97
THERMOCHEMISTRY OF BORON AND SOME OF ITS COMPOUNDS
The Thermochemistry of Boron and Some of Its Compounds. The Enthalpies of Formation of Orthoboric Acid, Trimethylamineborane,
and Diammoniumdecaborane'p'
by W. D. Good and M. MaSnacJon Contribution No. 141 from the Thermodwmtb Laboratory of the Bartleaville Petroleum Reeeurch Center, Bureau of Miwa,U.5. Department of the Intsrio7, B a r t W l e , OkEahoma ( M v d June 14,1966)
A rotating-bomb calorimetric technique was developed to solve the problem of determining accurate values of the enthalpies of combustion of boron and boron compounds. The enthalpies of combustion of crystalline boron, trimethylamineborane [ (CHa)aNBHs], and diammoniumdecaborane [B1,&Na] were determined. The combustion products were carbon dioxide, water, fluoroboric acid in excess aqueous HF, and (in the case of the boron compounds) gaseous nitrogen. The enthalpy of solution of orthoboric acid was measured in a hydrofluoric acid solution chosen to yield the same fluoroboric acid solution obtained in the boron combustion experiments. The derived values of the enthalpies of formafrom graphite, crystalline boron@, rhombohedral), and gaseous hydrogen tion, AHj0298.15, and nitrogen are: orthoboric acid(c), -261.47 f 0.20 kcal. mole-'; trimethylamineborane(c, 111),-34.04 f 0.55 kcal. mole-'; and diammoniumdecaborane(c), -85.84 f 2.50 kcal. mole-'. The value of the enthalpy of formation of orthoboric acid was used with numerous other thermochemical data from the literature to derive values of the enthalpies of formation of the boron oxides, diborane, boron trichloride, and boron tribromide referred directly to crystalline boron. The value of the enthalpy of formation of trimethylamineborane was used with other literature data to derive another value of the enthalpy of formation of diborane.
Introduction The lack of a method for the accurate determination of the enthalpies of combustion and formation of boron compounds has been an important gap in the methods of modern thermochemistry. Boron and the boron compounds, even if reacted completely with oxygen in the combustion bomb, yield a poorly defined, hygroscopic oxide. Even if water can be excluded from the reaction vessel, the physical form of the oxide remains uncertain. Prosen and co-workers* have pointed out the difficulty of the combustion of boron in oxygen and the divergence of the values of the enthalpy of formation of &Oa that have come about from such measurements. The difEculties are multiplied when the boron is combined in a compound with other elements such as hydrogen, carbon, and nitrogen. In addition to the
problems of the boron oxide alone, numerous other combustion products a r i ~ e . 4 ~ In recent research of this laboratory, a method developed previously for the determination of the enthalpies of combustion of silicon and organic silicon (1) Thh work waa reported in part a t the Sympdum on Thermodynamics and Themoohemistry, Lund, Sweden, July 1963, COsponsored by the Commission on Thermodynamics and Therm+
chemistry, IUPAC, and the Swedish Chemiaal Assoohtion. (2) The work waa supported by the Chembtm Office of the Advanced Research Projects Agency under Contract No. CSO-69-9, ARPA Order No. 416. (3) E. J. Prosen, W. H. Johnson, and F. Y.Pergiel,J . Rea. NatZ. Bur. & 62, I. 43 , (1969). (4) M. V. Kilday, W. H. Johnson, and E. J. Prosen, ibid., A65, 101 (1961). (5) W. H. Johnson, M. V. Kilday, and E. J. Prosen, &id., A65, 216 (1961).
Volume 70, Number 1 Janumy 1066
98
compounds6 was extended to determine accurate enthalpies of combustion of boron and boron compounds. The method involves combustion of boron or a boron compound mixed with a fluorine-containing combustion promoter, vinylidene fluoride polymer. The combustion bomb initially contained an aqueous solution of HF in such concentration that the boron appeared in the reaction products in a homogeneous solution, approximately HBFr. 14.5HF. 58.5H~O. The method was applied to pure crystalline boron@, rhombohedral), trimethylamineborane, and diammoniumdecaborane. The enthalpy of solution of orthoboric acid was measured in a hydrofluoric acid solution chosen to yield the same fluoroboric acid solution obtained in the boron combustion experiments. The measured enthalpies of combustion and solution permit calculation of the enthalpies of formation of the boron compounds referred directly to crystalline boron.
