The standard heat of formation values of SF6(g), S(g) and F(g) are −1100, 275 and 80 J mol−1 respecitvely. The S−F bond energy in SF6 is :
SF6(g)→S(g)+6F(g)
ΔH of reaction = 275+3×80+1100+3×80 = 275+240+1100+240
= 1615 kJmol−1+240=1855 kJmol−1
6×(S−F) bond energy = 1855
(S−F) bond energy =18556 ≃ 309.2 kJ