Heats of Formation for Unknown Compounds

Born-Haber cycles for CaCl [n = 1,2,3), used to calculate the AWf for these three compounds. 

The results for both the Hj s are shown in Table 8.5. Note that CaCl is a 

With the availability of thermochemical lattice energies (see Table 8.6 for some representative values) for salts involving polyatomic anions and cations, Kapustinskii s equation can be used to estimate the radii of these ions. As an example, let s take sodium perchlorate. The thermochemical lattice energy for NaC104 is given in Table 8.6 as —648 kj/mol. We use this value as the lattice energy in the Kapustinskii equation as shown in Equation (8.22) and solve for r . 

Solving for yields values of 3.32 A and 0.38 A, the latter value being physically impossible. Consulting Table 7.5, we find the Shannon-Prewitt radius for the sodium cation to be 1.16 A (C.N. = 6). Since r is the sum of r(Na ) and r(ClOT) we can calculate a value of 2.16 A for the radius of the perchlorate anion. 

Radii derived in this manner from thermochemical cycles and calculated lattice energies are known as thermochemical radii. Some representative values are presented in Table 8.7. These are values averaged over a series of salts involving the ion listed. Interpretation of these results must be approached with caution, but they do give some indication of the effective size of polyatomic ions. 

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