Thermochemistry of magnesium oxysulfate

The enthalpies of solution of magnesium oxysulfate compounds MgSO4·5MgO·7H2O and MgSO4·5MgO·8H2O in approximately 1 mol dm−3 aqueous hydrochloric acid were determined. From these results and the enthalpies of solution of MgSO4·7H2O and MgO in HCl(aq) respectively, the standard molar enthalpies of formation of −(6496.00±0.73) kJ mol−1 for MgSO4·5MgO·7H2O and −(6800.17±0.55) kJ mol−1 for MgSO4·5MgO·8H2O were obtained by using the standard molar enthalpies of formation of MgO(s), MgSO4·7H2O(s) and H2O(l). The standard molar entropies of formation of MgSO4·5MgO·7H2O and MgSO4·5MgO·8H2O were calculated from the thermodynamic relation with the standard molar Gibbs free energy of formation of MgSO4·5MgO·7H2O and MgSO4·5MgO·8H2O computed from a group contribution methods.