NOTES
949
mation whereas determination of E (31-H) requires only the ionization potential and the well established AHfo(H). From calorimetry, on the other hand, the heat of formation is obtained directly, but calculation of E(M--H) requires use of the heat of sublimat ion. For SiH4,using AHn = 11.7 e.v. or 270 kcal. and I = 189.4,1st21E(Si-H) is 72 kcal. For CeH4, using HI = 10.7 e.v. or 247 kcal. and I = 183.218(converted from OOK. to 25'C.), E(Ge-H) is 68 kcal. Saalfeld and SvecZ2also measured appearance potentials of Si+ and Ge+ from Siz&, Ge2HB,and Ge3H8, assuming the processes
+ Mo(g) + 3Hz hl+(g) + 2M0(g) + 4Hz
MzHe(g) --+ M+(g)
(7)
M,Hs(g)
(8)
+
From eq. 7 AHrO(;LIzHs) = 2A.Hr0(34,g)
+ I ( l / I ) = AH7
(9)
Since
E(M-M)
2AHr0(M,g)
+ 6AHf0(H) -
A.Hf'(MzH6) - 6E(M-H)
(10)
substituting 6 and 9 into 10 gives
E(M-M)
=
A H , - "'AH1
+ '/J(M)
E(M-M)
+ I ( M ) - AH8 - AH1 + ' / J ( I / I )
Acknowledgments. We wish to thank Professor William Jolly for providing the trigermane, and 3lr. Virgil DuVal for performing the gas chromatographic purification of trisilane. (21) C. E. Moore, "Atomic Energy Levels," Val. 111, National Bureau of Standards Circular 467, U. S. Govt. Printing Office, Washington, D. C., 1958. (22) F. E. Saalfeld and H. J. Svec, Inorg. Chem., 2, 50 (1963). (23) W. C. Steele and F. G. A. Stone, J . Am. Chem. Soc., 84, 3600 (1962). (24) W. C. Steele, L. D. Nichols, and F. G. A. Stone, ibid., 84, 444 (1962). (25) F . FehBr, G. Jansen, and H. Rohmer, Angew. Chem., 75, 859 (1963). (26) C. N. Cochran and L. M . Foster, J . Phys. Chem., 66, 380 (1962). (27) W. D. Good, ibid., 66, 380 (1962).
(11)
For Siz&,,using AH7 = 15.2 e.v. or 350 kcal., E(Si-Si) is 40. For Ge2]H6,using AH7 = 13.3 e.v. or 307 kcal., E(Ge-Ge) = 28. For Ge3H8,by similar reasoning AHfO(M3Hs)
the heats of formation of SiH*(g), SizH6(g),and Si;HB(l) at 20' from combustion calorimetry. The calculations are apparently based on -205.4 for the heat of formation of quartz,'* and the derived heats of formation would be even more negative if the better value of -217.5 for quartz were u ~ e d . ~ 6 *These ~7 large discrepancies apparently illustrate the difficulty of obtaining correct results for silicon compounds by ordinary combustion calorimetry.
