THERMODYNAMIC STABILITIES OF H4R404(g) AND H4B607(g)
87 1
Thermodynamic Stabilities of H4B404( g) and H4B,0,(g)
by Suresh K. Gupta and Richard F. Porter Department of Chemistry, Cornell University, Ithaca, N e w York
(Received October 7, 1 9 6 5 )
The molecular species and H4B607 have been identified mass spectrometrically as products of high-temperature reactions in the H-B-0 system. For the reactions 4/3H3B303(g) = H4B404g) and H4B404(g) 4- B&(l) = H4Bd&(g), A H O I ~ O O = ~ K -2.5 f 2.0 and -0.9 f 2.0 kcal/mole, respectively. Heats of formation of H4B404(g) and H4Be07(g) are -392 4 kcal/mole at 298°K and -691 h 5 kcal/mole at 1200"K, respectively.
*
Introduction Gaseous products identified in high-temperature reactions of H20(g) with boron include H3B303 (boroxine) and H3B304(hydroxyboroxine).2a With H2 pressures between 10-lo and atm and temperatures near 1400°K, H3B303 is the major product while H3B304 is lower in abundance by an order of magnitude. Thermochemical data for H3B303 and H3B304 have been reported earlier.2 Infrared spectral data3s4 have been obtained for H3B303(g)which can be isolated at ordinary temperatures for a short period of time. The infrared spectrum of H8B3O3 indicates a six-membered ring structure of alternating boron and oxygen atoms. The inference that H&04 is the monohydroxy derivative is reasonable in view of the reported work on trihydroxy b ~ r o x i n e . ~In this paper the thermodynamic stabilities of species in the H-B-0 system containing more than three boron atoms are reported. Experimental Section The experimental apparatus including the mass spectrometer,6 the high-temperature Knudsen cell assembly, and the gas inlet system2a have been described earlier. The cells which served as containers for boron samples were constructed of molybdenum. These were mounted to a stainless steel inlet tube through which water vapor could be introduced externally through an all-metal valve. The flow rate of H 2 0 was arbitrarily set to maintain a pressure in the cell below about atm. At the temperatures of these exDeriments the gas - effusing- from the cell is mostly The Oven was heated by radiation and hydroge;. bombardment from a tungsten Temperatures were measured with a platinum-
platinum-rhodium thermocouple. Boron samples were in the form of pressed pellets. The use of pellets tended to minimize the contact area between the boron and the molydenum oven. However, after several high-temperature experiments with a single sample, embrittlement of the container was noted, indicating that reaction had occurred between the boron and molybdenum. For certain calculations to be noted later, precise knowledge of the thermodynamic state of boron is not required. I n some cases, however, it was advantageous to note the effects resulting from the addition of excess B203to the system.
Results Mass spectra of the gaseous species produced in the HzO(g)-B reaction were obtained over a range of cell temperatures between 1100 and 1500OK. 11lustrated in Figure 1 are the high-mass regions of the spectra for the fully hydrogenated and fully deuterated products. The latter were observed when D20 was used in place of H 2 0 in the reaction scheme. The ion of highest intensity in the hydrogenated spectra is always a t mass number 83 corresponding to H2B303+,a fragment from H3B303. Ion intensities in the high(1) Work supported by the Army Research Office (Durham) and the Advanced Research Projects Agency. (2) (a) W. P. Sholette and R. F. Porter, J. P h y s . Chem., 67, 177 (1963); (b) R. F. Porter and S. K. Gupta, ibid., 68, 280 (1964). (3) G. H. Lee, W. H. Bauer, and S. E. Wiberley, ibid., 67, 1742 (1963). (4) S. K. Wason and R. F. Porter, ibid., 68, 1443 (1964). (5) D. J. Meschi, W. A. Chupka, and J. Berkowitz, J . Chem. Phys., 33, 530 (1960). (6) R. F. Porter and R. C. Schoonmaker, J . P h y s . Chem., 62, 234 (1958).
V o l u m e YO, A-umber 3
March 1966
872
SURESH K. GUPTAAND RICHARD F. PORTER
Deuterated Spectrum
100-
60 40 80
in110
80 -
100
120
Hydrogenated
I30
140
150
160
170
180 H W 7 +
Spectrum
HAC;
60 -
HE405+ and H,B405+
fragmentation from the same precursor as H3Bs07+ H3B404+ arises from a different parent molecule. The two major species in the high-mass region are assigned the molecular formulas H4B607 and H4B404. Parent ion intensities for H4B607+ and H4B404+are observable but are very small as expected by analogy with the mass spectrum of H3B303.2a The molecular precursor for H3B405' is tentatively identified as H3B404(OH). For the gas phase reaction 4/AB303(g) = H&04(g)
(1)
we may write 110
120
130
140
150
160
170
180
m/e
Figure 1. Mass spectra of high molecular weight species in the H-B-0 and D-B-0 systems.
