C. A. GOY,A. LORD,AND H. 0. PRITCHARD
1086
Kinetics and Thermodynamics of the Reaction between Iodine and Fluoroform and the Heat of Formation of Trifluoromethyl Iodide
by C. A. Goy, Allan Lord, and H. 0. Pritchard Centre for Research i n Experimental Space Science, York Univereity, Toronto, Canada (Received September 28, 1966)
+
+
The equilibrium CFPH I 2 $ CF3I H I has been studied in the gas phase over the temperature range 653402°K. The heat of this reaction is calculated to be = 17.10 f 0.17 kcal/mole. Assuming AHr02ss(CF3H, gas) = - 165.1 kcal/mole, this gives AHf029~ (CF31, gas) = -139.4 kcal/mole. The equilibrium is established via the reaction I CF3H HI CF3, which has a rate constant k = 1013.6exp[(-36.3 3)103/RT] cc mole-' sec-' over the temperature range 589-672°K. --f
+
*
In this paper we report measurements on the iodination of fluoroform which follow very closely our previous experiments on the iodination of methane. An over-all equilibrium CF3H
__
+ I:,
CFJ
+ HI equilibrium constant =
KT
is set up via the reaction sequence
I2
I
+ I equilibrium constant = K , I + CF3H H I + CF3 ki
k2 ka
CF3
+ I2 J'CF3I + I kc
Since kB has zero activation energyJ2 the over-all activation energy for the formation of CF31 will be 1/2D(12) E'. This work was complicated by the fact that commercial fluoroform contains about 0.2% CF3Cl, which catalyzes the formation of CF3I through the formation of C1 atoms. Thus, using the impure fluoroform, the over-all activation energy was about 40 kcal and rose to a value around 55 kcal as the CF3Cl content was successively reduced.
+
Experimental Section The experimental procedure used was identical with that used previously in the methane experiments (with the exception that in the kinetic runs a 1200-ml Pyrex vessel was used to increase the product yield somewhat). The Journal of Physical Chemistry
+
Known amounts of CFaH (5-10 cm pressure) and I2 (0.25-1.0 g) were sealed in the reaction vessel and heated for the required period of time (3-350 hr for equilibrium runs, 3-135 hr for kinetic runs). The reaction was terminated by pumping the contents of the vessel into a trap containing ICN and converting the H I to HCN. The products were analyzed gas chromatographically. Fluoroform was obtained from Matheson of Canada Ltd., and after removal of water and C02, mass spectrometer analysis showed it to contain 0.2% CF3C1. This was removed by passing the gas repeatedly over sodium heated t o 350" and after about 60 purifications the CF3C1 content became unobservable, Le., less than 5 PPm. Iodine was Baker's Analyzed reagent grade and was purified by sublimation in vacuo. No chlorine could be detected in the product by mass spectrometer analysis. Equilibrium Measurements. In our previous experiments on methane, the equilibrium constant was calculated from the ratio [CH3II2/[CH~] [Iz]. I n the fluoroform experiments, however, the yields of products were much smaller and it was found that the CF3I and H I (Le., HCN) were never equivalent. There appears to be a mechanism whereby the surface causes a small but variable loss of HI. Consequently, the equilibrium constants, K T Jwere evaluated as the ratio (1) C. A. G o y and H. 0. Pritchard, J . Phys. Chem., 69, 3040 (1965). (2) J. C. Amphlett and E. Whittle, Trans. Faraday Soc., 6 2 , 1662 (1966).
