1114
STUART R. GUNNAND JOHN H. KINDSVATER
The Heats of Decomposition of Some More Boron Hydrides'
by Stuart R. Gunn and John H. Kindsvater Lawrenee Radiation Laboratory, University of California, Livermore, California (Received September SO, 1966)
The heats of decomposition of decaborane-14, decaborane-16, and hexaborane-12 have been measured calorimetrically by explosion in mixtures with stibine a t 150, 130, and 25', respectively. Derived standard heats of formation of the gases a t 25" are, for B10H14, +4.4 kcal mole-', in excellent agreement with previous results by other methods; and for BloHls and B1H12, +34.8 and +26.5 kcal mole-', respectively, in good agreement with calculations from previously derived thermochemical bond energies.
The heats of decomposition of BZH6,' B2Ds,3and B4H10,B5H9,B5&, and BsHlo4have previously been measured in this laboratory by exploding the gases in mixtures with stibine in a calorimeter. Prosen and co-workers have measured the heats of decomposition of B2He,5B5H9,5and BloHd by pyrolysis in a furnace enclosed in a calorimeter. Results of the two methods for B2He and B5H9 are in agreement if the reasonable assumption is made that the energy of the amorphous boron produced is different in the two types of experiments. A set of bond energies was derived4 which is reasonably consistent with all the heats of formation except for B10H14. Accordingly, a calorimeter capable of operation at higher temperatures was developed to permit measurements of this compound by the stibine explosion method. I n recent years several other higher boron hydrides have been prepared; their heats of decomposition can be determined by the present method provided that they undergo little decomposition in an hour or so as pure gases at a temperature where the vapor pressure is ca. 0.1 atm or more. The upper temperature limit is probably between 150 and 200', imposed by the rapid pyrolysis of stibine. In the present work, the method has been extended to BlOHlsand B6H12.
walled capillary passing through the top axially, extending inside to the center of the cell and above the cell extending 16 in. upward to a stopcock and ball joint, which were just above the thermostat lid. Tungsten leads were sealed through the top of the cell. For the &OH14 runs and the first few BlOH16 runs a platinum fuse, 0.002 in. in diameter and 1 to 2 cm long, was mounted a t the end of the capillary inside the cell; this was fired with a 24-v storage battery. For the remaining work, the tungsten wires were crossed in springing contact a t the same position and sparked with a 0.1-pf condenser charged to 1000 v. The firing leads were No. 36 copper for a length of 5 in. above the cell and then KO.26 to the outside of the calorimeter. A heater of about 150 ohms of No. 35 manganin was wrapped bifilarly on the outside of the cell and covered with cellophane tape. The current leads were 5 in. of No. 36 copper above the cell and then No. 26 outward; potential leads were attached 2.5 in. above the cell and consisted first of 2.5 in. of No. 36 copper and then No. 26. For the B10H14and BloHlework, the capillary and leads were surrounded by a copper rod 0.75 in. in diameter, 7 in. long, with a slot which was closed by a copper
Experimental See tion The aneroid copper-block calorimeter described elsewhere' was used, with an oil thermostat at the elevated temperatures. Thermistors were used for work at 150" and a copper thermometer for the B10H14 BloHleat 130" and B8HI2at 25'. The reaction cells, about 90 cc in volume, were Pyrex tubes 2.8 cm o.d., 20 cm long, with a 1-mm i.d. thick-
(1) This work was performed under the auspices of the U. S. Atomic Energy Commission. (2) S. R. Gunn and L. G. Green, J . Phys. Chem., 6 5 , 779 (1961). (3) S. R. Gunn and L. G. Green, J . Chem. Phys., 36, 1118 (1962). (4) 5. R. Gunn and L. G. Green, J . Phys. Chem., 65, 2173 (1961). (5) E. J. Prosen, W. H. Johnson, and F. Y. Pergiel, J. Res. Nail.
The Journal of Physical Chemistry
Bur. Std., 61, 247 (1958). (6) W. H. Johnson, M. V. Kilday, and E. J. Prosen, ibid., A64, 521 (1960). (7) S. R. Gunn, Rev. Sci. Instr., 35, 183 (1964).
