Direct Determination of Standard Gibbs Energies of the Formation of 4CaO·P₂O₅ and 3CaO·P₂O₅ by Transpiration Method

Abstract

The standard Gibbs energy changes of the following reactions have been measured by using transpiration method;
4CaO∙P₂O₅(s) + 5C(s) = 4CaO(s) + P₂(g) + 5CO(g)
ΔG^o/kJ = 1624 – 0.953T (± 2.7), (1373 < T/K < 1573)
4(3CaO∙P₂O₅)(s) + 5C(s) = 3(4CaO∙P₂O₅)(s) + P₂(g) + 5CO(g)
ΔG^o/kJ = 1946 – 1.193T (± 0.9), (1448 < T/K < 1523)
The standard Gibbs energies of the formation of 4CaO∙P₂O₅ (ΔG^o_{f,(4CaO∙P2O5)}) and 3CaO∙P₂O₅ (ΔG^o_{f,(3CaO∙P2O5)}) have been derived as follows;
ΔG^o_{f(4CaO∙P2O5)}/kJ mol^–1 = – 4765 + 0.965 T, (1373 < T/K < 1573)
ΔG^o_{f(3CaO∙P2O5)}/kJ mol^–1 = – 4200 + 0.912 T, (1448 < T/K < 1523)
The reasons for discrepancies from reported values were discussed from the view point of whether the activity of phosphorus is measured directly or not.