HEATSOF
May, 1961
~~'OI~MATION F
UNSTABLE G a s ~ ~ HYDRIDES vs
779
results into eq. 15 yields
on polyisobutylene in benzene and with the data of Krigbaum" on polystyrene in cyclohexane; As* V 1.6535 - 0.6535eSH - 0.7310pH (16) (from these data we deduce that ( H / k ) = 182'K. Discussion forpolyisobutylene in benzene, and ( H / k ) = 188'K. If the attractive forces vanish, 8 = 0 and eq. for polystyrene in cyclohexane, numbers which 16 reduces to Az" = 1, as required for hard spheres. are at least reasonable). The data cited above fall Sumerical solution of eq. 16 for the Flory tempera- on or nearly on the curve computed from eq. 18 ture, 8 , givm for the special, reduced case of a. = 1. In this treatment, as in the theories, for example, 8 = 1.63 of Flory and Krigbaumlz and of Grimley,13 the so that eq. 16 can be written in the alternative Flory temperature does not depend on molecular weight. Isihara and Koyama3 suggest that 8 form might depend on molecular weight and cite Krige =1,* 'v 1.6535 - 0.6535 exp 0.6125 - + 0.9489 (18) baum's datal0 for support; the data of Krigbaum and Floryg on polyisobutylene in benzene do not In the region 7;ery near to 8 the graph of Az* exhibit a dependence of 8 on molecular weight. us. temperature has a positive slope but cannot It would be interesting to examine in detail easily be made strictly linear, in contrast to the experimental values of A 2 us. temperature in the result of Isihara and Koyama. region of the Flory point. The behavior of Az* described above is in general (11) W. R. l i n g b a u m . %bid.. 76, 3758 (1954). agreement with the data of Krigbaum and Flory'O (12) P. J. Flory and \I. R. Krigbaurn, J . Chem. Phgs., 18, 1086
(F)
[
TBI
(10) W. R. Krigbaum and P. J. Flory, J . Am. Chem. SOL,7 6 , 1775, 5254 (1953).
(ISSO); 20. 873 (1952). (13) T. B. Grimley, Proc. Roy SOC.(London), &la, 339 (1952).
THE HEATS OF FORMATIOS OF SOME UNSTABLE GASEOUS HYDRIDES' BY STUilRT R. GUKY A S D LEROYG. GREEN University of California, Lawrence Radiation Laboratory, Livermore, California Receiaed October $4, 1960
The heats of explosive decomposition of PHI, PZH4, SiH4,SizH6, GeH4,GezH6, SnH4and &He, either pure or in mixtures with SbH,, have been meaeured. Heats of formation and thermochemical bond energies are derived.
The success previously attained2 in the determination of the heats of formation of stibine and arsine by use of the explosive decomposition of pure stibine and of mixtures of stibine and arsine suggested that these methods might be generally applicable to measurement of heats of formation of other unstable gaaeous hydrides where compound formation in the product system is not a serious problem. For many of these hydrides thermochemical data are either non-existent or of doubtful accuracy. It was in fact found in this work that hydrides having only slightly positive heats of formation, such as phosphine, could be decomposed to the elements in good yield by explosion in mixtures with stibine; that the decomposition yield, or the minimum ratio of stibine to the second component necessary to give quantitative decomposition of mixtures, correlated well with the heat of formation; and that compounds such as stannane and digermane having heats of formation similar to that of stibine would themselves decompose quantitatively or nearly so upon ignition with a platinum fuse without admixture of stibine or other "promoter." The apparatus and experimental procedures were R C ~previously described2 except as herein specified. f 1 ) Work was Lerformed under auspices of t h e U. S. Atomic Energy Commission. (2) S. R G u n n , W. L , Jolly a n d L. G . Green, J. P h y s . Chem., 64, 12.34 (1QGO).
