Solution enthalpies halides

Although the data for the silver halides suggest that silver(I) fluoride is likely to be more soluble than the other silver halides (which is in fact the case), the hydration enthalpies for the sodium halides almost exactly balance the lattice energies. What then is the driving force which makes these salts soluble, and which indeed must be responsible for the solution process where this is endothermic We have seen on p. 66 the relationship AG = — TAS and... 

One way of determining carbocation stabilities is to measure the amount of energy required to form the carbocation by dissociation of the corresponding alkyl halide, R-X - R+ + X . As shown in Figure 6.10, tertiary alkyl halides dissociate to give carbocations more easily than secondary or primary ones. As a result, trisubstituted carbocations are more stable than disubstituted ones, which are more stable than monosubstituted ones. The data in Figure 6.10 are taken from measurements made in the gas phase, but a similar stability order is found for carbocations in solution. The dissociation enthalpies are much lower in solution because polar solvents can stabilize the ions, but the order of carbocation stability remains the same. 

Complex chlorides of plutonium (34, 41) do not present such a wide range of formulae as the complex fTuorides but we have at hand thermodynamic information on two important species which have also been characterized with other actinides. In table II we have disregarded the complex halides for which no thermodynamic data are available. The enthalpy of formation of Cs2NaPuClg(c) (55) and Cs2PuClg(c) (56) have been obtained from enthalpy of solution measurements."The selected (8) values are AHf(Cs2NaPuCl6,c) =... 

Enthalpy of formation, solution, and dehydration, halides and halo-... 

Caicium halides are relatively stable there is the possibility of a violent reaction in the presence of a more electropositive alkaline metal eg detonation on impact of a mixture of calcium bromide and potassium. Calcium chloride has a very high enthalpy of solution in water. When dissolved in large quantities in hot water this causes the solution to boil vigorously, creating emissions. 

Fig. 8. Correlation between Pearson s hardness parameter (7P) derived from gas-phase enthalpies of formation of halide compounds of Lewis acids (19), and the hardness parameter in aqueous solution (/A), derived from formation constants of fluoride and hydroxide complexes in aqueous solution (17). The Lewis acids are segregated by charge into separate correlations for monopositive ( ), dipositive (O), and tripositive ( ) cations, with a single tetrapositive ion (Zr4+, ). The /P value for Tl3+ was not reported, but the point is included in parentheses to show the relative ionicity of Tl(III) to ligand bonds. 

Apart from the qualitative observations made previously about suitable solvents for study, the subject of solvates has two important bearings on the topics of thermochemistry which form the main body of this review. The first is that measured solubilities relate to the appropriate hydrate in equilibrium with the saturated solution, rather than to the anhydrous halide. Obviously, therefore, any estimate of enthalpy of solution from temperature dependence of solubility will refer to the appropriate solvate. The second area of relevance is to halide-solvent bonding strengths. These may be gauged to some extent from differential thermal analysis (DTA), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) solvates of "aprotic solvents such as pyridine, tetrahydrofuran, and acetonitrile will give clearer pictures here than solvates of "protic solvents such as water or alcohols. 

For enthalpies of solution of rare-earth salts in a really wide range of solvents, one has to look outside the halide family to nitrates. Enthalpies of solution of lanthanum(III) nitrate have been measured in 15 solvents, under comparable conditions. Unfortunately, it was the hexa-hydrate rather than the anhydrous nitrate that was used (227). 

ENTHALPY DATA FOR HALIDE ION FORMATION IN AQUEOUS SOLUTION ... 

Later Fajans and Karagunis (64) showed that for alkali halides, enthalpies of solution, osmotic coefficients, and the tendency to form solid hydrates follow gradations rather analogous to those of the solubilities. 

Hydrogen fluoride in aqueous solution is a weak acid, characterized by its pKa value of 3.2. By comparison, the other hydrogen halides are extremely strong acids in aqueous solution all three are fully dissociated in dilute solution, and their pA", values may be estimated by thermochemical cycle calculations. The thermochemical cycle shown in Figure 3.1 represents the various processes as the aqueous hydrogen halide, HX, is converted to a solution containing hydrated protons and hydrated halide ions. The enthalpy of acid dissociation of the HX(aq) compound is given by ... 

Table 3.9 Standard Gibbs energy, enthalpy and entropy changes of solution for the sodium halides...

Figure 3.5 Plots of the enthalpies of solution, AsothT, of the Group 1 halides against cation radius...

In recent years we have undertaken a systematic investigation of the volumes and heat capacities of transfer of alkali halides and tetraalkylammonium bromides from water to mixed aqueous solvents (1-6). These properties are important because, when combined with enthalpies and free energies, they can be used to calculate the temperature and pressure dependences of various equilibrium properties of electrolytes in mixed solvents. Since the properties of electrolytes in mixed aqueous solvents are closely related to the corresponding properties of the nonelectrolyte in an electrolyte solution, infor-... 

One difference in behavior between the hydrophilic alkali halides and hydrophobic solutes like the larger tetraalkylammonium halides in water is expressed by the enthalpy. The enthalpies of solution of the larger tetraalkylammonium halides in water are more exothermic than those of the corresponding alkali halides but in other solvents, e.g., several amides, propylene carbonate (PC), and dimethylsulfoxide (DMSO), the reverse is true. Generally, this phenomenon is attributed to an enhanced hydrogen bonding in the highly structured solvent water in the vicinity of the tetraalkylammonium ions (hydrophobic hydration) (i). This idea is substantiated by the absence of the effect in solvents like N,N-dimethylformamide (DMF), PC, and DMSO (2), where specific structural effects are not present in the pure solvents. 

For the calculation of the enthalpies of formation of the tribromides and triiodides, the same reaction cycles were used as for the trichlorides. However, as the halide ion in the compounds are different from those in the solution (e.g., Lnh in HCl(aq)) the calculation of ArH2 becomes a bit more complex as we have to deal with the ternary system H2O-HCI-HX. In that case, it is assumed that the apparent enthalpy of formation of HI and HBr in HC1 solutions are the same as in HBr and HI solutions of the same molality. 

The lattice enthalpy U at 298.20 K is obtainable by use of the Born—Haber cycle or from theoretical calculations, and q is generally known from experiment. Data used for the derivation of the heat of hydration of pairs of alkali and halide ions using the Born—Haber procedure to obtain lattice enthalpies are shown in Table 3. The various thermochemical values at 298.2° K [standard heat of formation of the crystalline alkali halides AHf°, heat of atomization of halogens D, heat of atomization of alkali metals L, enthalpies of solution (infinite dilution) of the crystalline alkali halides q] were taken from the compilations of Rossini et al. (28) and of Pitzer and Brewer (29), with the exception of values of AHf° for LiF and NaF and q for LiF (31, 32, 33). The ionization potentials of the alkali metal atoms I were taken from Moore (34) and the electron affinities of the halogen atoms E are the results of Berry and Reimann (35)4. 

The heat of ionization in aqueous solution, AH q, represents the enthalpy change for the following reaction M(+anq) -f- nX<-aq) = MXn(aq). Although much AHaq data exist for class (b) metal chlorides, bromides and iodides, few data are available for class (b) fluorides and class (a) halides in general. This is because MXn(aq) in these cases is not a stable species. It is therefore difficult to compare class (a) and (b) halides in aqueous solution in a manner which is entirely consistent with AHion(g). It is easy to show, however, that in aqueous solution most metal ions, which are class (b) by... 

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