mass region (Figure 1) constitute only a few per cent of the total. Mass identification was made by counting background peaks from mass 83. Checks on the identification of the highest masses were made by mass calibration with a small quantity of C3F4C14which provided an easily recognizable fragmentation pattern. The number cf boron atoms in each ion group was established from the isotopic structure assuming a normal loB/l1Bratio of 0.25. The number of hydrogen atoms in each species was determined from the mass shift observed on deuterium substitution. The oxygen compositions were then obtained by subtracting the boron and hydrogen mass contributions from the mass number of the ion. Important mass groups for the hydrogenated species in the m/e ranges 109-112, 122-129, 150-154, and 177-182 were identified as H3B404+,a mixture of HB405+and H3B405+, H2B506+, and H3B&+, respectively. The presence of two species in the mass group 122-129 is more evident in the deuterated spectra where the boron isotope structure is clearly resolved. Precursor relationships among the ions were determined by noting variations in ion current ratios as a function of temperature and H2 pressure. For a temperature between 1000 and 1200' the ratios 1 1 g 0 / 1150 and 1180/1124 were constant within the limits 2.2 fi 0.2 and 4.0 i 0.4, respectively, for an ionizing electron energy of 75 v. An obvious variation was observed for the IISO/~III ratio. I n one set of experiments with a fresh boron sample this ratio changed from 0.95 to 0.28 as the temperature was raised from 900 to 1100'. As we note later, the ratio IISO/~III was found also to be sensitive to HzO leak rate. These observations indicate that while H2BsOe+and HB405+are formed by The Journal of Physical Chemistry
where C incorporates a number of factors including ionization cross sections and ion detection efficiency terms. The temperature-dependent factor in C was considered negligible in view of the over-all uncertainties in measuring the temperature coefficients for the reaction. The ionizing electron energy was maintained a t 75 v. As a test of the equilibrium conditions for reaction 1, changes in the relative pressures of H3B303 and H4B404 were noted from ion current measurements as the flow rate of H20 was changed while the cell was held a t constant temperature. These results are shown on a logarithmic plot in Figure 2. The experimental points fall on a curve with a slope close to the theoretical value of 0.75. A series of temperature dependence measurements for reaction 1 is shown in Figure 3. The three sets of data shown were obtained with relative flow rates of HzO in the same range as those corresponding to the conditions illustrated in Figure 2. A rapid drop-off in ion intensities was observed when the temperature of the effusion cell was decreased below about 100O'K. Since this behavior was observed when either boron or B-B203 mixtures were used, it appears that the effect is not due to a sudden change in the thermodynamic state of the condensed phase. Most probably, the reaction between H20 and boron at the lower temperatures is too slow to maintain equilibrium conditions. From a series of five temperature dependence determinations we obtain for reaction 1 a value of AH'12500K = -2.5 f- 2.0 kcal/mole, where the limits are maximum deviations. With a small estimated Aq, of R cal/mole deg and the heat of formation of H3B303 (-291 f 2 kcal/mole), we calculate for the heat of formation H4B404a t 298'K -392 f- 4 kcal/ mole.
THERMODYNAMIC STABILITIES OF H4B404(g) AND H4BaO&)
873
‘5.5
t
I
For evaluation of the thermodynamic stability of H4B607it was convenient to consider the reaction H4B404(g)
+ BzOa(1)
= H4BsOv(g)
PH4BdOd(aBlOs)
=
10.0
9.5
9.0
(IH~B~o, +)
+I
(IHsBIO~ (UBlOs)
1.01
0
I
7.0
I
+
8.0
l
I
I
9.0
(3)
For this reaction P%Bs07
8.5
x 104
Figure 3. Temperature dependence data for the reaction ‘/sHaBaOa(g) = H&B,Ol(g).
lo
llll Figure 2. Dependence of ion intensity of HaB404+ (m/e 111) on ion intensity of HzBsOa+ (m/e 83). The solid line defines a theoretical slope of 0.75 for the reaction P/8H3BaOs(g)= H4B404(g). Cell temperature was 1380°K.
=
8.0
f
lu;O 2 0 -
Keq
7.5
7.0
10.0
1
11.0
I .O
104
Figure 4. Temperature dependence data for the reaction BzOa(1) = H ~ B B O ~ ( Ion ~ ) . intensities were H4B404(g) observed for H3B404’ and HsB607+ a t m/e 111 and 180, respectively.
