599
Electron Capture Reactions in Mixtures of HCI and HBr
Electron Capture Reactions in Mixtures of HCI and HBr SurJit S. Nagra and David A. Armstrong' Department of Chemistry, University of Cafgary, Calgary, Alberta T2N 1N4, Canada (Received November 11, 1976) Publication costs assisted by the National Research Council of Canada
The gas phase 6oCoradiolysis of mixtures of HC1 and HBr has been studied at room temperature. Using SF6 as a thermal electron scavenger, the rates of electron capture determined by competition kinetics were higher by a factor of 3 or more than the combined contributions of (HC1)2and (HBr), dimers. The net increase in the rates is proportional to the product of the concentrationsof HC1 and HBr in the mixtures and can be explained by contributions of mixed (HC1, HBr) dimer species. The magnitude of the electron capture rate constant, s-' cm6 cm6molecule-' s-l, of the mixed dimers is very similar to 1.0 f 0.1 X 1.4 f 0.2 x observed in pure HBr, and it is suggested that the main reaction of the electron-mixed dimer ions is dissociation to H + ClHBr-. Introduction Although reaction 1is likely to be the most important e- + HX H + X(X = I, Br, C1) (11 electron reaction in gaseous hydrogen iodide,' previous studies in this labor at or^^-^ and elsewhere5 have shown that it is relatively unimportant for HBr and HC1. The capture of low energy electrons in these system is of higher order in hydrogen halide and obeys the stoichiometry of reaction 2. A mechanism based on the addition of +
e-
+
2HX --* H,
+
other products
(2)
electrons to (HX), dimer molecules as the initial event has been shown to hold for both halide^.^'^ However, the two molecules differ to the extent that dissociation of the transient (FIX);* complex dominates when HX = HBr, while stabilization to (HX),- and reactions with additional HX are most important in the case of HC1. This investigation was undertaken with the object of examining the kinetics of electron capture in mixtures of HBr and HC1, where mixed dimers may be expected to occur. If one assumes that hydrogen bonding forces dominate: then it is conceivable that the two species HCI-HBr and HBr-HC1 may be near linear and thus distinguishable. To describe electron capture by these species the mechanism given in ref 4 is generalized below by replacing one HX of the original (HX), dimer by HY. K 3HX,HY
HX t HY -2
(HX-HY)
(3)
(HX-HY)-*
(4)
k
e- t (HX-HY)& ha
(HX-HY)-*
!% H t
XHY'
+ HX + XH-Y--HX
(HX-HY)-* t H X - (HX-HY). (HX-HY)-*
+
HX
(HX-HY)-*
+ +
HY + (HX-HY)-
(HX-HY)-*
--*
H
HY + H
+
HY
+ XH-Y--HY
H+HY+H,+Y H + H X - H, t X
(7b)
As in ref 4 the rate of capture by hydrogen halide dimer species is defined as the effective first-order rate constant: ~ H X , H Y= -(d[e]/dt)/[e] s-'. For the (HBr), dimers (i.e., HX = HY = HBr) k, >> k, > (k6a+ k6b)[HBr] at normal pressure^^'^ and k , is simply equal to KsHBrtHBrk [HBr]', where K3Bp@%c = 1.0 f 0.1 X cm6 mol;: cule-2 s-l a t 298 K. For the case of HX = HY = HC1 on
the other hand k, > (k6, + k6b)[HC1] >> k, at normal pressures, and the rate of capture, kHCIJfC1, is K3Hc'.Hc'kc(k6a 4- k6b)k,1[HC1]3 = 10.4 f 0.7 X [HC1I3S-' a t 298 K. With the mixtures studied here the observed value of kHX,HY will be the sum of these two contributions plus those of the mixed dimers HBr-HCl and HC1-HBr, which will be designated as ~ H Band~ k ~H mc r ~,respectively. The total rate was measured by the competition technique with sulfur hexafluoride as the scavenger in reaction 8. The e- + S -,S(8) contributions of (HCl), and (HBr), dimers, calculated with the above rate expressions, were then subtracted to determine the following: (a) whether mixed dimers played a significant role; and (b) the rates of their reactions, and whether they resembled (HCl), or (HBr), in their kinetics. Experimental Section The source and methods of purifying and storing the hydrogen halides and sulfur hexafluoride were described in ref 2 and 4. Mixtures of the gases of the required compositions were prepared in a mercury-free vacuum line and introduced into the 10-cm diameter 360-cm3volume pyrex irradiation cells.'s4 Irradiations were carried out at 298 f 2 K in a 6oCoGamma cell 220, supplied by Atomic Energy of Canada Ltd. Dose rates were determined as discussed in ref 2 and lay in the range 1.8-2.0 X 10l8eV g-I h-', while typical absorbed doses were 0.9-1.0 X 10l8 eV g-l. After irradiation noncondensibles were transferred to a gas-measuring system, where the hydrogen content was determined. Results The total yields of hydrogen in the mixtures, G,(H,), were measured in molecules per 100 eV of absorbed energy for several different compositions. In the absence of sulfur hexafluoride the increase of G,(H,) from 8.0 molecules per 100 eV in HC1 to 9.8 molecules per 100 eV in HBr' was within experimental error (f0.2 molecules per 100 eV) a linear function of 2, the stopping power fraction of HBr defined in ref 2. As previously s h o ~ n >the ~ , competition ~ for low energy electrons between sulfur hexafluoride and the hydrogen halides may be described in terms of expression i, where {AG(H2)}-' = &?e.$-'
+ hHX,HY/h8[Sl 1
AG(H2) is the reduction in hydrogen yield caused by capture of electrons via reaction 8 and g, the yield per 100 eV of scavengable low-energyelectrons. Plots of (AG(HZ))-' The Journal of Physical Chemistry, Vol. 81, No. 7, 1977
600
S. S. Nagra and D. A. Armstrong
TABLE I: Rates of Electron CaDture in HCI-HBr Mixtures Concn
[ HBr 1 0.320 0.640 0.960 0.640
[HCII 1.94 1.94 1.94 3.89 a
Observed total rate.
O2
Electron capture rates,
(lo1' molecule cm-,) 1.0 2.2 4.0 5.0
1.9 3 5.2 i 10.6 11.3 i _+
i
L 05
15
10
0.2 0.5 1.1 1.1
20
25
[SF6]'1/10-"
30 0
A 04
08
12
16
20
cm3 molecule-'
Figure 1. Reciprocal of AqH,) vs. the reciprocal of SFBconcentration for mixtures of HCI and HBr: Concentrations of HCI and HBr in molecules ~ m were - ~ respectively: (a) 1.94 X 10lgand 0.32 X IO1' (0), 3.89 X IO" and 0.64 X IO" (A);(b) 1.94 X IO1' and 0.64 X (O), 1.94 X IO" and 0.96 X lOI9 (0).
vs. [SF,]-' are shown in Figure l a and b for mixtures of four different compositions of HBr and HC1. The total rates of capture by hydrogen halide dimer species, kHX,HY, were obtained from the (slope) X (intercept)-' (= kHX,HYki') for each of these plots and with ka = 2.0 X cm3 molecule-' s-' as in ref 4. They are given in Table I along with the concentrations of HC1 and HBr in each mixture. Column four of this table presents the combined contributions of (HC1)2and (HBr),. These were calculated with the rate coefficients given in the Introduction for pure HC1 and HBr, but in the case of HC1 an additional rate + k6f)ka-lwas included contribution equal to K3Hc1,Hc1kc(kk in ~ H C ] , H Cto~ take account of reactions 6e and 6f. For this
k'HCI,HBr/[HC11' [HBr]( cm6 s-l) k"a, H B ~ ~ 7.7 t 1.2 1.2, i 0.3, 16.9 i: 2.5 1.3, t 0.2, 29.1 i 5.1 1.5, i 0.3, 33.8 5.1 1.3, i: 0.2, Mean 1.4 * 0.2 _+
Combined contributions of (HCl), and (HBr),.
t
'0
t
~HCI,HCI k ~ ~H Br ~,
~ H X , H Y ~
9.6 + 22.1 t 39.7 i 45.1 c
lo9 s-'
Net contribution of mixed dimer species.
