What is Hess's Law, and how is it used to calculate enthalpy changes?

• A. Enthalpy changes are additive; you can sum enthalpies of steps to obtain the overall ΔH°rxn.
• B. Enthalpy changes depend on the path and cannot be added.
• C. Enthalpy is always conserved; sum of reactants equals sum of products in energy terms.
• D. Enthalpy changes are only for formation reactions; cannot be used otherwise.

Answer: (B)

Hess's Law is about enthalpy as a state function: the total heat change of a reaction depends only on the initial and final states, not on the path taken. Because of that, you can break a reaction into steps, assign each step an enthalpy change, and add them to get the overall ΔHrxn. In practice, this is why we can use enthalpies of formation or enthalpies of combustion for individual steps and sum them to find the total enthalpy change for the overall reaction.

So, the idea that enthalpy changes depend on the path is not correct; enthalpy changes are additive regardless of how the reaction proceeds. The notion that enthalpy is only for formation reactions or that it simply balances reactants and products in energy terms are not accurate descriptions of Hess's Law.