Contributions to the Data on Theoretical Metallurgy: [Part] 7. The Thermodynamic Properties of Sulphur and its Inorganic Compounds Page: 72

                                
DATA ON THEORETICAL METALLURGY.-VII

The dissociation pressure of CdSO4"8/3 H2O=CdSO4"H20+5/3
HO0(g) at 298.10 was found to be 2.24X10-2 and 2.32X10-2, respec-
tively, by Ishikawa and Murooka (50) and Carpenter and Jette (16).
The free-energy change corresponding to this pressure, AF= - R T
In PH2o6/3, is AF298.1=3,700. So,
CdSO4H20 +5/3HO (g) = CdSO4.8/3H20,
AF298.1= - 3,700;
and
CdSO4"H20 + 5/3H2O0(1) = CdSO4.8/3H2O,
AF298.1= -280.
As a consequence of the foregoing results,
CdSO4 + 8/3H2O(g) = CdSO4.8/3H20,
AF29s.1= -7,910;
and
CdSO4 + 8/3H20(1) = CdSO4.8/3H2O,
AH28.1= -8,080; AFO298.1= -2,440.
The result given for AH298.1 of the last reaction is derived from the
work of Thomsen (81, p. 1571).
Reaction 5CdSO4 = CdSO4.4CdO+4S02+202.-According to Marchal
(97), the first step in the dissociation of CdSO4 is the formation of the
compound CdSO4.4CdO. Marchal's equilibrium partial pressures of
SO2, 02, and SO3 may be used to obtain K= P4so2.P2o2 for the reaction
5CdSO4= CdSO4.4CdO+4SO2+202.       However, Z-function   values
from these data show marked curvature when plotted against 1/T.
That this is not attributable to the specific-heat increment employed
may be shown by varying the assumed AC,, within reasonable limits.
Consequently, no satisfactory free-energy calculations can be made
from these dissociation data.
CALCIUM SULPHATE
CaSO4 is known generally in two forms, anhydrite and soluble
anhydrite. In this section it is to be understood that only anhydrite
is being considered.
Values of the heat and free energy of formation of CaSO4 at 298.10
may be obtained from a variety of data. Unfortunately, the values
do not agree as well as is desirable and none is free from serious
objection. The entropy of formation is obtainable with considerable
certainty from the values (70) S298.1=9.95 for Ca, S298.1=7.64 for
S(rh), 5298.1=49.03 for 02, and S298.1=25.6 for CaSO4. The result is
AS298.1= --90.0 in which the maximum error is estimated as +0.5.
Thomsen (81, p. 1514) has obtained AH=--261,360 at room tem-
perature as the heat of the reaction Ca+S02+02=CaSO4. Assigning
this value to 298.10 involves an error which is entirely negligible in
comparison with the probable experimental error in Thomsen's deter-
mination. The heat of formation of SO2 and this result yield AH298.1=
-332,300 as the heat of formation of CaSO4 which, with the entropy
of formation, is seen to'correspond to AF298.1= -305,470.

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