Question

3. a) Explain the reason for the variation of the first ionization energies of the third period  elements. (4) b) Calculate the lattice energy (in Units kJ mol1 ) for ZnO crystal using Eq. 3.4 based on  electrostatic model and using a Born-Haber cycle. Compare the two answers and comment  on any difference. Useful data: Medelung constant (A) = 1.6411 Born Constant (n) = 8 Internuclear distance (a) = 199 pm Zn(s) + ½ O2(g)  ZnO(s) Hf =  350.5 kJ mol1 Zn(s)  Zn(g) HS = 130.4 kJ mol1 Zn(g)  Zn+ (g) I(Zn) = 906.3 kJ mol1 Zn+ (s)  Zn2+(g) I(Zn+ ) = 1733 kJ mol1 ½ O2(g)  O(g) ½Hd = 248.5 kJ mol1 O(g)  O  (g) EA(O) = 141 kJ mol1 O - (g)  O 2 (g) EA(O ) = 780 kJ mol1