Question:
Calcium carbonate (CaCO3) exists in two polymorphic forms, calcite and aragonite. The standard state enthalpy of calcite is -1207.37 kJ/mol, while that of aragonite is -1207.74 kJ/mol. The entropies of aragonite and calcite under the same conditions are 88 J mol-1 K-1 and 91.7 J mol-1 K-1, respectively. What is the stable polymorph at 25°C and 1 atm? Is there a temperature above which the other polymorph would be the equilibrium phase? If so, what is that T? If not, why not?