Experimental Combustion Calorimetry The basic procedures used in this research for combustion calorimetric measurements have been described.? Only innovations of the present techniques w i l l be described here. Preparation of Sample Mixtures. Bags of polyester film7 were used to prepare sample mixtures of crystalline boron, diammoniumdecaborane, or trimethylamineborane with vinylidene fluoride polymer, (CH2CF!Jn. Crystalline boron powder, 325 mesh particle size and finer, and vinylidene fluoride polymer, 100 mesh particle size and finer, were weighed into a previously weighed polyester bag. A “bubble” of air was intentionally sealed inside the bag. Manipulation of the bag in such a manner that the air “bubble” was pushed from end to end produced an intimate mixture of the two powders. The polyester bag with its contents was pressed flat in a vise; subsequently, the bag was pricked with a needle to allow the air to escape. The bag was rolled, with the opening made by the needle inside, and compacted with a pellet press. Subsequent weighing of the pellet revealed that it could be prepared without signi6cant loss of mass. Pellets of finely ground diammoniumdecaborane and vinylidene fluoride were prepared by the same procedure. Pelleted mixtures of trimethylamineborane and vinylidene fluoride polymer in polyester bags were prepared in the same way except that the slightly hygroscopic trimethylamineborane was handled inside a drybox. Trimethylamineborane is a waxy solid, and the mixtures obtained were not so intimate as obtained with the other two solids. Because trimethyl-
W.D.GOODAND M. MANEBON
amineborane is slightly volatile, the pellet was sealed inside a polyester envelope. The polyester bag technique is an invaluable aid in combustion calorimetry of fluorine-containing materials. Recognition of the problems of the hygroscopicity of the polyester and of its tendency to become statically charged must be emphasized. Recent experimenta have shown that several compounds, including water, permeate the polyester film very slowly, and appropriate precautions in its use must be taken. The Chemistry of ule Bomb Process. The pellets described in the previous section were burned under 30 atm. of oxygen in the presence of excess aqueous HF. In several noncalorimetric experiments, the bomb was discharged and opened as quickly as possible after ignition of the sample. Solid oxidation products never were found. The boron oxide and/or fluoride dissolve rapidly in the HF solution to form aqueous HBF4. The chemical reaction was selected with cognizance that oxygen-containing acids such as HBFaOH exist and might possibly affect the results. Wamsel.8 has shown that, in the presence of large excesses of HF, the only important boron-containing ion in solution is BF4-. Unknown effects associated with some small equilibrium concentration of BFaOH- should be minimized by the comparison experiments’ designed to produce the same final solution. The maas of pure boron, diammoniumdecaborane, or trimethylamineborane was used as a measure of the amount of reaction. Precise determination of 1 mole of HBF, in the presence of 15 moles of H F was found to be difEcult. Fluoride ion was precipitated and BF4ion hydrolyzed by the addition of excess calcium chloride followed by boiling under reflux. The total fluoride ion, whether from HF or HBF4, was titrated as HC1 with standard caustic (methyl red end point). The s m d amount of HNOa was neutralized in the same titration. Finally, mannitol was added to the cooled sluny, and the H&Oa was titrated with standard caustic (phenolphthalein end point). It was possible to demonstrate that 99+% of the expected boron was in solution. It only can be inferred from the equilibrium datas that the boron was in solution as BF4-. The exact find state of the boron in solution is not critical as long as the same state is reached in all experiments. Nitric acid was determined quantitatively.’ The gas from selected experiments was checked qualitatively (6) W.D. Good, J. L.L ~ SB., L. DePrater, and J. P. McCdough, J. Phys. Chem.,68,679 (1964). (7)W.D.Good and D. W. Scott, E&. Thsnnochsm., 2, 16 (1962). (8)C. A. Wamser, J . Am, C h . SOC.,70, 1209 (1948).
THERMOCHEMISTRY OF BORON AND SOME OF ITSCOMPOUNDS
for CO, and none was detected. Mass spectrometer analysis of the HF-free gas from selected experiments failed to show the presence of any gaseous combustion products other than C02 and H20. There was no evidence for chemical attack on the platinum crucible. Comparison Experiments. Comparison experiments were used to minimize errors from inexact reduction to standard states caused by lack of data necessaxy to correct for such effects as the solubility and enthalpy of solution of COz in a rather concentrated solution of HF and HBF,. For the boron comparison experiments, the sample consisted of benzoic acid and hydrocarbon oil. For the experiments with trimethylamineborane and diammoniumdecaborane, the sample was hydrocarbon oil. The amounts of these materials were so selected that the COz produced and (as nearly as possible) the energy evolved in the comparison experiments were the same as in the companion combustion experiment. The bomb initially contained an aqueous solution of H F and HBFl which, upon dilution with the water produced by the combustion of the sample, gave a solution of nearly the same amount and concentration as the combustion experiment. Calorimeter and Bomb. The rotating-bomb calorimeterlS laboratory designation BMR 111, and the platinum-lined bomb, laboratory designation Pt-5, with internal volume 0.353 l., have been described.
