NOTES
the formation of hydrogen polysulfides. Furthermore, it appears that the polysulfides so obtained are mainly, if not exclusively, of the higher members of the homologous series, H2S6and above. A thermodynamic treatment of the chemical equilibria involved indicates that the number-average chain length of hydrogen polysulfides in the system is about 27 sulfur atoms a t 127" and increases with rising temperature. 3. Shorter chain H2S, species (z = 2-5) may be formed as transient intermediates in the formation of H2S, species on dissolution of HZS in molten sulfur. 3. I t has been demonstrated that molten sulfur ran serve as a useful solvent in nmr spectroscopy as well as infrared spectroscopy. Consequently, nmr should facilitate the direct study of numerous organic and inorganir reactions which can take place in liquid sulfur and thus provide a better insight into the chemistry of molten bulfur. (4) T . K. Wiewiorowski, 11. F. >fatson, and C. T. Hodges, A n a l C'hem., 37, 1080 (1965).
Dissociation of Palladium Oxide] by Wayne E. Bell, R. E:. Inyard, and XI. Tagami General Atomic D i i i s i c n of General Dynamics Corporation, J o h n J a y H o p k i n s Laboratory for Pure and Applied Science, S a n Diego, Califorma (Re& ed .May 11, 1966)
Wohler2 and Schenck and K ~ r z e n ,using ~ static methods, measured the dissociation pressure of PdO(s) in the range 680 to 875". Data of the two investigations are in agreement arid yield the value of about -26 kcaljmole (at 298°K) for the heat of formation of PdO(s). This value does not agree with the -20.4 kcaljmole obtained by Rohler and Jockum4 using a calorinietrir method. Wohler and also Schenck and Kurzen noted a dependence of dissociation pressure on oxygen content of the solid oxide. Wohler attributed this behavior to appreciable solubility of P d in PdO, whereas Schenck and Iiurzen attributed the behavior to impurities in the palladium metal. The present investigation was undertaken (1) to determine the composition range of PdO(s) a t temperatures around 800", (2) to measure the dissociation pressure of the compound using both static and dynamic methods. and ( 3 ) to derive thermodynamic values from the pressure data.
Experimental Section The coniposition (O,/Pd ratio) of PdO was measured
3735
as a function of oxygen pressure using apparatus consisting of a small, closed-end quartz reaction tube mounted in a furnace and connected to a mercury manometer. Means were provided for evacuating the system and pressurizing it with oxygen. I n conducting the experiments, a sample of PdO(s) was placed in the reaction tube, which was sealed in place. The system was evacuated, oxygen was added to a desired pressure, the system was sealed off, and the reaction tube was heated to temperature. Incremental quantities of oxygen were then removed through a vacuum-type valve. Sufficient time (more than 6 hr) was allowed between increments for the pressure to stabilize. The oxygen content of the oxide sample was determined during the course of the experiment by using a material balance calculation. The data used in the calculation were (1) the initial weight and oxygen content of the sample, (2) the quantity of oxygen in the reaction system (exclusive of that in the oxide sample) initially and a t the end of each increment, and (3) the amount of oxygen removed from the system in each increment. The quantity of oxygen in the reaction system was determined from pressure-volume measurements using appropriate temperature corrections. The quantity of oxygen removed in each increment was measured by collecting the oxygen over mercury in an evacuated bulb of known volume and measuring the pressure in the bulb. Dissociation pressures were measured using a static method and a transpiration method essentially as described earliere5 I n the static method, the oxide sample was contained in a dead-end quartz reaction tube and oxygen pressures were read on a mercury manometer. A small sulfuric acid manometer showed when the pressure in the mercury manometer and the pressure in the reaction tube were equal. In the transpiration method, helium carrier gas flowed over an oxide sample contained in a 7-mm i d . quartz tube. Diffusion barriers were located on each side of the sample. The effluent helium-oxygen mixture was analyzed by use of a gas chromatograph. Dissociation pressures were independent of flow rate a t the flow rates used (around 1.5 ml (STP)/min). The metal used was palladium sponge (99.995% purity, Johnson-Matthey). The solid oxide (PdO) used (1) This work was supported in part by the U. S. Atomic Energy Commission under Contract AT(04-3)-164. (2) L. Wohler, 2. Elektrochem., 11, 836 (1905). (3) R. Schenck and F. Kursen, 2. Anorg. Allgem. Chem., 220, 97 (1934).
(4) L. Wohler and N. Jockum, 2. Phusik. C h m . , A167, 169 (1933). (5) W. E. Bell, M.C. Garrison, and U. AIerten, J . Phys. Chem., 64, 145 (1960).
