Thermochemistry of the Bromination of CC14
2709
(6) J. Janik, Acta Phys. Poion., 23, 487 (1963). (7) H . H. Reamer. B. H. Sage, and W, N. Lacey, ind. Eng. Chem , 42, 140 (1950) (8) F. T. Selleck, L. T Carmichael. and 8. H. Sage, Ind. Eng. Chem., 44, 2219 (1952). (9) G. G. Devyatykh, A , D. Zorin, and i. V. Runovskaya. Doki. Akad. NaukSSSR, 188,1082 (1969). 10) E. A. Guggenheim, J. Chem. Phys , 13,253 (1945). 11) 0. R. Quayle, Chem. Rev.. 53,439 (1953). 12) E. W. Haugh, B. 13,Wood, and M. J. Rzasa, J. Phys. Chem.; 56,
996 (1952). J. C.Ericksson, Acta Chem. Scand . 16,2199 (1962). R . H . Wright and 0. Maass. Can. J. Res., 6, 94 (1932). E.C.W. Clarke and D. N. Glew, Can. J. Chem.. 49, 691 (1971). M .P. Burgess and R. P. Germann.AIChEJ., 15, 272 (19691. P. B. Stewart and P. Munjal, J. Chem Eng. Data, 15, 67 (1970). Y. D. 2el'venskii.J. Chem. ind. ( U S S R ) . 14. 1250(1937). J. W. Belton and M.G. Evans, Trans. Faraday Soc.. 41. 1 (1945) I. Prigogine and J, Marechai,J. CoiioidSci. 7, 122 (1952). (21) G. L. Gaines. Jr., Trans. Faraday Soc., 65,2320 (1969)
(13) (14) (15) (16) (17) (18) (19) (20)
Thermochemistry of the Bromination of Carbon Tetrachloride and the Heat of Formation of Carbon Tetrachloride
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G . D. Mendenhall,' D. M. Golden, and S. W. Benson" Department of Thermochemistry and Chemicai Kinetics, Stanford Research institute, Menio Park. Caiifornia 94025 (Received May 29, 7973)
+
+
At 560°K the equilibrium constant for the reaction Brz CC14 c BrCl CBrC13 was determined as 0.0046 f 0.001 by combined optical and chromatographic techniques. From this result and literature data we calculate the heat of formation difference AHf"(CC14.g) - 1Hf"(CHC13,g)= 2.27 f 0.3 kcal/mol at 298", independent of calorimetry. Based on AHf"(CHC13.g) = -24.66 f 0.3 kcal/mol, we obtain AHf"(CC14,g) = -22.4 & 0.4 kcal/mol, a higher value than obtained from many earlier studies. The CCl3-Cl bond strength is estimated as 70.4 f: 1 kcal/mol.
Experimental Section
The Brz partial pressure was determined from the optical densities a t 550 and 570 nm, while the OD (340 nm) gave the BrCl pressure after subtracting out the contribution from Brz a t this wavelength. Mixtures of Clz and Brz were added to the cell to obtain 0: f P ( B r C 1 Torr) /OD (340 nm). In this calibration and in some of the reaction runs a correction was made for the presence of free Clz from the equilibrium 2BrC1 e Brz + Clz, which has a value of 0.375 a t 560OK.4 At the partial pressures of our experiments, pressureindependent c ( A nm) were Brz: 64.5 (550), 95.2 (570); BrC1: 48.6 (340); Clz: 49.1 (340); CBrC13: 174.4 (300). We did not relate these to published values because our instrument slit width varied over a wide range. The corrosive nature of the halogens made special handling necessary. Bromine (and probably Clz) reacted with hot silicone grease, so all stopcocks in direct contact with the hclogens were unheated and were lubricated with a silica gel-halocarbon mixture.9 The ratio CBrC13/CC14 was determined directly by condensing out the reaction mixture a t -196" followed by vpc analysis by peak height ratio (F & M Model 720, thermal conductivity detector, 2 ~1 samples on a 10 ft x 0.25 in. column of 20% di(2-ethylhexyl) sebacate on 60-80 m Chromasorb a t 140"). Mixtures of known composition of the two halocarbons were injected for calibration, and it was shown that added Br2 or Clz did not effect the ratios from these standard mixtures. The partial pressures were obtained from the ratio and the relation P'(CBrC1,) + Pe(CC14) = PO(CC1,)
The spectrophotometric procedure and apparatus ( a modified Cary Model 14) has been described previously.8
The analytical procedure was tested by adding known pressure of CBrC13 and cc14 to the cell, condensing out as
Introduction Values for the heat of formation of CC14 in the literature encompass an unfortunately wide range. Cox and Pilcher2a and Stull, Westrum. and Sinke2b have combined a number of calorimetric experiments with newer data and arrived a t Afffo298(CC14,gas) = -25.2 =k 1.5 and -24.0 kcal/mol, respectively. Many of the early values depended on 2,Ht"(CHC1s). Hu and Sinke3 recently reported results of rotating bomb calorimetry which lead to an assigned AHf"(CC14,g) = -22.9 f 0.5 kcal/moL4 Lord and Pritchard,5 however, arrived a t a value of -27.4 kcal/mol from an equilibrium study of COC12. cc14, and COz in the presence of excess Clz. Their determination required a rather elaborate analytical procedure. Sullivan and Davidson6 accurately measured the equilibrium constant for the reaction Br2
+
CHCl:,
A
e
HBr
+
CBrCl,
and the thermochemistry of the system has been evaluated by B e n ~ o n We . ~ can estimate the heat of formation difference between CHCl3 and cC14 from K A and the equilibrium constant for Br2
+
CC1,
B
-+
BrCl
+
CBrC1,
From a choice of AHf"(CHC13,g) one obtains a precisely related AHf"(CC14).
