1562
Vol. 66
NOTES
dunng the warmup period. The argument of competition suggests that the upper limit of the titers be used. This yields a reduction/oxidation ratio of 5/4 (or roughly l/l), while average values (for the runs in which the reagent was added in bulk and which meet the significance level cited above) lead to a 211 ratio. These results seem to rule out a chain reaction decomposition of hydrogen peroxide, and, coupled with the high mole fractions, a direct correlation with the species causing paramagnetism. On this basis we are led to suggest that the species is molecular. The isomeric peroxide postulated by Geib and Harteck7 fits the 1/1 ratio. and the quasimolecular H203is coiisistent with 211. No direct relation bet.cveen oxygen evolution and the difference in titers was found. However, the few cases in which oxygen evolutioii was measured concurrently with the oxidation of the Eroaen samples show a curious effect. The total oxygen values may be attributed to the oxidation reaction. Were the evolved oxygen independent of the cr.0genic species an additional 0.6 mmole would be expected. The cause of this deficit during oaridation is not clear, further studies being indicated. Acknowledgment.--W'e are indebted to Professor Clark h1. Bricker and Dr. John D. 11IcMinley, Jr., for many helpful suggestions. H. 31. G. thanks the National Woodrow IVilson Fellowship Foundation for its support during the period of this work. The support of the V.S. Air Force Office of Scientific Research under Contract AF 18 (603)-134 with Princeton Univer4ty is gratefully acknowledged.
HEAT OF REACTION OF FLUORINE WITH GRAPHITE1 BY RICHARD P. PORTER^
ASD
DAVIDH. SXITH N. Y .
Department of Chemistry, Corndl UnLuerszty, Ithaca, Recezved March 10, 1962
Fluorine is known to react with graphite to form condensed compound^.^ Thermochemical data for the reactions are not generally available. At temperatures between 500 and IOOO'K., CFaCCl,(g) reacts XT-ith graphite with the elimination of one F atom and one C1 atom to form CF2= CCl,(g).* Since this reaction appeared to provide a suitable means for determining the heat of reaction of F2(g! with graphite, a further quantitative examination of the reaction was undertaken. Experimental The experimental procedure has been described.? Reactant CF&Cla(g) was passed into a graphite effusion cell packed with small chips of graphite. Reactant and product gabes leaving the cell Rere then analyzed inass spectrometricallv. The leak rate was regulated to provide a pressure of CFS-CCl, between about and atm. within the cell. At these pressures the gas is assumed to obev Knudsen flow conditions. A small impurity of CFZClCFClz m7as present in the reaction gas and it was therefore
tieceswy to observe a fragnieiit ion, CCl,', as a rnea8ure of the CFa-CCls(g) rather than the C,F,ClQ+ ion which is common to both 18onm-s. For CFa=CCh, the CJ?2C12+ ion was observed.
Results and Discussion Pressure dependence data are shown in Fig. 1. The curve shows a simple first-order pressure drpendence at low pressures, but tends toward :+ lower order a t high pressures. The stoichionwt of the reaction is best described by the equation
+ graphite --+ CFZ=CCl,(g) + (F + C1)graphite
CF,-CC&(g)
where the last term in the equation represents F and C1 atoms chemically bound to graphite. Second law heats for reaction 1 were obtained for a series of flow rates. The quantity log I c ~ F ~ c ~ ~ + , I ' I c c ~ ~ which is proportional to log Kea is plotted 2's. 1 T in Fig. 2. Reliable values of AHlocould only be obtained when the operating pressure of CYdCClR was in the range corresponding to the firstorder region (see Fig. I). During these experiments, an increase in C2F2C12+intensity Tvith increase in cell temperature was followed by a decrease in CC13+ intensity, indicating that the total pressure was remaining nearly constant. From these determinations, m-e obtain Awlo = 18.7 2.0 kcal. 'mole reactant in the temperature range 840-1140'K. A trend toward higher values (2028 kcal.) was observed for higher flow rates of CF3CCL. These data were not considered significant since the reaction apparently is not at equilibrium under these conditions. This presumably is the reason for the high d u e (29 kcal.) originally reported. The reaction of the isomer CFCl2-CF2C1 x i t h graphite was also investigated. In this case the reaction a t low pressure of CF2C1-CFCl2 is represented by the equation 1,
*
CF2Cl-CFG12(g)
+ graphite +CF2=CFCI(g) + 2Cl(graphite) ( 2 )
For temperature dependence measurements, intensities of C2F3C1+and CFC12+were observed for product and reactant, respectively Second law data for this reaction also are illustrated in Fig. 2. To obtain the heat of reaction of fluorine with graphite, we consider reactions 1 and 2 and the subsequent thermochemical data Reaction I : ANin= 18.7 rt 2.0 kral. 'mole (this work) Reaction 2: AN20 = 10.5 + 1.5 kcal./'inole (this woi k)
Clz(g)
+ CF,=CFCI(g)
+Cl?&-CFC12(g)
AN30 = -48.8 kcal. 'moleb
_____
(1) Supported by the Advanced Research Prolects Agency ( 2 ) Alfred P Sloan Fellow (3) W. Rddorff and G. Rhdorff, Chem. Bar., 80, 417 (1947) (4) D. R Bzdtnesti find R. F. Porter, J . Am. Chern. S e a , , Ea, 3787 (LBAl).
