8.7 - Enthalpy Change and Bonding

• bond enthalpy

  amount of energy needed to break one mole of the bonds in a substance in the gaseous state
• mean bond enthalpy

  energy needed to break one mole of bonds in gas phase averaged over many different compounds

  needs to be an average because bonds of the same type in different compounds don't have the amount of energy
• what does mean bond enthalpy take into account

  bond enthalpy for a certain covalent bond varies slightly from compound to compound

  successive bond enthalpies aren't the same in some compounds
• bond dissociation enthalpy

  energy needed to break one mole of the bond to give separated atoms in gas state
• bonds breaking

  absorbs energy

  endothermic

  enthalpy change is positive
• bonds forming

  releases energy

  exothermic

  enthalpy change is negative
• enthalpy change of reaction =

  sum of bond enthalpies of reactants - sum of bond enthalpies of products
• limitations of using estimated enthalpy change values

  different from experimental value due to variations in strength of a bond in a certain compound or when reactant/product isn't in the gaseous state