Energetics of Ionic Compounds

In summary, in addition to allowing simple calculations of the energetics of ionic compounds, the Bom-Haber cycle provides insight into the energetic factors operating. Furthermore, it is an excellent example of the application of thermodynamic methods to inorganic chemistry and serves as a model for other, similar calculations not only for solids, but also for reactions in solution and in the gas phase. 

Note The actual formation of NaCl need not follow the above steps. The above steps are just a simplified and convenient way to analyse the energetics of ionic compound formation.]... 

In principle, we can use the Born-Haber cycle to predict whether a particular ionic compound should be thermodynamically stable, on the basis of calculated values of U, and so proceed to explain all of the chemistry of ionic solids. The relevant quantity is actually the free energy of formation, AGf, and this is calculable if an entropy cycle is set up to complement the Born-Haber enthalpy cycle. However, in practice AHf dominates the energetics of formation of ionic compounds. 

Triazoles are five-membered aromatic heterocycles that contain three nitrogen atoms located at the 1,2,3 or 1,2,4 positions in the ring. A large munber of ionic compounds that contain a triazole derivative are described as energetic materials. 

To date there is no evidence that sodium forms any chloride other than NaCl indeed the electronic theory of valency predicts that Na" and CU, with their noble gas configurations, are likely to be the most stable ionic species. However, since some noble gas atoms can lose electrons to form cations (p. 354) we cannot rely fully on this theory. We therefore need to examine the evidence provided by energetic data. Let us consider the formation of a number of possible ionic compounds and first, the formation of sodium dichloride , NaCl2. The energy diagram for the formation of this hypothetical compound follows the pattern of that for NaCl but an additional endothermic step is added for the second ionisation energy of sodium. The lattice energy is calculated on the assumption that the compound is ionic and that Na is comparable in size with Mg ". The data are summarised below (standard enthalpies in kJ) ... 

Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e... 

This outstanding behavior of bis(pentamethylcyclopendienyl)stannylene has been explained by the energetically favorable formation of the ionic compound 79 which contains the 6-membered cluster C5Sn 174>. The structure of the boron tetrafluoride compound is illustrated in Fig. 15 the tin atom in the cation is located at the apex of a pentagonal pyramide 173). 

An antisite defect is an atom on an inappropriate site in a crystal, that is, a site normally occupied by a different chemical species. In a compound of formula AB the antisite defects that can occur are an A atom on a site normally occupied by a B atom, or a B atom on a site normally occupied by an A atom. Antisite defects are not very important in binary ionic compounds, as the misplacement of an ion is energetically costly, and so unfavorable. In ternary ionic compounds, however, such as spinels, AB204, the transfer of A ions to B sites and vice versa, is not... 

In general, overlap of incompletely filled p orbitals results in large deviations from pure ionic bonding, and covalent interactions result. Incompletely filled / orbitals are usually well shielded from the crystal field and behave as essentially spherical orbitals. Incompletely filled d orbitals, on the other hand, have a large effect on the energetics of transition metal compounds and here the so-called crystal field effects become important. 

The simple triplet-triplet quenching mechanism requires that at low rates of light absorption the intensity of delayed fluorescence should decay exponentially with a lifetime equal to one-half of that of the triplet in the same solution. Exponential decay of delayed fluorescence was, in fact, found with anthracene, naphthalene, and pyrene, but with these compounds the intensity of triplet-singlet emission in fluid solution was too weak to permit measurement of its lifetime. Preliminary measurements with ethanolic phenanthrene solutions at various temperatures indicated that the lifetime of delayed flourescence was at least approximately equal to one-half of the lifetime of the triplet-singlet emission.38 More recent measurements suggest that this rule is not obeyed under all conditions. In some solutions more rapid rates of decay of delayed fluorescence have been observed.64 Sufficient data have not been accumulated to advance a specific mechanism but it is suspected that the effect may be due to the formation of ionic species as a result of the interaction of the energetic phenanthrene triplets, and the subsequent reaction of the ions with the solvent and/or each other to produce excited singlet mole-... 

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