From the Elements

Vapor-solid reactions are carried out in sealed evacuated Vycor tubes. These are designed to maintain physical separation of liquid Se and solid M to preclude a rapid violent reaction. Se vapor is distilled over M filings at temperatures slowly increasing up to 950°C over a period of 4 to 50 h. The temperature is then held there for an additional14to 150 h. Miller etal. [1,2], Miller, Himes [4], also see [3]. Forth preparation of M2S 3 with M = Sc, Y, Tb, Dy, Ho,Tm, Yb, the rare earth metals were placed in an AI2O3 boat to prevent reaction with the quartz ampule used as an envelope. In the case of Sc, the reaction was accelerated with I2 as a transport medium, Dismukes, White [5]. The direct synthesis was also used by Guittard et al. [6, 7], Muir [8], Kleber et al. [9], and Klemm, Koczy [16]. MI3 is used as flux for M = Ce, Pr, Nd byTakeshitaet al. [10]. 

High temperature-high pressure synthesis has been used to make the Th3P4 polymorphs of heavier rare earth sesquiselenides from stoichiometric amounts of the powdered elements. Possibly, they are nonstoichiometric, Se-deficient products. The following compounds were obtained at 1800°C  

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Boron trichloride, BCI3. Colourless mobile liquid, m.p. — 107°C, b.p. 12-5°C. Obtained directly from the elements or by heating B2O3 with pels in a sealed tube. The product may be purified by distillation in vacuo. It is extremely readily hydrolysed by water to boric acid. TetrachJoroborates containing the BCJ4 " ion are prepared by addition of BCI3 to metal chlorides. 

Tin IV) bromide, SnBr4. M.p. 33°C, b.p. 203 C, prepared from the elements. Fonns many complexes, including [SnBr ] ions. 

Figure 11.2. Formation oj jluoride and chloride iom from the elements Key ...

First, we would like to ehange the reference state from the isolated nuelei and eleetions to the elements in their standard states, C(graphite) and H2(g) at 298 K. This leads to the energy of formation at 0 K AfEo, whieh is identieal to the enthalpy of formation AfHo at 0 K. The energy and enthalpy are identieal only at 0 K. Next we would like to know the enthalpy ehange on heating propene from 0 to 298 K so as to obtain the enthalpy of formation from the isolated nuelei and eleetions elements This we will eonvert to from the elements in their standard... 

Equations (1) and (2) are the heats of formation of carbon dioxide and water respectively Equation (3) is the reverse of the combustion of methane and so the heat of reaction is equal to the heat of combustion but opposite in sign The molar heat of formation of a substance is the enthalpy change for formation of one mole of the substance from the elements For methane AH = —75 kJ/mol... 

Electron density represents the probability of finding an electron at a point in space. It is calculated from the elements of the density matrix. The total electron density is the sum of the densities for alpha and beta electrons. In a closed-shell RHF calculation, electron densities are the same for alpha and beta electrons. 

The values of fH° and Ay.G° that are given in the tables represent the change in the appropriate thermodynamic quantity when one mole of the substance in its standard state is formed, isothermally at the indicated temperature, from the elements, each in its appropriate standard reference state. The standard reference state at 25°C for each element has been chosen to be the standard state that is thermodynamically stable at 25°C and 1 atm pressure. The standard reference states are indicated in the tables by the fact that the values of fH° and Ay.G° are exactly zero. 

A major advantage of this hydride approach lies in the separation of the remaining elements of the analyte solution from the element to be determined. Because the volatile hydrides are swept out of the analyte solution, the latter can be simply diverted to waste and not sent through the plasma flame Itself. Consequently potential interference from. sample-preparation constituents and by-products is reduced to very low levels. For example, a major interference for arsenic analysis arises from ions ArCE having m/z 75,77, which have the same integral m/z value as that of As+ ions themselves. Thus, any chlorides in the analyte solution (for example, from sea water) could produce serious interference in the accurate analysis of arsenic. The option of diverting the used analyte solution away from the plasma flame facilitates accurate, sensitive analysis of isotope concentrations. Inlet systems for generation of volatile hydrides can operate continuously or batchwise. 

Ions of five different m/z values shown entering five elements of a ten-element array. All five ions would be recorded at the same time, with electrical outputs from the elements into which ions had entered and no output from those elements into which no ions had entered. 

