ORNL-1771 - Oak Ridge National Laboratory

AMP QUARTERLY PROGRESS REPORT
of two reasons. First, the equilibrium concentra-
tion of the speries in question maybe insignificant.
Second, the rate of formation of the species in
question may be so slow that it does not approach
equilibrium in any time period which may be con-
sidered. Since in the following discussion the
thermodynamic approach is used, only the first
reason wi I I be treated.
There are three particularly useful theoretical
nwosures of the significance of a decomposition
reaction. They are the equilibrium constant, the
standard free energy change, and the concentration
of the products of decomposition. The third measure
is the most directly related to experiment and hence,
at the present stage, the most useful. The only
concentration in a fused hydroxide which can be
measured at temperature by present methods is the
partial pressure of a gaseous product. Hence,
considerable emphasis is placed on possible de-
composition reactions which produce gaseous
products and on an expression of their equilibrium
concentrations in terms of the decomposition
pres S ure.
The decomposition pressure of a pure substance
with regard to a gaseous product is referred to here
as that partial pressure of the gaseous product
which, when in equilibrium with the condensed
phase, is required to maintain the over-all composi-
tion afthe condensed phase equal to the composition
of the pure substance. According to the phase
rule, the pressure so defined is unique for a given
temperature. If there a~e several gaseous products,
there is a decomposition pressure for eoch such
product.
The standard states used here are those con-
ventionally chosen in defining the standard free
energy of formation. Although this is not the usual
choice of stardard states in the case of solutes,
it is definitely the most convenient choice for the
computations which follow.
The four possible decomposition reactions which
give a gaseous product at the temperatures at which
the hydroxides are I iquid are considered here.
They are as follows:
hf7; ,
K
(3) 2MQH y3 MQ, + M i H, , A1;: ,
1 04
K
(4) 2MOH =4 MQ, + MH + bH2 , AF; I
where all substances are partitioned among all
phases present and where Ki and Ab': are the
equilibrium constants and standard free energies
of reaction respectively,
The sparsity of reliable inforniotion on the alkali
metal -oxygen cornpounds raises serious questions
about the application of all the above equilibria to
all the alkali metals. This subject is treated by
Brewer' and will not be repeated here. However,
it is the essential point of this treatment to de-
termine whether unsaturated oxygen ions such as
the peroxide and superoxide ions may occur as
decompos it ion products, in addition to the saturated
oxide ion. The existence of soline sort of unsatu-
rated oxygen ion is certain for all the alkali metals,
and it is known that the stability of the unsaturated
species increases in going from lithium to cesium.
Therefore, the basic competition referred to below
between hydrogen and the mono-oxide for water
should be a characteristic of all hydroxide de-
composition equilibria and should lead to a water-
hydrogen equilibrium in the gas phase of the type
discussed. The data for sodium compounds are
reasonably re1 iable. Hence, sodium hydroxide wi I I
be treated separately.
8y definition of the equilibrium constant,
where ax is the activity of substance X in the
hydroxide phase and NX and yx are the corre-
sponding mole fraction and activity coefficient,
respectively. Let the mole fractions be those
which are in equilibrium with the decomposition
presswre. 'Then NMz0 = NHZOI and it is possible
to write
Likewise, for Eqs. 2 through 4,
yM20
. ..... -
yH20
y,u202
YH2
I