REACTION BETWEEN IODINE AND FLUOROFORM
1087
0.0
[CFII][HCN]/ [CF3H][It]. Since the analysis of HI as HCN is not as reliable as the analysis for CF31 and because of the much smaller quantities of products obtained, the results presented in Table I have a scatter about ten times larger than the corresponding methane results. The van't Hoff plot of these equilibrium constants leads to a value of AHoZg8 of 17.01 kcal/mole, which is in fortuitous'y good agreement with the more reliable value derived in Table I by third-law methods. It is known3 that CF3 radicals attack glass and quartz to give SiF4, but in the present system, owing to the large concentration of I2 present, this did not present Table I:" The Equilibrium CFaH
+
A[-(F'T
T, OK
12
F? CFJ
-0.5
2
-1.0
3
-1.5
+ HI
- H0as)/T1,
os1 mole-1
AH0298,
Log K T
deg-1
cal mole-1
-4.7688 -4.9371 -4.9928 -4.8934 -4.9150 - 5.1458 -5.1645 -5.2647 5,4513 5.5002 5.6479 -5.5950 -5.6438 5.5997 -5.9060 -5.8659
-0.530 -0.560 -0.570 -0.595 -0.600 -0.618 -0.645 -0.665 -0.695 -0.710 -0.730 -0.750 -0.760 -0.780 -0.820 -0.825
-2.0
802 787 787 765 762 751 737 726 714 708 698 688 684 672 655 653
-
-
17,082 17,346 17,396 16,668 16,692 17,231 16,953 17,015 17,309 17,316 17,525 17,098 17,140 16,702 17,172 16,997 Av 17,103 Std dev A168
a Free-energy functions for CFaH, 1 2 , and HI taken from ref 4; for CFaI from P. R. McGee, F. F. Cleveland, A. G. Meister, and C. E. Decker, J. Chem. Phys., 21, 242 (1953). Further details of experimental conditions, etc., can be found in A. Lord, Ph.D. Thesis, York University, Toronto, 1967.
any serious difficulty. The production of SiFl was only noticed above 770"K, resulting in the progressive loss of CF31with time (about l%/hr a t 800°K); however, when the experiments were continued for just long enough to establish equilibrium (3 hr a t SO2OK), constants consistent with those found a t lower temperatures were obtained. The heat of formation of fluoroform given in the "JANAF Tables" is AHr"zs8 = - 165.1kcal/mole. Our measurements show that there is a substantially larger difference in heat of formation between CF3H and CF3I than between CH4 and CH3I and we find AHrozgs (CF31, gas) = -139.4 kcal/mole, subject to the same
I
I
I
I
1.50
1.60
1.70
101/T.
Figure 1. Arrhenius plot for the reaction I HI CFI.
+
+ CFaH
-c
uncertainties as AHrozgs(CF3H,gas) with respect to the heat of formation of HF.496 Kinetic Measurements. The results of 15 kinetic experiments carried out in the temperature range 589-672°K give an Arrhenius plot for the rate of formation of CF3I having an over-all activation energy of 54.5 f 3 kcal/mole. Removing the temperature dependence of the I atom concentration4 from the equations through K , (assuming perfect-gas behavior) gives k~ = 1013e6exp[(-36.3 f 3)103/RT] cc mole-' sec-l; the results are shown graphically in Figure 1. A rather high frequency factor of 1014.95was obtained for the I CH4 reaction and after allowing for a factor of 4 in the number of available hydrogens and a factor of 2 in the ratio of the collision frequencies, one might expect to get a value of A for I CF3H of around lo1* cc mole-' sec-l if the same factors are operative. The activation energy, E2, of the reverse reactions is 0.5 kcal/mole in the temperature range 333-523°K. Assuming Ez is independent of temperature, we have D(CF3 - H) = 35.8 D(H - I) = 106.8 a t about 625"K, or about 105.6 kcal/mole a t 298°K. This is
+
+
+
(3) G. 0. Pritchard, H. 0. Pritchard, H. I. Schiff, and A. F. TrotmanDickenson, Trans. Faraday SOC.,52, 849 (1956). (4) "JANAF Interim Thermochemical Tables," Dow Chemical Co., Midland, Mich. (5) J. D. Cox and D. Harrop, Trans. Faraday SOC.,61, 1328 (1966). (6) E. Whittle, personal communication.
Volume 71,Number
4 March 1967
C.A. GOY,A. LORD,AND H. 0. PRITCHARD
1088
Table 11: Dissociation on Energies Calculated from Pairs of Opposed Reactions Diseooiation Reaction
(a) (b) (c) (d) (e) (f)
CH3 CH3 CHa CHa CHI CF3 ( 9 ) CFa (h) CF3 (i) CF3
Efarwsrd,
Ereverse,
energy (calcd), kcal/mole
kcal/mole
kcal/mole
at 298OK
+ HZ& CH4 + H + HC1+ CH4 + C1
+ HBr + CHd + Br + HI e CHI + I + Iz CHaI + I + Iz CFJ + I + HCl a CFsH + C1 + HBr + CFSH + Br + HI & CFaH + I (j) CFa + CH4 Ft CFsH + CHa (k) CF3 + CHaBr & CFaBr + CH3 (1) CF3 + CHJ CFJ + CHa
10.Oh W2.1j 1.4'
1.3' O*
;=?