HEATSOF DECOMPOSITION OF SOME MOREBORONHYDRIDES
bar taped in place over the capillary when assembled. The lower end of the rod was 6 in. above the top of the cell. A heater was wrapped on its surface, and its temperature was maintained equal to that of the oil bath by a differential thermocouple, amplifier, and controller. This served to eliminate the effect of varying heat conduction along the leads and capillary and to heat stibine entering the cell. The space between the cell and the copper rod, around the capillary and leads in the calorimeter well, was stuffed loosely with tissue paper to minimize convective heat transfer. A weighed sample of the borane was condensed in the cell through :t side arm, which was then sealed off. With B10H14and BloHla an equal amount of xenon was added to the cell. The purpose of this was to prevent condensation of decaborane in the short length of cool capillary below the stopcock. Calculation indicated that in the interval-about 1 hr-between placing the cell in the calorimeter and performing the reaction, diffusion of borane through the xenon in the capillary would be negligible. With B6HI2,one-tenth to onethird as much xenon was added, serving only to separate the borane from the stopcock grease. In performing the runs, the cell was placed in the calorimeter and heated to 0.3-0.5" below the bath temperature. A weighed bulb of stibine was connected to the cell through a linkage communicating with the vacuum system. The temperature foredrift was recorded. The stibine was kept frozen with liquid nitrogen, to minimize absorption in the stopcock grease, until a few minutes before reaction. It was then warmed to room temperature and admitted to the linkage. The cell stopcock was opened for 3 or 4 sec and closed, and the stibine remaining in the linkage was condensed in the weighing bulb, which was later reweighed. With BI0Hl4 and B10H15,firing was performed immediately after admitting the stibine; it was not possible to wait a few minutes to permit better mixing of the gases as was done with lower boranes at 25" because of rapid decomposition of the stibine at the higher temperatures. After the reaction, the calorimeter was calibrated twice by introduction of about the same amount of electrical energy. After the run, the hydrogen was pumped off through liquid nitrogen traps and measured. Mass spectrometric analysis for xenon in the hydrogen was performed in all cases, but none was ever found. Commercial BI0Hl4was purified by two sublimations at room temperature. Analysis of the melting curves indicated a purity of 99.87 mole %, and this may be low, since some discoloration occurred when the sample was melted into the melting point bulb and possibly during the melting point measurement. The
1115
material was handled in a drybox; samples were sealed in small bulbs and weighed. These bulbs were then sealed in side arms attached to the reaction cells, and the system was reevacuated overnight. Then the bulb was broken and the decaborane sublimed into the cell. BloHle was prepared as described by Lipscomb's groups,10 and purified by trap-to-trap sublimation and use of a low-temperature fractional sublimation column. Purified samples were stored overnight in bulbs with strips of gold foil to reduce mercury contamination. Vapor pressure measurements were well fitted by the following equations. These imply a logp,, logp,,
10.357 - 3103/T = 7.959
- 2254/T
(40-81") (81-126')
melting point of ca. 81"; this is consistent with a poor melting-curve analysis of a less pure sample. Only a small amount of noncondensable gas was evolved in the vapor pressure apparatus during several hours in the higher temperature region. X-Ray diffraction analysis of the purified material showed the known structure of BI0H16and no other lines; estimated sensitivity for B10H14, the most likely impurity, was 5%. Calorimetric samples were weighed in a U tube with greased stopcocks. BaH12 was prepared as described by Gaines and Schaeff er" and purified by gas-liquid partition chromatography.12 The infrared spectrum agreed with those previously reported.113'2 An elemental analysis of a sample pyrolyzed 15 min a t 800" gave 99 f 1% of theoretical hydrogen and 101 f 1% theoretical boron. A sample showing a vapor pressure of 20 mm a t 0" gave a molecular weight of 79 (calcd, 76.96). The compound appeared to be adequately stable as a gas, giving only traces of hydrogen upon standing 1 hr a t room temperature. Calorimetric samples were weighed as a gas in 100-cc bulbs with greaseless valves.
