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COMMUNICATIONS TO THE EDITOR
structure proposed by Wasserman., Furthermore, it appears that the e.s.r. signal may not arise from the major colored species but may be associated with parallel reactions of the parent material or subsequent reactions of the colored modifications. The spectra of the negative ion and of the low-temperature photolytic species both indicate structures in which the unpaired spin is effectively localized in one half of the molecule and are probably skewed conformations.
fourth vibrational level and above. These molecules are energetic enough to dissociate fluorine via reaction 3, although de-excitation via reaction 4 seems more likely. In the reaction between hydrogen atoms and chlorine, 1.3% of the HC1 is excited to the fourth level or above. This would seem a conservative estimate for reaction 2, since the heat of reaction is larger relative to the size of the vibrational quantum. Thus we might expect CY 0.013-0.02. By applying the steady-state approximation to the AIR FORCE MATERIALS LABORATORY LARRYA. HARRAH hydrogen and fluorine atom concentrations and also the WRIQHT-PATTERSON AIR FORCE BASE,OHIO concentration of vibrationally excited HF*, the followDEPARTMENT OF CHEMISTRY RALPHBECKER ing rate expression for the kinetics is easily obtained UNIVERSITY OF HOUSTON
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HOUSTON, TEXAS
d[HFI 2~~12k3[H212[F21 dt k5[~~wl(ks[F:,I k4[&1(4)1)
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A Suggested Mechanism for the Hydrogen-Fluorine Reaction
S i r : Two recent papers by Levy anL Copeland1s2 report experimental studies of the rate of reaction of hydrogen and fluorine. These authors were not able to devise a mechanism to explain their data quantitatively, although they did discuss their results in a qualitative manner. I t is the purpose of this communication to suggest a mechanism which quantitatively accounts for the results of Levy and Copeland; it is hoped that this will provide a useful working hypothesis for planning future work. In their first paper,' Levy and Copeland investigated the reaction of hydrogen-fluorine mixtures diluted with nitrogen in a flow system a t 110". They found the reaction rate to be proportional to fluorine concentration and independent of hydrogen concentration. This result can be explained by the reactions
+ Hz -% HF + H H + Fz --%aHF* + (1 - a ) H F + F HF* + Fz -% H F + 2F HF* + -% H F + M(4) 2F + &I --% ,,) Fz + (or perhaps F
(1)
(2)
(3)
M(4)
(4)
&)
(5)
2HF for
= H,)
The first two reactions-both exothermic-are analogous to steps occurring in the hydrogen-bromine and hydrogen-chlorine reactions. In the case of the reaction H Clz --t HC1 C1 nearly 25% of the product HCl is vibrationally e ~ c i t e d . ~It seems reasonable to expect the same phenomenon in reaction 2, and HF* represents hydrogen fluoride molecules excited to the
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The Journal of Physical Chemistry
This corresponds to the observed kinetics provided k4[M(,)] >> kI([Fz]and provided the inert collision partners M(4)and are both preferentially hydrogen. The condition k4[M(4)]>> k3[Fz]is almost certainly satisfied in Levy and Copeland's experiments, since their fluorine concentrations were always low (mole fraction less than 0.05). Furthermore, reaction 4,a vibrational relaxation, is expected to be very fast, whereas reaction 3 may have a very low steric factor. The assumption that M(4) is preferentially hydrogen is also reasonable. Reaction 4 is a vibrational de-excitation, and light molecules seem to be especially effective in robbing diatomic molecules of excess vibrational energy. Thus Millikan and White's4 correlation predicts that hydrogen should be thirty times as effective as nitrogen and over forty times as effective as fluorine in relaxing H F from the first vibrational level. I n addition, the fundamental vibrational frequencies of H F and HZ differ by only about 6%, so that there is the possibility of de-excitation by transfer of vibrational quanta from H F to Hz. On the other hand, there is no a priori reason to expect that is preferentially hydrogen; hydrogen does not seem to be especially effective in recombining other halogens.5 It is possible to provide an ad hoc explanation by postulating an intermediate with a few kilocalories of stability, so that recombination occurs stepwise HzF 11 F Hz 31 HzF F --+ products
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J. B. Levy and B. K. W. Copeland, J . Phys. Chem., 67, 2156 (1963). (2) J. B. Levy and B. K. W. Copeland, ibid., 69,408 (1965). (3) J. T.Airey, R. R. Getty, J. C. Polanyi, and D. R. Snelling, J. Chem. Phys., 41, 3255 (1964). (4) R.C.Millikan and D. R. White, ibid.,39,3209 (1963). (6) D. L. Bunker and N. Davidson, J . Am. Chem. SOC.,80, 5086 (1958). (1)
COMMUNICATIONS TO THE EDITOR
