Chapter 5, Problem 103SCQ

You want to determine the value for the enthalpy of formation of CaSO₄(s), but the reaction cannot be done directly.

Ca(s) + S(s) + 2 O₂(g) → CaSO₄(s)

You know, however, that (a) both calcium and sulfur react with oxygen to produce oxides in reactions that can be studied calorimetrically, and (b) the basic oxide CaO reacts with the acidic oxide SO₃(g) to produce CaSO₄(s) with △_rH° = −402.7 kJ. Outline a method for determining △_fH° for CaSO₄(s), and identify the information that must be collected by experiment. Using information in Appendix L, confirm that △_fH° for CaSO₄(s) = −1433.5 kJ/mol.