Exemplar Problems

Problem 11 : Calculate the standard enthalpy change (∆H°) for the combustion of propane (C₃H₈) to form carbon dioxide (CO₂) and water (H₂O) vapor, given the following standard enthalpies of formation:

• ∆H°f(C₃H₈) = -103.9 kJ/mol
• ∆H°f(CO₂) = -393.5 kJ/mol
• ∆H°f(H₂O) = -285.8 kJ/mol

Solution :

∆H° = Σ(∆H°f(products)) - Σ(∆H°f(reactants))

∆H° = [∆H°f(CO₂) + ∆H°f(H₂O)] - ∆H°f(C₃H₈)

∆H° = [(-393.5 kJ/mol) + 2(-285.8 kJ/mol)] - (-103.9 kJ/mol)

∆H° = (-393.5 kJ/mol - 571.6 kJ/mol) + 103.9 kJ/mol

∆H° = -965.1 kJ/mol

So, the standard enthalpy change for the combustion of propane is -965.1 kJ/mol.