3AHro(M,g)
(12)
= '/2AH8
(13)
Using AH%= 16.3 e.v. or 376 kcal., E(Ge-Ge) = 33. The bond energies from the appearance potential work are more consistent with the calorimetric work if the lower values of the heats of sublimation of Si and Ge are used. However, recent work rather strongly supports the higher value in both cases. Stone and co- worker^?^!^^ have reported appearance potential work indicating that the bond dissociation energies D(H-SiH3) and D(H3Si-SiH3) are much higher than the thermochemical bond energies E(Si-H) and E(Si-Si) from our work. This seems surprising, since for the carbon analogs the values are quite similar In general, the thermochemical measurements leading to E are much less ambiguous than appearance potential measurements leading to D; a reinvestigation of D might well be in order. Feher, Jansen, and RohmerZ5have recently reported values of -11.3. -36.2, and -54.4 kcal. mole-' for
The Heats of Formation of HzSe and H2Te. Correlations of Simple Covalent Hydrides' by Stuart R. Gunn Lawrence Radiation Lahoratory, University of California, Livermore, California (Received September 13, 1963)
Measurements of the heats of formation of several hydrides of groups IV and VZm4and of boron5p6by the method of explosion in mixtures with stibine have been previously reported. This method appeared rather unattractive for H2Te and H?Se because of compounds in the product Sb-Te and Sb-Se systems? with unknown heats of formation. However, SnHl had been found to explode by i t ~ e l f ,more ~ violently than SbH3, and it appeared possible that H2Te, lying ~
~~
~-
(1) This work was performed under the auspices of the U. S. Atomic Energy Commission. (2) S. 12. Gunn, W. L. Jolly, and L. G. Green, J . Phys. Chem., 64, 1334 (1960). (3) S. R. G u m and L. G. Green, ibid, 6 5 , 779 (1961). (4) S. R. G u m and L. G. Green, to be published. (5) S. R. Gunn aiid L. G. Green, J . Phys. Chem.. 6 5 , 2173 (1961). (6) S. R. Gunn and L. G. Green, J . Chem. Phys., 36, 1118 (1962). (7) M. Hansen, "Constitution of Binary Alloys," McGraw-Hill Book Co., Inc., New 1-ork, N. T.,1958.
Volume 68,.Vxmber 4
A p r i l , 19Rg
NOTES
950
in the same row of the periodic table, might also explode by ignition with a fuse. although extrapolation would suggest it to be less explosive than SbH8. Preliminary experiments showed this indeed to be the case; it does not explode as completely a t low pressures or propagate as readily into narrow tube sections as does SbH3, although its spontaneous slow decomposition is considerably more rapid.
Experimental Materials. HzTe and HzSe were prepared by the hydrolysis of Al2Te3and Al2Se3,essentially according to the procedure of Watkins and Shutt.8 The yield of HzTe was much poorer, perhaps 10% that of HzSe, Purifications were performed by bulb-to-bulb distillations, after a preliminary passage through CaClz and PzOF,in the case of HzTe, The principal criterion of purity was equality of vapor pressure of initial and final small fractions distilled from the total purified batch. Measurements of HzTe were especially poor because of deposition of Te metal in the manometers; the value was approximately 95 mm. in a chlorobenzene slush bath (-45.2’); the vapor pressure equation of Kelleyg gives 105 mm. The vapor pressure of HsSe was about 230 mm. in a chloroform slush bath (-63.5’) ; Kelley’s equationg gives 247 mm. Chemical analysis of the Te metal produced in the calorimeter runs showed less than 0.1% Se. Calorimetry. The reaction tubes were as previously described2 except for being 10 cm. long instead of 20 cm. and having a volume of about 45 ml. ; this was done because of the difficulty of preparation of HzTe and the apparently improved decomposition yield a t higher pressures. The bulbs were equipped with ungreased Teflon-tipped high vacuum needle valves.’O The gases were measured by weighing in the reaction tubes, HzSe being loaded first when used. There appeared to be some evidence that HzTe was decomposed by the room fluorescent lighting, so it was handled largely in the dark. Some decomposition of HzTe still occurred during the weighing, so after completion of the weighing the gases were frozen out with liquid nitrogen and the hydrogen was pumped off and measured; this usually amounted to about 0.2 or 0.3’%. The tube was then sealed off and placed in the calorimeter; further decomposition occurring during the somewhat shorter interval before firing is neglected in the calculations below. Copper block calorimeter XXIX, described elsewhere,” was used. It was calibrated six times using a dummy glass cell, similar to the reaction cells, with a heater wound on its outer surface. Temperature intervals used were 0.25 to 0.5’ (70 to 150 cal.); the The Journal of Physical Chemistry
total spread of the calibrations was 0.15%. After reaction runs, the cells were weighed and a small correction was applied to the calibrated heat capacity to allow for the weight difference of the experimental cells and the calibration heater cell. Runs were performed a t 25.0 f 0.2’. After the runs, the hydrogen was transferred with a Toepler pump through traps a t -196’ and measured; in the case of H2Te-HzSe runs, the traps were then warmed and the condensable gas also was measured.