Lavoisier beUeved he could distinguish acetic acid from acetous acid, the hypothetical acid of vinegar, which he thought was converted into acetic acid by oxidation. Following Lavoisier s demise, Adet proved the essential identity of acetic acid and acetous acid, the latter being the monohydrate, and in 1847, Kolbe finally prepared acetic acid from the elements. 

Heating elements operating <760°C are almost always of a chrome—nickel resistance alloy and are ia the form of ribbon, cast alloy, open wire cods, or sheathed constmction. Several alloys are suitable ia this temperature range and all are satisfactory if properly appHed. In general, the more expensive alloys are used when physical space limitations dictate higher watts per area dissipation from the element. 

Halides. Gold(III) chloride [13453-07-1] can be prepared directiy from the elements at 200°C (167). It exists as the chlotine-bridged dimer, Au2Clg ia both the soHd and gas phases under an atmospheric pressure of chlorine at temperatures below 254°C. Above this temperature ia a chlorine atmosphere or at lower temperatures ia an iaert atmosphere, it decomposes first to AuCl [10294-29-8] and then to gold. The monochloride is only metastable at room temperature and slowly disproportionates to gold(0) and gold(III) chloride. The disproportionation is much more rapid ia water both for AuCl and the complex chloride, [AuCy, formed by iateraction with metal chlorides ia solution. 

Hafnium Boride. Hafnium diboride [12007-23-7] HfB2, is a gray crystalline soHd. It is usually prepared by the reaction of hafnium oxide with carbon and either boron oxide or boron carbide, but it can also be prepared from mixtures of hafnium tetrachloride, boron trichloride, and hydrogen above 2000°C, or by direct synthesis from the elements. Hafnium diboride is attacked by hydrofluoric acid but is resistant to nearly all other reagents at room temperature. Hafnium dodecaboride [32342-52-2] has been prepared by direct synthesis from the elements (56). 

Because the reaction takes place in the Hquid, the amount of Hquid held in the contacting vessel is important, as are the Hquid physical properties such as viscosity, density, and surface tension. These properties affect gas bubble size and therefore phase boundary area and diffusion properties for rate considerations. Chemically, the oxidation rate is also dependent on the concentration of the anthrahydroquinone, the actual oxygen concentration in the Hquid, and the system temperature (64). The oxidation reaction is also exothermic, releasing the remaining 45% of the heat of formation from the elements. Temperature can be controUed by the various options described under hydrogenation. Added heat release can result from decomposition of hydrogen peroxide or direct reaction of H2O2 and hydroquinone (HQ) at a catalytic site (eq. 19). 

Interest has continued in on-site manufacture of hydrogen peroxide from the elements, particularly for remote sites located considerable distances from wodd-scale anthraquinone processes. However, no commercial-scale direct combination plants have been constmcted as of this writing. 

The heat of formation of ammonium chloride from the elements is 317 kJ /mol (75.8 kcal/mol) it is 175 kJ /mol (41.9 kcal/mol) from gaseous ammonia and gaseous hydrogen chloride. The heat of formation of ammonium bromide from the elements, bromine in the Hquid form, is 273 kJ /mol (65.3 kcal/mol) for ammonium iodide, the corresponding heat of formation is 206 kJ /mol (49.3 kcal/mol). Iodine is in the soHd state. 

There is Htde evidence of the direct formation of sodium carbide from the elements (29,30), but sodium and graphite form lamellar intercalation compounds (16,31—33). At 500—700°C, sodium and sodium carbonate produce the carbide, Na2C2 above 700°C, free carbon is also formed (34). Sodium reacts with carbon monoxide to give sodium carbide (34), and with acetylene to give sodium acetyHde, NaHC2, and sodium carbide (disodium acetyHde), Na2C2 (see Carbides) (8). 

Nitrogen and sodium do not react at any temperature under ordinary circumstances, but are reported to form the nitride or azide under the influence of an electric discharge (14,35). Sodium siHcide, NaSi, has been synthesized from the elements (36,37). When heated together, sodium and phosphoms form sodium phosphide, but in the presence of air with ignition sodium phosphate is formed. Sulfur, selenium, and tellurium form the sulfide, selenide, and teUuride, respectively. In vapor phase, sodium forms haHdes with all halogens (14). At room temperature, chlorine and bromine react rapidly with thin films of sodium (38), whereas fluorine and sodium ignite. Molten sodium ignites in chlorine and bums to sodium chloride (see Sodium COMPOUNDS, SODIUM HALIDES). 

