Answer to Question #98727 in General Chemistry for Hassan Fallous

Question

Consider a hypothetical ionic compound AB (comprised of A+ and B- ions).

Given the following enthalpy data and using a Born-Haber cycle calculation, predict ΔHlattice in kJ mol-1.

A (s) + B (s) → AB (s) ΔrH = -418 kJ mol-1

A (s) → A (g) ΔrH = 103 kJ mol-1

B (s) → B (g) ΔrH = 118 kJ mol-1

First ionization energy of A (gas phase) = 470 kJ mol-1

Electron affinity of B (taking into account it's exothermic, gas phase) = -302 kJ mol-1

Answer 1

from the Born Haber cycle,

- ΔHf = delta H (sublimation of A) + First ionization energy of A (gas phase) + ΔH (sublimation of B) - Electron affinity of B + ΔHlattice

or

- 421 = 108 + 475 + 118 - 389 + ΔHlattice

or

ΔHlattice = - 733 KJ / mole.