Using Hess's law and standard enthalpies of formation, which expression gives ΔHrxn for the formation of products from reactants?

• A. ΔHrxn = ΣΔHf(reactants) − ΣΔHf(products)
• B. ΔHrxn = ΔHf(reactants) − ΔHf(products)
• C. ΔHrxn = ΣΔHf(products) − ΣΔHf(reactants)
• D. ΔHrxn = ΔHf(products) − ΔHf(reactants)

Answer: (C)

The main idea is using Hess’s law with standard enthalpies of formation: the enthalpy change for a reaction equals the total formation enthalpies of the products minus the total formation enthalpies of the reactants, each multiplied by its stoichiometric coefficient. Since ΔHf is the energy to form 1 mole of a compound from elements in their standard states, you add up ΔHf for all product moles and subtract the sum for all reactant moles. This gives ΔHrxn for forming the products from the reactants. As a quick check, if all reactants and products were elements in their standard states, their ΔHf would be zero and the reaction enthalpy would be zero, which matches intuition.