Self-ionization equilibria solvents

GermannIS found that aluminium trichloride reacts with carbonyl chloride to give a solution which yields KAICI4 on addition of potassium chloride. He considered aluminium chloride as a solvo-acid and potassium chloride as a solvobase, according to the following self-ionization equilibrium of the pure solvent (which is unlikely to be correct) ... 

Arsenic(III) fluoride is similar in its solvent properties to iodine(V) fluoride. Association in the liquid state seems to be due to fluorine bridging. The occurrence of F-exchange has been concluded to take place from the results of n.m.r. measurementsi05. A self-ionization equilibrium is assumed to be present in the pure liquid due to autofluoridolysis ... 

Carbonyl chloride is of historical interest since it was the first oxyhalide known to behave as an ionizing solvent " . Complex formation between aluminium chloride and calcium chloride was explained by assuming a self-ionization equilibrium of the solvent molecules. 

Self ionization equilibria are known to exist in various other solvents and it may be proposed that their presence should, in principle, be admitted for any liquid system even if such ions were not detectable. The extent of the self-ionization equilibrium appears related to the strength of solvent-solvent interactions and hence to their amphoteric properties in order to mediate the cooperative effects, to the ease of heterolysis of bonds within solvent molecules and to the value of the dielectric constant. In a liquid with unfavourable conditions for the production of SMM-centres by self-ionization such as carbon tetrachloride, SMM-centres must be made available in other ways in order to provide for the existence of the molecular liquid. 

Molten I2CI6 has been much less studied as an ionizing solvent because of the high dissociation pressure of CI2 above the melt. The appreciable electrical conductivity may well indicate an ionic self-dissociation equilibrium such as... 

Ion product — A temperature-dependent constant related to pure substances that can dissociate forming ions and remain in equilibrium with them. It is the product of the ion activities raised to the stoichiometric coefficients of such ionic species in former pure substance. Since the concentration of the pure substance is practically a constant, it is not included in this equilibrium expression. Common pure substances characterized by an ion-product constant are -> amphiprotic solvents, and those salts that are partially dissolved in a given solvent. In the latter case, the ion product is synonymous with solubility product. The following table (Table 1) summarizes self-ionization ionic products and - autoprotolysis constants of some - amphiprotic solvents [i]. 

Some ionizing solvents are of major importance in analytical chemistry whilst others are of peripheral interest. A useful subdivision is into protonic solvents such as water and the common acids, or non-protonic solvents which do not have protons available. Typical of the latter subgroup would be sulphur dioxide and bromine trifluoride. Non-protonic ionizing solvents have little application in chemical analysis and subsequent discussions will be restricted to protonic solvents. Ionizing solvents have one property in common, self-ionization, which reflects their ability to produce ionization of a solute some typical examples are given in table 3.2. Equilibrium constants for these reactions are known as self-ionization constants. 

Increasing basicity or acidity of the solvent displaces the equilibria (3-8) and (3-10) to the right. The addition of these two equations gives a new equilibrium describing the self-ionization (autoprotolysis) of the solvent. 

Unlike other self-ionization equilibria that we shall discuss, reaction 9.14 requires the separation of doubly charged ions, and on these grounds alone, the establishment of this equilibrium must be considered improbable. Its viability is also questioned by the fact that thionyl chloride, SOCI2 (the only reported acid in the solvent), does not exchange or O with the liquid SO2 solvent. Selected properties of SO2 are given in Table 9.3, and its liquid range is compared with those of other solvents in Figure 9.2. 

Many chemical processes involve heterogeneous reactions in which reactants or products are in different phases. The concentrations of pure solids and liquids do not change, and by convention are not written in the equilibrium expression. Also, in a system involving acids and bases, when a solvent such as water is in an equilibrium equation, it is not included in the equilibrium expression. In an earlier chapter, the expression for used this convention, and the concentration of water is not included in the expression. The reaction representing the self-ionization of water is... 

However, one can imagine hypothetically a solvent being not prone to self-ionization, which possesses its intrinsic add-base equilibrium without breakdown to ionic components. An example of such a solvent and process of its acid-base self-dissociation may be expressed by the following equation ... 

In this contribution, we describe and illustrate the latest generalizations and developments[1]-[3] of a theory of recent formulation[4]-[6] for the study of chemical reactions in solution. This theory combines the powerful interpretive framework of Valence Bond (VB) theory [7] — so well known to chemists — with a dielectric continuum description of the solvent. The latter includes the quantization of the solvent electronic polarization[5, 6] and also accounts for nonequilibrium solvation effects. Compared to earlier, related efforts[4]-[6], [8]-[10], the theory [l]-[3] includes the boundary conditions on the solute cavity in a fashion related to that of Tomasi[ll] for equilibrium problems, and can be applied to reaction systems which require more than two VB states for their description, namely bimolecular Sjy2 reactions ],[8](b),[12],[13] X + RY XR + Y, acid ionizations[8](a),[14] HA +B —> A + HB+, and Menschutkin reactions[7](b), among other reactions. Compared to the various reaction field theories in use[ll],[15]-[21] (some of which are discussed in the present volume), the theory is distinguished by its quantization of the solvent electronic polarization (which in general leads to deviations from a Self-consistent limiting behavior), the inclusion of nonequilibrium solvation — so important for chemical reactions, and the VB perspective. Further historical perspective and discussion of connections to other work may be found in Ref.[l],... 

In the above equilibrium, extractant dependency studies have indicated that n = 1 for Am" and Bk " and = 2 for Cf ". These stoichiometries have been observed for extractions into chloroform, and the self-adduct formation with trivalent actinides has been possibly cormected with tetrads where Cm is one of the minima. Extractions into xylene, however, leads to the formation of selfadducts with all four actinides due to better distribution coefficients in xylene over chloroform. The formation of self-adducts is due to ligand concentration, ionization constant of the ligand, basicity of the bound ligand, solvent identity, and oxidation state of the metal ion. 

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