Ionic bonding lattice energy

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. 

Design a concept map that shows the relationships among ionic bond strength, physical properties of ionic compounds, lattice energy, and stability. 

Born-Haber cycle A thermodynamic cycle derived by application of Hess s law. Commonly used to calculate lattice energies of ionic solids and average bond energies of covalent compounds. E.g. NaCl ... 

There is a lively controversy concerning the interpretation of these and other properties, and cogent arguments have been advanced both for the presence of hydride ions H" and for the presence of protons H+ in the d-block and f-block hydride phases.These difficulties emphasize again the problems attending any classification based on presumed bond type, and a phenomenological approach which describes the observed properties is a sounder initial basis for discussion. Thus the predominantly ionic nature of a phase cannot safely be inferred either from crystal structure or from calculated lattice energies since many metallic alloys adopt the NaCl-type or CsCl-type structures (e.g. LaBi, )S-brass) and enthalpy calculations are notoriously insensitive to bond type. 

The table shows the lattice energy for some ionic compounds. Based on these data, which of these compounds would require the most energy to separate the bonded ions ... 

Qualitatively, the dipole-dipole interactions between the macro-molecular chains and the halide salt compensate for the lattice energy of the halide crystal and tend to decrease the interactions existing in the glass between the oxide macroanions. This decrease is probably the reason for the significant drop in the glass transition temperature resulting from the addition of a halide salt (Reggiani et al, 1978). Furthermore this type of reaction is consistent with the fact that dissolution of a halide salt in a vitreous solvent requires the existence of ionic bonds provided by a network modifier. 

The ability of an ionic solid to dissolve depends on its lattice energy, as well as the degree to which its ions can become hydrated. The lattice energy of an ionic crystal is a measure of the strength of its three-dimensional network of bonds. If these interactions are weaker than the solute-solvent attractions, the ionic bonds will be easily disrupted by water molecules. 

The lattice energy of a crystalline substance U with purely ionic bonds and negligible polarization effects is given by equation 1.68, changed in sign and with the subtraction of an energetic term known as zero-point energy ... 

Now if the fluorides are compared with the oxides, the latter have a still greater lattice energy and the V versus n curve should be steeper for the oxides. Thus, if there were no lower fluorides, lower oxides would be still more unstable. From the existence of TiO, it can be concluded that this compound must be stabilized by some kind of non-ionic bonding. The same argument holds for the lower nitride TiN, and for the hydride TiH2. Since the H ion is about the same size as the F ion, the lower hydride could not be stable if the fluoride is not. 

This energy is termed the Madeiurg energy since it represents a lattice energy internal to the molecule with a Madelu rg constant, of course, equal to 1.00. It is a maximum in a purely ionic bond (5 Z4) and decreases to whatever extent the charges on X and Y decrease. 

---