Consider an ionic compound, MX , composed of generic metal M and generic, gaseous halogen X .

Question

1. The enthalpy of formation of MX is Δ𝐻∘f=−537 kJ/mol.
2. The enthalpy of sublimation of M is Δ𝐻sub=109 kJ/mol.
3. The ionization energy of M is IE=407 kJ/mol.
4. The electron affinity of X is Δ𝐻EA=−311 kJ/mol. (Refer to the hint).
5. The bond energy of X2 is BE=163 kJ/mol.

Determine the lattice energy of MX.

Answer 1

Hi Bri L.,

So what you need to solve this is to write the two paths from M(solid) +X2 -> MX (solid) . (Not balanced yet!!)

One path is direct (delta H⁰_f) .

The other path includes all the other specified (or unknown, as the answer) energy changes. Take care to include coefficients sufficient to make exactly one MX as product!

These two paths have the same total energy change, so equate them and solve for your unknown.

-- Cheers, -- Mr. d.

(P.S. It's a piece of cake, except that cake usually includes also an energy of digestion term, at least in our household.)