Heat of Formation of Decaborane Walter H

JOURNAL O F RESEARCH of the National Bureau of Standards- A. Physics and Chemistry Vol. 64A. No. 6, November- December 1960 Heat of Formation of Decaborane Walter H . Johnson, Marthada V. Kilday, and Edward J. Prosen (August 8, 1960) The heat of formation of crystall in e decaborane has been deter mined by calorimetric measlIrement of the heat of decompoF ition: B lOTI j4(c) ...... lOB (am) + 7H2 (g), MT(25 °C) = S2.8 ± 5.9 kj/ mole, (l9.8 ± 1.4 kcal/mole). With t he heat of t ransition of crystalline to flmol'phollS boron taken as 0.40 kcal/mole we obtain for t he heat of formation of decaborane : ' M-Tf; ••. I0= - 66.1 kj/ mole, (- J5.8 keal/mole). 1. Introduction bustion of deeaborane. The heat of formation of decaborane [2] was tal<:en as 8 kcal/mole, calculated Combustio n meLhods afford a con venien t and from his results. precise meLhod fo1' determination of the heats of The decomposition of crystalline decaborane into formation of a large number of compounds. In the amorphous boron and gaseous hydrogen proved t o case of decaborane, however, preliminary measure be the best m ethod for calorimetric determination of ments of t he heat of combustion of dccaborane llave the heat of formation. The results of this investi been found Lo be vcry unreliable. CombusLion ex gation are presented here. perimen Ls in an oxyge ll bomb failed to yield resulLs of t he required precision ; the sample detonated in 2. Source and Purity of Materials practically all cases and the combusLion products in chlded varying amounLs of elemenLal bo[,on and The cl ecaborane was obtained from the General higlw r bo],on h:vdricies. There was also some 1111 (,e'1' E lectric R esear ch Laboratory through th e courtesy tainty in the amount of dissolved boric acid and the of A. E. Newkirk and A.L. Marshal1. Jt was prepared possibility of t he presence of boron oxides. In ge ll by th e pYl'olysi of diborane and purified by several eral, the eXpel'inlents we're performed with the deca v:w uum sublimations, the last two of' which produced bo1'an e sample' in tbe form of a pcllet. I n some materials wi tIt no . measUl'able differ ence in m elting cases, in which the sample was burned in Lh e form point. Prio], to this investigation it was further of loose crystals or of crysLals intcrmixed wi tl) quartz purifi.ed b.v Lwo successive vacuum sublimations and fib ers, t he combustion residue also illcluded deca stored in a desiccator over magnesium perchlorate. borane. In other experimen ts a solu tion of deca A m ass spectrometric analysis of the helium2 used borane in toluene was burned ; the resulting quan tity showed it to contain less than 0.01 volume percent of carbon dioxide, however, indicated the formation or ail' ; the helium was further treated as described eitber of elemental carbon or of boron carb ide. m t he n ext section of this paper. The quantity of boric acid found in the watel' sohlble portion of the combustion products varied from 75 to 95 percent of the theoretical amount cal 3. Apparatus culated from the mass of sample. The resulting heat of combustion of decaborane (to crystalline boric The isothermal jacket calorimeter has been de oxide and liquid water) was - 2004 kcalfmole. A scribed previously [3]. The calorimetric system reason able estimate of the probable errol' in the beat consisted or the niclcel-plated copper calorimeter can of combustion of decaborane, as determined by the and covel' into which was placed a weighed quantity oxygen bomb process, is about 1 percent. In view of water , the calorimetric reaction vessel, and the of this information, it is obvious that the heat of platinum r esistance thermometer. The apparatus for formation calculated from the heat of combustion is measuring calorimeter temperatures and electrical subject to a relatively large un certainty. energy have been described previously [3, 4, 5]. Liebhafsky [1]1 also determined the heat of com- 2 Tbis analysis was perform ed by V. H . Dibelcr of the Mass Spectrometry 1 Figures in brackcts indicate the literature references at the end of tbis paper. Scction of the Atomic and Radiation P hysics DivisiolJ. 