Journal of Research of the National Bureau of Sta nda rds Vol. 62, No.5, May 1959 Research Paper 2956

Heat of Formation of Boron Trichloride Walter H. Johnson, Richard G . Miller, ! a nd Edward J. Prosen

The heat of formation of gaseou boron t richloride has been determined by t he direct reaction of gaseous chlorine with amorphous boron in a calorimeter. B (amorph) + 3/2 CIz (gas) = BCh(gas) AHfO(25° C)= - 407.98 ± 1.34 kj/mole (- 97.51 ± 0.32 kcal/moJe). By utilizing t he valu es previously reported for the hea ts of formation of boric acid, dibor­ ane, and pentaborane, t he heat of hydrolysis of boron t richloride and the heats of reac­ tion of dibora ne a nd pentaboran e wit h chlorine have been obtained. By the use of an estimat ed value for t he heat of sublimation of boron, the average bond energy of t he B- CI bond in boron t richloride is found to be 105. 2 k cal at 0° K . The data on the heats of formation of diborane, boric oxide, boric acid, and boron trichloride now form a co n­ sistent set of values.

I. Introduction 3. Materials and Apparatus The heat of formation of boron trichloride has The boron was prepared by the thermal de compo­ long been uncertain because of the fact that calcu­ sition of diborane by pa sing diborane diluted with lation of it has been dependent upon uncertain values helium through a quartz tube heated to 600 0 C [1] . of the heat of formation of boric oxide or boric acid . The amorphous boron was removed from the tube, Since new values have been determined for the heaL place d in a tantalum combustion boat, and heated of formation of boric oxide through Lhe heat of de­ for 1 hI' at about 700 0 C at a pressure of Ie s than composition of diborane [1] 2 and the heat of r eac­ 1 X 10- 5 mm of mercury. Spectrochemical examina­ tion of diborane with water [2], it seemed desirable tion of the boron by the Spectrochemistry Sec tion to make a direct determination of Lhe hea t of for­ of the Bureau showed only negligible Ll'accs of metal mation of boron trichloride. This latter value, impurities . The eff ect of a small amount of hydrogen when combined with the heat of hydrolysis of boron remaining in the boron was calculated Lo be less than trichloride and with auxiliary data, yields an inde­ the es timated uncertainty of Lh c measuremen ts. pendenL value for the heat of formaLion of boric The chlorine, obLained from Lh e MaLheson Com­ oxide. The consistency in the values thu s obtained pan~', was about 99 percent pure ; Lh e chief impurity is very strong evidence for the accuracy of the heaLs was 0.9 percenL of carbon dioxide. It was dried by of formation of boric oxide, boric acid, diborane, and passin g over phosphorus pentoxidc. The helium was boron trichlorid e. pUl'ifi.ed by heating it to 600 0 C in a copper-oxide An acc urate value Jor th e heat of formation of furnace alld directing it successively Lhl'ough Ascarite, BCla provides useful information r egarding th e en­ anhydrous magnesium perchlorate, and phosphorus ergy of the B-Cl bond and permiLs t ile calculation of pentoxide. the heat of r eaction of boron hydrides with chlorine. A section of the calorimetric vessel is shown in figure 1. It con sisted of a quartz tube .fl closed at 2. Method the bottom and fiLted with a manganin h eating coil B surrounded by a silver shield G and a vacuum The substitution method of calorimetry was used. jacket D. The boron sample was placed in the The quantity of electrical energy required to produce quartz crucible E , and the chlo)'in e introduce d a certain rise in the temperature of Lh e calorimeter through the quartz tube F . The exit gases passed ys tem was determined in one series of experiments. through tb e glass helix 0, in which they weTe cooled In a second series the temperatUl'e rise was dupli­ to calorimeter temperature. ca.ted, but a substan tial par t of the energy was The calorimeter was of the isothermal jacket type derived from a precisely measUl' ed quantity of the described in a previous paper [2] . Calorimeter tem­ chemical reaction. The calorimeter system was peratures were measUl'ed by means of a pIa tinum identical in boLh series excep t for the reacting chem­ resistance thermometer in conjunction with a G- 2 icals. By this method Lh e ell ergy liberated by the Mueller bridge and a galvanometer of high sen si­ chemical l'eaclion is obLained direc Lly in terms of tivity. R eadings on the galvanometer scale wer e electrical energy. made to Lhe neares t 0.5 mm which was equivalent to 50 J.Ldeg C. The electrical energy was obtained from a 120-v storage battery from which no other curren t I Prescnt address : Union O~ r b i c1c & Oarbon Ol· p ., X CIV York, N.Y . , Figures in brackets indicate the liLerattlre references at the end of this papcr. was drawn dming these experiments. The quantity 213 f------I allowed to come to equilibrium. T emperat ures were 2cm . then observed at regular in tervals during a final rating period. T he r ates of flow of the helium and chlorine were determined wi th capillary flowmeters. The temper­ atures of the chlorine a nd helium wer e taken as the I~------F mean room temperaturcs during the time of flow . The flow rates and temperaturcs of the chlorine and helium were required only for malcing the small heat. capacity correc tion . The a moun t of chcmical reac­ tion was determined from the difference between the mass of boron sample and th at of th e residue r emain­ ing in the crucible at the en d of the experimen t. \--1-+----D The system was calibrated in the same m ann e l~ except for the omission of the boron and chlorine. In each case the quantity of electrical encrgy was.