Volume 70, Number 11
Soaember 1906
3736
NOTES
-
From heat capacity equations given by Kelley,6 AC, = -6.08 12.32 X lO+T 0.20 X 105T+ for reaction 1 ; using these values, one derives AH"298 = -26.8 kcal/mole and ASo298 = -23.6 eu for reaction 1. Estimated uncertainties are =t 2.0 kcal/mole for AH"298 and A 2 . 0 eu for AS"298. Combining AS"298 with standard entropies for palladium (9.06 f 0.05 eu) and oxygen (49.01 =t 0.01 eu) given by Kelley and King' yields S O 2 9 8 PdO(s) = 9.9 f 2.0 eu. This value may be compared with 13.2 eu derived from Latimer's tables.8 (Comparison of entropy values for 1.5 monoxides given by Kelley and King with values for the same monoxides calculated from Latimer's tables shows a maximum deviation of 3.5 eu and an average deviation of 1.3 eu.)
I .O
+
0.5
I
-5 U w w
(3.1
0.05
3
g z
001
0.005
w W
>
g
0.001
0.0005
'
i
0.0001
I
0.9
I I .o IT
I 1.1
1.2
x lo3
+
Acknowledgment. The authors wish to thank Dr. J.
Figure 1. Dissociation pressure of palladium oxide,
H. Xorman for helpful discussions.
was prepared by heating the metal in oxygen for several days at 600". The resulting oxide had an O/Pd ratio of 1.00 f 0.01 as determined by hydrogen reduction analysis. Temperatures are uncertain by h 2".
(6) K. K. Kelley, Bureau of Mines Bulletin 584, U. S. Government Printing Office, Washington, D. C., 1960. (7) K. K. Kelley and E. G. King, Bureau of Mines Bulletin 592, U. S. Government Printing Office, Washington, D. C., 1961. (8) W. 11.Latimer, J . Am. Chem. Soc., 7 3 , 1480 (1951).
Results and Discussion The composition of PdO was measured as a function of oxygen pressure at 780" over the range 0.50 to 0.16 atm, and at 834" over the range 0.94 to 0.48 atm. The lower oxygen pressure at each temperature represents the dissociation pressure ( i . e . , the oxygen pressure in equilibrium with PdOis) Pd(s)). Results show that the oxygen content (O/Pd ratio) of the samples did not change with oxygen pressure until the dissociation pressure point was reached. The uncertainty of the measurements %as equivalent to O/Pd = 0.01, which, coupled with the uncertainty in the PdO analysis, means that the O/Pd ratio of the oxide in contact with metal was l.0.98. On the basis of this result and the observation that the dissociation pressure of PdO is linear with 1 T (see below), it is concluded that the composition range of PdC! is small. Oxygen pressures in equilibrium with PdO(s) Pd(s) measiired over the range 552 to 872" are plotted against 1, 7 in Figure 1. The data are linear within experimental error and show that the oxygen dissociation pressure rewhes 1 gtni nt 870". On a log Pol us. 1 T plot, the pressure values reported by TVohler2 fall on the line drawn through our data; those of Schenck and Kurzen3 straddle the line, indicating good agreement jn the results. The line in Figure 1 corresponds to AH0960 = -25.8 kcal/mole and L L S O ~ ~ ,=, -22.5 eu for
+
+
Pd(q) -4-
'/2Oq
T h e Journal of Phusical Chemtstry
=
PdO(sj
(1)
Dissociation of Iridium Trichloride'
by Wayne E. Bell and M. Taganii General Atomic Dizision o f General D y n a m i c s Corporation, J o h n J a y H o p k i n s Laboratory f o r Pure and Applied Science, S a n Diego, California (Received M a y 11, 1966)
In previous investigations, Wohler and Streicher2 claimed to have demonstrated the existence of IrCls(s), IrClz(s), and IrCl(s). They reported dissociation pressure data for all three chlorides. Remy and Kohn3 reported dissociation pressure data for IrCL(s). The present investigation was undertaken to verify the reported existence of the lower chlorides of iridium and to measure chlorine dissociation pressures as a function of temperature.
Results and Discussion Condensed-Phase Studies. It was found that the reaction of iridium sponge in a stream of chlorine gas at 600" for 12 hr produces a finely divided, olive-green material with the composition Cl/Ir = 2.99 =t 0.01, as (1) This research was supported in part by the U. 8. Atomic Energy Cornmission under Contract hT(04-3)-164. (2) L. Whhler and S. Streicher, Chern. Ber., 46, 1577 (1913). (3) H. Remy and 11. Kohn, Z . Anorg. Allgem. Chem., 137, 365 (1924).