'
7.4' 3.8& 17.gm** 35.0' 19. Snip 17.6g 8.4j 21.1' 36.3"
Oe"
5.1' 2.gc 0.5'
N102 -105 103.5 103.2 54.3 53.5 106.3 106.0 105.6 102.5 -72 -59
'
'
a This work. Ref 1. Ref 2. Ref 3. Ref 6. Ref 7. Ref 9. E. Whittle and E. W. R. Steacie, J . Chem. Phys., 21, 993 (1953). J. W. S. Jamieson and G. R. Brown, Can. J. Chem., 42,1638 (1964). j R. J. Cvetanovic, F. Raal, and E. W. R. Steacie, &id., 31, 171 (1953). H. 0. Pritchard, J. B. Pyke, and A. F. Trotman-Dickenson, J. Am. Chem. Soc., 77, 2629 (1955). G. C. Fettis and A. F. Trotman-Diekenson, J. Chem. SOC.,3037 (1961). G. C. Fettis, J. H. Knox, and A. F. Trotman-Dickenson, ibid., 59, 1609 (1963). E. O'Neal and S. W. Benson, J. 4177 (1960). P. Corbett, A. M. Tarr, and E. Whittle, Trans. Faraday SOC., M. I. Christie, PTOC.Roy. SOC. (London), A244, 411 (1958). * hl. C. Flowers and S. W. Benson, Chem. Phys., 36, 2196 (1962). J. Chem. Phys., 38, 882 (1963). A. M. Tarr, J. W. Coomber, and E. Whittle, Trans. Faraday Soc., 61, 1182 (1965). R. E. Dodd and J. W. Smith, J. Chem. SOC.,1465 (1957). ' P. B. Ayscough, J. C. Polanyi, and E. W. R. Steacie, Can. J . Chem.,33, 743 (1955). W. G.Aleock and E. Whittle, Trans. Faraday Soc., 61, 244 (1965). " G. 0. Pritchard and R. L. Thommarson, J. Phys. Chem., 68, 568 (1964). D. M. Tomkinson and H. 0. Pritchard, &id., 70, 1579 (1966).
'
O
u,
in excellent agreement with the values obtained' from the bromination (106.0 kcal) and chlorination (106.3 kcal) of fluoroform, but the limits of error on our determination are unfortunately large. It seems therefore that a value of D(CF3 - H) of 106.0 kcal/ mole can now be accepted with some confidence. Our equilibrium measurements give the heat of formation of CF31relative to that of CFaH and also the heat of formation of CzF6has recently been quoteds relative to CF3H. Hence we may calculate D(CF, - I) = 53.7 kcal/mole and D(F3C - CF3) = 97 kcal/mole, both independent of any uncertainty in the heat of formation of HF; the former value is very close to that obtainedg-'0 from a study of the pyrolysis of CF3I (53.5kcal) and the latter is consistent with recent shocktube studies" on the dissociation of CzF493 =t4 kcal).
Discussion There has been considerable confusion over the value of D(CF3 -- H) in recent years, with values ranging from 102 to 109 kcal being calculated from the difference in activation energy of pairs of opposed reactions. The present situation in respect to pairs of opposed reactions is summarized in Table 11. The table seems t,o divide into two parts those reactions where one species involved is an atom and those in which radicals The Journal
of
Physical Chemistry
are involved on both sides of the equation. Reaction pairs a-d give reasonably consistent values for D(CHI - H), e and f give acceptable values for D(CH3 I) and D(CF3 - I), respectively, and pairs g-i, as we have already noted, give remarkably consistent values for D(CFs - H). However, reactions j , k, and 1 give unacceptable values for D(CF3 - H), D(CF3 Br), and D(CF3 - I), respectively, the error in each case being about 4 kcal, which is considerable in comparison with the individual activation energies involved. In each case, however, the reverse reaction in each pair has a frequency factor which may be regarded as "abnormal," and we feel that there may be some as yet undetermined experimental effect which causes (7) J. C. Amphlett, J. W. Coomber, and E. Whittle, J. Phys. Chem., 70,593 (1966). (8) G. C. Sinke, ibid., 70, 1326 (1966). (9) R. K. Boyd, G. W. Downs, J. S. Cow, and C. Horrex, ibid., 67, 719 (1963). (IO) The referee has pointed out that our measurement of the quantity [AHrOm(CFaH) AHr02~8(CFd)]= -25.7 f 0.2 kcal, taken together with the value of D(CF8 - I) = 53.5 f (?) kcal from ref 9 (cf. also ref 2),constitutes it determination of D(CF8 - H) = 105.8 f (?) kcal. We prefer to interpret it as establishing the mutual consistency of the kinetic studies on CFsH and C F d ; certainly it inspires considerable confidence in them. (11) E.T.Roux, J. Chem. Phys., 43, 2251 (1965).
