Born-Haber cycles were used to obtain the first reliable values for electron affinity by considering the EA value as the unknown and using a theoretically calculated value for the lattice energy. Use a Born-Haber cycle for KF and the following values to calculate a value for the electron affinity of fluorine:
K(s) → K(g) ΔH° = 90kJ K(g) → K⁺(g) + e⁻ ΔH° = 419kJ
F₂(g) → 2F(g) ΔH° = 159kJ
K(s)+ (1/2)F₂(g) → KF(s) ΔH° = -569kJ
K⁺(g) + F⁻ (g) → KF(s) ΔH° = -821kJ