Lithium lattice energy

Explain why the lattice energy of lithium chloride (861 kj-mol ) is greater than that of rubidium chloride (695 kj-mol ), given that they have similar arrangements of ions in the crystal lattice. See Appendix 2D. 

Consider figure 2.5, which is a thermochemical scheme involving crystalline lithium methoxide (LiOCI I3) as the starting substance. The lattice energy, Aiati/°(LiOCI I3), is the internal energy associated to the following process, at 298.15 K ... 

Heats of solution, hydration energies and lattice energies are discussed in reference (77). For oxygen and nitrogen donor atoms, only a few compounds of potassium, rubidium, and caesium are known, but several have been characterised for the smaller cations, sodium and lithium. 

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+. 

The band picture of metals developed by physicists accounts very well for conduction and other electric and magnetic properties. The valence bond description of the bonds in metals related to the concepts of chemistry explains much better than the former theory such properties as lattice energies and bond distances. Today, however, the V.B. picture does not lend itself well to a priori quantitative calculations of these properties and it seems doubtful to what extent a bond in solid lithium with a bond order of o. 11 (with respect to the bond order one in a gas molecule) has any fundamental meaning. There is no doubt, however, that in less typical metals and compounds Pauling s theory is valuable as a counterpart to the band picture, just as the V.B. and the M.O. methods are both of great importance for the description of the constitution of organic molecules. 

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,... 

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