Metal chlorides, lattice energies

These trends are apparent In the values of lattice energy that appear in Table Notice, for example, that the lattice energies of the alkali metal chlorides decrease as the size of the cation increases, and the lattice energies of the sodium halides decrease as the size of the anion increases. Notice also that the lattice energy of MgO is almost four times the lattice energy of LiF. Finally, notice that the lattice energy of Fc2 O3, which contains five ions in its chemical formula, is four times as large as that of FeO, which contains only two ions in its chemical formula. 

FIGURE 17.11 Lattice energies ofthe chlorides of the + 2 metal ions ofthe first transition series. 

These were developed in an endeavor to expand the range of metals that could be incorporated into an ionic liquid. The presence of waters of hydration decreases the melting point of metal salts because it decreases the lattice energy. Hence, as Figure 2.4 shows, hydrated salts should be more likely to form mixtures with quaternary ammonium salts that are liquid at ambient temperature than anhydrous salts. Table 2.5 shows a list of some of the metal salts that have been made into ionic liquids with choline chloride and the freezing point of a lChCl 2metal salt mixture. 

X-ray and neutron diffraction studies show that in these hydrides the H ion has a crystallographic radius between those of F and Cl". Thus the electrostatic lattice energies of the hydride and the fluoride and chloride of a given metal will be similar. These facts and a consideration of the Bom-Haber cycles lead us to conclude that only the most electropositive metals can form ionic hydrides, since in these cases relatively little energy is required to form the metal ion. 

In the sulphides, selenides, tellurides and arsenides, all types of bond, ionic, covalent and metallic occur. The compounds of the alkali metals with sulphur, selenium and tellurium form an ionic lattice with an anti-fluorite structure and the sulphides of the alkaline earth metals form ionic lattices with a sodium chloride structure. If in MgS, GaS, SrS and BaS, the bond is assumed to be entirely ionic, the lattice energies may be calculated from equation 13.18 and from these values the affinity of sulphur for two electrons obtained by the Born-Haber cycle. The values obtained vary from —- 71 to — 80 kcals and if van der Waal s forces are considered, from 83 to -- 102 kcals. 

Imides, lattice energies of, 196 Indenyl compounds, eighth-group elements and, 73,75 Intercalation, metal chlorides, graphite and, 254-259 metal oxides, graphite and, 260-262 metal sulfides, graphite and, 260-262 Intercalation compounds, graphite, comparative survey of, 263-264... 

Figure 20.27 shows a plot of experimental lattice energy data for metal(II) chlorides of first row J-block elements. In each salt, the metal ion is high-spin and lies in an octahedral environment in the solid state. The double hump in Figure 20.27 is reminiscent of that in Figure 20.26, albeit with respect to a reference line which shows a general increase in lattice energy as the period is crossed. Similar plots can be obtained for species such as MF2, MF3 and [MFg], but for each series, only limited data are available and complete trends cannot be studied. 

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