Lithium fluoride lattice energy

The fifth step is to let the lithium ions and the fluoride react and form solid lithium fluoride. The energy change here corresponds to the lattice energy for LiF which is 1047 kJ/mole. Thus when 1 mole of LiF is formed from the gaseous ions, 1047 kJ will be released. 

Properties of the alkali fluorides.—The anhydrous alkali fluorides crystallize in the cubic system.8 Lithium fluoride forms regular optohedrons and nacreous plates sodium fluoride crystallizes in cubes, but in presence of sodium carbonate, the crystals are octohedrons. The cubic crystals are frequently en tremies. H. Schwendenwein has discussed the space lattice of the alkali fluorides, and K. Fajans and H. Grimm estimated the distance of the atoms apart in sodium and potassium fluorides to be respectively 2 34 X 10 8 and 2 67 X10-8 cm. and the respective lattice energies to be 210-4 and 192 2 Cals, per mol. The taste of potassium fluoride is acrid and salty. 

From Fig. 6 it is seen that in the group of the alkali halides the heat of formation always increases from iodine to fluorine and also from lithium to caesium, this latter with the exception, however, of the fluorides. In this group the sequence is just reversed in this case, in view of the small radius of the negative fluorine ion, the decrease of the lattice energy predominates over that of the ionization energy, which decreases much more slowly than proportional to i /r+. 

In discussing the energetics of the formation of solid lithium fluoride, we emphasized the importance of lattice energy in contributing to the stability of the ionic solid. Lattice energy can be represented by a modified form of Coulomb s law,... 

Reference has already been made to the high lattice energies of crystalline compounds containing small ions lithium fluoride is such a compound, and its high heat of formation and low solubility are not unexpected. 

We can also determine lattice energy indirectly, by assuming that the formation of an ionic compound takes place in a series of steps. This procedure, known as the Born-Haber cycle, relates lattice energies of ionic compounds to ionization energies, electron affinities, and other atomic and molecular properties. It is based on Hess s law (see Section 6.5). Developed by Max Bom and Fritz Haber, the Bom-Haber cycle defines the various steps that precede the formation of an ionic solid. We will illustrate its use to find the lattice energy of lithium fluoride. 

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