Bond enthalpies

• Average Bond enthalpy is the energy required to break one mole of a specified type of bond in a gaseous molecule.

  • energy is always required to break bonds
  • bond enthalpies are always endothermic
  • bond enthalpies always have a positive enthalpy value
• The actual bond enthalpy can vary depending on the chemical environment of the bond.
• Energy is required to break bonds, bond breaking is endothermic ΔH is positive.
• Energy is released when bonds form, bond making is exothermic ΔH is negative.
• For a reaction involving gaseous molecules of covalent substances:

  ΔrH =  Σ(bond enthalpies in reactants) - Σ(bond enthalpies in products)