Results The calorimetric program for each of the three boron hydrides consisted of a comparison of two series of runs, In one, stibine was introduced into a cell containing boron hydride and xenon and was exploded. I n the other, stibine was introduced into a cell containing a larger amount of xenon, comparable to the (8) S. R. Gunn, Anal. Chem., 34, 1292 (1962). (9) R. Grimes, F. E. Wang, R. Lewin, and W. N. Lipscomb, Proc. Natl. Acad. Sci. U.S.,47, 996 (1961). (10) R. N.Grimes and W. N. Lipscomb, ibid., 48, 496 (1962). (11) D.F. Gaines and R. Schaeffer, Inorg. Chem., 3, 438 (1964). (12) C. A. Luta, D. A. Phillips, and D. M. Ritter, ibid., 3 , 1191 (1964).
Volume 70, Number 4
April 1966
1116
STUART R. GUNNAND JOHN H. KINDSVATER
sum of boron hydride and xenon in the other series, and exploded. In this manner some possible systematic errors, such as absorption of stibine in stopcock grease, errors in weighing stibine, etc., could be made to cancel. The millimoles of SbH, given in Tables I-VI represents the weight loss of the stibine bulb; this will be designated a. Part of this, a‘, is trapped in the bore of the stopcock at the top of the reaction cell and is assumed not to decompose significantly before the hydrogen determination is performed. The volume of the bore is 0.05 cc. The factor a‘/a is calculated from the temperature of the calorimeter (since the stopcock ~
Table I: SbH3Runs at 150’
In the boron hydride runs, Tables 11, IV, and VI, a’ and a” are calculated as before, and for each run the amount of heat due to SbH3is calculated by rearrangementof eq2 & y ( a - a’
- a”)
(4) where the values of & and Y used are the averages from the preceding tables. The amount of boron hydride, B,H,, decomposed, d, is calculated from @bHa
d = 2[c - 1.5Y(~ - a’)]/%
(5) The per cent boron hydride decomposed is d/b, b being the millimoles of boron hydride determined by weighing. The decomposition energy of the BIOHI~ and BIOHI~ is then calculated from
- qSbHs)/d
(6) This implicitly assumes that hydrogen not liberated is Ha, Q, Y m m o l e s -fo YOof q* kcal present as the unchanged original boron hydride, or Xe SbHa Ha theory ea1 mole-’ an energetically equivalent form; one check upon this 91.84 3.952 99.91 35.03 2.639 0.4 is whether AE varies systematically with d / b , which 99.94 90.65 34.95 2.611 3.911 0.4 normally varies directly as a/b. 99.84 86.39 35.02 2.488 3.723 0.8 The results for B10HI4 are given in Tables I and 11. Av. 99.90 35.00 For the SbH3 only runs, AH is -33.74 kcal mole-’. Using our value2,’ of - 34.68 kcal mole-’ for AHtg8and enthalpy data for SbH3 from Sunderamla and for remains essentially at room temperature) and the ratio, antimony and hydrogen from Stull and Sinke14 we r, of stibine introduced to boron hydride plus xenon calculate -33.93 kcal mole-’ for AH423. There are originally in the cell; it ranges from 0.0006 to 0.0011, two possible mechanisms to explain the discrepancy. A larger amount, a”, in the 0.4-cc capillary volume First, and more probably, the entering stibine may above the cell explodes but does not contribute heat not be fully heated in the upper part of the to the calorimeter. This volume is assumed to be at the calorimeter; the deficiency in AH corresponds to calorimeter temperature, and hence the factor a”/a stibine entering the cell at 133” instead of 150”. is calculated from r only; it ranges from 0.0050 to Experiments in which air was admitted to a similar 0.0060. The hydrogen produced is designated c, cell at 150” indicated a temperature deficiency and the per cent of theoretical Hz, Y, in Tables I, of 15”. Secondly, a significant amount of stibine might 111,and V is given by be decomposed in the capillary while entering the cell. In either-event, the effect would tend to be similar Y = ( 2 / 3 ) [ ~ / (~ a’)] in the decaborane runs. In the B10H14runs about two-thirds of the hydrogen The heat of explosion per mole of SbHa, &, uncorrected deficiency was evolved by flaming the cells to the for compressional heat, is then softening point of Pyrex. This could be either unQ = ~ / [ Y (-u a’ - a”)] (2) changed decaborane or higher polymeric hydrides; lower volatile hydrides would have escaped from the where q is the observed heat, corrected for fuse energy cell and are thus indicated to be rather low in amount. in the earlier work. Subtracting RT to correct for the There is no significant trend of AE with the percentage pV compressional heat of entering stibine, changing of decomposition. Accordingly we take the average, the sign, and adding 0.5RT to convert from constant 4.8 kcal mole-’, for A E 4 2 3 of the reaction volume to constant pressure, the heat of explosion of stibine is given by B1OH14(g) +10B(am) 7H&) (7) AH = -(& - 1.5RT) (3) (13) S. Sunderam, Can. J . Phys., 39, 370 (1961). Ah? = (p