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ysis of these ions, and precipitation in the form of one hydrate (either throughout the solution or on the C3S particles), it is apparent that no more than one peak in the concentration of any species can be understood. A number of subsequent peaks, as reported by Greenberg and Chang for the silicic acid concentration (see Figure 3 of their paper), is incomprehensible on the basis of such a solution theory alone. Greenberg and HF*(U=I~ HF(,=o)--+ HF(b=3) HF(,=i, Chang, for instance, interpret the peak in silicic acid Thus we should anticipate strong inhibition by the reacconcentration, occurring in suspensions of 1.25 g. of tion product. The data in Levy and Copeland’s first C3S/1. after about 35 min., as follows: “some time (40 paper are generally for a small extent of reaction and min.) elapses before the silicic acid concentration IS frequently with a large excess of hydrogen, so it is not sufficient to cause rapid crystallization” ; they dispossible to draw conclusions as to possible inhibition by regard the silicic acid decrease up to about 20 min. HF. However, the first figure of their second paperhydration showing that the silicic acid concentration shows two stoichiometric runs carried past 60% compeak after about 35 min. is in reality the second peak, pletion. These two runs suggest very strong product the first being situated shortly after addition of the C3S inhibition. Indeed, they fit this mechanism very nicely to water. by assuming k 4 , m 10k4 H?. As far as the present authors can see, the only way of In summary, the mechanism proposed here provides a understanding these data is to assume either recrystalquantitative explanation df the kinetics of the reaction lization of calcium silicate hydrates or a changing between hydrogen and fluorine. The most unique feareactivity of the C3S (or both). Conversion of a first ture of the scheme is the assumption of an energy transformed hydrate (F.H.) into a second one (S.H.) has fer chain branching step. The most questionable feabeen found by Kantro, et ~ l .on, ~the basis of analyses ture is the assumption that hydrogen is especially efof the hydrates formed and of surface area measurefective in recombining fluorine atoms. ments; the present authors reported? the influence of amorphous silica on C3S hydration, both in suspensions Acknowledgment. The author wishes to thank Frank and in pastes, to be consistent with the following E. Belles for helpful suggestions, especially as to the role mechanism: the F.H. adheres firmly to the C3S of hydrogen in reactions 4 and 5 . surfaces and strongly retards its hydration, the S.H. RICHARD S.BROKAW LEI+IS RESEARCH CENTER does so to a much lesser extent; conversion of F.H. AND SPACE NATIONAL AERONAUTICS into S.H. is accelerated by the presence of S.H. nuclei ADMINISTRATION and is accompanied by an increasing reactivity of C3S. CLEVELAPZD, OHIO 44135 This mechanism explains Greenberg and Chang’s RECEIVED JUNE1, 1965 data, notably the silicic acid concentration data referred to, much more substantially than the simple solution theory as outlined above. I t should be noted that some Remarks on the Hydration of electron microscopical evidence for the ultimate conTricalcium Silicate version of S.H. has been adduced3 which together with the formation of F.H. and the conversion of F.H. into Sir: Recently, Greenberg and Chang’ reported measS.H. might be correlated with the three c.oncentration urements on the hydration of C3S (3Ca0.SiOz) that peaks recorded by Greenberg and Chang i n suspensions according to the summary of the paper concerned, of 1.25 g. of C3s/1. The shift of the qecond silicic acid “substantiated the solution theory of hydration.” concentration peak toward later hydration times i n I t is the purpose of this note to show that Greenberg C3S suspensions of higher solid content (5 g. ’1.) IS and Chang’s data can be combined with phenomena readily understood from this mechanism, since lower reported by other research workers into a much more calcium and hydroxyl ion concentrations in the water coherent picture of C3S hydration than that offered by phase during the early stages of the reaction seem to these authors. It is not quite clear from the paper what exactly is (1) S. A. Greenberg and T. N. Chang, J Phys. Chem.. 69, 553 (1965). (2) D. L. Kantro. S. Brunauer, and C. H. Weise. ibid., 66, 1804 understood by the “solution theory of hydration.” (1962). As long as it is meant to represent no more than disso(3) H. N. Stein and J. M . Stevels, J . A p p l . Chem. (London), 14, 338 lution of calcium and silicate ions from the C3S,hydrol(1964).
Such a scheme has been proposed5 to explain the remarkable efficiency of iodine molecules in recombining iodine atoms. One further fact seems to support this mechanism. We might expect that hydrogen fluoride would be especially effective in deactivating HF* by transfer of vibrational quanta, for example
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Volume 69, -\-umber 7
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J u l y 1.966
