Lesson 6: Bond Energies

• Bond Dissociation Energy

  The quantity of energy required to break one mole of covalent bonds in a gaseous species, usually expressed in kJ/mol
• Average Bond Energy

  The average of bond-dissociation energies for a number of different species containing a particular covalent bond
• ΔH for reactions

  Can be found using bond energies
• The ΔH calculated by using bond energies will be different than that found using Hess's Law or that found experimentally
• Bond energies found in tables are average values for a particular bond type
• Determining Bond Energies

  • The bond energy for an H-H bond, or an F-F bond can be determined to a high degree of precision because H2 and F2 molecules contain single bonds of a particular type
  • The exact bond energy for a C-H bond cannot be determined exactly because there is no such thing as a C-H molecule
  • The C-H bond always has other atoms bonded to it which affect the bond energy
• The C-H bond energy in CH4, CH2Cl2 and CH3F are all slightly different since the C-H bond is in different environments
• This is because of the electrons and intermolecular forces, so the bond energies are different
• ΔH values calculated using average bond energies are therefore slightly different than the values calculated by some other method
• Bond formation is an exothermic process while bond breakage is an endothermic process
• To find ΔHrxn from average bond energies

  Bond energies of all reactant bonds broken - Bond energies of all product bonds formed
• bond energies are less accurate because they are based on the average of the energies from various compounds