Enthalpy is an extensive property. In general, if enthalpy of an overall reaction A→B along one route is and
,
,
….. represent enthalpies of intermediate reactions leading to product B. What will be the relation between
for overall reaction and
,
etc. for intermediate reactions.
According to Hess’s law,
This is so because during the reaction , B’s formation undergoes various intermediate reactions, with the overall value of the enthalpy being
.