Experimental Solution Calorimetry Four enthalpy-of-solution experiments were performed in which crystalline orthoboric acid was dissolved in a hydrofluoric acid solution chosen to produce the same final solution as obtained in the boron combustion experiments. The boric acid was contained in a methyl methacrylate vessel with a flat, smoothsurfaced lid. This vessel, with its contents, was floated in the HF solution inside the bomb. The bomb was placed inside the calorimeter without the usual inversion. The calorimeter was heated to an initial temperature about 0.5" below the temperature of the isothermal jacket (25"). Timetemperature measurements were made for an initial period, and rotation of the bomb was begun. When the bomb turned, the contents were mixed. The accompanying heat effect was rapid and was observed by t i m e temperature measurements that were continued until a steady rate of temperature change was obtained again. About 0.15 mole of H&Oa was dissolved in the HF solution with an energy evolution of approximately 2000 cal. The enthalpy of solution was calculated from the known heat equivalent of the system, E (calor.) = 4032.3 cal. deg.-l, and heat capacities of the contents.
99
Materials Boron. The crystalline boron was supplied by the Eaglepicher Co. This material had been prepared by hydrogen reduction of pursed boron tribromide with deposition on boron filaments. The boron was subsequently purified by zone refining. The crystalline form of the material was p-rhombohedral. The boron had been crushed with a so-called "diamond" mortar. It was necessary to sieve the powder to obtain material of 325 mesh particle size, and smaller, in order to obtain complete combustion. The sieving was done with stainless steel sieves used as gently as possible. The sieved material was leached with concentrated hydrochloric and hydrofluoric acids, water washed, and vacuum dried. Carbon content was less than 50 p.p.m. Emission spectrographic analysis of the powder showed 0.02% iron and 0.01% silicon as the only detectable impurities. Trimethglamineborane. The sample of trimethylamineborane was provided by the Ethyl Corp. through the courtesy of Mr. W. E. Foster. It was purified by recrystallization from diethyl ether at -70" under nitrogen. The material was dried in a vacuum oven equipped with nitrogen flushing. The purity of the sample, as evaluated by study of the equilibrium melting temperature as a function of the fraction melted, was 99.95 f 0.03 mole yo. Unpublished measurements of this laboratory have shown that trimethylamineborane exists as three crystalline forms. Crystal 111,the crystals studied in this research, is the form stable at 25" and lower temperatures. Diammoniumdecaborane. Two samples of diammoniumdecaborane (Figure 1) were studied. The first sample (I) was obtained from the Dow Chemical Co. through the courtesy of Dr. G. C. Sinke. This sample had been recrystallized from water, washed with 95% ethanol, and vacuum dried. The second sample (11) was obtained from the Rohm and Haas Co. through the courtesy of Dr. A. R. Pitochelli. This sample had been recrystallized from water, washed with ethanol and ether, and dried at 80" for 4 or 5 days. Both samples were dried before use by maintaining them at 110" under high vacuum for 72 hr. At the end of this time, the loss of mass had ceased. The mass of each sample decreased by about 0.5% with this treatment. The volatile material was trapped, and mass spectrometric analysis of the trapped material indicated only water. Sample I regained no water on exposure to laboratory air (50% relative humidity). Sample 11, (9) (a) W. D. Good, D. W. Scott, and G. Waddington, J . Phye. Chena., 60, 1080 (1966); (b) J. L. Lacina, W. D. Good, and J. P. McCullough, ibid., 65, 1026 (1961).
Volume 70,Number 1 Januaty 1066
W. D. GOODAND M. MANSSON
100
Auxiliary Substances. The benzoic acid was National Bureau of Standards Sample 39h, certified to evolve 26.434 =t 0.003 abs. kjoules/g. of mass when burned under specified conditions. The polyester film,’ auxiliary ~il,~’and cotton thread fuse’” have been described.