The Journai of Physical Chemistry, Voi. 77, No. 22, 7973
2708
G . R. Mendenhall, R. M. Golden, and S. W. Benson
TABLE 1: Equilibrium
Constants for the Reaction CC14 + Brz FI CBrCl3 + BrCl
Run 1
CCln
57 25
Br2
2
cc14
55 20.5
Br2
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3
Pe.Torr
Temp, "C (time, hr)
ZBre/ZBrca
Ke
53.7 23 CBrC13 3.3 5rCI 2.4 52.6 18.3 CBrC13 2.4 BrCl 1.8 51.4 22 CBrC13 2.1 BrC! 2.4 2.6 CC14 29.4 Br2 15.6 BrCl 0.95 47.8 3.55 CBrCI3 0.88 BrCl 0.73 12.9 46.5 CBrC13 1.6 1.2 BrCl
(15) 284 1
1.03
0.0064
(17.5) 285 f 1
1 .oo
0.0045
287
1.04
0.0045
(13.5) 285 f 1
1.02
0.0054
(41.5) 287 f 1
0.92
0.0038
1.04
0.0032
P o . Torr
cc14 Br2
53.5 23.4
4
CBrC13 Clz
32 17
5
CC14 Br2
48.7 4.8
6
CCI4 B ~ z
14.5 46
CCI4 CBrCI3 CCI4 CBrC!3
36.1 0.87 44.7 2.8
*
(8.5) i1
(18) 287 5 1
Av 7 8
0.0046
f 0.001
28 7
36.3 0.74 44.4 3.1
(22.5) 287 (1.5)
Equilibrium Br concentration, Initial Br concentration calculated as Br
TABLE 1 6 : ThermodynamicProperties of Species in Equilibria A and Sa
__
______
1 H f "(298") Br26 BrCIb
HEr" CHCI3
cc14 CBrC13
7.387 3.50 -8.66 - 2 4 . 6 6 f. 0.3b - 2 2 . 4 f 0.4d -10.0 f. 0.3d
a Gas phase. A/+,' in Kcal 'mol, So in gibbs/mol. SOC..75, 5259 (1953)
S"(298")
C, (298")
58.647
8.62
57.337 47.44 70.66c 74.12' 79.8e
8.36 6.96 15.71' 20.02' 20.42'
Reference 4.
Reference 2a.
before. and analyzing (Table I, runs 7 and 8). Mixtures of Brz and Glz and of Brz, CC14, and excess Clz (and CBrCl3 alone at 300 nm) were shown to undergo no significant change in OD at the three wavelengths tested under reaction conditions. KO significant peaks were seen in the vpc trace of the equilibrated reaction mixture other than those of CBrC13 and CC14. (The halogens were probably destroyed on the column.) With Brz and CC14 mixtures above 360", three additional peaks of longer retention times did appear successively in the vpc trace, and their intensities increased with reaction time. These were probably the polybromo products from CC14. Bromine (Mallinckrodt). CC14 (Mallinckrodt), and CBrCl3 (Matheson Coleman and Bell) were of reagent grade and were degassed before use. CC14 was stored over molecular sieve (Linde 4A). About 20% of each Brz sample was distilled off to remove 6 1 2 and other impurities. Chlorine (Ailatheson) was added t o the vacuum system, The Journal of Physical Chemistry. Voi. 77, No. 22, 1973
C, (500")
C, ( 6 0 0 " )
8.78
8.86
8.91
8.61 6.98 17.83C 22.04' 22.25f
8.74 7.03 19.34c 23.28c 23.40f
8.83 7.14 20.44c 24.07' 24.001
C,(400")
This work. e Reference 7. 1 E. Gelles and K. S. Pitzer. J. Amer. Chem.
condensed out a t -196", and degassed several times before use.
