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 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
• An average bond enthalpy is calculated from the actual bond nthalpies in different chemical environments
• Energy is required to break bonds - bond breaking is endothermic, ∆H is positive
• Energy is released when bonds form - bond making is exothermic, ∆H is negative
• The difference between the energy required for bond breaking and the energy released by bond making determines whether an overall reaction is exothermic or endothermic
• Exothermic = the energy released when making bonds is greater than the energy required when breaking bonds
• Endothermic = the energy required when breaking bonds is greater than the energy released when making bonds
• For a reaction involving gaseous molecules of covalent substances:

  $∆_rH =$$∑(bond.enthalpies.in.reactants) - ∑(bond.enthalpies.in.products)$