Contributions to the Data on Theoretical Metallurgy: [Part] 11. Entropies of Inorganic Substances: Revision (1948) of Data and Methods of Calculation Page: 21
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ENTROPIES OF INORGANIC SUBSTANCES AT 298.16 K.
the general procedure has been to consider that part of the entropy
given by actual measurement separately from the part that is entirely
extrapolation. The assumed accuracy of the experimental portion
depends partly upon the estimated errors in the heat-capacity data
as given by the investigator responsible and partly upon information
obtained by intercomparison of results of several workers. The error
in the extrapolated portion generally is taken as about 5 to 10 percent,
which seems a reasonable procedure, as the uncertainty should in-
crease with the magnitude of the extrapolation and in instances in
which the heat capacity has been measured down to very low temper-
atures an error of this magnitude in the extrapolated part of the
entropy is only a negligible portion of the total entropy at 298.160K.
It should be emphasized that the estimate of error in the third-law
calculations applies only to the existing heat-capacity data and the
necessary extrapolations as made. Of necessity, it usually is as-
sumed that the substance is in such energy condition at the lowest
temperatures reached in the heat-capacity measurements that a normal
type of extrapolation may be made. The data and procedure for
Gd2(SO4)3.8H1120, for example, illustrate the difficulties that may
arise in computing entropies of paramagnetic substances. Moreover,
the data and calculations for such substances as CO, H20, and N20O
show that correct entropies are not obtained in all instances by indis-
criminate application of the usual methods of calculation.
In some instances heat-capacity values of several experimenters
have been averaged, while in others the results of a single investigator
have been adopted. The bibliography at the end of this bulletin
contains a virtually complete (to November 1948) list of references
pertaining to true heat-capacity data at low temperatures of the ele-
ments and inorganic compounds. A table of heat-capacity values of
the substances whose entropies have been calculated from the third
law of thermodynamics is given immediately following this entropy
Element.-The low-temperature heat capacity of Al (c) was meas-
uted by Giauque and Meads (182) (150-3020), Kok and Keesom (304)
(10-200), Maier and Anderson (344) (540-2970), Nernst (373, 375)
(320-890), and Nernst and Schwers (377) (190-800). The computed
entropy is S298.16= 6.77 0.02. The extrapolated portion, below
1.110, is negligible, ca. 3 X 10-4 unit.
Spectroscopic data for Al (g), listed by Moore (364), show that
only two energy levels, 2P1/2 and 2P3,2, separated by 112.04 waves per cm.
and having quantum weights of 2 and 4, respectively, need be con-
sidered in deriving the entropy at 298.160. These states add 3.492
to the entropy given by the Sackur equation, to yield S298.1-= 39.31
0.01 for Al (g).
Aluminum Ion.-Latimer, Pitzer, and Smith (325) have obtained
S 9s,16, - 76 10 for Al+++(aq.), based upon data for the reaction CsAl
(SO4)2.12H20 (c) = Cs+ (aq.) +Al+++ (aq.) + 2SO4-- (aq.) 12H20 (l).
Oxides.-Anderson (24) (530-2950), Parks and Kelley (383) (910-
2920), and Simon and Swain (443) (300-2800) have measured the
heat capacity of granular alundum, sapphire, and corundum, respec-
tively. These are three varieties of the same crystalline modification
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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/m1/25/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.