__.
+
although this function is not needed for calculating the borane runs. The Journal of Physical Chembtry
(14) D. R. Stull and G. C. Sinke, “Thermodynamic Properties of the Elements,” Advances in Chemistry Series, No. 18, American Chemical Society, Washington, D. c.,1966.
HEATSOF DECOMPOSITION OF SOMEMOREBORON HYDRIDES
1117
Table 11: B,oHta Runs a t 150' mmoles of
r
Xe
hHl4
SbHa
0.2 0.2 0.3 0.3 0.4 0.4
0.160 0.193 0.318 0.331 0.419 0.429
2.653 2.627 2,522 2.517 2,401 2.422
-
%
0.6 0.6 0.5 0.5 0.5 0.5 0.5
2.328 2.497 2.594 2.689 2.633 2.652 2.603
0.60 0.87 1.02 1.04 1.89 1.30
88.9 87.2 71.7 71.8 68.6 70.6
4.2 5.2 4.5 4.4 6.6 4.3
-PBIOH:(S
Hz
cal
cal
4.970 5.112 5.373 5.433 5.607 5.748
91.62 90.44 86.61 86.41 81.51 82.83
92.22 91.31 87.63 87.45 83.40 84.13
In the BlOHl6 runs, about half of the missing hydrogen was liberated by flaming the cells. Again, there is no significant trend of AE with the percentage of decomposition; the average is -26.0 kcal mole-' for AEM3 of the reaction
Q,
H2
Hz , % of theory
cal
koa1 mole-:
3.483 2.734 3.865 4.020 3.928 3.957 3.888
99.83 99.77 99.41 99.74 99.53 99.55 99.65
80.27 86.95 89.45 93.29 91.06 92.05 90.38
34.76 35.13 34.90 34.99 34.96 35.08 35.06
-
AB, kcal mole-:
PBBH1-
Table 111: SbHs Runs a t 130'
7-mmolea of Xe SbHa
csl
BlOHl4 decompd
Rot,
qs
Av. 99.64
BloHdg)
-
10B(am)
+ 8Hk!
(8)
Converting to constant pressure, AH403 is -20.4 kea1 mole-'. Estimating H403 - H298 for BloHle to be 5.67 kcal mole-', slightly more than BI0Hl4 and slightly less than twice B6Hg,'5 AH298 is 23.8 kea1 mole-'. Converting to crystalline boron and changing the sign, AHto(B10Hl6(g)) is +34.8 kea1 mole-'.
34.98
~~
Table IV: BloHlsRuns a t 130' %
AB,
kcal mole -1
26.5 25.5 25.5 26.3 27.9 24.0 26.8
Xe
BioHis
SbHa
Hl
cal
csl
cal
BioHie decompd
0.35 0.2 0.3 0.26 0.3 0.35 0.4
0.184 0.231 0 275 0 291 0 311 0 387 0.444
2.656 2.597 2.552 2.503 2.476 2.407 2.298
5.390 5.685 5.892 5.825
96.71 95.76 95.03 93.54 93.38 91.43 89.80
92.00 90.00 88.39 86.68 85.74 83.32 79.54
4.71 5.76 6.64 6.86 7.64 8.11 10.26
96.7 97.8 94.6 89.7 (88) 87.5 (86)
,
mmoles of
1
... 6.304
...