B ------+----r--~\I carefull y measured and the initial and fin al tempera­ tures of the calorimeter were r eproduced as closely as possible. 1+----14-+----- C 5. Results

The results of the electrical calibration experiments are given ill table 1. E is the electrjcal energy added to the system. i1R c is the temperature rise corrected for heat leakage and hea t of stirring and E-----+-----+---UI expressed in ohms increase in resistance of the platinum resistance thermometer as measured on the resistance bridge. E s is the energy equivalent of the calorimeter expressed in joules/ohm .

T A RLE l. Results oJ the electrical calibration ex periments

F I GUR E 1. Calorimetric reaction vessel. E xperim ent num bel' a R c E E, 1------·1------ohm j jfohm 1 ______0.28()106 59088. 3 206526 of electrical energy was determined from the elec­ 2 ______. 28()667 59161. 3 206376 3 ______. 284978 58829. 9 • 206373 trical curren t through the heating coil, the potential L ______drop across the coil and the time of passage of the .2R7584 59334.5 206321 curren t. The curren t and poten tial measuremen ts lI1ean ___ . ______206399 were obtained from the po ten tial drop across appro­ Standard deviation of the mean______± 44 priate standard r esistors included in the power circuit. The poten tials were measured with a W'enner poten · • Tl1is value incl udes a correction of -64. 0 j/ohm because of a ch an ge in tbe tiometer which was balance d against a th ermostated calorimetric systelll . Weston standard cell [3]. Timing of the experiments was accomplished by refere nce to the standard Tbe results of the calorimetric reaction experi­ second signals produced at th e Bureau. All equip­ men ts are given in table 2 where i1Rc is the corrected men t was calibrated in the Bureau laboratories. temperature rise of the calorimeter system, q, the product of E s and ~R c , is the total heat absorbed 4. Procedure by the system , E is the electrical energy added, qg is the correction for the heat capacities of the re- A sample, 0.2 5 to 0. 50 g, of th e finely divided amorphous boron was pressed into a pellet, placed T ABLE 2. R esults oJ the cctlorimetric reaction ex periments in the weigh ed quartz crucible, and the apparatus assembled. A constant stream of helium was passed Experiment bo Rc q E q , Boro n - anf through the system during the en tire experiment. number reacted (250 C ) After th e initial ra ting period , during which temper­ Ohm j j j q kjfmole atures wer e observed at regular intervals, the vessel L ______0. 287323 59303. 2 49172. 7 - 0.4 0. 268 76 407.86 2 ______c__ .302977 0 62534. I 46119. 0 -2.8 . 43684 406. 65 was heated electrically t o abou t 400 C and th e 3- ______.291356 60135.6 47954. 0 - 3.5 . 322 16 409. 24 chlorine introduced. The reaction started and pro­ 4______. 3055.10 63065. 2 44643.5 -5.8 .48890 407.83 ceeded smoothly. When th e desired calorimeter temperature was reach ed th e electric current was interrupted, the chlorine bypassed, and the system 214 ~1C tant and products in converting to the isothermal BCIs liq) + 4003 H20 (liq) = [HsBOs + 1000 process at 25° C, and - t.Rf is the heat evolved by H20] (soln)+3 [HCl+ lOOO H 20] (soln), (6) the chemical reaction at 25 ° C. The mean value for the heat of reaction as given t.H0298.l5=-287.36 ± 1.84 kj/mole, in table 2 was converted to lecal/mole, using 10S2 =-68 .68 ± 0.44 kcal/mole. for the atomic weight of boron and 4.1S40 j as the equivalen t of 1 cal. A correc tion of 0.02 kcal/mole By combining the value recently obtained for the was applied Lo convert the gases to their thermody­ heats of decomposition of diborane and pentaboranp namic standard states. The following heat of [1] with the value from the present investigation and reaction wa s obtained : the heat of formation of gascous hydrogen chloride [6], we obtain for the heats of reaction of diborane B (amorph) + 3/2 C12 (gas)=BCb (gas), (1) and pentaborane with chlorine:

.6.H 02US.15= - 407.9S ± 1.34 lej /mole, B2He (gas)+6C12 (gas) = 2BCls (gas) + 6f-I Cl (gas), (7) = - 97.51 ± 0.32 lecal/mole. .6.Ho298 .15=-139S .00 ± 2.89 k:j/mole, The uncertainty has bee11 takeD as twice the standard =-334.13± 0.69 kcal/mole, and deviation of the mean of the experimental values combined with reasonable estimates of all known B5H9 (gas) + 12 C12 (gas) = 5BCIs (gas)+9 HCl sources of error. (gas), ( ) 6 . Derived Data .6.H 029S. 1S =-2925 .08 ± 3.43 lej /mole, =-699 .11 ± 0.S2 kcal/mole. By the use of the heat of transition of crystalline to amorphous boron as 0.4 kcal/mole (estimated [4]), We have combined the value given fo r eq (2) with we obtain for the heat of formation of boron trichlor­ the followin g valu es for H0298 .15- Hg in kcal/mole : ide from crystalline boron: BCIs (gas) 3.362 [7] B (cryst) + 3/2 C1 2 (gas)=BCls (gas), (2) B(cryst) 0.290 [7] C12 (gas) 2. 194 [S] .6.H ~9S.15=-406 .3 1 ± 1.34 lei /mole, =-97 .11 ± 0.32 kcal/mole. and have obtained - 96.S9 kcal/mole for the heat of forma tion of BCIs (gas) at 0° K. By the combination of (1) and (2) with the heat of The heat of sublimaLion of boron has been calcu­ vaporization of boron trichloride (5 .6 ± 0.1 kcal/mole lated from vapor pressure data [9] to be 133 ± 4 [4]) there is obtained for the liquid: kcal/mole at 0° K. If the heaL of dissociation of C12 (gas) is taken as 57. 08 kcal/mole [8], these data give B (amorph) + 3/2 C12 (gas) = BCb (liquid), (3) for the reaction :

t.H ~98. 15 =-4 3 1.41 ± 1.42 lej /mole, B (gas) + 3Cl (gas) = BCls (gas), (9) =-103. 11 ± 0.3 4 kcal/mole, and .6.H g=-315.51 kcal/mole,

B (eryst)+3/2 C12 (gas)=BCls (liq), (4) from whi ch the average bond energy is calculated to be 105.2 kcal. .6.R ~98.15=-429.74 ± 142 kj/mole, =-102.71 ± 0. 34 kcal/mole. 7 . Prior Investigations

