Hess' law and enthalpy cycles

• Hess' law states that, if a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
• Hess' law comes from the idea of conservation of energy
• using enthalpy changes of formation, ∆fH,

  $∆_rH =$$∑∆_fH products - ∑∆_fH reactants$
• using enthalpy changes of combustion, ∆cH,

  $∆_rH =$$∑∆_cH reactants - ∑∆_cH products$