Alkali metal halides, properties

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... 

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... 

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. 

Bromo-, 3-chloro-, 3-fluoro- and 3-iodo-oxetanes have all been prepared in good yield by the reaction of 3-oxetanyl tosylate with alkali metal halides in hot triethylene glycol (equation 70). Substitution reactions of the halogen atom have not been reported, except for the reaction of 3-iodooxetane with diethylamine. A low yield of 2-diethylaminooxetane was obtained from this reaction at 200 °C, but its chemical properties are not known (73JOC2061). 

The solubilities of alkali metal halides in various solvents are shown in Table 11.2 [2], In the table, water can dissolve all of the halides listed. Because water has a high permittivity and moderate acidic and basic properties, the hydration energies of the halides are large enough. Polar protic solvents like MeOH, HCOOH, FA and NMF can also dissolve many of the halides to considerable extents. However, in polar protophobic aprotic solvents like AN and Ac, halides... 

Rare earth oxides are useful for partial oxidation of natural gas to ethane and ethylene. Samarium oxide doped with alkali metal halides is the most effective catalyst for producing predominantly ethylene. In syngas chemistry, addition of rare earths has proven to be useful to catalyst activity and selectivity. Formerly thorium oxide was used in the Fisher-Tropsch process. Recently ruthenium supported on rare earth oxides was found selective for lower olefin production. Also praseodymium-iron/alumina catalysts produce hydrocarbons in the middle distillate range. Further unusual catalytic properties have been found for lanthanide intermetallics like CeCo2, CeNi2, ThNis- Rare earth compounds (Ce, La) are effective promoters in alcohol synthesis, steam reforming of hydrocarbons, alcohol carbonylation and selective oxidation of olefins. 

Generally, any electrolyte is composed of a mixture of alkali metal halides, which serve as solvent, and the compound of the deposited metal. In addition, there may be other additives, which may improve the properties of electrolyte or enhance the metal deposition. 

Alkali metal halides, mainly sodium and potassium fluorides and chlorides, are usually used as solvents for salts of multivalent metals, which are deposited on electrolysis at the cathode. Alkali metal halides improve the physico-chemical properties of the electrolyte and in many cases, it is the only possibility to deposit the desired metal altogether. 

Several attempts to calculate the properties of alkali metal halides on the basis of the equation of state can be found in the literature. Reiss et al. (1959, 1960) estimated the reversible work needed for creation of a spherical cavity in liquids of rigid spheres and derived the equation of state for these liquids... 

Therefore, Fellner and Dangk (1974), for molten alkali metal halides, assumed that the minimum distance between the ions of the same sign could be expressed in terms of aF, where the value of the factor F will be lower than V2. For the contact correlation function g a) of oppositely charged ions, which is important in the calculation of the transport properties, it holds... 

Properties of the metal-metal halide systems have been studied since the first Davy s observations of colored melts near the cathode at the electrolysis of alkali metal hydroxides. However, already the knowledge of these molten systems was substantially improved 50 years ago by the pioneering work of Bredig et al. (1958), who investigated the phase equilibria in a number of alkali metal-alkali metal halide and earth alkali metal-earth alkali metal halide systems. Further interest was also aroused in the case of... 

Ip values for Lil, LiBr, Nal, KI, Rbl and Csl are lower than for HF. In contrast, Qa values for all alkali metal halides are higher than for HF, and this appears more realistic with respect to the physical and chemical properties of the said compounds. The relatively high ionic character of HF is reflected in its strong tendency to association via hydrogen bonds. The ionic character of HCl, HBr and HI is much smaller than that of HF with Qa values generally higher than Ip. 

Ryabchuk V. K., Basov E. E. and Solonytsin Yu. P. (1989), Dependence of photoadsorption and photocatalytic properties of alkali metal halides on the spectral region of the excitation , Khimiches. Fiz. 8, 1475-1482. 

Results of analyzing Gibbs energy of transfer data on the basis of equation (4.9.8) for five alkali metal halides and TATB from water to various non-aqueous solvents are summarized in table 4.11. Acceptable fits to equation (4.9.6) are obtained for these systems, the correlation coefficient r decreasing with increase in the size of the alkali metal cation and halide anion. However, there is a problem in assessing the properties of the fit on the basis of the response factors and p. ... 