I Figure 1. Diamoniumdecabrane.
if opened to laboratory air, regained approximately the same 0.5% lost in the vacuumdrying procedure. The dried sample I burned smoothly in the combustion bomb with reproducible results. Sample 11, when burned dry, detonated to cause considerable damage to the crucible and internal fittings of the bomb. When allowed to reabmrb water by exposure to air, sample I1 also burned smoothly although not quite as reproducibly as sample I. If the values of the enthalpy of combustion of the “wet” sample I1 were corrected for the mass of abmrbed water, they were essentially the same as the values for sample I. No valid explanation can be given for the difference in combustion behavior of the two samples. Orthoboric Acid. Commercial reagent grade boric acid was recrystalIieed three times from distilled water and air dried at room temperature. The finely ground material was stored in sealed containers until used. Titration with standard alkali in the presence of mannitol indicated 100% HpBOa. Vinylidae Flumide Polyme7. Vinylidene fluoride polymer (a difierent sample than used in earlier work with silicon compounds) was supplied by the Pennsalt Chemicals Corp. The value of aEco/M for this material was -3527.79 f 0.64 cal. g.-I (mean and uncertainty interval) for combustion according to the reaction
+
+
(CH&FZ).(S, polymer) 2Odg) 20H200 = 2COzk) 2[HF.lOHz010
+
0
Mass spectrometric examination of the HF-free gas resulting from combustion of the polymer did not reveal the presence of CF, or other fluorine-containing molecules. As nearly as could be ascertained from the HF recovery, the composition of the polymer was exactly (CH&Fz),,.
Calorimetric Results Units of Measuremat and Auziliary Quantities. The results reported are based on a molecular weight of 61.833 for orthoboric acid, 72.947 for trimethylamineborane, 154.267 for diammoniumdecaborane, and an atomic weight of 10.81111 for boron and the relations 0°C. = 273.15’K. and 1 cal. = 4.184 abs. joules (exactly). Thermochemical data from the literature were adjusted to the 1961 atomic weight scale” as necessary. All electrical and mass measurements were referred to standard devices calibrated at the National Bureau of Standards. For use in reducing weights in air to those in uaeuo, in converting the energy of the actual bomb process to the isothermal bomb process, and in reduction to standard states, the values in Table I were used for density, p, specific heat, c,, and (W/N’)T of the substances. Table I: Physical Properties at 298.15’H. CPI
(aE/aPh
od. der.
4. atm.-
E. ml.’
g.-1
g.-
2.31 0.82 1.0
0.245
0.43
Negligible -0.002 Negligible
1.755
0.33
-0.00189
0.87
0.53 0.315 0.289
-0.W60
0.
Boron Trimethylamineborane Diamoniumdecabw
0.569
rane
Vinylidene fluoride polymer Auxiliary oil Polyester iilm Benzoic acid
1.38 1.320
-0.W069 -0.0028
Bwon Cmnbustion Rewlts. Ten pairs of satisfactory combustion and comparison experimentswere obtained. Eleven experiments were attempted in which 21% of the energy came from boron, and nine were successful with no evidence of incomplete combustion. Three experimentswere attempted in which 30% of the energy came from boron, and only one experiment was successful. Data from a typical combustion experiment and the corresponding comparison experiment are sum(10) W.D. Gwd, D. R. D o d i n , D. W.Scott, A. George, 3.L. Lwina. J. P. Dawson, and G. Waddingbon, J. Phys. Chem.. 63, 1133 (1959). (11) A. E. Csmeron snd E. Wiehers, J . Am. Chem. Sa.,84, 4176 (1962).
THERMOCHEMISTRY OF BORON AND SOMEOF ITSCOMPOUNDS
101
Table II : Summary of Typical Calorimetric Experiments" Combustion experiments Boron
0.11090 1.61864 0.11225(45) 0.58804 0.14220 1.998770 -8060.94 -30.75 -13.37 0.67 10.06 1.06 4.62 5699. 30d 613.42 0.24 1775.69
m (compd.1, g. m (vinylidene fluoride), g. m (polyester), g. (% relative humidity)
ni (HzO),mole ni (HF), mole A&, deg. Gppp (calor.)( - At,), cal. E (cant.)( - ~ t , ) , cal." AE (concn. of HF), cal." AEk,,,cal. AE,cor. to std. states, cal. U r d , , (moa),ad. -mAEc"/M (fuse), cal. -mAEc"/M (vinylidene fluoride), cal. -mAEc"/M (polyester), cal. AE, cor. for Si and Fe, cal. meFcD/M(compd.), cal.
