ORNL-1771 - Oak Ridge National Laboratory

K, I r3
(G2
PERIOD ENDING SEPTEMBER 70, 7954
YM 0 yhf
three values of the decomposition pressure with
respect to hydrogen. Except over unique temper-
2 ature ranges, one of these three decomposition
uizl Q H
pressures will be much larger than the other two
- I I', = - I
4:
d2
YMO, YMMH
K =r--l
4 4
I'4 - 4----"
4 O H di
The above equations are exact. However, a rrtore
useful form can be obtained by assuming that the
extent of decomposition is very small. As will be
shown later, this assumption involves a negligible
error for all four reactions. From this assumption
two conclusions can be drawn: (1) the activity of
the hydroxide as used in the above equations may
be taken to be unity because of the choice of
standard states; (2) the partial pressures of water
and hydrogen in the gas phase may be taken to be
equal to the fugacities in the gas phase and hence
equal to the fugacities in the liquid phase. Finally,
by definition, the fugacity of the gases in the
standard reference state is unity. Substituting
these conclusions into the above equations, solving
for the partial pressures, and applying the reaction
isotherm give
2/5
where (pH- ) is to be read as the partial presswe
[ ..2Ii
of hydrogen provided that no other hydrogenproducing
reaction is operative save the Ith.
From the original choice of mole fr~cti~ns it
may be noted that the ubove partial pressures are
1
'
and hence, by the law of mass action, will stabilize
the others. Thus, Qnly one of the three pressures
will be the decomposition pressure of ,llOH with
regard to hydrogen.
Because of the lack of thermodynamic data on
all the alkali-metal hydroxides except sodium, it
is necessary, in most applications, to use ratios
of decomposition pressures. Thus, it IS assumed
that reactior~s 1 and 2 are simultaneously at equi-
librium and that they determine the gaseous water
and hydrogen pressures, respectively. By dividing
Eq. 56 into 5a, the decomposition pressure ratio
becomes
where
The concentrations of the oxide in reaction 1 and
of the peroxide in reaction 2 are not independent
but are related through the equilibrium
4"s" .
Furtkrrrrore, the concentrations of oxygen in this
equilibrium atid of hydrugen in reuction 2 are
related through the equilibrium
the decomposition pressures and that there ure (8) AF,. a