Materials The hydrides were prepared and handled in conventional vacuum lines, and purified by passage through a sequence of U-traps cooled by slush baths. Vapor pressures were measured in most cases, but the principal criterion of purity was the stoichiometric yield of hydrogen from a weighed sample in the calorimetric decomposition measurements. Stibine was prepared essentially as previously described .$ Two runs using 4.0 and 5.0 g. of KBH, and keeping all other quantities constant each yielded 40 mmoles of SbH3, compared with the 25 previously obtained. Phosphine was prepared by the reaction of PCl, with LiAlH, in diethyl ether. Twenty-six mmoles of PCll was added slowly to 58 mmoles of LiAlH, in 50 ml. of diethyl ether, the product being slowly swept with nitrogen through traps at -196". The yield, after purification, was 18 mmoles. The vapor pressure, measured in volumes of both 25 and 580 ml. was 171 mm. a t the carbon disulfide melting point (lit.3 171 mm.). Biphosphine was prepared by the neutral hydrolysis of calcium phos hide, essentially according to the procedure of Evers and &met., The material is quite unstable, decomposing in the liquid phase to a product having roughly the composition P2H,4the reaction for this composition being 5PzH4 +6PH3 2P2H (1) We condensed in a bulb enough phosphine to give a pressure of 2 cm. a t room temperature and observed, after warming, a rate of decomposition of about 1% per minute. The reaction did not appear to be autocatalytic on the solids produced since the rate was approximately constant with
+
(3) R. T. Sanderson. "Vacuum Manipulation of Volatile Compounds," John T i l e y and Sons, Inc., New P o r k . N Y., 1948. (4) E . C. Evera and E H, Street, Jr.. J . Am. Chem. Soe., 7 8 , 5726 (1QbG).
780
STUART R. GUNNAND LEROYG. GREEN
time. The experiment was repeated with a clean bulb but transferring the P2HI through a goldfoil tra and a cold trap tp prevent condensation of mercury in the {ulb. After warrmnd, the decomposition rate was about 0.1% per minute. hen this bulb was shielded from light, the decomposition rate dropped to less than 0.01% per minute. Thus both mercury and light seem to catalyze the decomposition. Accordingly, final purification of the material was done in the dark and the samples were kept free of mercury and handled in the dark. Vapor pressure measurements of biphosphine are rather impreciee because of the rapid decomposition. The combined data of Evers and Street‘ and Royen and Hillbyield a value of about 200 mm. at 25O, which would correspond to about 1 mmole of gas in our reaction tubes. Silane was prepared by substantially the same procedure used for hosphine, the yield of urified product relative to Sic&ieing about the same. $he preparation also haa been described by Finholt, et al.6 Disilane was prepared by the reaction of magnesium silicide with hydrochloric acid. The disilane fraction produced also contained monochlorosilane or disiloxane; it was treated with LiAlH4 in diethylene glycol and then fractionated. The germane was prepared in this Laboratory by D . Shriver using the method of Piper and Wilson.’ After standing two years in a blackened flask a t room temperature only a small amount of hydrogen had been formed. It was fractionated once. Digermane was prepared by a procedure similar to that given by Jolly,B consisting of addition of an alkaline solution of Ge(1V) and NaBH4 to aqueous sulfuric acid. It was found that addition of an antifoaming agent (Dow Polyglycol P-1200) reduced the problem of foaming and improved the yield. The vapor pressure of 5.6 mmoles of purified material measured a t 0” in volumes of both 40 and 380 ml. was 236.5 mm. Stannane was prepared by the slow addition of a solution of 50 mmoles of SnC14 in 80 ml. of dry diethylene glycoldimethyl ether to 500 mmoles of LiH and 240 mmoles 2f LiAlH4 in 140 ml. of the same solvent cooled to -10 , the product being swept through traps by the hydro en also produced. The yield after purification was 20 mmoyes, the product showing a vapor pressure of 17.5 mm. a t the carbon disulfide melting point (lit.3 18 mm.). Stannane is cfte unstable, the decomposition being autocatalytic on t e tin produced. An earlier batch of material, prepared by the method of Finholt, et al.,B decomposed too rapidly to permit accurate calorimetric measurements. Based on our experience with biphosphine, we loaded this material through gold-foil and cold traps to eliminate mercury and handled it in the dark. Neither during the weighing, requiring some 30 minutes, nor during the equilibration period of a similar length of time in the calorimeter before firing was there any evidence of decomposition. The calorimetric reaction vessel contained a platinum fuse, tungsten leads, steel connecting screws, and sharp corners on a Pyrex electrode spacer. Later investigation, however, showed that samples sealed in Pyrex tubes both with and without care to exclude mercury and exposed to room (fluorescent) lighting showed only slight traces of solid deposit in one hour, and very slow acceleration of the decomposition rate for several hours thereafter. Possibly the impurities in the preparation (see “Results”) inhibited the decomposition. Diborane was from the batch described as preparation C in an earlier paper;g it contained about 0.2 mole yo impurity, probably ethane. After storage for a year a t -196’ it showed no decomposition and was used without further purification.