521 A diagr am of th e calorimetric decomposition vessel is given in figure 1. The vessel consisted of a quar tz tube, wound with a 183-ohm nichrome heating coil , surr ounded with a silver r adiation shield, :wd a vacuum jacket. The decaborane sample was placed t u in the Vycor crucible at the lower end o( the quar tz tube. H elium entered at th e top t hrough a coarse capillary immediately above the vessel, fl owed around the crucible and out through t he glass h eli x. A nichrome wire attached to the crucible was led out of th e vessel through the inlet tube and fastened to a circularly ground stopcock in the helium line. By turning the stopcock, the crucible could be raised without interrupting the £l ow of helium or permitting air to enter the vessel. An aluminum cover (no t shown in the diagram) which fi tted over the top of the vessel served to preven t loss of energy by radi ation through the opening in the calorimeter can cover. A di agram of t he gas train is shown in figure 2. The helium was purified by passing it successively ovel· titanium, copper , and copper oxide in a Vycor tube which was heated to 600 DC and through absorbers con taining Ascarite, anhydrous magnesium perchlorate, and phosphorus pen toxid e. The exi t gases wer e passed successivel y through a capillary flowmeter, a trap cool ed with liquid nitrogen, a Vycor tube con Laining copper oxide at 600 DC, two water-absorption tubes, E'ac h con taining magnesiu m FIG URE 1. Decomposition ve.ssel. HELI UM CAL ORIME TE R FIGURE 2. Ga s train used in decomposition of decaborane. 522 perchlorate and phosphoru pen toxid e, and a r e period. Calorimeter temper atures wer e observed iLt cording gas-volume meter. i-minute in tervals during the en tire 1-houl" "r eac The capillary flowmeter ser ved for the approxi Lion " peri od. The helium flow was then directed to miLte adjustment of the ra Le of How of helium and by-pa s the decomposition ve el ~tncl Lh e r ecording also to indica te the l"<tte of Lh e decomposition process. gas-volume meter, and calorimeter temper ature The cold trap Wfl,S included to collec t any traces wer e ob erved at 2-minute intervals during a fin al of vol atile boron hydride . The hydrogen evolved 20-minute m tin g period. in the decomposition wn. burned to water in the After the experimen t the decomposition vessel copper-o x.ide heater and ciLrried, as vapor, into the \vas r emoved from the calorimeter iLnd di sassembled. weighed absorption tube by th e str eam of dry helium. Al though most of the boron r emained in the crucible, The portion of the water which collected as liquid iL significiLn t amount was transported to the walls of in the bulb b elow the heater was evaporated by the vessel. It was necessary to place a plug of glass warming the bulb sligh tly. The second absorption wool in the helix in order to prevent some of the t ube ser ved only to protect the first from moisture boron from being carried out of the calorimeter by in the atmospher e. The total quan tity of helium the helium and hydrogen. Some small crystals of was m easured by means of the r ecording gas-volume decaborane, apparen tly formed by sublima tion, were meter at the exit end of the gas triLln . The approxi found in the upper part of the vessel. These usually mate temperature of the helium which entered the became di slodged a nd fell into the crucible during system dUTin g the experiment was m easured by removal of th e crucible from the vessel. It was means of a thermometer placed in con tact wi th the ther efor e no t possible to collect, quantitatively, all inlet tube of the decomposition vessel. of the boron produced in the decomposition. The quan tity of decaborane decomposed was cal 4. Procedure culated from the weigh t of hydrogen, as water, pro dllCed by the process. The accuracy of this deter The decabol'fLl1 e sample wa.s pl aced in the weighed mination WiLS checked by dissol ving a weighed quan Vycor crucible and melted by h eating wi th CiLr e in tity of zin c in dilute sulfuric acid while a slow str eam iL belium atmospher e. When cool, t he crucible plus of h elium was bubbled tlu-ough the solution and sample was weighed, transferred to the decompo directed through the gas train. The water vaporized sition vessel, and the vessel assembled. The vessel from the solution WiLS retained in the cold trap ; the was placed in th e calorimeter can, which con tain ed mass of water produced by burning the hydrogen 5,653 g water at 25. 0 °C. The calorimeter assembly agreed within 0.2 percen t wi th the theoretical was lower ed into the water bath, the temperature quan ti ty calculated from the mass of zinc.

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