Results Four runs were performed with HzTe only. The amounts in millimoles, corrected for decomposition during weighing, were 1.809, 2.041, 2.607, and 3.348. Observed heats, q, corrected for fuse energies, were 43.18, 48.32, 61.82, and 79.30 cal. The hydrogen found was 1.807, 2.024, 2.597, and 3.336 mmoles, or 99.9, 99.2, 99.6, and 99.6% of theoretical, respectively. The molar heats of reaction. AH, calculated with respect to the hydrogen found, are then -23.90, -23.87, -23.81, and -23.77 kcal. mole-’, respectively. XRay diffraction observations of the powder produced gave strong, clear patterns for the known structure of Te. Five runs were performed with HzTe-H2Se mixtures. The data are summarized in Table 1. The amounts of H2Se and HzTe are calculated from the sample weights, the HzTe being corrected for decomposition during weighing. “HzSe decomposed” is taken as Hz found minus 99.5% of the H2Te. The value qtot is the observed heat corrected for fuse energy; Q H ~ is S ~qtot minus 0.995 times 23.83 kcal. mole-1 times the amount of HzTe (0.990 was used instead of 0.995 for run no. 3, where firing was considerably delayed). AHH,s~ is then qH& divided by HzSe decomposed. The sum of Hz and condensable gas found agrees quite well with the sum of HzSe and HzTe taken. Chemical analysis of Se and Te in the powder produced gave results which, while not very precise, were consistent with the gas measurements except for run no. 5 , where the chemical ratio of Se to Te was significantly higher than calculated from the gas measurements. The condensable gas from all runs was condensed in tubes which were then sealed off and stored in light a t room temperature. Run no. 5 developed (8) G. R. Watkins and R. Shutt, “Inorganic Synthesis,” Vol. 11 W. C. Fernelius, Ed., McGraw-Hill Book Co., Inc., New York. N. P., 1946, pp, 183-186. ( 9 ) K. K. Kelley, Bureau of Mines Bulletin 383, U. S. Govt. Printing Office, Washington, D. C., 1935. (10) Fischer and Porter Co., Warminster, Pa. (11) S. R. Gunn, Rsz. Sci. Instr., in press.
951
NOTES
Table I : Heats of Explosion of H2Te-HzSe Mixtures HzSe Run
mmoles
HzSe, mmoles
1 2 3 4 5
2.991 2,264 2.506 2,264 1.655
0,661 0.919 1.329 1.562 2,068
HzTe,
Hz found,
3.289 2,694 3.114 2,958 2.160
mmoles
Condensables, mmoles
0.360
... 0.713 0,861 1,542
decomposed
-AHHzs~, kcal. mole-’
QAzSe,
Ptot,
mmoles
cal.
0.313 0.441 0,621 0.705 0.513
78.92 56.11 62.08 56.47 41.08
cal.
6 39 5.50 4.73 3 97 3.59
2.00 2.43 2.94 2.80 1.84
-
Table 11: Heats of Formation of Simple Hydrides CH4, -17.89 (f25.2) SiHI, $7.3 (+14.3) GeH4, +21.6 ($17.3) SnH4, +38.9
(+6.8)
(-5.7)
-11.04 (fl2.3) PH3, $1.3 (+14.6) A ~ l l s ,$15.9
(-4.2)
($18.9) Sbll3, +34.7
(-6.0)
“3,
H20, -57.80
( -46.8)
(-6.1) (
-8)
( - 10.9)
