Enthalpy is an extensive property. In general, if the enthalpy of an overall reaction A→B along one route is Δr H and Δr H1, ΔrH2, ΔrH3 ..... represent enthalpies of intermediate reactions leading to product B. What will be the relation between ΔrH for overall reaction and ΔrH1, ΔrH2..... etc. for intermediate reactions.
o For the reaction, A→B the formation of B goes through several intermediate reactions with different enthalpy values Δr H1, ΔrH2, ΔrH3....., and the overall enthalpy change is Δr H.
Being an extensive property means a property which depends on the quantity of the size of substance present in a system (like enthalpy, mass, volume or internal energy).
And
o According to Hess’s law: “If a reaction takes place in several steps then its standard reaction enthalpy is the sum of the standard enthalpies of the intermediate reactions into which the overall reaction may be divided at the same temperature.”
and that means the intermediate enthalpies will be additive in nature to give the overall enthalpy for the given reaction :
Hence, ΔrH= Δr H1 +ΔrH2 + ΔrH3 +.......... and it can also be represented as :