-
-16,011.6
AEco/M (compd.), cal. g.-1
Trimethylamineborane
Diammoniumdecaborane
0.36394 0.81528 0.18447 (32) 0.25137 0.06760 1.986188 -8010.17 -17.64 -7.37 0.60 5.21 5.68 4.54 2869. 618 1008.68
0.159177 1.470058 0.112039 (57) 0.57872 0.14676 2.005903 -8090.33 -30.53 -12.77 0.69 8.72 2.70 5.56 5175.90' 611.92
... -4140.86
... -2328.14 -14,626.1
-11,377.9
Compuison experimenta
m (auxiliary oil), g. m (benzoic acid), g.
ni (HzO),mole ni (HF), mole ni (HbFd), mole At,, deg. mAEc"/M (auxiliary oil), caL mAEc"/M (benzoic acid), cal. meFc"/M (fuse), cal. -U f d e o (HNO,), tal. -AE,cor. to std. state, cal. -AEign, C J . - A E (diln.), cal.' E (cant.)( atc),cd." E., (calor. )( At,),
-
&pp
0.60326 0.22933 0.55924 0.15171 0.01026 1.999625 6626.09 -1447.76 -4.33 -1.70 -8.10 -0.57 -4.91
(calor.), cal. deg.-l
0.721177
...
...
-
0,24535 0.07309 0.00499 1.897041 -7649.01
0.55689 0.15141 0,01032 1.961546 -7921.23 -5.08 -1.25 -6.90 -0.60 -4.91
-8084.40
-5.10 -2.96 -3.95 -0.71 -5.53 16.63 -7650 * 63
-7911.43
-4032.96
-4032.93
-4033.26
29.06
d.
0.69635
...
...
28.54
" The symbols and terminology are, except as noted, those of W. N. Hubbard, D. W. Scott, and G. Waddington, "Experimental i , Ed., Interscience Publishem, Inc., New York, N. Y., 1956, Chapter 5, pp. 75-128. &i (cont.)(t* Thermochemistry," F. D. M 25") (cont.XZ5" It Ataor). Correction for the change in concentration of the original HF in the bomb caused by the HF and HaO formed in the combustion reaction. For combustion of vinylidene fluoride to HF-3.064Eld. 'For combustion of vinylidene fluoride to HF.3.016HaO. For combustion of vinylidene fluoride to HF.2.743H20. # Correction for dilution of the HF and HBF, initially in the bomb by the water formed in the combustion reaction.
+
&f
-
- +
'
marized in Table 11. Values of AEco/M for aJl of the successful experiments are summarized in Table 111. Values of AEc"/M refer to the reaction B(c)
+ 0.7502(g) + 18.674HF*57.219H20(1) HBF4*
[email protected] (11)
The energy of reaction I1 was found to be
AEc"/M = -15,998.7 A 6.7 tal. g.-' (mean and standard deviation)
A&"
= -172.96
* 0.20 kcal. mole-l (mean and uncertainty interval)
AHc" = -173.41
f
0.20 kcal. mole-' (mean and uncertainty interval)
Trimthylamineborane Combustion Results. Eight pairs of satisfactory combustion and c o m p h n experiments were obtained in nine attempts. About 35% of the energy came from trimethylamineborane. Data Volume 70, Number 1 Janucrry 1866
W. D. GOODAND M. MANSSON
102
+
Table ITI : Summary of Experimental Results: Values of AEc"/M (in cal. g.-I at 25') Trimethyl-
Boron, reaction I1
amineborane, reaction I11
-15,976.7 16,000.9 16,018.3 16,027.9 16,011.2 16,011.6 15,990.5 15.961.5 15,978.6 16.009.5 Mean SM.dev.
- 15.998.7
Diammoniumdecaborane, reaction IV
-11,377.9 11,379.5 11,381.6 11,384.7 11,390.5 11,381.3 11,387.1 11,384.0
- 14,626.1
- 11,383.3
-14,628.6
14,623.0 14,622.9 14,636.0 14,633.2 14,630.2
f2.2
from a typical combustion experiment and the corresponding comparison experiment are summarized in Table 11. Values of AEco/M for all successful combustion experiments are summarized in Table 111. The energy of the reaction
+
+
C3HizNB(c, 111) 6.75Oz(g) 18.674HF.51.21gHz0(1) = 3coz(g) 0.5Nz(g)
(111)
was found to be
AEc0/M = -11,383.3 AEc"
=
-830.38
L\Hco = -832.30
1.5 cal. g.-' (mean and standard deviation)
f
f 0.46 kcal.
