Question

7) For the ionic solid AlzOs a) Determine its lattice energy using the appropriate Born-Haber cycle and the following values. All values in kJ/mol: IEi (A)-557.5:IE2 (A)-1817; IEs (A)-2745; IE(Al) 11580 E (0)-1314; IE2 (0) 3388; IEs (O)-5300 ΔΗ"a (O) =-141 (first electron affinity) ; ΔΗ'EA AH (Al) 330; AHa (O)-249;AH (Al Os)--1669.8 (o)- 798 (second electron affinity) b) Al:O, crystallizes in a corundum structure. How does the above lattice energy compare to the lattice energy determined by an electrostatic treatment? e) Given the melting points of Al-O, (2345 K) and Fe:0, (1838 K), what can we infer about the lattice energy of Fe0s?

Answer 1

Lattice Energy = Lattice energy of an ionic solid is defind as the the energy required to dissociate a solid into its gaseous ions.

(c) Higher lattice energy result in higher melting points  and increased hardness because more thermal energy is required to break the bonds of solid to make them into gaseous ions.  

Therefore, as given, Al2O3 has more melting point than Fe2O3, inspite of being the same cation, Al2O3 will have higher lattice energy than Fe2O3.

So the lattice energy of Fe2O3 will be less than that of Al2O3.