Contributions to the Data on Theoretical Metallurgy: [Part] 11. Entropies of Inorganic Substances: Revision (1948) of Data and Methods of Calculation Page: 2
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2 CONTRIBUTIONS TO DATA ON THEORETICAL METALLURGY
available thermodynamic data sometimes has been expensive and is
to be deplored.
Four general methods are available for obtaining free-energy
changes accompanying reactions:
1. The reaction equilibrium may be studied, in which instance the
reaction free energy is given by (335):
AF0= -R Tln K, (1)
in which AF; is the standard free-energy change at the temperature
TOK., R is the gas constant per mole, and In K is the natural logarithm
of the equilibrium constant.
2. The reaction may be allowed to take place reversibly in a galvanic
cell and the electromotive force measured. Then (335),
AFT= -nf o0, (2)
in which n is the number of chemical equivalents involved, eo is the
standard electromotive force of the cell, andf is Faraday's constant.
3. The free energy may be obtained in the form AF-AH0 directly
from spectroscopic data by methods analogous to those discussed later
for calculating entropies. This method alone is applicable at present
only to reactions for which all the reactants and products are gases.
In conjunction with other methods, however, it often is useful for
reactions in which at least one reactant or product is a gas. In either
case data other than spectroscopic ordinarily are required for evaluat-
4. Use may be made of the expression for the second law of thermo-
AFT= AH} - TASr, (3)
in which AH; is the heat of reaction and AS0 is the entropy of reaction,
both at temperature ToK.
Only a few reactions have been made to take place in reversible
cells free from objectionable features; and, generally, experimental
difficulties at high temperatures almost preclude adequate equilibrium
measurements for many reactions of greatest metallurgical interest.
These and the limitations already indicated for method (3) mean that,
in the great majority of instances, method (4) is the only one available.
Consequently, it is not surprising that several times as many free-
energy values have been obtained from method (4) as from the other
three methods combined. The evaluation of entropies for this usage
is therefore a problem of considerable importance.
METHODS OF CALCULATING ENTROPIES FROM EXPERIMENTAL
CALCULATIONS BASED UPON LOW-TEMPERATURE HEAT-CAPACITY DATA
Of the various methods of obtaining entropies that based upon
low-temperature heat-capacity data is of greatest importance. This
method is intimately connected with the third law of thermodynamics.
As the substances to be considered by this method are, with one ex-
ception, crystalline at low temperatures, the discussion is restricted
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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/m1/6/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.