Alkali-metal Halides

The above Born-Haber cycle represents the enthalpy changes in the formation of an alkali metal halide MX from an alkali metal (Li. Na, K, Rb. Cs) and a halogen (Fj. CI2. Br2 or I2). 

A signiflcairt property of the alkali metal halides is the solubility of the metals in their molten halides. Typical values of the consolute temperatures of metal-chloride melts are 1180°C in Na-NaF, 1080°C in Na-NaCl, 790°C... 

The compounds which most nearly fit the clas-sicial conception of ionic bonding are the alkali metal halides. However, even here, one must ask to what extent it is reasonable to maintain that positively charged cations M+ with favourably... 

The extent to whieh eharge is transferred baek from the anion towards the eation in the alkali metal halides themselves is diffieult to determine preeisely. Caleulations indieate that it is probably only a few pereent for some salts sueh as NaCl, whereas for others (e.g. Lil) it may amount to more than 0.33 e" per atom. Direet experimental evidenee on these matters is available for some other elements from teehniques sueh as Moss-bauer speetroseopy, eleetron spin resonanee speetroseopy, and neutron seattering form faetors. ... 

The alkali metal halides are all high-melting, colourless crystalline solids which can be conveniently prepared by reaction of the appropriate hydroxide (MOH) or carbonate (M2CO3) with aqueous hydrohalic acid (HX), followed by recryslallization. Vast quantities of NaCl and KCl are available in nature and can be purihed if necessary by simple crystallization. The hydrides have already been discussed (p. 65). 

Figure 4.3 Melting point and boiling point of alkali metal halides.

Figure 4,4 Standard enthalpies of formation (A// and lattice energies (plotted as —t/O for alkali metal halides and hydrides.

The Bom-Haber cycle is also useful in examining the possibility of forming alkali-metal halides of stoichiometry MX2- The dominant term will clearly be the very large second-stage... 

Ternary alkali-metal halide oxides are known and have the expected structures. Thus Na3C10 and the yellow K3BrO have the aqti-perovskite structure (p. 963) whereas Na4Br20, Na4l20 and K4Br20 have the tetragonal anti-K2NiF4 structure. 

The alkali metal halides, particularly NaCl and KCl, find extensive application in industry (pp. 71 and 73). The hydrides are frequently used as reducing agents, the product being a hydride or complex metal hydride depending on the conditions used, or the free element if the hydride is unstable. Illustrative examples using NaH are ... 

More complex is the hydrolytic disproportionation of the molecular halogens themselves in aqueous alkali which is a commercial route to several alkali-metal halides ... 

The compounds can therefore be used as nonaqueous ionizing solvent systems (p. 424). For example the conductivity of ICl is greatly enhanced by addition of alkali metal halides or aluminium halides which may be considered as halide-ion donors and acceptors respectively ... 

By incorporation of alkali metal halides in the reaction mix, materials of composition M4-[MfiXig] can be produced in which each MgXn unit has a further six X atoms attached to its apices, so forming discrete clusters. 

In the second part of the 20th century, the tantalum capacitor industry became a major consumer of tantalum powder. Electrochemically produced tantalum powder, which is characterized by an inconsistent dendrite structure, does not meet the requirements of the tantalum capacitor industry and thus has never been used for this purpose. This is the reason that current production of tantalum powder is performed by sodium reduction of potassium fluorotantalate from molten systems that also contain alkali metal halides. The development of electronics that require smaller sizes and higher capacitances drove the tantalum powder industry to the production of purer and finer powder providing a higher specific charge — CV per gram. This trend initiated the vigorous and rapid development of a sodium reduction process. 

Ionic bond, 287, 288 dipole of, 288 in alkali metal halides, 95 vs. covalent, 287 Ionic character, 287 Ionic crystal, 81, 311 Ionic radius, 355 Ionic solids, 79, 81, 311 electrical conductivity, 80 properties of, 312 solubility in water, 79 stability of, 311... 

Fluorination of a mixture of alkali metal halide and an appropriate ruthenium or osmium halide affords cream MRuF6 (M = alkali metal, Ag /ieff = 3.5—3.8 /l b) or white MOsF6 ... 

Other RhX(CO)(PPh3)2 compounds can be made as shown in Figure 2.23 metathesis with an alkali metal halide or pseudohalide is often convenient, but the most versatile route, as with the iridium analogues, is a two-stage process in which the fluoro complex is first prepared, the fluorine then being readily displaced. 

This reaction is a principal method of forming IIIB-transition-metal cr bonds. The formation of thermodynamically favored alkali-metal halides or related salts and acids HX enhances the easy formation of those bonds. A second possible interaction between anionic metal bases and group-IIIB halides is a simple acid-base relationship without elimination of halide anions. However examples of this are rare, and they have been described often for group-IIIB compounds without halogen ligands ( 6.5.3.2). 

Limitations common to both salt elimination methods 1 and 2 are (a) the required product may be difficult to separate from the alkali metal halide, (b) reactions are best carried out in the solvent (usually an ether) in which the initial alkali metal derivative is prepared, (c) difficulties may arise through metal-halogen exchange (207), and (d) the range of starting anions is limited [e.g., X3Si compounds are only readily formed when X = H or Ar,... 

The auxiliary electrolyte is generally an alkali metal or an alkaline earth metal halide or a mixture of these. Such halides have high decomposition potentials, relatively low vapor pressures at the operating bath temperatures, good electrolytic conductivities, and high solubilities for metal salts, or in other words, for the functional component of the electrolyte that acts as the source of the metal in the electrolytic process. Between the alkali metal halides and the alkaline earth metal halides, the former are preferred because the latter are difficult to obtain in a pure anhydrous state. In situations where a metal oxide is used as the functional electrolyte, fluorides are preferable as auxiliary electrolytes because they have high solubilities for oxide compounds. The physical properties of some of the salts used as electrolytes are given in Table 6.17. 

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

Starting from the corresponding hydroxymethyl-benzocrown, it has been possible to generate the immobilized system (186) by reacting the above precursor with chloromethylated polystyrene (which is available commercially as Merrifield s resin). Typically, systems of this type contain a polystyrene matrix which has been cross-linked with approximately 1-4% p-divinylbenzene. In one study involving (186), a clean resolution of the alkali metal halides was achieved by HPLC using (186) as the solid phase and methanol as eluent (Blasius etal., 1980). In other studies, the divalent alkaline earths were also separated. 

The sample is dissolved in 1-5 % of the solvent and it is then placed in a solution cell consisting of transparent windows of alkali metal halides. A second cell containing pure solvent is then placed in the path of reference beam to cancel out solvent interferences. 

Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

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