PbO2
apparently requires four stages of decomposition to reach
normalPbO
. Based on thermogravimetric studies the intermediate products appear to be
5PbO2·4normalPbO
,
4PbO2·5normalPbO
, and
Pb3O4
. Although the first three stages seem irreversible, they too come to a steady state in decomposition, but no recombination takes place.
normalΔH°
and
normalΔS°
values have been calculated for the four stages.
Although earlier investigators showed the dissociation of
Ag2O
to be reversible and equilibrium points had been obtained, there was a range from 191° to 203°that had not been studied. This region has now been studied. From all the data available and heat capacity data
normalΔH°298
,
normalΔS°298
, and
normalΔG°298
have been calculated to be 14.69 kcal, 31.92 kcal, and 5.17 kcal per mol O2respectively, for the dissociation reaction
2Ag2O→4normalAg+O2
.
Oxygen pressures of the pyrolusite form of
MnO2
have been determined in a temperature range of 441°–642°C. The dissociation reaction
4MnO2false(normalcfalse)⇄2Mn2O3false(normalcfalse)+O2false(normalgfalse)
has been found to reach a steady state in the forward direction; reaction in the reverse direction was exceedingly slow and did not reach a steady state.
normalΔH°
and
normalΔS°
for the reaction at 813°K are 37.87 kcal and 51.28 cal/deg, respectively. With little error these values can be used in the equation −4.5756logpO2 false(normalatmfalse)=37,870/T−51.28 to calculate the pressure for any temperature in the above mentioned range. Conversion to 298.15°K resulted in
normalΔH°298=38.8 normalkcal
and
normalΔS°298=52.8 normalcal/normaldeg
for the dissociation. The limits of uncertainties are not given here because equilibria were not attained by recombination.
The equilibrium represented by the equation 6Mn203(c) ~ 4Mn804(c, a) O2 has been studied by measuring the equilibrium pressures of oxygen over Mn203-Mn~O4. From the data obtained in the 1132~176 range studied AH ~ and AS ~ were calculated for the reaction at 1200~ to be 41.2 ___ 1.2 kcal and 32.9 • 1.0 cal/deg, respectively. These constants can be used in the equation --4.5758 log pO2 (atm) ----41,200/T-32.9to relate log poe and temperature of the system in the above-mentioned range with no appreciable additional error. A slight extrapolation showed poe ~ 1 arm at 1252~ The above hH and aS values were converted to 298. 15~ and compared with those in the literature. A new entropy 8~ ~ 36.8 cal/deg for Mn304 and a new heat of formation • z --228.7 kcal for Mn203 are indicated from analysis of data in the literature and the present data. This value of entropy of Mn304 is higher by 1.3 cal/deg (roughly Rln2) than the value taken from the literature. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 157.182.150.22
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