Vol. 65
these values of -AE with those reported earlier* (34.99, 34.82 and 34.91), gives an average of 34.98 f 0.10 and for AHfO, 34.68 f 0.10 kcal. mole-’. This value of AE was used in subtracting the SbH, heat contribution in all mixed hydride runs. Phosphine.-Results of the phosphine-stibine runs are given in Table 1.
+
TABLE I HEATOF DECOMPOSITION OF PH3 Run
1 2 3
PHs SbHa -millimoles-
0.859 1.200 1.327
2.901 2.996 2.594
PHa decom osed. q(PH:), cal.
8
95.8 93.2 81.2
1.07 1.85 1.89
-AE(PHa) kcal. mole-1
1.30 1.65 1.75
The amount of PHI decomposed is calculated from the total observed hydrogen minus that expected from the stibine. The condensable gas recovered in all runs agreed within 0.003 m o l e with the amount of undecomposed phosphine expected from the deficiency in hydrogen. The vapor pressure of this condensable gas from run 3 was checked in several volumes and agreed well with that of pure phosphine; it is thus evident that that portion of the phosphine not decomposed to the elements was unchanged, and AE is calculated by dividing 47(PHa) by the amount of PHa decomposed. X-Ray diffraction examination of the solid product, transferred in an argon-atmosphere dry box, showed only a normal antimony pattern with the lattice constants unchanged. According to HansenlO compound formation in the Sb-P system is doubtful and the solid solubility of P in Sb is very low. Upon admission of air to the solid products a white flame appeared briefly; evidently the phosphorus is a t least largely of the white form, which is the thermochemical standard state. A value of - 1.6 f 0.4 is taken for AE and 1.3 f 0.4 for AHfo(PH3). Biphosphine.-Results of the biphosphine-stibine runs are given in Table 11.
+
TABLE I1 HEATOF DECOMPOSITION OF PIH4 Run
1 2 3 4
P2H4 SbHs -millimoles-
0.599 .782 .612 .584
3.039 2.863 2.029 1.205
-
AEP2HI (P2H4). decomposed, ~ ( P P H I ) , kcal. % cal. mole-1
95.5 84.4 80.1 5.8
3.43 3.44 3.35 1.96
5.73 4.40 5.47 3.36
The amount of PzH4 decomposed is calculated from the excess of hydrogen over that contributed Results by the stibine. AE, however, is calculated by Stibine.-Three more runs with pure stibine dividing q(P2H4) by the total sample of PzH4, gave values for -AE of 34.93, 35.09 and 35.14 since it is evident that some exothermic.heat effect kcal. mole-’ with hydrogen yields of 99.91, 100.10 is associated with the biphosphine not decomposed and 100.06% of the theoretical. Combining to the elements; it is probable that a t least part of this decomposes roughly according to equation 1. (5) P. Royen and K. Hill, Z. anorg. alloem. Chem., 229,97 (1936). That the solid deposit was different from that pro( 6 ) A. E. Finholt, A. C. Bond, Jr., K. E;. R‘ilabaoh and H. I. Schlesinger, J. Am. Chem. Sac., 69,2692 (1947). duced in the phosphine runs was supported by the (7) T. S. Piper and M. K . Wilson, J . Inorg. Nuclear Chem., 4, 22 observation that it flamed more violently upon (1967). (8) W. L. Jolly, J . Am. Chem. Sm..8 3 , 335 (1961). (9) 5. R. Gunn and L. G. Green, J. Phys. Chem., 64, 61 (1960).
(10) M. Hansen, “Constitution of Binary Alloys,” McGraw-Hill
Book Co., h a . , New York, N. Y., 1968.
