ORNL-1771 - Oak Ridge National Laboratory

UNCLASSIFIED
ORNIL-LR-DWG 3032
r--
300 500 700 900 1100 1300
TEMPERATURE (OK)
Fig. 6.22. Free Energy of Formation Data as a
function of Temperature for the Hydroxide, the
Peroxide, and the Superoxide of Sodium.
sodium hydroxide' and the entropies of sodium
hydroxide," hydrogen, oxygen, and sodium in their
standard reference states. The computation shows
that AF;,,(NaOH) = -91.0 kcal/mole. At higher
temperatures, values of the standard free energy of
formation hove been given in the literature for only
sodium oxide,' ' Howeverp values have been esti-
mated for sodium peroxide, sodium superoxide, and
sodium hydroxide in the following way. The curves
of the standard free energy of formation vs temper-
ature for the class of compounds dealt with here
are known to a fairly good degree of approximation
to consist of segmented straight lines with dis-
continuous changes in slope at the transition
points of the compound and of its component ele-
ments. One point on each of these curves can be
fixed from the values for the standord free energy
of formation data at 25"C, mentioned above. The
'Unless otherwise specified, literature values used
herein are taken from F. D. Rossini, Selected Values o/
Cb emzcal The mzodynumzc Prope rt ze s, Notion al Bureau of
Standards, Washington, 0. C., 1952.
'OJ. C. R. Kelly and P. E. Snyder, J. Am. Chern. Soc.
73, 41 14 (1951).
"F. D. Richardson and J. H. E. Jeffes, J. Iron Steel
Insf. (London) 160, 261 (1948).
PERIOD ENDING SEPTEMBER 70, 1954
slope at this point is determined from values of
the standard entropy of formation by using the
1 relation
These entropy data are obtained from the standord-
state entropy values. The change in slope at each
transition point is determined from the transition
entropy, which, of course, is readily computed from
the heat of trans it ion.
Recently obtained datal2 on the heat capacity of
liquid sodium hydroxide will permit a more precise
evaluation to be made of the slope of the free
energy curve from the melting point up to 1000°C.
The heat of fusion of sodium peroxide (TLM
= 980°K)
was not available, but it should introduce only a
small change in slope, which may be considered to
be negligible for the computations which follow.
Decomposition of Hydroxides. In considering any
solvent as a reaction medium it is important to de-
termine whether the solvent tends to decompose to
an appreciable extent, particularly if any of the
products of the decomposition take part in the re-
action. The classic example is, of course, water.
Here the covalent water molecules decompose
slightly into ions. The resulting equilibrium fre-
quently exerts a controlling influence on the course
of aqueous reactions. The following is an outline
of the results obtained to date in a theoretical
search for the significant decomposition equilibria
in fused alkali-metal hydroxides. It should be
emphasized that the work presented here is in no
sense definitive or complete.
The most important result of the treatment given
here is the evidence for the possible occurrence of
appreciable quantities of peroxide as the result of
decomposition equilibria. The way in which this
peroxide may play a decisive role in mass transfer
and corrosion will be shown in a later report.
In a fused alkali-metal hydroxide in an inert
environment such that the volume of any gas phase
is small compared with that of the liquid phase,
the fused hydroxide may be regarded thermodynami-
cally as being in equilibrium with every chemical
species composed of nothing more than oxygen,
hydrogen, and the alkali metal. In practice, many
of these chemical species can be ignored for either
"W. D. Powers and G. C. Blalock, Enthalpies md
Speczftc Heats of Alkali and Alkaline Earth Hydroxzdes
at ffzgh Temperatures, ORNL-1653 (Jan. 7, 1954).
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