mole-' (mean and uncertainty interval)
f
0.46 kcal. mole-' (mean and uncertainty interval)
By combination of the enthalpies of reactions I1 and I11 with data for the enthalpies of formation of COz(g)12 and HzO(1)12and dilution enthalpy for aqueous HF,12 the enthalpy of formation of trimethylamineborane = -34.04 i 0.55 (c, 111) was found to be AH~OZM.IS kcal. mole-1 (mean and uncertainty interval). Diammoniumdecaborane Combustion Results. Six pairs of satisfactory combustion and comparison experiments were obtained in six attempts (sample I), About 29% of the energy came from the diammoniumdecaborane. Data from a typical combustion experiment and the corresponding comparison experiment are summarized in Table 11. Values of AEco/M for all the experiments are summarized in Table 111. Values of bEco/M refer to the reaction The J m r d of Physical Chemistry
bEco/M
=
-14,628.6
f
2.2 cal. g.-l
(mean and standard deviation)
AEc"
=
-2256.7
f
1.5 kcal. mole-' (mean and uncertainty interval)
=
-2263.2
f
1.5 kcal. mole-l (mean and uncertainty interval)
The enthalpies of reactions I1 and IV were combined with the enthalpy of formation of water12 and appropriate dilution enthalpy data for HF12 to derive the enthalpy of formation of crystalline diammoniumdecaborane, A H p 2 9 8 . 1 6 = -85.84 f 2.50 kcal. mole-' (mean and uncertainty interval). Enthalpy of Solution of Ha03 in Aqueous HF. Four experiments were performed in which orthoboric acid was dissolved in aqueous H F according to the reaction
+
+
+ HBF,. 14.674HF.58.71gHzO(l)
+
The energy of reaction IV was found to be
AHc"
f1.5
f6.7
+
BioHieNz(c) 12Oz(g) 10[18.674HF* 56.31gHz0 ] (1) = Nz (g) lO[HBF*.14.674HF*58.719HzO](l) (IV)
H~BO~(C)18.674HF.55.71gHzO(l) =: HBF4. 14.674HF.58.71gHzO(l) (V) The enthalpy of reaction V (at 298.15"K.) was found to be 14.60 i 0.01 kcal. mole-' (mean and standard deviation). From combination of the enthalpies of reactions I1 and V with the enthalpy of formation of waterI2 and dilution enthalpy for HF,I2 the enthalpy of formation of crystalline HaBOa at 298.15OK. wa8 found to be -261.47 f 0.20 kcal. mole-' (mean and uncertainty interval).
Derived Results The enthalpy of formation of orthoboric acid, a "key" boron compound,was referred directly to crystalline boron by the measurements of this research. Earlier experimental measurements on crystalline boron have been few because of the difEculties of securing good samples and obtaining complete reaction of the samples. Orthoboric acid is an important compound because enthalpy-of-solution and enthalpy-ofhydrolysis measurements relate the enthalpies of formation of many other boron compounds to its enthalpy of formation. The derived value of the enthalpy of formation of orthoboric acidwaa used to derivevalues of the (12) F. D. Rossini, D. D. Wagman, W. H. Evans, S. Levine, and I. J d e , National Bureau of Standards C ~ c u l m600, U. S. Government Printing Office, Washington, D. C., 1962.
THERMOCHEMISTRY OF BORON AND Sora OF ITSCOMPOUNDS
103
enthalpies of formation of several other important boron compounds. An exhaustive literature survey was not made. The results for orthoboric acid simply were extended to other areas of thermochemical research where the authors were aware of recent definitivework on "key" compounds. Where two measurements of the same reaction enthalpy existed, no attempt was made to assess which measurement was better or to weight the values to give a single value. The enthalpies of formation of the boron compounds formerly have been referred to amorphous boron through two thermochemical cycles. In one, the enthalpy of decomposition of diborane into amorphous boron and gaseous hydrogen was m e a ~ u r e d . ' ~This ~~~ measurement was extended to other boron compounds by hydrolysis of diborane to give aqueous boric acid and gaseous h y d r ~ g e n . ~ JIn~ the second cycle, the enthalpy of formation of gaseous BCl3 was determined by direct reaction of gaseous chlorine with amorphous boron,I6 and this work was extended to other boron compounds through measurement of the enthalpy of hydrolysis of BC13.16117 The enthalpies of formation of the boron compounds were related to crystalline boron by means of an estimate of the enthalpy of formation of amorphous boron3 from crystalline boron, ' ~ reported a 0.4 kcal. g.-atom-'. Gross, et ~ 1 ,have difference of about 0.8 kcal. g.-atom-' for the enthalpies of fluorination of two samples of boron, one of these being a highly refined crystalline form and the other the amorphous form derived from the decomposition of diborane. Although there is some uncertainty in appending experimental measurements for one particular amorphous boron to the experimental measure ments of other workers involving other amorphous borons, it will be shown in the following treatment that use of the value, 0.8 kcal. g.-atom-', for the enthalpy of formation of amorphous boron improves the general agreement of all results. Diborune. Two modern measurements of the enthalpy of hydrolysis of diborane e~ist.~3
formation of waterlZto derive two values of the enthalpy of formation of gaseous diborane.