HEATSOF FORMATION OF UKSTABLE GASEOUS HYDRIDES
May, 1961
admission of air. The amount of condensable gas recovered varied from 10 to 40% greater than the “missing” biphosphine; equation 1 predicts 20%. However, the material in the traps pumped off only slowly; it is possible that some undecomposed biphosphine was left in the gases passed through the trap and decomposed upon warming, giving I”, trapped in ‘T2H.” Plotting Ah‘ vs. percentage decomposition and extrapolating to 100% gives a value of -5.6 1.0 for A E and +5.0 f 1.0 for AHfO. Silane.-Results of the silane-stibine runs are given in Table 111.
*
TABLEI11 HEATOF D E C O M P O S OF ~ OSi N& Run
1 2 3
SbHa
SiHc deeom oaed,
2
q(SiHc), eal.
2.924 2.919 2.462
99.8 99.2 98.7
7.38 11.57 14.49
SiHi --millimoles-
0.971 1.469 1.852
- A.E(SiHd, kcal. mole-’
7.62 7.94 7.93
The condensable gas from run 3 measured 0.026 mmole; the hydrogen deficiency corresponds to 0.024 mmole of Si&. Strong heating of tube 2 with a torch after pumping out the gas produced a negligible additional amount of gas. Hence that part of the silane not decomposed was unchanged and a negligible amount of hydrogen was incorporated in the solid residue; A E was calculated from the amount decomposed. X-Ray diffraction analysis of the product showed only a normal Sb pattern; the silicon presumably is too finely divided to give a pattern. The NBS tables” give 1.0 kcal. mole-’ for the heat of formation of “amorphous” silicon, but this is probably a function of the method of preparation; we shall neglect the correction. Hansen’O indicates the solid solubility in the S b S i system to be slight. We shall take -7.9 0.3 for AE and 7.3 rt 0.3 for AHfO. Disilane.--Results of the disilane-stibine runs are given in Table IV.
+
+
*
TABLEIV HEAT01’ DECOMPOSITION OF SizHe Run
1 2 3
4
%€Is S3Ha -m~llimoles-
1.009 1.062 1.652 1.541
2.026 2.079 1.677 1.536
H, from SiqHe, % theo.
q(SiJIcf, cal.
99.0 98.8 90.9 91.9
18.22 19.20 27.96 25.77
TABLEV HEATOF DECOMPOSITION OF GeH, GeHi
Run
1 2 3 4 5
(11) F. D. Roasini, et al., “Selected Values of Chemical Thermodynamic Properties,” Ciroular of the NBS,No. 500. 1952.
*
SbHa
-millimoles-
1.711 1.742 2.571 2.575 2.712
2.252 2.281 1.523 1.515
0.890
GeH4 decomposed,
q(GeH3,
- AE(GeHi), kcal./
yo
csl.
mole
99.4 99.7 99.6 99.7 98.5
34.03 35.59 54.82 54.99 57.75
20.01 20.50 21.40 21.42 21.63
The condensable gas from run 5 measured 0.041 mmole, which agrees exactly with the amount calculated from the hydrogen deficiency. Strong flaming of tubes 1 and 2 released a negligible amount of gas. Hence the germane not decomposed to the elements was unchanged, and AE is calculated from the amount decomposed as determined by the hydrogen measurement. X-Ray diffraction analysis of the product from both these and the digermane runs gave the same result: a pattern of pure Ge, and a pattern of a solid solution of Ge in Sb which has the same structure as pure Sb but in which the constants a and c are shifted from 4.307 and 11.273 to 4.242 and 11.315 A., respectively. The values of aE were plotted tis. the Sb:Ge atom ratio; they lie fairly closely on a line having a slope of 1.46 kcal. mole-’ and extrapolating to 22.18 a t a ratio of zero. A line of this same slope is a much closer fit to the plot of 0.5 AE(GezHe) us. Sb:Ge, and implies that the heat of formation of the solid solution (which may be metastable a t room temperature, having been quenched out of the gas phase at a higher temperature) is 1.46 kcal. per mole of Sb. We take -22.2 0.5 for AEand 21.6 0.5for A H f O . Digermane.-Results of the digermane-stibine runs are given in Table VI.
