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

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

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