BzHa(g)
Sa., 61, 247 (1968).
+ 6HzO(1) + [2000H20(1)1
=
2[H$Oa* 1000H2O(soln)I AHO29g.16
=
-111.46
AHo2s.16 = -112.22
A
+ 6H2(g)
Orr)
0.46 kcal. mole-' (Prosen, et d.8)
f 0.10
kcal. mole-' (Gum and Green16)
These measurements were combined with the enthalpy of formation of crystalline orthoboric acid from this research, the selection of Prosen, et U Z . , ~ for the enthalpy of solution of orthoboric acid, and the enthalpy of
mf02sdB2H6, g) = 8-77 f 0.68 kcal. mole-' (after ref. 3)
*
mf0298.16(B2HB,g) = 9-53 0.42 kcal. mole-' (after ref. 15) These values may be compared with the values, 7.53 0.5613and 7.53'' kcal. mole-', both derived from the decomposition of diborane into amorphous boron m d gaseous hydrogen. It should be noted that, although these two earlier values agree exactly, the agreement was forced by use of two very different values of the enthalpy of formation of amorphous boron, 0.4 f 0,113and 1.255'' kcal. g.-atom-'. If 0.8 kcal. g.-atom-l is used for the enthalpy of formation of amorphous boronr8 and these values still are forced to agree, the enthalpy of formation of diborane becomes 8.33 kcal. mole-'. Another mmewhat less direct approach to the enthalpy of formation of diborane is through the enthalpy of the reaction
*
(CHa)aN(g)
+ 0.5Bz&(g)
AH0273
= -31.27
=:
(CHa)aNBHo(c)"
WI)
* 0.15 kcal. mole-'
By use of appropriate heat capacity data, the enthalpy of reaction at 25' was calculated to be -31.40 ked. mole-'. The enthalpy of formation of trimethylamine borane from this research, the value of Jaffe20for the enthalpy of formation of gaseous trimethylamine, A€?,,Z~S.W = -5.9 kcal. mole-', and the enthalpy of reaction VI1 were combined to derive a value of the enthalpy of formation of gaseous diborane, M l j O 2 S . 1 1 1 = 6.5 0.6 ked. mole-'. The enthalpy of reaction MI has been remeasured recently by Gunn2' to be -32.33 f 0.06 kcal. If this value is used instead of
*
(13) E.J. Prosen, W. H. Johnaon, and F. Y. Pergiel, J . Res.Natl. But.
(14) (a) 8. R. Gunn and L. G. Green, J . Phys. Chem., 65,779 (1961); 6)S. R. Gunn and L. G. Green, J . C h . Phgs.,36, 1118 (1962). (16) S. R. Gunn and L. G. Green, J . Phys. C h . ,64, 61 (1980). (16) W.H.Johnson, R. G. Miller, and E. J. Prosen, J . Rea. Natl. Bur. Std., 62,213 (1969). (17) H.A. Skinner and N. B. Smith, Tram. Faradag Soc., 49, 801 (1963). (18) H. A. Skinner, "Thermodynamics and Thermochemistry," Plenary Lectures presented at the Symposium on Thermodynamics and Thermochemistry, Lund, Sweden, July 18-23, 1983, Butterworth and Co. Ltd., London, 1964, p. 119. (19) R. E. McCoy and 8. H. Bauer, J . Am. Chem. Soc., 78, 2061 (1966). (20) I. Jaffe, M.S. Thesis, University of Maryland, 1968. (21) 8. R. Gu, J . Phy8. chem., 69, 1010 (1966).
Vo1um.e 70, Number 1 January 1966
104
W. D. GOODAND M. MANSSON
that of McCoy and B a u ~ , the ~ s enthalpy of formation of gaseous diborane is 8.4 kcal. mole-', in good agreement with the values obtained from the diborane hydrolysis. The Boron O&. The value of the enthalpy of formation of crystalline HBOs from this research was combined with the values selected by Prosen, et U J . , ~ for the enthalpies of solution of HBOS and amorphous BtOs, the enthalpy of transitiona of amorphous %Os to crystalline B~OS,and the enthalpy of formation of liquid water12 to calculate values of the enthalpy of formation of amorphous B208 and crystalline B~OS AZ~~O~W.E(B~OS, amorph) = -299.74 AH~O~W.IS(&OS, c) = -304.10
f
0.40 kcal. mole''
f 0.40 kcal.
