5.3 Bond enthalpies

Bond Enthalpy

  • Bond (dissociation) Enthalpy: The energy required to break 1 mol of bonds in gaseous covalent molecules under standard conditions
    • Bond breaking is endothermic process and has a positive enthalpy value
    • Average values found in section 11 of the data booklet
  • Bond length
    • As atomic radius increases, so does bond length. This results in decrease on bond strength
  • Bond Strength
    • Bond enthalpy reflects strength of covalent bond
    • As number of bonds increase (single, double, triple), the bond strength increases, and bond length decreases
  • Bond Polarity
    • Determined by difference in electronegativity


  • Ozone, O3 is created and destroyed in stratosphere. UV rays split O2 molecules into single oxygen atoms, then these radicals combine with oxygen molecules to create ozone.
  • The importance of ozone is that it is very effective at blocking harmful long and short wavelength UV radiation. Without ozone, radiation would harmfully damage cells on plants and animals.
  • Ozone molecule is decomposed more readily than oxygen molecule because it is at higher energy state and less stable


Bond Enthalpy

  • Note: The answer calculated will vary from actual answer since the bond enthalpy values are an average, and this does not take into account intermolecular forces which are broken as well

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