How do you compute the enthalpy change from standard formation enthalpies?

• A. ΔHrxn° = Σ ΔHf°(reactants) − Σ ΔHf°(products).
• B. ΔHrxn° = ΔHf°(products) − ΔHf°(reactants).
• C. ΔHrxn° = Σ νr ΔHf°(reactants) − Σ νp ΔHf°(products).
• D. ΔHrxn° = Σ νp ΔHf°(products) − Σ νr ΔHf°(reactants), using the stoichiometric coefficients as multipliers.

Answer: (D)

Use the standard formation enthalpies as building blocks. Each product’s ΔHf° tells you how much energy is needed to form 1 mole of that product from elements. To get the overall reaction enthalpy, multiply each ΔHf° by how many moles of that species participate (the stoichiometric coefficient), sum these for the products, and subtract the sum for the reactants. In formula form: ΔHrxn° = Σ (νp × ΔHf°(product)) − Σ (νr × ΔHf°(reactant)). The overall sign tells you if heat is released or absorbed.

Example: for 2 H2 + O2 → 2 H2O, ΔHrxn° = [2 × ΔHf°(H2O)] − [2 × ΔHf°(H2) + 1 × ΔHf°(O2)]. Since ΔHf°(H2) and ΔHf°(O2) are zero, this simplifies to 2 × ΔHf°(H2O), which is negative because formation of water is exothermic.