Contributions to the Data on Theoretical Metallurgy: [Part] 11. Entropies of Inorganic Substances: Revision (1948) of Data and Methods of Calculation Page: 33
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ENTROPIES OF INORGANIC SUBSTANCES AT 298.16 K.
(extrapolation) and S298.6-S53 .09=40.27 (measured), making S298.16=
Oxalate.-Latimer, Schutz, and Hicks (326) (190-3000) have stud-
ied the heat capacity of CaC204.H20(c). From their data Ss98.16-
S7.s=.37.23 is computed and S17.78=0.20 is extrapolated, making
S98.16=- 37.4 0.2.
Phosphates.-Southard and Milner (450) (150-2980) have investi-
gated the heat capacities of a- and -Ca3(PO4)2. The entropy values
are, respectively, S 98.16= 57.6 0.2 and S98 .16= 56.4 0.4. The extra-
polation for the a-form is 0.09 below 14.130, while for the 0-modifica-
tion 0.20 is the extrapolation below 17.780.
Sulfide.-The heat capacity of CaS(c) was measured by Anderson
(12) (580-2950). The data yield S298.16=13.50.3, of which S56.2=
1.32 is extrapolation below 56.20
Sulfite.-Kelley and Moore (294) (530-2960) measured the heat
capacity of CaSO3(c). The extrapolated portion of the entropy
below 50.120, is 1.68, and the measured portion is 22.55. The total
is S298.16= 24.2 0.2.
Sulfates.-Anhydrous or nearly anhydrous CaSO4 exists in three
forms having different but definite thermal properties. The variety
obtained by heating gypsum or hemihydrate to temperatures around
9000 C. is distinguishable by its low solubility in water and is identical
with the naturally occurring anhydrite. It is designated here by the
symbol CaSO4 (insol.). The other two varieties are obtained by low-
temperature dehydration of the two forms of the hemihydrate men-
tioned below. Both are several times as soluble in water as CaSO4
(insol.) and they have different heats of solution and hydration.
These forms are symbolized by CaSO4 (sol. a) and CaSO4 (sol. 0), the
a and 3 corresponding to the hemihydrates from which they are
Anderson (298) (530-2960) has studied the low-temperature heat
capacity of CaSO4 (insol.). His data yield Ss98.16-25.50.4, of which
1.51 is extrapolation below 50.10 and 23.95 is measured between 50.10
and 298.160. The entropy of this substance may be approximated
from the data of Kamiike (237), who obtained AS~g,8.= 11.1 for the
reaction CaSO4.2H20 (c) = CaSO0 (insol.) + 2H1120(1). This figure cor-
responds to Ss.98.1= 24.0 2.0 for CaS04 (insol.). No weight is given
this result in comparison with the value from Anderson's work.
Kelley (298) (540-2950) has measured the heat capacities of the
two soluble varieties of CaSO4. Although appreciable differences are
apparent in their heat capacities, compensation occurs and the two
forms, CaS04 (sol. a) and CaSO, (sol. p), have virtually the same
entropy. The results obtained are S9.16= 25.9 0.3 for CaS04
(sol. a), of which 1.76 is extrapolation below 53.10, and S9s.=--
25.9 0.3 for CaS04 (sol. 0), of which 1.57 is extrapolation below 53.10.
Calcium sulfate hemihydrate, CaSO4.1/2H20, exists in two forms
having different but definite thermal properties. For the present
purpose, macrocrystalline hemihydrate, obtained by dehydrating
gypsum in the presence of liquid water, is designated as CaSO4-1/2H20
(a). The ordinary variety, prepared by dehydrating under condi-
tions where the water is gaseous, is microcrystalline and is designated
as CaSO4. 1/2H20 (0).
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Kelley, K. K. Contributions to the Data on Theoretical Metallurgy: [Part] 11. Entropies of Inorganic Substances: Revision (1948) of Data and Methods of Calculation, report, 1950; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc12637/m1/37/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.