+
(4)
where C contains the relative ionization cross sections is similar to that for H3B303 where the major ion fragof H4B607and H4B404and ion detection efficiency ment, aside from HzB303+, is HB2OZ+. terms. In this case only ratios of ion intensities are Although the structures for H4B404 and H4B& are needed. According to eq 4, I H ~ B ~ ~ +depends /IH~B,o,+ unknown, plausible configurations may be inferred. on uB2oa,the thermodynamic activity of Bz03 in the Cagelike structures are possible but it seems more condensed phase. For a fixed cell temperature, an reasonable to assume that these molecules are derivaincrease in IHaBa@+/IHsB,04 + was observed over a period tives of H3B303, Likely configurations for H4B404 of time as boron oxide is produced in the cell by reand H4B607are action of boron with water vapor. For temperature dependence measurement the flow of HzO was adjusted to give the highest value in the intensity ratio. The data shown in Figure 4 were taken under these conditions. The temperature coefficients give substantially the same values of AHoas that observed in a separate experiment in which B203 had been added to the cell. For either set of conditions only a very slight temperature dependence was noted. From the data = -0.9 f 2.0 kcal/ in Figure 4 a value of AHOI~WOK mole is obtained for reaction 3. By combining the heat for reactions 1 and 3 with the J A N A P data for H3B303,we obtain for H4B607 a heat of formation of respectively. I n the organic sense H4B607 would be -691 i 5 kcal/mole at 1200’K. structurally analogous to diphenyl ether and H4B404 Discussion would be somewhat similar to methyl phenyl ether. The low-mass ion fragments from H4B607 are separated by 28 mass units, which is equivalent to an (7) “JANAF Thermochemical Data,” Thermal Laboratory, The HBO group. In this respect the fragmentation pattern Dow Chemical Co., Midland, Mich., 1960. ~
~
~
~~
Volume 70, Number 3 March 1966
R. A. KENT,J. D. MCDONALD, AND J. L. MARGRAVE
874
The molecules need not have over-all planar symmetry but planarity would probably be retained in the individual boroxine rings. We can visualize the formation of H4B607 as elimination of a H2 molecule in a reaction of H3B303 at a B-H bond with HzB303(0H) at the 0-H bond. Similarly, H4B404 could be formed from H3B303 and HzB(0H) (hydroxyborane) .* Ion currents of HBO+ and HBOH+ are always observable in these experiments but, because these ions are formed by fragmentation from a number of precursors, it is difficult to assess the importance of the molecular contribution of HBO and HzBOH. These species may be
important in the kinetics of reaction as precursors to boroxine. The pressure dependences of reactions 1 and 3 show that the equilibrium concentrations of H4B404and H4B60 7 should increase relative to H3B303 as the pressure of H3B303 increases. Relatively high concentrations of H4B404 and H4Be07should be possible in a static system where the partial pressure of Hz over B-Bz03 mixtures is of the order of 1 atm at temperatures near 1400'K. (8) R.F. Porter and S. K. Gupta, J . Phys. Chem., 68, 2732 (1964).
Mass Spectrometric Studies at High Temperatures.
IX.
The Sublimation
Pressure of Copper(I1) Fluoride
by R. A. Kent, J. D. McDonald, and J. L. Margrave Department of Chemistry, Rice University, Houston, Texas 77001 (Received October 8, 1965)
Mass spectrometric studies of CuFz sublimation from a Knudsen cell have established CuFz(g) as the major vapor species and AH"~~~[sublimation] = 63.9 f 1.0 kcal mole-'. The dissociation energy of CuF(g) is 87 f 9 kcal mole-' (3.8 f 0.4 ev).
I. Introduction have reported mass Previous papers in this spectrometric and microbalance measurements of Knudsen and/or Langmuir vaporization and/or sublimation rates for various transition metal fluorides. In this investigation, the sublimation rate of CuFz has been measured, and the vapor species have been identified by the Knudsen technique employing a mass spectrometer.
11. Experimental Section The mass spectrometer has been described previ0us1y.~ Temperatures were measured with an infrared pyrometer as described previously. The CuFz sample, 99.8% purity, was generously provided by Professor J. W. Stout of the University of Chicago. The JOUTnd of Physical Chemistry
Choice of the Knudsen cell material presented a slight problem inasmuch as CuFz has one of the weakest metal-fluorine bonds of any of the first-row transition metal fluorides, and, thus, CuFz is an effective hightemperature fluorinating agent. Preliminary experiments showed that CuFz reacted with tantalum at high temperatures to produce Cu(s) and TaFs(g). Finally, the CuFz was contained by using an MgO (1) R. A. Kent, T. C. Ehlert, and J. L. Margrave, J . Am. Chem. SOC.,86, 5090 (1964). (2) T. C. Ehlert, R. A. Kent, and J. L. Ahfargrave, ibid., 86, 5093 (1964). (3) R.A. Kent and J. L. Margrave, ibid., 87,3582 (1965). (4) R. A. Kent and J. L. Margrave, ibid., 87, 4754 (1965). (5) G. D.Blue, J. W. Green, R. G. Bautista, and J. L. Margrave, J . Phys. Chem., 67,877 (1963).
