Question

9.15) Use the following data to calculate average values of B(Se-F) in SeF4 and

SeF6. Comment on your answers in the view of corresponding values for B(S-F) in SF4 = +340 kJ mol-1 and SF6 = +329 kJ mol-1.
∆aHθ (Se) = +227 kJ mol-1 and ∆aHθ (F) = +159 kJ mol-1

∆fHθ (SeF6,g) = -1030 kJ mol-1 and ∆fHθ (SeF4,g) = -850 kJ mol-1

Answer 1

Consider the reaction
Se (g) + 4 F (g) --> SeF4 (g)

The reaction enthalpy is
deltaH = -850kJ/mol - (+227 kJ/mol + 4(+159 kJ/mol) ) = -1713 kJ/mol

Four Se-F bonds are formed in the rxn, so the bond energy is about

E(Se-F) = (0 - (-1713 kJ/mol)) / 4 = 428.25 kJ/mol

S (g) + 6 F (g) --> SF6 (g)
deltaH = -1030 kJ/mol - (+227 kJ/mol + 6(+159 kJ/mol) ) = -2211 kJ/mol
E(S-F) = 2211 kJ/mol / 6 = 368.5 kJ/mol

The obtained bond energies of S-F obtained are greater than the given values