(1)
Cl,(g)
(3j
+ CF2=CC12(g) AH40
=
--+ CF*Cl-CCl?(p) (4) -41.1 kcal./mole5
Combination of AH20 and AHaogives for the reaction
August, 1962
1563
NOTES
(CF3-CCI3) = 46.8 ked. Combining this latter quantity with values of AH8 and A116O, we obtain for the reaction i-
~
Fp(g) -5 graphite -+ 2F(graphite) AHy0 = - 100.2 kcal. hiole FZ
(7)
The uncertainty in AH,O is estimated to be aboul =t6 kcal., and results largely from the use of empirical bond energies in the computation. In view of this, heat capacity corrections to AHI0and AH2O were considered insignificant. The availability of experimental heats of formation of CF3--CC13and CF2=CC12 should diminish the uncertainty. In the reaction of CF2C1-CFC12 with graphite, the only observable product in addition to CF2= CFCl was HCl(g). This is attributed to a reaction with hydrogen in graphite, which can diffuse into the reaction zone. The pressure of HF(g) in the reaction of CF3-CC13with graphite, however, was small compared to that of HCl(g) (as noted by the Fig. 1.--Pressure dependence dztta for the reaction of CF'?-CC18(g)with graphite. Electron energy = 50 v., re- HF+/€ICl* ratio). Thus it appears that most of the reacted fluorine remains in the graphite. It :wtion temperature 91 5°K. should be noted that the total amount of CFJCCl,(g) that has reacted during the course of the experiment was less than 10-3 mole, although a t least a mole of solid carbon was available for reaction. Since so little fluorine is finally present in the graphite, we cannot demonstrate whether a new phase such as C4F has been formed. For our purposes, it is perhaps only meaningful to consider that the reaction involves the bonding of fluorine atoms to active sites in the graphite, or to active -0.2 surface sites. Comparison of these data with 0.61-0.4 calorimetric heats may provide information that will allow us to distinguish between a surface re-0.6 N+ action and one involving internal sites. --OB It should be noted that reaction 1 must have a positive entropy change of the order of 20-25 e.u. -4.0 -g 0.0a t 1000°K. This probably is due in part to -0.2break-up of the graphite structure on the bonding of fluorine and chlorine atoms. The ease of removal -0.4of an F atom in reaction 1 must reflect the ease of -0.6removal of chlorine on the adjacent carbon atom. If we assume that the rate determining step is the -0.8transfer of a chlorine atom to graphite, an upper * , -''oO.B . L limit of 30 kcal. for the activation energy is ob1.0 1.2 1.4 1.6 1.8 tained by combining AH6O with a dissociation energy fO000). of C1, of 58 kcal. and a bond strength of 78 kcal. in Fig. 2.-Temperature dependence data for the reactions chloroethanes. of Cli'a-CCl,(g) and CF2Cl-Cl?C12(g) on graphite. Ionizing Acknowledgrnent.-'We wish to thank Professor electron energies were 50 and 75 v., respectively. A small correction has been applied to the intensities of C2F2C12+ W. T. Miller for samples of CF3-CClB and CF2C1and C*F&l+ due to ion fragmentation of react,ant gam. CFClz used in this work. Squares and! circles represent independent sets of data.
o.8t
I
I
+
2Cl:graphite) -+Clz(g) graphite ( 5 ) A H 8 = $38.3 kcal./moIe Combination of data for reactions 1 and 4 gives for the reaction
+
+
CFd!CIJ(g) graphite Cl,(g) + CFZCl-CC13 (F C1)graphite AHeO = -22.4 kcal./mole
+ +
-
(5) J. R. Lacher, J. J. McKinley, C. Talden, K. Lea, and J. D Park, J. Am. Chem. SOC., 71, 1337 (1949). (6) C. R. Patrick, "Sdvances in Fluoline Chemistry," Vol. 2, Butterworthi, 1961.
THE VAPOR PRESSURE OF GERMS1\JIITAI TELLURIDE' BY
(6)
CHIKARA HIRAYAM-4
Westznghouse Research Laboratorzes, Pzttsburgh 86, Pennsglaanla Received March 28, 18622
Heats of formation computed from bond additivity The germanium-tellurium system and the therCQnsid@ratjOnB6 givg. AHfo (CFa-CICC&) A l l t o moel~~trio prapertisa ~f germanium telluride have