a much stronger mirror than the others; it is evident that in this run considerably more HzTe was undecomposed. It may be noted that the per cent of HzSe decomposed, calculated from the gas measurements, ranges from 48 to 45% in the first four runs, tending to decrease with increasing HzSe:HzTe ratio, but is 25y0 for run no. 5. X-Ray diffraction analysis of the products showed them to be crystalline solid solutions. l 2
Discussion The results for H2Te seem to be straightforward and adequately precise; the explosions give well defined crystalline tellurium. The standard heat of formation, AHfO, is f23.83 kcal. mole-l, with *0.20 as an estimated over-all limit of error. Bichowsky and Rossini13give +34.2 (ah 18’) based on work of Berthelot and Fabre14; Rossini, et aZ.,’5 give 4-36.9, referring to the same work. Awad‘6 has proposed 23.1 for AFf(HzTe)a t 30’; neglecting heat capacities and using tabulated entropies, l5 this corresponds to +26.9 for A H f O a t 25’. Our values for HzSe show a regular trend, slightly curved, when plotted against the HzSe: HzTe ratio, which extrapolates to about +8 kcal. mole-1 for A H f O a t a ratio of aero. This could be rationalized if an increasing fraction of HzTe were undecomposed as the H2Se:HzTe ratio in the runs was increased. However, there is no direct evidence for this except in the case of run no. 5 . Tellurium and selenium are
(+53.0) HzS, -4.82 (+13) H2Se, +8 1 (+16) H2Te, $23.8
(-6.4)
HF, -64.2 (+42.1)
( - 17.2)
HC1, -22.06
( -17)
HBr, -8.66
( - 17.6)
HI, +6.20
( +13.4)
($14.8)
isomorphous and completely miscible as solids’; the heat of solution would not be expected to be large. Bichowsky and RossiniI3 give $18.5 (at 18’) for AH*’ (HzSe) based on several investigations. Fabre” measured the heat of oxidation of H2Se by FeCI3; this gives 18.5 for AHr’. This is the same method that gave a high value for HzTe. Fabrel’ also measured the heat of reaction of HzSe with aqueous SeO,; this gives 19.8 for A H t O . Equilibrium data of Rolla18 for the reaction of HzSe with Iz and of Pelabonlg for the dissociation of HzSe to the elements give lG.9 and 19.0 for AHfO, respectively. Rossini, et d , 1 5 give 20.5 for AHfO, referring only to Fabre.” More recently, Kapustinskii and KankovskiiZ0have reported a value of 4-18.16 from measurements of the heat of combustion of HzSe. Table I1 gives standard heats of formation (as gases) (12) E. Grison, J . Chem. Phys., 19, 1109 (1951). (13) F. R. Bichowsky and F. D. Rossini, “The Thermochemistry of the Chemical Substances,” Reinhold Publishing Co., New York, N. Y., 1936. (14) M. Berthelot and M. C. Fabre, Ann. Chim. Phys., (6) 14, 92 (1888). (15) F. D. Rossini, et al., National Bureau of Standards Circular 500, U. S. Govt. Printing Office, Washington, D. C., 1952. (16) 9. A. Awad, J . I’hys. Chem., 66, 890 (1962). (17) C. Fabre, Ann. Chim. Phys., (6) 10, 472 (1887). (18) L. Rolla, Atti. Accad. Nazl. Lincei, Mem. Classr Sci. Fis., Mat. N a t . S e z . , 2 1 11, 278 (1912). (19) H. Pelabon, Ann. Chim.PhzJs., (7) 25, 365 (1902) (20) A. F. Kapustinskii and R. T.Kankovskii, Zh. Fiz. Rhim., 33, 722 (1959).
