The standard heat of formation values of SF6(g) ,S(g) and F(g) are: −1100,275 and 80 KJ mol−1 respectively. Then the average S - F bond energy in SF6 is:
309 KJ mol-1
S(g) + 6F(g) → SF6(g); ΔHr = −1100 KJ mol−1
S(S) → S(g); ΔH = +275 KJ mol−1
12 F2(g) → ΔH = 80 KJ mol−1
Therefore heat of formation = Bond energy of reactant - Bond energy of product - 1100 = [275 + 6 × 80] − [6 × S − F]
Thus bond energy of S − F = 309 Kj mol−1