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UNIF, = yN should be unity (a pure solid has unit
activity). The apparent activity is 9 at 600°C and
20 at 70O0C, however, which indicates that the
saturating phase is something less soluble than
NiF,. The discrepancy amounts to a factor of
10 in the activity coefficient or the solubility and
to about 5 kcal/mole in the value of AFF;,F2. In
computing the AF" for the formation of the complex
compound from the elements, the free energy esti-
mates given in Table 2.2.13 were employed.20
,OL. Brewer et aL, p 104-115 in The Chemistry and
Metallur o Mkcehneous Materials, Thermod namics.
NNES Ivlds, ed. by L. L. Quill, McGraw-dll, New
York, 1950.
TABLE 2.2.13. SELECTED VALUES OF
FREE ENERGY OF FORMATION
bo (kcal/mole)
At 600°C At 700°C
Fe F2 -137.5 -133.9
Cr F2 -1525 -1 48.9
Ni F2 -127.9 -1248
Zr F4 -383.4 -376.8
PERlOD ENOlNC JUNE IO, 1956
An independent check on the differences in the
standardfree energies of formation of the MF,*ZrF4
complex from the elements is afforded by previously
published data obtained for bimetallic couples in
saturated half To a first approximation,
the cel'l reaction was
M + M'F2*ZrF4 = MF2*LrF4 + M' .
This reaction neglects transport and junction poten-
tials, which are small if the saturated solutions
are sufficiently dilute. From the relation
En3 = -AF
far the cell reaction, where E is the electromotive
force, R is the number of equivalents transferred,
and 3 is the number of Faradays passed, an approx-
mate value of the difference in the free energy of
formation from the elements is found. The com-
parison is shown in Table 2.2.14.
The values of AFcalc in Table 2.2.14 were ob-
tained by subtracting the values of AFo given for
these complexes in the last column of Table 2.2.12.
While the agreement is good, the effect of the high
solubility of GF, at 700°C is probably responsible
for the larger of the discrepancies.
,'L. E. Topol, ANP Quat. frog. Rep. Sept. 10, 1955,
ORNL-1947, p 85.
TABLE 2.2.14. COMPARISON OF FREE ENERGIES OF FORMATION OF MF2*ZrF4
OBTAINED BY TWO METHODS
FeF2*ZrF4-GF2*ZrF4
Difference Between bM F2,fr F4 and &M'F2*ZrF4
(kea Vmo le)
At 600°C At 70OoC
-En 3 . 15.5 16.8
AFcalc 15 15
Fe F2*Zr F4-N i F2* Zr F4
-En 3 -19.5 -20.0
~colc
GFZ*ZrF4-NiF2*ZrF4
-20 -20
-En 3 35.1 -36.3
3 5 3 5
107