Volume 68, Number
4 April, 1964
KCTOTES
952
from Rossini, et a1.,15 and our earlier2r3 and present work. The values in parentheses are the differences between successive compounds. For periods 3, 4, and 5 these differences are remarkably regular, both horizontally and vertically, and a value near +20 for H2Se would be entirely out of line. All in all, a value of around +8 kcal. mole-’ for A H E O (H2Se) appears most probable, with an unknown but large uncertainty. Using Cottrell’sZ1values for the heats of formation of the gaseous atoms: H, 52.1; Se, 49; Te, 48. the therniochemical bond energies E(Se-H) and E(Te-€1) are 7 3 and 64, respectively, forming a regular trend with the values of 110.6 for E(0-H) and 83 for E(S-H). The 4 X 4 matrix of heats of formation of simple gaseous binary hydrides given in Table I11 should provide a valuable test of theory when and if theoretical chemistry becomes capable of giving reasonably rigorous a priori calculations of molecular bonding energies. An attempt has been made to correlate the thermochemical bond energies derived from these heats of formation with electronegativity scheme^*^-^^ A
=
E(?(.I-H) - ’/,[E(AI-hl) - E(H-H)]
The Free Energy of Formation of Niobium Dioxide between 1100 and 170OoIL1 by Wayne L. Worrell Inorganic Materials Research Division, Lawrence Radiation Laboratory, Unisersity of California, Berkeley, California (Receiiied October 7 , 1.965)
The free energy of formation of Kb02 has been determined by measuring the carbon monoxide equilibrium pressure of reaction 1between 1050 and 12OOoK. NbzOj(s)
+ C(gr)
?;bzOb(s)
Results are unsatisfactory, except for the halogens. For Si, Ge, and Sn, A is negative, and for groups V and VI the E(1.I-R4) values to be used are poorly defined. The regular trends shown in Table I suggest that fairly good estimates may be made for period 6. Assuming that the differences between periods 6 and 5 will be somewhat greater than those between 5 and 4, as those between 5 and 4 are greater than between 4 and 3, reasonable values would be PbHd, 60; BiH3, 55; H2Po, 45; HAt, 25. However, the intervening lanthanide contraction might perturb this extrapolation somewhat. Saalfeld and SvecZ6have given values of 60 and 66 for A H f O for SnHl and BiH,, respectively, from mass spectrometric appearance potentials; this method in general is subject to fairly large experimental uncertainties, although their values for several other hydrides are in rather good agreement with ours.
(21) T. L. Cottrell, “The Strengths of Chemical Bonds,” 2nd Ed., Butterworth, London, 1958. (22) L. Pauling, “The Nature of the Chemical Bond,” 3rd Ed., Cornell University Press, Ithaca, N. I-., 1960. (23) A. L. Allred, J . Inorg. S7rcZ. Chem., 17, 215 (1961). (24) H. 0. Pritchard and €1. A. Skinner, Chem. Re?.. 5 5 , 745 (1955). (25) F. E. Saalfeld and H. T. Svec, Inorg. Ch,em., 2 , 46 (1963).
The Journal of Phgsical Chemistry
2Kb02(s)
+ CO(g)
(1) Combining these results with recent thermal data2 and with another equilibrium study3 yields free energy of formation values for KbOz which are more precise than those tabulated by Elliott and Gleiser4 and by Coughlin.5 To obtain an expression for the free energy of formation of NbO, between 1100 and 17OO0K.,the standard free energy change of the reaction F=+
+21\;bOZ(s) + O.SOn(g)
(2)
was calculated from the results of the three investigations. I n this study a solid pellet of SbOz-Sb206-C was equilibrated with a carbon monoxide atmosphere a t elevated temperatures. The experimental apparatus and procedures are described e l ~ e w h e r e . ~ ,The ~ amount of CO (usually >94%) was determined by Orsat analysis,8 and an X-ray diffraction pattern of the remaining pellet indicated that only the initially charged phases were present. The results are summarized in Table I, in which A F O , , , and AFO,,, are the standard free energy changes of reactions 1 and 2, (1) This note is based on a thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology, May, 1963. The work was supported principally by the Kational Science Foundation and the U. 5. Atomic Energy Commission. (2) E. G. King and A. 0. Christensen, U. S. Bur. Mines Rept. Invest. 5789, U. S. Govt. Printing Office, Washington, D. C., 1961. (3) 1’. I. Larmnt’ev, 1’. I. Gerasimov, and T. N. Itezakhina, Dokl. A k a d . A’aitk S S S R , 136, 1372 (1961). (4) J. F. Elliott and M .Gleiser, “Thermochemistry of Steelmaking,” Vol. I, Addison-Wesley Publishing Co., Inc., Reading, Mass., 1960. (5) J. P. Coughlin, U. S. Bur. Mines Bull. 542, U. S.Govt. Printing Office, Washington, D. C.. 1954. ( 6 ) W.L. Worrell and J. Chipman, J . Phys. Chem., 68, 800 (1964). (7) & Gleiser ‘I. and J. Chipman, ibid., 66, 1539 (1962). (8) “Manual for Gas Analysis,” Catalog No. 80, Burrell Technical Supply Co., Pittsburgh, Pa.
