bond enthalpy - enthalpy change for breaking of a bond

• strength of covalent bond determined by amount of energy needed to break it
• always positive value
• DHrxn = (bond enthalpies of bonds broken) - (bond enthalpies of bonds formed)
• derived for gaseous molecules, only averaged values

Find the DH for 2C2H6 + 7O2>> 4CO2 + 6H2O  

• C2H6
• CO2
• H2O
• Given bond enthalpies:
  • C-H 413 kJ/mol
  • C-C 348 kJ/mol
  • C=O 799 kJ/mol
  • H-O 463 kJ/mol
  • O2 495 kJ/mol
• 2 C-C, 12 C-H, 7O2 bonds broken
• 8 C=O, 12 H-O bonds created
• [2(348) + 12(413) + 7(495)] - [8(799) + 12(463)] = -2831 kJ

Find the DH for N2H4 >> N2 + 2H2  

• N2H4
• N2
• Given bond enthalpies:
  • N-H 391 kJ/mol
  • H-H 436 kJ/mol
  • N=N 941 kJ/mol
  • N-N 163
• 1 N-N, 4 N-H bonds broken
• 1 N=N, 2 H-H bonds created
• [163 + 4(391)] - [941 + 2(436)] = -86 kJ