Heat and Entropy of Hydration of α-NiSO4·6H2O to NiSO4·7H2O. Their Low-Temperature Heat Capacities

The heat capacities of α-NiSO4·6H2O and NiSO4·7H2O have been determined over a range 13° to 300°K. These measurements have been combined with available heat-capacity data on these hydrates extending to 1°K. Heats of solution of the two hydrates give α-NiSO4·6H2O+H2O(l)=NiSO4·7H2O , ΔH298.15°K=−1836±25 cal/mole. For the reaction NiSO4·7H2O=α-NiSO4·6H2O+H2O(g) , at 25°C P(equil.)=21.0 torr, ΔF°=2126 cal/mole, ΔH°=12 356 cal/mole and ΔS°=34.31 gibbs/mole. From the heat-capacity data, the known entropy of H2O(g) and the third law of thermodynamics, ΔS°=34.47 gibbs/mole at 25°C, in good agreement. Tables of CP°, S°, (H°–H0°)/T, and (F°–H0°)/T of α-NiSO4·6H2O and NiSO4·7H2O are included.