The individual steps need not be realizable in practice: they may be hypothetical
reactions, the only requirement being that their chemical equations should balance.
The thermodynamic basis of the law is the path-independence of the value of ΔrH7
and the implication that we may take the specified reactants, pass through any
(possibly hypothetical) set of reactions to the specified products, and overall obtain
the same change of enthalpy. The importance of Hess’s law is that information about
a reaction of interest, which may be difficult to determine directly, can be assembled
from information on other reactions.