Correlation between standard enthalpy of formation, structural parameters and ionicity for alkali halides

Abstract

The standard enthalpy of formation (ΔHo) has been considered to be an interesting and useful parameter for the correlation of various properties of alkali halides. The interrelation between ΔHo and structural parameters for the halides of Li, Na, K and Rb has been thoroughly analyzed. When cationic component element is kept constant in a homologous series of alkali halides, the negative value of ΔHo has been observed to decrease linearly with increase of interionic distance (d) and accordingly following empirical equation ΔHo = α + βd (where α and β are empirical constants) has been established. However, for common anionic series of alkali halides an opposite nonlinear trend has been observed with the exception of common fluorides. The correlation study on the standard enthalpy of formation has been extended in term of radius ratio and also discussed in the light of ionization energy of the metal, electron affinity of the halogen, size of the ions, ionic character of bond and lattice energy of the compound

Estimation of thermodynamic and physicochemical properties of the alkali astatides: On the bond strength of molecular astatine (At2) and the hydration enthalpy of astatide (At−)

“…It had been emphasized by several authors that the trend in lattice binding energy is very regular among the alkali halides 23,31,32 . As can be seen from Figure 2, the LBE 's show an inverse proportionality with d ( LBE = 1810.8/d + 142.29 [R 2 = 0.9972]), which reproduces LBE to an average of 6 kJ/mol (with maximum deviations of +12 kJ/mol for LiI, −9 kJ/mol for KBr and +9 kJ/mol for CsCl, experimental minus calculated values) but note that this line does not go through the origin.…”

“…It has been observed by several authors that the trends in heats of formation of the alkali halides are not as regular as the trends in lattice energies 23,32 . However, and as noted by Nasar, 23 for the subfamilies containing a common metal cation, very good linear correlations are observed, with R 2 exceeding 0.999. This is shown in Figure 3B for the solid alkali halides.…”

“…Lattice binding energies (LBE) can be found in various (review) papers, for example, by Kaya, 27 It had been emphasized by several authors that the trend in lattice binding energy is very regular among the alkali halides. 23,31,32 As can be seen from Figure 2, the 33 ).…”

“…However, for a common halide anion an opposite, non-linear and much smaller trend with d is observed and thus only the size of the halide anion plays a decisive role in the heats of formation of the alkali halides, as was discussed in an earlier paper. 23 The linear relationships observed in Figure 3B, where the metal ion is kept constant, may be exploited for estimation purposes of the heats of formation of the solid alkali astatides, provided their bond lengths can be assessed, as discussed below.…”

“…In the present paper, we have collected from various sources the thermodynamic properties of the solid and gaseous alkali halides. It has been reported previously 23 that the interionic distance in (MX) s is a good parameter for relevant thermodynamic properties of (MX) s such as their heats of formation, lattice binding enthalpies and sublimation enthalpies. We have extended this finding to gas-phase species and have used this approach to (re-)estimate various thermodynamic properties of the alkali astatides, from which ΔH f (At À ) g is derived using Equations ( 1) and (2).…”