+
+
*
*
+
TABLEV I HEATOF DECOnlPOsITION OF DIGERMANE
mole-1
A E was calculated from tJheamount of SizHe decomposed as indicated by the amount of hydrogen produced. The condensable gas from run 2 equalled one-third of the hydrogen deficiency, but that from runs 1 and 4 was somewhat greater; that from run 3 was lost. The data can be reconciled by assuming that most of the SizHS not completely decomposed to the elements was partially decomposed to give Si, SiHc and Hz. If this assumption is made, AE becomes somewhat lower, but the value extrapolated to 100% decomposition is the same.
+
*
We take - 18.3 0.3 for AE and 17.1 0.3 for AHf‘. Germane.-Results of the germane-stibine runs are given in Table V.
- AE(SizHs), kcal. 18.24 18.30 18.61 18.19
78 1
Run
1
2 3 4
GerHp -millimoles---.
0.965 1.5% 1.540 1.899
SbHr
1.875 1.552 1.001 0
Qe2Hs decomposed, ”r,
99.8
99.7 99.2 97.95
q(CenHs), ral.
35.65 58.15 59.68 74.12
- AE(GerIls), kcal. mole - 1
37.0’2 38.26 39.06 39.85
The condensable gas in all runs, including run 4, agreed well with one-third of the hydrogen deficiency, indicating that most of the digermane not decomposed to the elements was unchanged. The four values of AE when plotted against the Sb:Ge ratio lie closely on a straight line; we take -39.9 f 0.3 for A E and 38.7 0.3 for AHfo. Stannane.-Stannane explodes upon ignition with a platinum fuse without addition of stibine. The explosion is more violent than that of stibine; in one run, with about one millimole in the ca. 90 ml. reaction tube, the tube was shattered. Three runs were performed with samples of
+
*
STUART R. G u m AND LEROYG. GREES
782
-
about 1 mmole each. The values of AE were 39.11, 39.35, and 39.52 kcal. mole-', referred to the sample weight. The hydrogen yield in the first two runs was 101.7 and 101.201, of the theoretical, respectively; the gas from the third run was lost. The first two runs showed ea. 0.6% condensable gas, referred to the sample. The first sample was taken from the top of the 20 mmole batch, the second after half of it had been distilled away, and the third after half of the remainder had been distilled away. It is probable that the material contained some decomposable hydride of lower molecular weight, most likely silane, and also carbon dioxide, the vapor pressure of which is very close to that of stannane. We shall take - 39.5 f 0.5 for AE and 38.9 f 0.5 for AHSO. X-Ray diffraction examination of the solid showed a normal pattern for 8-tin, the thermochemical standard state. Diborane.-It was found that in mixtures of stibine and diborane, the stibine decomposed rather rapidly, several per cent. per hour, while the diborane remained unchanged. This rate was not affected by elimination of mercury; elimination of light was not tried. This same catalytic effect, a t a still higher rate, was found in ammonia-stibine mixtures but not with any of the other mixtures studied in this work. (Kone of the ammonia in ammonia-stibine mixtures is decomposed upon explosion.) Thus it was necessary to employ a reaction system in which the gases were mixed only shortly before firing. The reaction vessel was similar to the usual type but included an axial capillary tube passing through the top to a point in the middle of the vessel and extending to the top of the calorimeter then horizontally a few inches and downward to a stopcock and ball joint. The stibine was weighed in an auxiliary weighing flask and then transferred quantitatively to the reaction vessel. A small amount of mercury was raised above the stopcock t o prevent contact of the stibine with the grease; a n enlargement in the tube above the stopcock permitted the diborane to be bubbled through the mercury without sweeping it through the capillary. The diborane was measured volunietric~ally and transferred to a storage hidb equipped n i t h :L mercury reservoir. The experimental procedure then con placing the reaction tube in the calorimeter, connecting the diborane bulb to it and evacuating the intervening linkage and, when the reaction vessel had come to equilibrium with the calorimeter, warming the frozen diborane to about 25" and admitting mercury to force all the dihoraiw and stibine in the diborane bulb and connecting tubes into the reaction vessel. Thc gases were allowed to mix five to seven minutes and the fuse then was fired. The temperature-time curve was integrated from the time a t which mixing of the gases was started, and a correction for the PV work of compression was subtracted from the observed heat. This correction was RT times the number of moles of gas initially outside of the calorimeter, which was slightly greater than the amount of diboranc. Results of the diborane-stibine runs are gi\ ('11
+
Yo]. 65
in Table VII. The percentage of BzHs decomposed is calculated from the hydrogen produced. The fifth column represents the condensable gas, probably largely unchanged B2H6,mixed with the hydrogen, expressed as a percentage of the B2Hs sample. TABLE 1-11 HEATOF DECOMPOSITIOS OF B2He BzHs SbHa Run c--millimoles-
1 2 3 4 5
0.823 1.457 0.997 1.023 1.532
1.910 2.552 1.491 1.469 1.215
B2Hs decomposed,
%
99.0 97.5 97.5 97.4 93.4
Condens- Hydroable gen in gas, solids, (B&S), r0 c?'' cal.
