How is ΔHrxn defined in terms of formation enthalpies when using Hess's law?

• A. The enthalpy change per mole of product formed
• B. The enthalpy change per mole of reactants
• C. The enthalpy change per mole of solvent
• D. The enthalpy change per mole of reaction as written

Answer: (D)

The main idea is that Hess’s law lets you compute the heat of the reaction from the formation enthalpies of the species involved, and the quantity you get is for the reaction as it is written, per mole of that reaction. In practice, you combine the standard formation enthalpies of the products with their stoichiometric coefficients and subtract the formation enthalpies of the reactants with their coefficients:

ΔHrxn = Σ (stoichiometric coefficient × ΔHf° of products) − Σ (stoichiometric coefficient × ΔHf° of reactants).

This means you add the formation enthalpies of all products (each multiplied by how many moles of that product appear) and subtract the same for all reactants. For a simple one-to-one reaction, it reduces to the product’s formation enthalpies minus the sum of the reactants’ formation enthalpies. For example, a reaction forming CO2 and H2O from CH4 and O2 uses ΔHrxn = ΔHf°(CO2) + 2ΔHf°(H2O) − [ΔHf°(CH4) + 2ΔHf°(O2)], noting that ΔHf° for elements in their standard state is zero. The result is the enthalpy change per mole of reaction as written.