What is Hess's law and how is it used to determine reaction enthalpies?

• A. Enthalpy is a state function; the total ΔH for a process is the sum of the enthalpies of its steps; use known enthalpies of formation or combustion to assemble the desired ΔHrxn.
• B. Enthalpy depends on path; the total ΔH for a process is the sum of the enthalpies of its steps; use bond energies only.
• C. Enthalpy is mass times specific heat; determine by q = m c ΔT.
• D. Enthalpy is only defined for gases; use PV work to compute.

Answer: (A)

Hess's law rests on enthalpy being a state function. This means the enthalpy change of a chemical reaction depends only on the starting and ending states, not on the path taken between them. Because of that, you can add together reactions whose enthalpies are known to build up the target reaction, and the enthalpies simply add to give the overall ΔHrxn. In practice, you use standard enthalpies of formation (or combustion) for the substances involved: you sum the formation enthalpies of the products and subtract the sum for the reactants, or you assemble a sequence of known reactions that totals the desired one and add their ΔH values. This is why Hess's law is so useful—it converts a difficult direct measurement into a matter of combining tabulated enthalpies.

The idea that enthalpy depends on the path is not correct, since enthalpy is a state function. Using bond energies is one way to estimate ΔH, but it’s an approximation and not the fundamental statement of Hess’s law. The q = m c ΔT relation describes sensible heat for a temperature change, not the enthalpy change of a chemical reaction. And enthalpy is defined for all phases, not only gases; relying on PV work alone misses the broader, state-function nature of enthalpy used in Hess’s law.