By combination of the value obtained for eq (1) In the folIo wing trea tmen t of the previously re­ with the values recently reported from oLir laboratory ported data , several corrections have been applied to for the heat of decomposition of cliborane (L~ H = reduce the data to a comparable basis. The data for - 6.73 ± 0.52 lecal/mole [1]) , its heat of reaction with the hydrolysis reaction (eq 6) have bee n converted water (.6.H =-l11.46 ± 0.54 kcal/mole [2]) , and with to 25° C virith an estimated tf'mperature coefficient the heat of formation of aqueou hydrochloric acid of - 150 cal/deg mole. The heats of formation of (HCl·1000H20 ; t.Hfo=-39.S0S ± 0.010 Rcal/mole HCl and H sBOs in the resulting solution have been [6]), we obtain for the heats of hydrolysis of boron taken as those of the aqueous acids of the same con­ trichloride: cen tration [6]; this assumes that the heat of mixing of these acids is negligible at high dilutions [10, 11 , BCls(gas) + 400 3 H20 (liq)=[HsBOs+ I000 12] . }' or case in comparison, the tabulated heats of H 20 ] (soln) +3 [HCl+ 1000 H20] (soln), (5) hydrolysis have been corrected to final concentra­ tions of H sBOs· I000[-I20 and HCl·1000H20 . All .6.H0298.15=-3 10. 79 ± 1.84 kj/mole, values are based on the 1956 International Atomic =-74 .2S ± 0.44 kcal/mole, and Weights [13]. 215 Troost and Hautefeuille [14] passed chlorine over tration, give - 69.3 kcal/mole for the heat of hydroly­ amorphous boron and hydrolyzed the resulting BCI3 · sis and - 102 .1 !ccal/mole for the heat of formation In a separate experiment they hydrolyzed liquid BC13 of liquid BCI3• to a solution of the same final composition (H 3B03· Kapustinskil and SamoYlov [19] have measured 3HCl·900H20 ). For the fil"st reaction they report as the heat of solution and hydrolysis of gaseous BCl3 the mea,n of SL\: experim ents: L1H =-lS3.2 kcal/mole at 25° C. Their data give - 90.7 !ccal/mole for the BC13 ; for the second, L1H =-79.2 kcal/mole. No de­ heat of formation of BCl3 (gas). tails are given by Troost and Hautefeuille as to the Skinner and Smith [20] determined the heat of temperature at which they carried out their experi­ hydrolysis of liquid BCl3 at 25° C to be - 69.2 kcal/ ments, however they used the same mercury calorim­ mole, which gives - 102.2 kcal/mole for the heat eter as Favre and Silberman [15] and apparently used of formatioll. tre same conditions. Favre and Silberman also do Lacher, Scruby, and Park [21] measured the heat not specify their working temperatures but there are of reacLion for the gas-phase chlorination of diborane, some indications in their papers that it was about to yield BCl3 and HCI aL about SO O C. We have 20° C . We have assumed this to be the case; thus, corrected this value to 25 ° C and obtained for t he the values obtained by Troost and Hautefeuille reaction : corrected to the specified conditions give ilH=-183.7 6CI2 (gas) + B2H6(gas) = 2BC1 3 (gas) + 6HCI(gas) , kcal/mole for the formation reaction and L1.H =-SO.O L1H = - 342.9 kcal/mole. kcal/mole for the hydrolysis reaction. The standard This leads to - 101.5 kcal/mole for the heat of for­ heats of formation of BCl3 (based on crystalline boron mation of B C!a (gas) . as the standard state) calculated from these data are The values for the heats of hydrolysis and the - 103.3 kcal/mole for the direct combination and - 91.4 corresponding standard heats of formation of liquid kcal/mole for the hydrolysis method. BCl3 are summarized in table 3. The crystalline Considerable doubt has been cast upon these data form of boron at 25 ° C has been taken as the standard from the observations by Berthelot [16] and Thomsen reference state. Values reported for the heat of [17] that Favre and Silberman