Predicting Stabilities of Ionic Compounds

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. 

1 Count the effective numbers of anions and cations contained within the zinc blende, fluorite, and rutile unit cells. 

2 With the aid of the data of Appendix F, predict the crystal structure of magnesium fluoride. (The observed structure is of the rutile type.) 

3 The structure of the sodium selenide crystal may be described as a cubic close-packed array of Se ions, with an Na+ ion in every tetrahedral hole. 

4 Estimate the lattice energy of sodium chloride, using the appropriate data from Appendix F and assuming that the Born exponent is 9. Compare your result with the value given in Eq. 4.8. 

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