Hess' law and enthalpy cycles

• Hess's 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.
• Enthalpy changes from standard ΔfH

  1. Construct the enthalpy cycle between the reactants the products and their elements. In the enthalpy cycle, the elements, form the common link between reactants and products. Using standard ΔfH, the reactants and products of the original reaction are formed from their elements and the arrows point upwards.
  2. Add ΔfH values and calculate the Unknown enthalpy change
• ΔEnthalpy changes from  standard ΔcH

  1. Construct the enthalpy cycle between the reactants the products and their common combustion products $CO_2(g)$and$H_2O(l)$. Using ΔcH, both the reactants and products of the original reaction react to form combustion products and the arrows point downwards.
  2. Add ΔcH values and calculate the unknown enthalpy change
• Using enthalpy changes of formation, ΔfH,

  ΔrH = ΣΔfH products - ΣΔfH reactants
• Using enthalpy changes of combustion, ΔcH,

  • ΔrH = ΣΔcH reactants- ΣΔcH products