ORNL-1771 - Oak Ridge National Laboratory

........
UNCLASSIFIED
ORNL-LR-OWG 3034
.
I
... .... i
300 500 700 900 1100 !300
TEMPERATURE (OK)
Fig. 6.24. Free Energy Differences as Functions
of Temperature,
There are no satisfactory heat of formation
data for any of the alkali-metal hydroxides other
than those for sodium. Therefore, only the rela-
tive importance of the decomposition equilibria
can be studied. Some of the entropy values for
the higher oxides have been estimated and are
reported in the literature. However, the heats
of formation for rubidium and cesium oxides
are quite uncertain, and it is presently believed
that the accuracy of the AFYIZ and AF';,, values
given in Table 6.12 would not be improved by
use of these entropy estimates. Therefore, only
enthalpy values will be used, but it will be shown
that this restriction is not so serious as it might
at first seem to be.
Comparing equilibria 1 and 2 shows
PERIOD ENDfNG SEPTEMBER lo, 1954
The degree of approximation can be checked by
using the data for sodium compounds. In this case
AFT,2 is -35.5 kcal, when using the approximation,
as compared with the correct value of -40.7 kcal.
Likewise, is -94.2 kcal compared with the
correct value of -94.9 kcal. The approximate
values of and AF;,3 at 25°C are tabulated
in Table 6.12 for all the alkali-metal hydroxides.
Table 6.12 shows that hydrogen and the peroxide
should be produced most readily in potassium hy-
droxide and least readily in lithium hydroxide.
Nevertheless, equilibrium 1 is predominant, and
equilibrium 3 and hence equilibrium 4 are insigni-
ficant.
TABLE 4.12. APPROXIMATE
AND AF;,3 AT 25OC
M A";, 2
(kcal)
Li -47
No -36
K -25
Rb -34
cs -35
Aq, 3
(kcal)
- 94
-76
- 73
- 70
Various investigations of corrosion and mass
transfer hove shown the fundamental role played
by hydrogen. The above computations show that
hydrogen is important in stabilizing sodium hy-
droxide at temperatures above 6QO"C, as shown in
All the data required in the above equation are
-__available,
except those for the entropy term. Since Table 6.11. This is the temperature range at which
this term represents a difference in entropy for two mass transfer becomes significant. These compu-
similar substances, it may be supposed that its tations also link the formation of hydrogen with
omission will not be serious. A similar situation the formation of unsaturated oxygen ions. As will
obtains for AFY,3. Hence, the following approxi- be shown in a subsequent report, the presence of
motions will be used.
these unsaturated ions may be necessary for mass
107