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 = -413 kJ mol-1

A (s) → A (g) ΔrH = 113 kJ mol-1

B (s) → B (g) ΔrH = 115 kJ mol-1

First ionization energy of A (gas phase) = 542 kJ mol-1

Electron affinity of B (taking into account it's exothermic, gas phase) = -394 kJ mol-1

You have 5 attempts at this

Answer 1

A (s) + B (s) → AB (s) ΔrH = -413 kJ mol-1

A (s) → A (g)        ΔrH1 = 113 kJ mol-1

B (s) → B (g)    ΔrH2 = 115 kJ mol-1

First ionization energy of A (gas phase)

A(g) ------>A^+ (g) + e^-   rH3 = 542KJ/mole

Electron affinity of B (taking into account it's exothermic, gas phase)

B(g) + e^- -----> B^- (g)     rH4 = -394Kj/mole

lattice energy

A^+(g) + B^- (g) ---------------> AB(g)     rH5 =

from Hess law

rH    = rH1 + rH2 + rH3 + rH4 + rH5

-413    = 113 + 115 + 542 -394 + rH5

rH5    = -789KJ/mole

lattice energy of AB = -789KJ/mole

ΔrH2