From Sullivan and Davidson's measurement of the equilibrium constant for
Br2+CHCl3⇌ lim AHBr+CCl3Br
at 442°K, and accurate CP° and entropies for the species involved, we find ΔSA°(298°K) = 2.2±0.2 gibbs/mole and ΔHA°(298°K) = −0.80±0.13 kcal/mole. Using known ΔHf° we calculate the unknown ΔH°f,298(CCl3Br) = −8.7±1 kcal/mole.
Analyses of the kinetic data together with reasonable estimates of S°(ĊCl3) and CP°(ĊCl3) permit assignments to be made of the Arrhenius parameters of the rate constants in the detailed mechanism. Analysis of the initiation and termination steps permit fixing limits on the activation energies for these steps as follows:
Br+CHCl3⇌ lim 21HBr+ĊCl3ĊCl3+Br2⇌ lim 43CCl3Br+Br.
E2=E3+0.22 kcal/mole≤2.0 kcal≈1.0±0.5 kcal/mole. These then lead to ΔH°f,298(ĊCl3) = 19.7 kcal/mole and DH°(CCl3–H) = 95.7±1.5 kcal/mole. Some other DH°(CCl3—X) bond strengths are listed.
1.
J. H.
Sullivan
 and
N.
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19
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1951
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2.
Thermodynamic properties of Br, Br2, HBr, CHCl3, CCl4 are taken from the JANAF Interim Thermodynamic Tables (Thermal Laboratory, Dow Chemical Company, Midland, Michigan, 1960–1965).
3.
Thermodynamic properties of CBrCl3 are taken from tables of
E.
Gelles
 and
K. S.
Pitzer
,
J. Am. Chem. Soc.
75
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5259
(
1953
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I have rechecked the calculations of CP and S for this compound using spectroscopic data of
J. P.
Zeitlow
,
F. F.
Cleveland
, and
A. G.
Meister
,
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18
,
1076
(
1950
), whose frequencies agree to within a few percent with those used by SD.
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Moments of inertia were computed from two independent sources of bond lengths and angles which agreed to within ±1% with the values used by SD. These all gave values of the product I1I2I3 about 4% larger than Gelles and Pitzer, which has a negligible effect on S. The Gelles and Pitzer values of S are in error by a constant amount over the entire temperature range. They are too low by 2.20±0.1 gibbs/mole suggesting a numerical error of Rln3 in their partition function. Their CP values agree to within ±0.05 gibbs/mole with my own calculations over the entire temperature range. See Errata in
J. Am. Chem. Soc.
76
,
6419
(
1954
).
4.
Natl. Bur. Std. (U.S.) Circ. No. 500 (1952).
5.
From the classical form of the RRK theory of unimolecular reactions [See S. W. Benson, Foundations of Chemical Kinetics (McGraw‐Hill Book Company, Inc., New York, 1960), Chap. XI] the lifetime t of the excited, nascent (CCCl3Br)* species formed from the recombination is given approximately by 1/t = A[(E−E0/E]S−1. Here A∼1013 sec−1, E is the total internal energy of the species, E0 is the bond dissociation energy at 0 °K, and S = 3n−6 = 9 is the number of internal coordinates of the molecule. Using E0∼55 kcal/mole and (E−E0) = (S−1)RT = 7 kcal/mole at 442 °K, we find t = 10−5.5 sec. Under the conditions used by SD the time between collisions is of the order of 10−9.2 sec so that a nascent (CCl3Br)* will experience about 104 collisions before it dissociates. This is more than adequate to ensure collisional deactivation.
6.
For purposes of simplification we have neglected the very slight HBr inhibition.
7.
See text given in Ref. 5 for compilation of data.
8.
G. C.
Fettis
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S. W. Benson, “Bond Dissociation Energies,” J. Chem. Educ. (to be published).
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J. H.
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29
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546
(
1958
); see especially p. 567.
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© 1965 American Institute of Physics.
1965
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