1. Bond Energy and Reaction Enthalpy
The bond dissociation energy is sensitive to its environment:
CH4(g) → CH3(g) + H(g) DH=435 kJ/mol
CH3(g) → CH2(g) + H(g) DH=453 kJ/mol
CH2(g) → CH(g) + H(g) DH=425 kJ/mol
CH (g) → C(g) + H(g) DH=339 kJ/mol C H
Total = 1652
Average = 1652/4
=413 kJ/mol
H-CBr3 → H + CBr3 DH=380 kJ/mol
H-CCl3 → H + CCl3 DH=380 kJ/mol
H-CF3 → H + CF3 DH=430 kJ/mol
H-CH2CH3 → H + CH2CH3 DH=410 kJ/mol

2. H2(g) + F2(g) → 2 HF(g) H-H +F-F → H-F H-F H H F F
Energy change: two bonds broken (DH > 0),
energy required: BE(H-H) +BE(F-F)
two bonds formed (DH < 0),
energy released: -BE(H-F)x2

3. CH4(g) + 2Cl2(g) + 2F2(g) → CF2Cl2(g) + 2HCl(g) + 2 HF(g) Cl F C
Broken bonds
4 C-H 4x413 kJ
2 Cl-Cl 2x239 kJ
2 F-F 2x154 kJ
2438 kJ
formed bonds
2 H-Cl 2x427 kJ
2 H-F 2x565 kJ
2 C-Cl 2x339 kJ
2 C-F 2x485 kJ
3632 kJ  C H Cl F