative weakness of the S-I bond an experimental value is not available but extrapolation from representative values for the bond energies of other S-X bonds leads to a value of 170kJmol ... 

Paradoxically, the most firmly established dihalides of the heavier chalcogens are the dark ruby-red P0CI2 and the purple-brown PoBr2 (Table 16.5). Both are formed by direct reaction of the elements or more conveniently by reducing P0CI4 with 8O2 and PoBt4 with H28 at 25°. 

The other tetrahalides can all readily be made by direct reactions of the elements. Crystalline SeCU, TeCU and -SeBr4 are isotypic and the structural unit is a cubane-like tetramer of the same general type as [Me3Pt(/Z3-Cl)]4 (p. 1168). This is illustrated schematically for TeCU in Fig. 16.13d each Te is displaced outwards along a threefold axis and thus has a distorted octahedral environment. This can be visualized as resulting from repulsions due to the Te lone-pairs directed towards the cube centre and, in the limit, would result in the separation into... 

Black OsCl4 exists in two forms. A high-temperature form is made by reaction of the elements... 

The boron halides are made either by direct reaction of the elements at a high temperature or from boron oxide. The most important is boron trifluoride, BF, an industrial catalyst produced by the reaction between boron oxide, calcium fluoride, and sulfuric acid ... 

In the Li-Rh system LiRh is prepared from rhodium metal foil and liq Li in a 25 at% excess of the 1 1 molar ratio. The mixture is heated in an iron crucible to 750-880°C in Ar. The direct reaction of the elements in a molybdenum crucible at 800°C for 7 d produces LiRh. Identical methods produce Lilr and Lilrj with which the rhodium compounds are isostructural . The reaction of Rh metal with LiH at 600°C gives the ternary hydrides Li4RhH4 and Li4RhH5. 

D. T. Burns, A. Townshend, A. H. Carter, Inorganic Reaction Chemistry, Vol. 2, Reactions of the Elements and Their Compounds, Part A AlkaU Metals to Nitrogen, Ellis Hotwood, Chichester, 1981, p. 243. 

In our design, divalent Ca was chosen to partially substitute the trivalent atoms, and La and Ce were selected for a trivalent element because their ionic size (rLas+ = l.SOA rce3+=l-48A) was close to that of Ca (rca2+ = T48A) [21]. Like La, the Ce element also generally shows a formal -i-3 oxidation state in in-termetallics. Erom the reactions of the elements, we have identified as major phases the electron-precise/deficient alloys, Ln5.xCaxGe4 (Ln=La, Ce x=3.37,... 

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