Given the Gibbs free energy at two temperatures, determine the maximum temperature for this heterogeneous reaction to occur:
2Ca(liq) + ThO2(crystal) ↔ 2CaO(crystal) + Th(crystal).
∆G° = -3.4 kcal/mol at 1000°C, and
∆G° = -2.0 kcal/mol at 1200°C.
Assume all phases are pure.