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From the table below (source: McMurry's Chemistry [1, p. 212]), it is evident that $\ce{AlCl_3}$ has a higher lattice energy than $\ce{AlF3},$ even though $\ce{F}$ is smaller than $\ce{Cl}$. Why is this so?

Table 6.3 Lattice Energies of Some Ionic Solids (kJ/mol)

However, the melting point of $\ce{AlF3}$ $(\pu{1291 °C})$ is larger than that of $\ce{AlCl3}$ $(\pu{192.4 °C}).$ I think the huge melting point difference is because $\ce{AlCl3}$ is more covalent, while $\ce{AlF3}$ is more ionic. That being said, does lattice energy account for the covalent and ionic character of compounds?

References

  1. McMurry, J.; Fay, R. C.; Robinson, J. K. Chemistry, 7th ed.; Pearson: Boston, 2015. ISBN 978-0-321-94317-0.
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Lattice energy is a function of the radius ratio, which when tends to 1, improves the packing efficiency of the molecule. Here the radius ratio of the chloride ions and aluminium ions is much closer than the fluoride ions.
You could think of this from the born habers cycle also. Where the electron affinity of chlorine is higher, which leads to overall higher lattice energy. However, the radius ratio logic is much more convincing as the habers cycle logic would require the other parameters like the heat of formation also

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  • $\begingroup$ "… packing efficiency of the molecule": what molecule are you talking about? Also, it's Born–Haber cycle, not "habers cycle". $\endgroup$ – andselisk Dec 28 '19 at 16:32

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