+ 1303H20(1) =
+ (n + 3)H20(1) = [H&Oa + 3HBr].nH20(soln)
(IX)
BBra in CClr was found to be negligible. The energy of mixing HBOa and HBr was assumed to be negligible, an assumption that appears to be valid in view of the finding of Gunn and Greeds for the enthalpy of mixing of HBOa and HCl. The value of the present research for the enthalpy of formation of crystalline HBOs was combined with the enthalpy of solution of HJ30a,8 the enthalpy of formation of water,12 and the enthalpy of formation of HBr(n./3)H2012 to calculate the enthalpy of formation of liquid BBra. AZ3f02~.~(BBrs, 1) = -56.8 kcal. mole-'
+
H a 0 3 * lOOOH20(soh) 3 [HCl*100H2O(soh)] (VIII)
Gunn and Green" find
BBrs(soh in CCb)
mole-'
Uncertainties expressed are the uncertainty interval. Prosen, et u Z . , ~ select -301.78 f 0.75 and -306.14 f 0.74 kcal. mole-' as the enthalpies of formation of amorphous B2Oa and crystalline B2Oa from amorphous boron. If 0.8 ked. g.-atom-l is used for the enthalpy of formation of amorphous boron, these values become -300.18 i 0.85 and -304.54 =t0.84 kcal. mole-', in excellent agreement with this research. Boron Trichloride. Two modern measurements of the enthalpy of hydrolysis of BC& (liquid) exist. BC&(l)
The direct measurement of Promn, et aZ.,lsgives - 103.1 f 0.34 kcal. mole-' for the enthalpy of formation of the liquid referred to amorphous boron and -102.71 f 0.34 kcal. mole-' for the enthalpy of formation of the liquid referred to crystalline boron (based on the estimateda 0.4 kcal. g.-atom-' as the enthalpy of t r d t i o n of crystalline boron to amorphous boron). If the value, 0.8 kcal. g.-atom-', is used, the enthalpy of formation of liquid BCla referred to crystalline boron becomes - 102.31 f 0.34 kcal. mole-'. Bmon Tribromide. The enthalpy of hydrolysis at 25' of boron tribromide has been measured.22 The hydrolysis was measured at such a dilution that n G 6000 in reaction IX. The enthalpy of dissolving
*
AZ3'20s.15 = -68.14 0.06 for the enthalpy of reaction VIII. Skinner and Smith" used a different ratio of water to liquid BCla, a mole ratio of about 5000. If the value of Skinner and Smith17is corrected for the enthalpy of mixing of HCI and H&03 according to Gunn and Greed6and reduced to the concentration of eq. VIII, their value for reaction VI11 becomes -68.6 kcal. From these two values of enthalpy of hydrolysis of BC13, the enthalpies of formation of HCl. 100H20and water,'2 the enthalpy of formac tion of crystalline HBOs from this research, and the enthalpy of solution of HBOs selected by Prosen, et d . , 3 the enthalpy of formation of liquid BCls was calculated.
AHf02W.ls(BCla, 1) = -102.3 kcal. mole-l (using value of Gunn and Greeds) AHf02~.ls(BC13, 1) = -101.9 kcal. mole-' (using value of Skinner and Smith")
Amorphous Boron. If the value of the enthalpy of formation of orthoboric acid from this research is accepted along with the values of the enthalpies of formation of BCla or B2H6derived from it, the enthalpy of formation of the amorphous boron derived from diborane decomposition would appear to be about 1 kcal. g.-atom-l, in good agreement with the 0.8 kcal. g.-atom-' determined by Gross, et al. These values differ considerably from the value derived from the work of Gunn and Green,'4 about 1.7 kcal. g.-atom-'. The amorphous boron obtained by Gunn and Green" in an explosive reaction could be quite different material than that obtained in more mild diborane decomposition. Acknowledgment. The authors wish to thank Dr. C. A. Wamaer for his helpful advice on analytical techniques. Mr. B. L. DePrater performed the analyses for part of the study. Appreciation also is expressed to Drs. J. P. McCullough and D. R. Douslin for their helpful advice and encouragement. ~~~
(22) H. A.
(1966).
~
Skinner and N. B. Smith, Tram. Far&g
Soc., 51, 19
