We’re being asked to calculate the heat of sublimation of potassium (K), given the lattice energy of KCl.
Recall that lattice energy is the energy required to combine two gaseous ions into a solid ionic compound:
Mx+(g) + Ny–(g) → MyNx(s)
To calculate for lattice energy, we need to do the Born-Haber cycle for KCl.
We start with the corresponding formation equation for KCl:
K(s) + ½ Cl2(g) → KCl(s), ΔH˚f
What is the enthalpy of sublimation for K, in kJ/mol?
Lattice energy of KCl = 699 kJ/mol
First ionization energy of K = 418.7 kJ/mol
Electron affinity of Cl = 349 kJ/mol
Bond energy of Cl-Cl = 242.7 kJ/mol
Enthalpy of formation of KCl = -435.87 kJ/mol
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