Thermochemical treatments of some ionic solids

We first look at the fluorides of barium. Only BaF2 is known, a typically ionic solid having the fluorite (8 4) structure. From Table 5.2, we see that the calculated lattice energy is very close to the experimental value in other words, we can calculate the enthalpy of formation of BaF2(s) almost within the limits of experimental uncertainty. Why have BaF3 and BaF not been prepared Presumably they are thermodynamically unstable with respect to other species. In order to verify this supposition, let us estimate the enthalpies of formation AHf of BaF(s) and BaF3(s), assuming these to be ionic. 

The formation of BaF3(s) from the elemental substances can be broken down as follows  

All the required data can be obtained from standard tabulations except for the lattice energy of BaF3(s), which has to be calculated or estimated. 

The Kapustinskii equation is unreliable for trifluorides (Table 5.2) but the lattice energy of BaF3 can be estimated to be a little more than twice that of BaF2, for which an experimental value is given in Table 5.2. Our estimate of —4800 100 kJ mol-1 is in line with the experimental value for LaF3 (Table 5.2). 

We now look at the unknown BaF(s). An obvious route to this substance would be to treat barium metal with a deficiency of elemental fluorine or some other fluorinating agent  

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