Final answer:
The enthalpy of a reaction with intermediate steps can be determined using Hess's law by summing the enthalpy changes of each step, since enthalpy is a state function. It can also be estimated using bond energies if enthalpies of formation are not known.
Step-by-step explanation:
The overall enthalpy of a reaction can be calculated even with intermediate steps present, by using Hess's law. This law stipulates that the enthalpy change for an entire reaction is simply the sum of the enthalpy changes of its intermediate steps, because enthalpy is a state function. The enthalpy change for the reaction (ΔHreaction) is the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants, with each multiplied by their respective stoichiometric coefficients.
Alternatively, when enthalpies of formation are not available, enthalpy changes can be estimated by considering the bond energies in the reactants and products. The enthalpy change can be found by subtracting the total bond energies of the bonds formed (products) from the total bond energies of the bonds broken (reactants).
The concept is similar to calculating the changing altitude when climbing a mountain, irrespective of the path taken. The enthalpy change is influenced by the initial and final states of the reactions, just as the altitude change depends on the base and the summit of the mountain, and not the path or rest stops taken to reach the top.