Qtot?
9SbH:t
PBlOHldv
Converting to constant pressure, AH423 is 9.84 kcal mole-'. Using the enthalpy data of Evans, Prosen, and Wagman16 for boron and decaborane and of Stull and SinkeI4 for hydrogen, AH298 is calculated to be 6.65 kea1 mole-'. Converting from our form of B(am)4 to B(c) by subtracting 1.1kcal (g-atom of B)-', the standard heat of decomposition is -4.4 kcal mole-' and AHfO (B10H14(g)) is +4.4 kcal mole-'. Results for BloHla are given in Tables I11 and IV. The SbHs only runs scattered more than usual, and hydrogen yields were also exceptionally low. The final result for AH403, -33.78 kcal mole-', compared with the calculated value of -34.04 kcal mole-', corresponds to a temperature deficiency of 23".
Table V: -mmoles Xe
0.2 0.2 0.2 0.2
SbH3 Runs at 25" -fo SbHa
H1
1.678 1.641 1.693 1.387
2.459 2.534 2.085
.,.
Q,
H2, %, of theory
cal
k0a1 mole-'
(99.9) 99.90 99.78 100.2
59.02 57.80 59.49 48.70
35.38 35.44 35.39 35.21
Av. 99.90
Q8
35.36
(15) W. H. Evans, E. J. Prosen, and D. D. Wagman, "Thermodynamic and Transport Properties of Gases, Liquids, and Solids," McGraw-Hill Book Co., New York, N. Y., 1959, p 226.
Volume YO, Number 4 April 1966
1118
STUART R. GUNNAND JOHN H. KINDSVATER
Table VI: Heat of Decomposition of BeHlz at 25' 70
mmoles of
- AE,
Xe
Banlz
SbHs
HI
cal
oal
cal
decompd
koa1 mole-'
0.03 0.1 0.04 0.03 0.02 0.06
0.210 0.298 0.168 0.209 0.213 0.214
2.022 1.770 1.118 1.331 1.278 0.899
4.176 4.255 2.567 3.110
75.94 68.70 42.75 51.25 49.31 35.84
71.04 62.13 39.25 46.75 44.87 31.58
4.90 6.57 3.50 4.50 4.44 4.26
91.1 89.7 89.5 89.1 (90) 90.0
23.3 22.0 20.8 21.5 20.8 19.9
... 2.503
Qtot,
98bHsv
QBIOHls,
BsHiz
Results for Be& are given in Ta,bles V and VI. Discussion The SbHa only runs give a value of -34.47 kcal mole-' The over-all uncertainty in the three standard for AHzgs,compared with a best value of -34.68 kcal heats of formation obtained in the present work may mole-'. be estimated as ca. 1 2 kcal mole-', aside from an For the runs with B6H12, unlike the other two boron additional uncertainty in the heat of conversion of hydrides, there is no trend of the percentage of the amorphous to crystalline boron which affects the abborane decomposed, as indicated by the hydrogen solute values of heats of formation but not calculayield, with the borane :stibine ratio in the initial tions of heats of interconversion of boranes. mixture. Flaming the cells in all cases evolved about Decaborane-14 is the most stable of the boron hyone-fourth of the missing hydrogen. It was finally drides; this is reflected in the relatively low percentages found, for runs 4-6, that the traps between the cell of decomposition achieved with it in the present work. and the gas buret had caught two fractions, subseIt is, indeed, the only hydride having a positive heat quently separated, measured, and identified by inof decomposition which has been successfully decomfrared spectrometry as BsHg and B2Hs in a mole ratio posed in this laboratory by the stibine explosion of approximately 2 : l and a total amount roughly method; however, the large value of A S of course corresponding to the missing hydrogen. Accordingly, makes AG negative. Johnson, Kilday, and Prosen6 A E in Table I11 is calculated from the weight of obtained -15.8 f 1.4 kea1 mole-' for AHrO(B10measured, q ~ ~ ~ the ~ ~average, / n -21.4 ~ ~ kcal ~ ~ H14(c)), ~ ; and Galchenko, Timofeev, and SkuratovlG mole-', is assumed to represent AE of the reaction obtained -14.0 f 1.0 kcal mole-', both by pyrolysis methods. Using f18.6 kcal mole-' for the heat of B&dg) --+ 5.4H2(g) 5.4B(am) sublimation,14 these values become +2.8 and $4.6 kcal mole-' for AH1"(B10H14(g)),in excellent agreeO.lBsHg(g) O.O5BzHe(g) (9) ment with the present result. Thus, as previously observed14Bl0HI4does not fit well in a thermochemical Converting to constant pressure, AH3 is -18.7 kcal bond-energy scheme with the lower boranes. mole-'; using AHf(B(am)) = +1.1 kcal mole-', The decaborane-16 molecule consists of two pyramiAHf"(B5H9(g))= f15.0 kcal mole-', and AHt"(B2H6) dal B5Hs groups, having essentially the structure of = +6.5 kcal mole-', AHfo(B6Hl2)is calculated to be B5H9 with the apical terminal hydrogen removed, +26.5 kcal mole-'. joined by a B-B bond a t the apical borons. Hence, If all the B6H12 were assumed to have decomposed one might expect the heat of the reaction to the elements, AHrO(BeH12) would be calculated as +25.0 kcal mole-'; if it were assumed that 90% decomposed and the remainder were unchanged, AHfO would be +27.4 kcal mole-'. If the missing 10% to be closely equal to E(H-H) E(B-B) - 2E(B-H), of hydrogen is assumed to remain all as BsHg or all which from the previously recommended set of bond as BZH6, AHf' becomes +26.6 or +26.3 kcal mole-', energies4 is -1.2 kcal. However, from the presently respectively. Only if the hydrogen remains in a obtained heats of formation, AHlois $4.8 kcal mole-'. thermochemically much more stable form does the calculated AHf"(B0Hl2) change much from that obIn view of possible perturbations of the skeletal shape tained by assuming reaction 3; even if it is all present as solid BlOH14, AHr" is calculated to be +23.1 kcal (16) G. L. Galchenko, B. I. Timofeev, and S. M. Skuratov, Dokl. mole-'. Akad. Nauk SSSR, 142, 1077 (1962).
+
+
+
+
The Journal of Phyaieal Chemistry
HEATSOF DECOMPOSITION OF SOME MOREBORON HYDRIDES
and electronic distribution of the two BSHs groups, this 6-kcal discrepancy is probably not surprising. From the nmr spectrum, Gaines and Schaeffer” deduced the most probable structure of BaHlz to be 4212 in terms of Lipscomb’s formulations.‘7 From our previous set of bond energies4 this leads to a calculated heat of atomization of 1411.7 kcal mole-l and a standard heat of formation of +24.8 kcal mole-’ (not +14.7 kcal mole-‘ as given by Gaines and Schaeff er), in excellent agreement with the present experimental results. Gaines and Sohaeff er observed a base-catalyzed decomposition of gaseous hexaborane-12 to give predominantly a mixture of pentaborane-9 and diborane-6
1119
in a mole ratio of roughly 2: 1. We observed the same to be one apparent course of decomposition in the stibine explosions. The calculated value of the heat of the rearrangement reaction, AHll, is -8 kcal mole-’. BsHiz(g)
----t
BsHs(g)
4- ‘/zB2Hs(g)
(11)
Acknowledgments. We thank Leroy G. Green for assistance in the B10H14work, Vernon G. Silveira for performance of the X-ray analyses, and Professor W. N. Lipscomb for providing additional information concerning the B10H16 preparation. (17) W. N. Lipscomb, “Boron Hydrides,” W. A. Benjamin, Inc., New York, N. Y . , 1963.
Volume 70, Number 4
April 1966