-
THERADIATION CHEMISTRY OF LIQUIDMETHANE
the Arrhenius parameters of these reactions to be distorted from their true values. It would not appear, however, to be simply “experimental error,” since in
1089
all three cases the experiments were carefully done and, in the case of reaction j , there is agreement between independent determinations.
The Radiation Chemistry of Liquid Methanel
by Hugh A. Gillis Radiation Research Laboratories, Mellon Institute, Pittsburgh, Pennsylvania, and Department of Chemistry, Western Reserve University, Cleveland, Ohio’ (Received October 4, 1966)
Liquid methane a t 112°K has been irradiated by Coaoy rays in the pure state and with small amounts of added ethylene or propylene. The effects of these olefins on products up to Ca can be interpreted by a simple hydrogen atom scavenging mechanism. The ratios kdis/kcomt, were estimated to be 1.0 for ethyl plus ethyl, 3.6 for isopropyl plus isopropyl, and 0.9 for methyl plus isopropyl. These are all much higher than gas-phase results. The isotope effect in the disproportionation reaction was found to be small ( k H / k ‘v ~ 1.1). In experiments with CH4-CD4 solutions, the isotopic composition of the product ethane showed that the methylene radical is an important intermediate. In an experiment with a C D r C a H s solution, a product of the ion-molecule reaction between CDb+ and CaHs, for the occurrence of which there is good evidence in the gas phase, was looked for and not found.
Introduction The radiation chemistry of gaseous methane has been investigated in a number of studies.a Good evidence that ion-molecule reactions are important above 0.01% conversion was obtainedlad-*and the important role played by product ethylene and acetylene in determining over-all product formation was stressed.aa-a I n studies of the radiation chemistry of solid methane, ionic reactions have been invoked to explain the production of polymer with a G( -CH4) of 0.3,4and a major role was assigned to methylene radicals in the formation of ethane.5 This study was undertaken to determine the significance of ion-molecule reactions in the irradiated liquid and to see if a correlation could be found with results obtained in esr studies.6 In the course of the investigation, evidence was found which bears on the interesting question of possible temperature and phase
effects on the ratios of disproportionation to combina tion of alkyl radicals.
Experimental Section Purijication. The starting material was Phillips (1) Supported in part by the U. S. Atomic Energy Commission. (2) Where correspondence should be addressed. The experimental work was performed at the first address. (3) For example: (a) L. W. Sieck and R. H. Johnson, J.Phys. Chem., 67, 2281 (1963); (b) R. W. Hummell, Discussions Faraday SOC.,36, 75 (1963); (0) J. F. Riley and T. C. Hung, Chemistry Division Annual Progress Report, Oak Ridge National Laboratory, ORNL3679, June 20, 1964, pp 39-41; (d) P. J. Ausloos and 5. G. Lias, J. Chem. Phys., 38, 2207 (1963); (e) P. Ausloos, 8. G. Lias, and R. Gorden, Jr., ibid., 39, 3341 (1963); (f) P. Ausloos, R. Gorden, Jr., and S. G. Lias, ibid., 40, 1854 (1964). (4) (a) D. R. Davis and W. F. Libby, Science, 144, 991 (1962); (b) D. R. Davis, W. F.Libby, and W. G. Meinschein, J . Chem. Phys., 45,4481 (1966). The author is grateful to one of the referees for making available a preprint of this work. (5) P. Ausloos, R. E. Rebbert, and S. G. Lias, ibid., 42, 540 (1965). (6) R. W.Fessenden and R. H. Schuler, ibid., 39, 2147 (1963).
Volume 7 1 , Numbm 4 March 1967
