Metals heat of sublimation

Fia. 23. Dependence of current density at constant overpotential (600 mv vs NHE) for the ethylene oxidation reaction at 80° on the latent heat of sublimation (Ls) of metal or alloy 58). 

We have seen that in a metal the atoms are close-packed, i.e. each metal atom is surrounded by a large number of similar atoms (often 12, or 8). The heat required to break up 1 mole of a metal into its constituent atoms is the heat of atomisation or heat of sublimation. Values of this enthalpy vary between about 80 and 800 kJ. for metals in their standard states these values indicate that the bonds between metal atoms can vary from weak to very strong. There is a rough proportionality between the m.p. of a metal and its heat of atomisation. so that the m.p. gives an approximate measure of bond strength. 

The vanadium alloy is purified and consoHdated by one of two procedures, as shown in the flow diagram of the entire aluminothermic reduction process presented in Figure 1. In one procedure, the brittle alloy is cmshed and heated in a vacuum at 1790°C to sublime most of the aluminum, oxygen, and other impurities. The aluminum faciHtates removal of the oxygen, which is the feature that makes this process superior to the calcium process. Further purification and consoHdation of the metal is accompHshed by electron-beam melting of pressed compacts of the vanadium sponge. 

Metafile arsenic can be obtained by the direct smelting of the minerals arsenopyrite or loeUingite. The arsenic vapor is sublimed when these minerals are heated to about 650—700°C in the absence of air. The metal can also be prepared commercially by the reduction of arsenic trioxide with charcoal. The oxide and charcoal are mixed and placed into a horizontal steel retort jacketed with fire-brick which is then gas-fired. The reduced arsenic vapor is collected in a water-cooled condenser (5). In a process used by Bofiden Aktiebolag (6), the steel retort, heated to 700—800°C in an electric furnace, is equipped with a demountable air-cooled condenser. The off-gases are cleaned in a sembber system. The yield of metallic arsenic from the reduction of arsenic trioxide with carbon and carbon monoxide has been studied (7) and a process has been patented describing the gaseous reduction of arsenic trioxide to metal (8). 

Bismuth triduoride is usually prepared by dissolving either Bi O or BiOF in hydroduoric acid to yield the addition compound bismuth triduoride ttihydroduoride [66184-11-0] 3 HF or H2(BiF ). Caredil evaporation of the solution permits isolation of a grey soHd, which upon heating loses HF to yield BiF. It may be purified by sublimation in a stream of HF at 500°C. Bismuth triduoride may also be prepared by direct duorination of bismuth by (/) reaction of Bi O with sulfiir tetraduoride, SF (2) treatment of metallic bismuth with HF at 350°C and (J) reduction of BiF in a dilute stream of hydrogen. 

It is interesting that the bond energy relative to the bond-forming state of the atoms shows the same monotonic trend for the alkaline-earth metals as for the alkali metals. The irregularity in the heats of sublimation at magnesium is due to the high... 

Surface Composition Measurements. The surface composition and metal dispersion for a series of silica (Cab-O-Sll) supported Ru-Rh bimetallic clusters are summarized In Table I. Surface enrichment In Rh, the element with the lower heat of sublimation, was not observed over the entire bimetallic composition range. In fact, to within the experimental limit of error of the measurements, surface compositions and catalyst compositions were nearly equal. A small local maximum In the dispersion was observed for the catalyst having a surface composition of 50% Rh. 

The dark spot on the inside of a burned-out light bulb is tungsten metal that has sublimed (vaporized from the solid) as a consequence of the heating of the tungsten filament to produce white light. 

As we have seen, several atomic properties are important when considering the energies associated with crystal formation. Ionization potentials and heats of sublimation for the metals, electron affinities, and dissociation energies for the nonmetals, and heats of formation of alkali halides are shown in Tables 7.1 and 7.2. 

Table 7.1 Ionization Potentials and Heats of Sublimation of Alkali Metals and the Heats of Formation of Alkali Metal Halides. ...

An early calculation of the 7S of metals was based29 on the equation Nip = MXS + 0.5 RT, in which the heat of sublimation per gram-atom was combined with the work ip of removing a surface atom from its nearest neighbor. The work of forming a new surface (per atom) was said to be, for instance, 1.5 ip for the 0001 face in dense hexagonal lattices. These hypotheses lead to 7S values for Mg 0001, ... 

If the compound dissociates in the vapor, the sum of the heat of sublimation and the heat of dissociation must be used instead of Xs alone. If also the lattice structure is known, the ys can be calculated for metal carbides such as TiC or ZrC31). For instance, ys of TiC at 298 °C should equal 3.05 J/m2 for the 100 face. 

Gas evaporation using Ar for the preparation of various sort of metal fine powders was first reported by Kimoto et al. in 1963 (5). The production chamber of this method is basically the same as that of a vacuum sublimation chamber. A target material is heated in this chamber with several torr inert gas atmosphere. The nanometer-sized particles are easily formed in the chamber space. However, by this method, it is difficult to get genuine nanoparticles whose sizes are several nanometers. This is because of the radiation heating in a production chamber, resulting particle coalescence on the chamber wall or particle collector, as well as the direct particle contact in the deposited particle layer (powders). Therefore the size becomes several tens to hundreds of nanometers. Several ultrafme metallic powders are now commercially available, including Cu, Ag, Al, Ni, Co, Fe, and Au, with a size of several tens of nanometers. 

---