obtained values for formation of BCI3 (gas) have been converted to the heats of neutralization of hydrochloric, hydro­ correspond to the liquid using 5.6 kcal/mole [5J bromic, hydriodic, and nitric acids by both potassium for the heat of vaporization. It is apparent that and sodium hydroxides that were much higher than the agreement among the results of Berthelot, the accepted values, although exhibiting reasonable Laubengayer and Sears, Skinner and Smith, and concordance among themselves. This suggests that the present investigatioll is good. there was some sysLematic error, probably in their calibration. An examination of their data [15] for these, as well as for other reactions indicates that TABI>E 3. H eals of hydrolysis and formation of BCl3 (liquid) the reported values should be reduced by about 11 percent. If Troost and Hautefeuille used the In vestigators aH h ydrol· same calorimetric system with the same calibration, ysis which they imply, then the same correction should be applied to their data. This would reduce the value 'l'roost and Hautefe uille [14]: keat /mole kcal/mole Direct r e9.ct ion ~ ______-so. 0 - 103.3 for the heat of formation of BCl3 (liq) obtained H y droly s i s ~_~.~ ~~~~ ~~~ _~ _ ~ ~ ~ _ ~~ .. ___ ~ __ _ - 91.4 Direct r C:Jc tioll, corrected ______- 91.9 from the combination of the two reactions to - 9 l. 9 HydrolYSiS, corrocLcd ______. __ .. __ _ - 71. 2 - 100. 2 B er thelot [16]. ______~_ . -68.2 - 103.2 kcal/mole. The heat of hydrolysis then becomes L aubcn gayer and Sears [18] ___ .______- 69. 3 - 102. 1 - 7l.2 kcal/mole, which leads to - 100.2 kcal/mole K a pustinskii and S1moIlo\· [19] ______. __ . - 75.1 -96.3 Skinner and Smith [20] __ ~ _~~~~ _____ . __ . __ _ - 69. 2 - 102.2 for the heat of formation of Jiquid BCI3. This cor­ L acher, Scruby, and l'ark [21] . __ ~~ .. __ _ - 107. 1 rection is admittedly only approximate and the Pr0'so:> nt inn~stigation ______- 102.7 values so obtained are quite uncertain. There is no information regarding the purity of the amorphous boron used by Troost and Hautefeuille; however, the boron available at that time was un­ 8. Discussion doubtedly quite impure. On the other hand, reason­ ably pure BCl3 could be prepared; for t hi s reason Most of the thermochemistry of the boron com­ their data for the hydrolysis reaction should be more pounds involves the heat of formation of boric oxide reliable than that which they obtained from the (or of boric acid); a reliable value for this constant direct combination. is therefore quite important. As we have stated Berthelot [16] obtained - 65.S kcal/mole for the previously [2], values reported for the heat of forma­ heat of hydrolysis of liquid BCl3 at 10° C. When tion of boric oxide range from - 2S0 to - 370 kcal/ corrected for temperature and concentration the' mole. Most of these values were obtained by the heat of hydrolysis becomes - 6S.6 kcal/mole, which combustion of amorphous boron in an oxygen bomb gives - 103.2 keal/mole for the heat of formation calorimeter; the difficulties and unreliability of such of liquid BCI3• experiments have been analyzed [2]. Nathan [22] Laubengayer and Sears [IS] studied the same obtained - 306 ± 3 kcal/mole for the complete reaction at 0° C in an ice calorimeter and reported combustion of boron in an oxygen bomb calorimeter. two values for different final concentrations. Both We have obtained -305.3 ± O.S kcal/mole for the values, when corrected for temperature and concen- standard heat of formation of boric oxide from the 216 heat of decom.position of diborane and the heat of [4] F . D . Rossini, D . D . Wagman, W . II. Evans, S. Levine, a nd 1. Jaffe, Selected values of chemical thermo­ reaction of diborane with water [2]. The latter dynamic properties, NBS Circ. 500 (U.S. Government value is in reasonably good agreement with that Printing Office, WashingLon 25, D.C., 1(52). reported by Roth, Borger, and Bertram [23]. [5] W. J I. Evans, D . D . Wagman, and E. J . Prosen, JB An independent value for the heat of formation of (private communication). [6] W. 11. Evans and D. D . Wagman, NB (private com­ boric oxidc may be obtained from the heat of chlori­ munication) . naLion of bOfOll and the heat of hydrolysis of Lhe [7] \V. ] r. Evans, E. J. Prosen, D. D. V\'agman, Thermo­ rcsulting BOla. The only previous data lcading to chemistry and thermodynamic functions of som e boron a value fol' Lhe heat of formation of BOla that do compou nds, thermodynamic and transport properties of gases, liquids, and solids, in ASME, Sympo ium Oil noL involve boric oxide are those of Troost and HauLe­ thermal propertics (McGraw Hill Book Co., New York, feuille, discusscd above. The uncertainties regard­ IT. Y ., 1(59). ing thei r materials, apparatus, and calibration make [8] \V. n. Evans, T. R. Munson, and D . D . Wagman, J . t hi s value very unreliable. R.esearch KBS 55, ]47 (J 955) RP2614. [9] D . D . vVagman and W. II. Evans, NBS (private com­ vVe have determined directly the heat of formation munication) . of BOla to obLain an independenL value for the heat [10] W . D . Davis a nd G. Stegeman, J. Am. Chem. Soc. of formaLion of B20 a, and to resolve the discrepancy 71,2775 (1949). in the values "eported for the heat of formation of [11] J . Smisko and L . S. Mason, J . Am. Chem. Soc. 72, 3679 (1950). BOla. If the value for the heat of formation of BOla [12] E. R . Van Artsdalen and E:. P. Anderson, J . Am . Chern. obtained in this investigation is combined with the Soc. 73,579 (195]). heaL of hydrolysis reported by Skinner and Smith [13] E. Wichers, J . Am. Chem. Soc. 78,3235 (1956). [20], Lhere is obLained -306.3 lecal/mole for the [14] L. Troost and P. lIautefeuille, Ann. chim. et phys. [5] standard heaL of formation of boric oxide. This is 9, 70 (1 76); Coml?,t. rend. 70, 185 (1870). [15] P . A. Favre and J . r . Silberman, Ann. chim. et phys. in good agreemenL with tbe value obtained from the [3] 340, 357 ( I 52); [3] 36, ] (J 852); [3] 37, 406 (1853). diborane daLa. [\ 6] M . Berthelot, Thcrmochimie, I I (GauLhier-Villars et In view of tile general agreemenL among the values Fils, Paris, ] 897); Ann. chim. et phys. [5] 15, ] 85 obLained by clifIerent meLhods it can b e stated that (1878) . [l 7] J . Thomsen, Thermochemische Untersuchungen, I the data available for tbe heaLs of formation of (V crJag J. A. Barth, Leipzig, ] 882). diborane, boric oxide, boric acid, and boron tri­ [18] A. W. Laubengayer and D . S. Sears, J. Am . Chem. chloride form a consistent scL of values. Soc. 67, 164 (1945). [19] A. F . Kapustin ski! and O. Ya. Samollov, I zvest. Akad. Nauk. S.S.S.R. Otdel. Khim . Kaule 1952, 218 ; Chern Abstr. 46, 9406 (1952). 9 . References [20] E . A. Skinner and N. B. Srnitll, Trans. Faraday Soc. 49, 60J (1953). [21] J . R . Lacher, n. E. Scrllb~T , a nd J . D . Park, J . Am . [11 E. J . Prosen, W . H . Johnson, and F . Y. P ergiel, J. Chern. Soc. 74, 5292 ( l952). Research NBS 61,247 (1958) RP2901. [22] C. C. Kathan, Thesis, University of Pittsbmgh (1948). [2] E. J . Prosen, W . H. Johnson, and F . Y. Pergiel, J . [23] W. A. Roth, E. Borger, and A. Bertram, Bel' 70B, Research NBS 62, 43 (J (59) RP2927. 97 J (1937). [3] E. J . Prosen , F. W. Maron, a nd F. D . Rossini, J. He­ search NBS 40 6,106 (1951) RP2J8J. W ASHINGTON, Januar~T 21,1959.

217 r-.-t)-(,-)-(.~-o_ C I _ C) _U~~t'_ ( I _tl_')~~C)_CI_(I_()_()-"-'C)_ (' _I_C' _O, i Important Notice I

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