0.1 .4 .7 .4 .1
.. ..
0.8 0.8 4.2
5.21 9.50 6.02 6.07 10.73
- 4E-
(BZH6), kcal. mole
6.33 6.52 6.04 5.93 7.00
After pumping off the gases, the lower t,wo-thirds of the length of t'he reaction tube was flamed wit'h a hand torch to the softening point of Pyrex. The gas given off mas largely non-condensable, presumably hydrogen, and is given in the sixt'h column as a percentage of the total hydrogen expected from the BzHs(three times t,heB2H6 sample). This flaming procedure apparently liberates a considerable part, but not all, of the hydrogen in the solid phase. The data indicate that' part of the diborane not decomposed to the element's is unchanged, but that more of it forms some non-volatile hydride. Run 5 suggests that the heat' of formation of this solid hydride is somewhat negative. A E is calculated from t,he total sample of diborane; rejecting run 5, the average is -6.2 f 0.4 and AHrO is +5.0 f 0.4 referred to amorphous boron. Using 0.4 for the est,imated heat of transition of amorphous to crystalline boron, AHrO is +5.8. X-Ray diffraction analysis of the solids from run 2 showed only a normal Sb pattern. Presumably the boron is amorphous. Hansen'O gives no data on t,he boron-antimony system. The explosion of diborane-stibine mixtures is more violent than that of pure stibine. h louder audible click is produced in the calorimeter, and in an exploratory experiment in a smaller t'uhe the glass was shattered. Discussion Cottrell12has reviewed briefly the previous data for PH3. There is considerable scatter in results by both equilibrium and calorimetric methods, even the sign of AHfO being uncertain. There appear t'o be no data for biphosphine. The most recent determination of the heat, of formation of silane is t,hat of Brimm and Huniphreys.13 They used a furnace ill a ca1orimet)er to pyrolyze t,he gas t,o the elements, this being the procedure used by Prosen, Johnson and PergielI4 to measure the heat of formation of diborane. Decomposition of silane was found to require a (12) T. L. Cottrell, "The Strengths of Chemiral Bonds," Botterworth's Scientific Publications, London. 2nd E d . , 1968. (13) E. 0 . Brimm and H. h l . Humphregs, J . Phiis. Cham.. 61, 829 (1937). ( 1 2 ) I;.J. Prosen, if'. H. Jotinson and 1:. Y.Pergiel: .I. Research Satl. BILI..Standards. 61, 247 (1058).