Other preparative methods include direct synthesis from the elements, reaction between gaseous hydrogen fluoride and titanium tetrachloride, and decomposition of barium hexafluorotitanate [31252-69-6] BaTiF, or ammonium, (NH 2TiFg. 

Titanium Dibromide. Titanium dibromide [13873-04-5] a black crystalline soHd, density 4310 kg/m, mp 1025°C, has a cadmium iodide-type stmcture and is readily oxidized to trivalent titanium by water. Spontaneously flammable in air (142), it can be prepared by direct synthesis from the elements, by reaction of the tetrabromide with titanium, or by thermal decomposition of titanium tribromide. This last reaction must be carried out either at or below 400°C, because at higher temperatures the dibromide itself disproportionates. 

ZrSe [12166-53-9] and ZrTe [39294-10-5] (138). Zirconium disulfide [12039-15-5] is made from the elemental powders and by the action of carbon disulfide on zirconium oxide above 1200°C (139) some ZrOS [12164-95-3] is usually also obtained. The higher sulfides disproportionate at ca 700°C synthesis reactions at 900—1000°C with S Zr ratios between 0.2 and 2.3 produced crystals that were identified as Zr S2 [12595-12-9] ... 

Several compounds such as BaZrS [12026-44-7], SrZrS [12143-75-8], and CaZrS [59087-48-8], have been made by reacting carbon disulfide with the corresponding zirconate at high temperature (141), whereas PbZrS [12510-11-1] was produced from the elements zirconium and sulfur plus lead sulfide sealed in a platinum capsule which was then pressurized and heated (142). Lithium zirconium disulfide [55964-34-6], LiZrS2, was also synthesized. Zirconium disulfide forms organometaUic intercalations with a series of low ionization (<6.2 eV)-sandwich compounds with parallel rings (143). 

Zirconium tetrabromide [13777-25-8] ZrBr, is prepared direcdy from the elements or by the reaction of bromine on a mixture of zirconium oxide and carbon. It may also be made by halogen exchange between the tetrachloride and aluminum bromide. The physical properties are given in Table 7. The chemical behavior is similar to that of the tetrachloride. 

Zirconium tetraiodide [13986-26-0], Zrl, is prepared directly from the elements, by the reaction of iodine on zirconium carbide, or by halogen exchange with aluminum triiodide. The reaction of iodine with zirconium oxide and carbon does not proceed. The physical properties are given in Table 7. 

Thermodynamic calculations for reactions forming carbon disulfide from the elements are compHcated by the existence of several known molecular species of sulfur vapor (23,24). Thermochemical data have been reported (12). Although carbon disulfide is thermodynamically unstable at room temperature, the equiHbtium constant of formation increases with temperature and reaches a maximum corresponding to 91% conversion to carbon disulfide at about 700°C. Carbon disulfide decomposes extremely slowly at room temperature in the absence of oxidizing agents. 

The earliest method for manufacturiag carbon disulfide involved synthesis from the elements by reaction of sulfur and carbon as hardwood charcoal in externally heated retorts. Safety concerns, short Hves of the retorts, and low production capacities led to the development of an electric furnace process, also based on reaction of sulfur and charcoal. The commercial use of hydrocarbons as the source of carbon was developed in the 1950s, and it was still the predominate process worldwide in 1991. That route, using methane and sulfur as the feedstock, provides high capacity in an economical, continuous unit. Retort and electric furnace processes are stiU used in locations where methane is unavailable or where small plants are economically viable, for example in certain parts of Africa, China, India, Russia, Eastern Europe, South America, and the Middle East. Other technologies for synthesis of carbon disulfide have been advocated, but none has reached commercial significance. 

Compiled from Daubert, T. E., R. R Danner, H. M. Sibiil, and C. C. Stebbins, DIPPR Data Compilation of Pure Compound Properties, Project 801 Sponsor Release, July, 1993, Design Institute for Physical Property Data, AlChE, New York, NY and from Thermodynamics Research Center, Selected Values of Properties of Hydrocarbons and Related Compounds, Thermodynamics Research Center Hydrocarbon Project, Texas A M University, College Station, Texas (extant 1994). The compounds are considered to be formed from the elements in their standard states at 298.15 K and 101,325 P. These include C (graphite) and S (rhombic). Enthalpy of combustion is the net value for the compound in its standard state at 298.15K and 101,325 Pa. 

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