Alkali metal halides lattice energies

Figure 4,4 Standard enthalpies of formation (A// and lattice energies (plotted as —t/O for alkali metal halides and hydrides.

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. 

TABLE 1.16 Lattice energies of some alkali and alkaline earth metal halides at 0 K... 

TABLE 3.7. Heats of Formation and Lattice Energies of Alkali Metal Halides ... 

Which of the following alkali metal halides has the largest lattice energy, and which has the smallest lattice energy Explain. 

Table 4.2.4. Lattice energies (in kJ mol 1) of some alkali metal halides and divalent transition metal chalcogenides...

The alkali metals react with many other elements directly to make binary solids. The alkali halides are often regarded as the most typical ionic solids. Their lattice energies agree closely with calculations although their structures do not all conform to the simple radius ratio rules, as all have the rock salt (NaCl) structure at normal temperature and pressure, except CsCl, CsBr and Csl, which have the eight-coordinate CsCl structure. The alkali halides are all moderately soluble in water, LiF being the least so. 

Intrinsic point defects are deviations from the ideal structure caused by displacement or removal of lattice atoms [106,107], Possible intrinsic defects are vacancies, interstitials, and antisites. In ZnO these are denoted as Vzn and Vo, Zn and 0 , and as Zno and Ozn, respectively. There are also combinations of defects like neutral Schottky (cation and anion vacancy) and Frenkel (cation vacancy and cation interstitial) pairs, which are abundant in ionic compounds like alkali-metal halides [106,107], As a rule of thumb, the energy to create a defect depends on the difference in charge between the defect and the lattice site occupied by the defect, e.g., in ZnO a vacancy or an interstitial can carry a charge of 2 while an antisite can have a charge of 4. This makes vacancies and interstitials more likely in polar compounds and antisite defects less important [108-110]. On the contrary, antisite defects are more important in more covalently bonded compounds like the III-V semiconductors (see e.g., [Ill] and references therein). 

The matrix isolation technique has been applied, in conjunction with the salt/molecule reaction technique, to model the high temperature gas phase reactions of alkali halide salt molecules. The reactions with Lewis acids such as SiFi, HF and CO2 yielded ion pair products which were quenched into inert matrices for spectroscopic study. Difficulties arising from lattice energy considerations in alkali halide salt reactions are minimized by the initial vaporization of the salt reactant. The reaction of such salt molecules with Lewis bases, including H2O and NH3, yielded complexes similar in nature to transition metal coordination complexes, with binding through the alkali metal cation to the base lone pair. 

The application of the Bom-Haber cycle to a theoretically determined lattice energy actually gives a value for A/f/ X" (g), or Ai / X(g) — E, and a knowledge of Aff/ X(g) is required before a value can be assigned to the electron aflSnity. Alternatively, if a value of the electron aflSnity is available from other sources a value for the enthalpy of formation of the free radical Afl/ X(g), can be obtained. However in some cases such as NO3, O2, NO2, X is not a free radical but a stable molecule whose enthalpy of formation is known and then the electron afiSnity can be found directly. In some other cases, for example, the alkali metal halides and the alkaline earth oxides, a bond... 

Lattice energies, alkali metal salt values, amides and, 196 azides and, 198-199 bifluorides and, 199 borofluorides and, 203 borohydrides and, 197 chalcogenides and, 192,193 cyanates and, 199-200 cyanides and, 196-197 halides and, 189, 190 hydrides and, 189, 191, 192 hydrosulfides and, 195-196 hydroxides and, 192,194, 195 nitrates and, 201 superoxides and, 197-198 thiocyanates and, 200 alkaline earth salt values, acetylides and, 198 carbonates and, 202-203 chalcogenides and, 192, 193 imides and, 196 peroxides and, 198 calculation uses, absolute enthalpies and, 206 electron affinity determination and, 203-204... 

Of the alkali metal halides, the fluorides have much the highest heats of formation. This is ascribed to the low heat of dissociation of the Fg molecule and the high lattice energies of the compounds themselves. In the fluorides the heats of formation fall as the size of the cation increases, in the other halides they rise ... 

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