RiIay, 1961
HEATSOF I ~ ~ H M A TOFI O T,*ssi> S ~ B L EGASEOL-S HYDRIDES
higher furnace temperature than diborane. Individual data were not reported, but a value of +7.8 3.5 kcal. mole-' was given for AHfO, in good agreement with our result. KO previous data are available for disilane, germane or digermane. value of +99 =t 4 kcal. mole-' has been repcrtedI4 l5 for AF~O(SnH,)based on observations of the minimum potential a t which staiinane is liberated a t tiiL cathodes in acid solution. Estimating 54 e.u. for the entropy of stannane, this corresponds to +81 for AHr'). The large error may be due to overvoltage effecats. The heat of formation of diborane is particularly important because it i h used, together with the heat of hydrolysis of diborane and the heat of solution of boric oxide, to deteimine the best values of the heats of formation of boric oxide and aqueous boric acid, to which the thermochemical data of most boron compounds are referred. Prosen, et aZ.,14obtained a value of +6.73 f. 0.52 for the heat of formation of diborane from amorphous boron by decomposing diborane in a furnace enclosed in a calorimeter. We obtain +5.0 for the same quaiitity, with about the same estimated uncertainty. While the boron produced by both methods is amorphous, it is possible that due to particle size or other effects its energy is not the same. I n particular, that produced in the exploRive decomposition3 inight consist of smaller particles and h a w a higher energy than that produced by the slower furnace pyrolysis; this would tend to make our heat of formation lower. In view of this uncertainty, it should perhaps be considered that the agreement between the two determinations is more significant than the disagreement. It may be noted, however, that our value of the heat of formation of diboraiie is less consistent with the value (of the heat of formation of BC13 given by Johnson, Xiller and 1'ro~e.n~~ than is the X'BS valuc. Using the heat of formation16 and heat of hydrolysis1' of FC1, one obtains -257.06 f 0.33 for AHf(H:BO3~1000H2O) (amorphous boron) 17; using the NBS xraluc for the heat of formation of diborane'j aiid the heat of hydrolysis," -257.70 =t 0.2816; using oiir valuc for diborane, 258.56 f. 0.22. -4possible error in our measurement of the heat of formation of diboraiie lies in the small amount of
*
\ 15) N. de ZJilbo> and E. Deltombe. "Proc. 7 t h International Cominittee of Electrocheinrcal Therinod> nainics and Kinetics," Lindau, Germany, 1955, Butter north's Scientific Pahllcations, London, 1957, p 240. (16) W.H .Johneon. R C, Miller and E. J. Proien, J Rezearch h'atl Bur. Standnrda 62, 213 (1959). (17) S. K. Gunn and 1,. Ci Green. J . Phys. Chern., 64, 61 (19130).
783
hydrogen remaining in the boron. However, the consistency of the results of runs 1 to 4 a t various diborane: stibine ratios and the rather small difference in A E for run 5 where the amount of hydrogen in the boron was several times larger indicates that this was not a serious effect. I t is of interest to calculate the thermochemical boiid energies of the group IV and T' hydrides and observe their progression with increasing atomic number in the groups. These values are given in Table VIII, along with the heats of formation of the compounds and the heats of formation of the gaseous atoms. All data are for 25". Standard heats of formation of the gaseous atoms, AH& (RI,g), are taken from Cottrell12 and heats of formation not measured in the present work arc from the KBS compilation." TABLE VI11 '~HERMOCHEhIIC~4L BONDENERGIES AHfO(LIHn) AHfa(M,g) E(b1-H)
....
-11.0 1.3 +15.9 +34.i
+
+22 ($5)
+ 5 . 0 (R.) -17.9
+ i.3 $21 . 6 $38.9 -20.2 +li.l $38.7
+ 5 0 f.0 . 4
52.09 112.9 85.3 60 61 ... ... 170.9 105 89 72
..
h'(A1-AI)
93.4 76.8 66.8 60.9
.. .. .. .. ..
..
38.0
.. 99.3 76.5 69.0 60.4
46.8
78.8
...
.. .. ..
37.9
...
..
..
... ...
_.
.. .. .. 46.4
With the exception of the hydraziiie-hiphosphine pair, the heats of formation and boiid energies all show a smooth trend of decreasing stability with increasing atomic number. The &I-H bond enerpy is essentially the same for the group IV and the group V hydrides in each of the four periods listed. The thermochemical P-P bond energy in P4(g) is 48; various estimates of steric strain have bemi made to derive a normal bond energy of 51 t o 54.12 This is 4 to 7 kcal. greater than that in PzH4. The C-C, Si-Si and Ge-Ge bond energies are all 6 to 8 kcal. lower than one-half of the heat of atomization of the elements, which can he taken to represent the bond energy in the element. Acknowledgments.--The authors wish to thaiik Prof. William L. Jolly for helpful discussioiis of various phases of the investigation and Xlr. Vernon G. Silveira for performance of the X-ray diffritctiori analyses.