Results and Discussion Good agreement in Keq values was found in six runs starting from both directions, and with final CC14(Brz) pressures covering a 4(13)-fold range (Table I). The error in the individual K ' s was about k0.0020, so all of them overlap the average. The error in K,, was assigned generously as *0.001; the standard deviation error is about half this. Two internal checks are present in the data. Except for run 4, we expect P(BrC1) = P(CBrC13), and this does obtain within error limits. The bromine balance a t equilibrium was generally 2-470 too high. The delay in mixing reactants in the narrow inlet tube partly accounts for the discrepancy in some runs, but the errors are small and we neglected them. For K,, we calculate
2909
Thermochemistry of the Bromination of CC14
AG"(559'K)
=
- R T In K,, = 5.98 f 0.3 kcal/mol
AGB0(559"K) = AH'(298) A q ( 5 5 9 - 298)10-3
A C In
(559/298)]
(Since
AG
+
- 0.559[AS0(298) + AH'(298) - 0.559AS0(298)
- only 0.03 gibbs/mol)
AHB0(298'K) = 5.98
+
= 52.1
DH"(CCl,-Br) = 19
AGB0(298'K) = 7.15 f 0.3 k c a l / m o l An exactly similar treatment starting from Sullivan and Davidson's Ki(442"K) = 1.94 i 0.19 gives A H A 0 ( 2 9 8 " K )= -1.41
f
0 1 kcalimol
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AGA0(298'K) = -0.79 f 0.1 k c a l / m o l where we assigned A T = 2.46 gibbs/mol from 298 t o 442°K. (The value of A H ~ ~ ( 2 9 8calculated ") in ref 7 has an error of 0.6 kcal/mol.) It is easy to show that AH, - AH, = AH,'(HBr) - AH,'(CI-ICl,) AHfo(BrC1) + A Ilfo(CC1,) t h e refore AHf'(CC1,) - AHf'(CHC1J = AHA - AHB AH,'(HHr) f A H f o ( B r C l ) = 2.27 f
-
0.3 k c a l / m o l a t 298'
If AHt."(CHC13) = -24.66 f 0.3 kcal/mol. the value derived4 from H u and Sinke's data.3 then AHr"(CC14) = -22.4 f 0 4 kcal/mol, in excellent agreement with the 0.3 kcal/mol obtained by the same auvalue of -22.9 thors. .4 With the newer chloroform data and the correct AHA, we obtain AHi."(CBrC13,298") = -10.0 k 0.3 kcal/mol,
*
I kcal/mol from Benson's
DH'(CC1,-C1) = 70.4 I 1 kcalimol (DH' = dissociation e n t h a l p y , 298°K) DH'(CC1,-H)
0.599AS0(298) = 8 84 f 0 . 3 kcal/mol
*
and A.Hf"(.CC13) = 19.0 analysis.? We also find
+
+
19
+
26.740
24.66 = 93.8 iz 1 kcal!mol
+
10.0 = 55.7 rt 1 kcal/mol
The latter two bond strengths are not changed from earlier estimates.? The heats of formation of a number of halomethanes are also based on AHf'(CCl4). Recently this situation was reviewed by Shaw,'o who assigned a number of heats of formation assuming AHf"(CC14) = -26.0 kcal/mol. This value was chosen partly for self-consistency with equilibrium results: and we have not attempted to reevaluate his work. References and Sotes Postdoctoral Research Associate. (a) J. D. Cox and G. Pilcher, "Thermochemistry of Organic and Organometallic Compounds," Academic Press. New York, N. Y . . 1970; ( b ) D. R. Stuli, E. F. Westrum, and G. C. Sinke, "The Chemical Thermodynamics of Organic Compounds." Wiley, New York. N. Y., 1969. A. T.Hu and G .C. Sinke, J. Cnem Jhermodyn.. 1 . 507 (1969). D. P. Stuil, Ed., "JANAF Thermochemical Tables Daw Chemical Company. Midland, Mich., 1963. A. Lord and H, 0. Pritchard, J . Chem. Thermodyn., 1, 495 (1969) J. H. Sullivan and N. Davidson. J . Chem. Phys , 19.143 (1951) S . W. Benson, J. Chem. P h y s . . 43, 2044 (1965). D. M . Golden, R. Walsh. and S. W. Benson, J. Amer. Chem. Soc.. 87, 4053 ( 1965). 25-58 grease from R. S. Hughes CO. R. Shaw in S. Patai's, "Chemistry of the Carbon-Halogen Bond," Wiley, New York, N. Y., 1973, Chapter 16.
The Journal of Physical Chemis?ry, Voi. 77. No.22. 1973