The change of halide ion results in weaker acidic properties for LnCl3 as compared with LnF3. This means that equilibrium (1.1.41) with the participation of alkali metal halide should be shifted to the left as compared with the fluoride complexes. That is, lithium chloride does not react with chlorides of the rare-earth elements with the formation of any compounds the binary phase diagrams are characterized by one simple eutectic. The same situation is observed for the binary diagrams for lithium- and rare-earth bromides. 

Concerning molten alkali-metal halides (which are referred to the solvents of the second kind) it should be emphasized that there is no levelling of acidic properties in them, and, therefore, it is possible to determine the relative strength of acids by measurements in these media. Nevertheless, the properties of strong bases are levelled to those of the oxide formed by the most acidic constituent cation of the melt. As a rule, it is assumed that this oxide is formed by the alkali metal cation of the smallest radius (i.e. Li+ in the KCl-LiCl eutectic, and Na+ in the KCl-NaCl equimolar mixture). 

The described levelling of the acidic properties in molten nitrates makes them unavailable for the determination of the constants of Lux acid-base equilibria where the strongest acids take part. The melts based on molten alkali metal halides are the most convenient solvents for this purpose. Since the most essential physico-chemical properties of ionic melts (density, charges and ionic radii) are close,1 application of the data obtained in molten chlorides for the description of acid-base equilibria in molten nitrates is more correct than is the use of a similar approach to room-temperature molecular solvents, since the properties determining their acid-base properties are numerous (e.g. dielectric constants, donor-acceptor properties). 

As follows from Part 1, the ionic melts based on molten alkali metal halides are referred to the solvents of the Second Kind (Kind II), and, therefore, the acid-base ranges for these media are half-open (see Fig. 1.1.1, scheme N3). Therefore, to form an idea of the relative oxoacidic properties of the studied chloride melts it is enough to know their oxobasicity indices. The necessary experimental parameters obtained at 600 °C are presented in Table 1.3.1. The data in this Table show that the KCl-LiCl eutectic melt possesses appreciable acidic properties, the corresponding oxobasicity index being equal to 3.2. 

A number of our works are devoted to the investigations of different kinds of acid-base equilibria in the ionic melts based on alkali metal halides in order to determine their oxobasicity indices at 700 and 800 °C. Unfortunately, none of the necessary experimental data have been published by other investigators. The equimolar KCl-NaCl mixture has been chosen to be the reference melt at these temperatures, although its oxoacidic properties differ by less than 0.1 from the CsCl-KCl-NaCl eutectic (see below), i.e. they are practically coincident. The solubilities of 11 metal oxides in the equimolar KCl-NaCl mixture are reported in Ref. [175]. Similar investigations in the molten CsCl-KCl-NaCl eutectic [188] allow us to conclude that the solubility products of the same oxide (in molar fraction scale) in both melts are close. This leads to the conclusion that both melts are suitable as reference ones, not only at 700 °C but also at other temperatures at which these media exist in the liquid state. 

To estimate the relative oxoacidic properties of molten alkali-metal halides, the carbonate-ion dissociation reaction was studied for the following sets of melts at 800 °C KCl-KBr-KI [183], CsCl-CsBr-CsI [197], NaCl-NaBr-Nal (830 °C) [198]. Also, in order to arrange all the melts in the oxoacidity scale at 800 °C, the solubility of MgO was determined for the following melts CsCl, KC1, NaCl, KCl-NaCl, KCl-LiCl, CsCl-KCl-NaCl, which allowed the estimation of the oxobasicity indices by the solubility method. 

The disposition of the molten alkali metal halides to the pyrohydrolysis (2.5.13) has been shown in Ref. [258] to decrease together with the increase of radius of both alkali metal cation and halide ion. Apart from the above-said constants, the authors of Ref. [258] estimated the activity coefficients of the solutions of OH- ions in molten alkali metal halides to approach unity (1), i.e. the properties of these solutions are close to ideal ones. Qualitatively the conclusions concerning the activity coefficients agree with the data of Hanf and Sole [255]. Nevertheless, the activity coefficients of hydroxides at the melting points of alkali metal halides are close to unity (1) and then they are reduced by a factor of two or three. 

Regarding the above dissociation reactions, it should be noted that the process of BaO dissociation results in the accumulation of foreign Ba2+ ions in the melts this, in turn, changes the chemical properties of the melt-solvent. Also, BaO possesses a limited solubility in molten alkali metal halides and its dissociation is essentially incomplete even in unsaturated solutions. Even if we neglect the incomplete dissociation, there are other limitations on the practical use of this base for studying oxoacidic properties of ionic melts. It cannot be used in the following cases ... 

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