Energetics

Energy in to break bonds, energy out to make bonds

• During a reaction:
  • Existing bonds in the reactants must be broken — this takes energy IN (an endothermic step)
  • New bonds in the products are formed — this gives energy OUT (an exothermic step)
• Whether the overall reaction is exothermic or endothermic depends on which of these is larger:
  • If energy released from forming new bonds > energy needed to break old bonds → exothermic (ΔH negative)
  • If energy needed to break old bonds > energy released from forming new bonds → endothermic (ΔH positive)
• Mnemonic for the direction: bond breaking is the END of the bond, so it is END-othermic; bond making is EX-othermic

Bond energy values

• The bond energy of a given bond is the energy needed to break one mole of that bond, in the gas phase, in kilojoules per mole (kJ mol⁻¹)
• Bond energies are quoted in data books as positive values (the amount of heat that has to be supplied to snap that bond apart)
• Stronger bonds have larger bond energies (e.g. the triple bond N≡N is much larger than the single bond N−N)

Calculating ΔH from bond energies

• General method:
  • Write out the equation with every bond shown explicitly (a "displayed formula")
  • Energy in = sum of the bond energies of every bond in the reactants
  • Energy out = sum of the bond energies of every bond in the products
  • ΔH = (energy in) − (energy out)
• A negative answer means the products are at lower energy → exothermic; a positive answer → endothermic