Discussion If one or more of reactions 6a-d were important for the ~/ [HBr] should be a mixed dimers, then It ' H C ~ , H B [HCI] function of the total hydrogen halide concentration. This is clearly not the case for the concentration regimes used here. Also the magnitude of the mean value of k'~~l,qp/[HCl][HBr] in column six of Table I, 1.4 f 0.2 x 10- cm6 molecule-* s-l, is very similar to 1.0 3t 0.1x cm6molecule-' s-' observed in pure HBr. It appears therefore that the kinetics of electron capture by the mixed dimer (or dimers) of HC1 and HBr resemble those (HBr), and are such that It, >> k, > (&a + ksb)[HX] (k6c + k6d)[HYIThe foregoing conclusion requires the absence of a significant energy barrier for reaction 5. As pointed out in ref 4,the energy change a E 2 g a o i h ) for this reaction is the same as that of reaction 5', which m turn can be calculated e t (HX-HY) e t HX
+
+
H
+
XHY-
HY + H t YHY-
from (aE298'(2,) - h E 2 9 8 O ( 3 ) ) . Yamdagni and Kebarle' have estimated DBr--HCl = 16 kcal mol-' at 298 K from their results for gas-phase ion-molecule association energies. From this and standard thermochemical datagand electron affinities" aE298'(2,) is --7 kcal mol-' for X = C1 and Y = Br. The energy change for reaction 5' will be less exothermic by the absolute magnitude of @ 2 9 8 O ( 3 ) , which may obviously be different for HC1-HBr and HBr-HC1 dimer configurations. However, since a E Z g 8 O ( 3 ) for HX = HY = HCl is --2.4 kcal mol-' and m ' g 8 O ( 3 ) for HX = HY = HBr smaller," it seems probable that 1aE2980(3)1 for both (HCl);* t HBr (HCl),- t HBr (6e) dimer configurations would be less than 7 kea1 mol-'. Based on current thermochemical data reaction 5 should (HCI);* t HBr H t ClH-Cl--HBr (6f) not therefore have a significant energy barrier. purpose (k6e h6f) = 0.88 x 10-~ cm3 molecule-' s-' was In summary the present observations are consistent with calculated from ion-molecule reaction rate t h e ~ r y . ~ the capture of electrons by mixed HC1,HBr dimers having Comparison with 1.16 X lo-' cm3molecule-' s-' calculated and K , similar to those for (HCl), and values of K3HX3HY in ref 4 for the corresponding reactions of (HC1) -* with (HBr),. The subsequent reactions of the transient mixed HC1 showed that the overall rate coefficient K3Hc19dC1k c (k6e dimer anions closely resemble those for the HBr system. kSf)k;' should be 0.76 K3HC1'HC1kc(k6a + k64ka-l for HX Acknowledgment. This research was supported by = HY = HC1. Since HC1 was usually present in excess the Canadian National Research Council Grant A3571. contributions of reactions 6e and 6f are not large in any case, and, in the absence of experimental values of + References and Notes k6f),the above procedure is therefore satisfactory. (1) D.E.Wilson and D.A. Armstrong, Radiat. Res. Rev., 2,297 (1970). For every mixture the magnitude of kHX,HYin column (2) S.S.Nagra and D. A. Armstrong, Can. J. Chem., 53,3305 (1975). by a factor of 3 or more. three exceeds (kHCl,Hc1+kHBr,HBr) (3) S.S.Nagra and D. A. Armstrong, J Phys. Chem., 79, 2875 (1975). (4) S.S. Nagra and D. A. Armstrong, Can. J Chem., 54,3580 (1976). Thus reactions of the mixed dimer species must make a (5) G.R. A. Johnson and J. L. Redpath, Trans. Faraday Soc., 66,861 substantial contribution to the rate of capture. The (1970). magnitudes of these contributions were obtained from the (6) See the discussion of (HX), dimers in ref 4. One may note that differences between the rates in columns three and four. HCI-HBr chains have been observed in crystals, but information on gaseous mixed dimers appears to be lacking. G. L. Hiebert and D. They are referred to subsequently as k Lc1,mrand are listed F. Hornig, J. Chem. Phys., 28,316 (1958). in column five of the table. (7) T. Su and M. T. Bowers, Int J. Mass. Spectrum Ion Phys., 12,347 Clearly the concentrations of both (HC1-HBr) and (1973),and references cited therein. (8) R. Yamdagni and P. Kebarle, J. Am. Chem. Soc., 93,7139 (1971). (HBr-HC1) dimers will be proportional to [HCl][HBr] and (9) JANAF Thermochemical Tables, Natl. Stand. Ref. Data Ser., Natl. column six of Table I gives the values of k ;IC1,HBr divided Bur. Stand., No. 37 (1971). by this product. The quotients are within experimental (10) L. G.Christophorou in "Atomic and Molecular Radiation Physics", Wiley, New York, N.Y., 1971. error equal and independent of [HBr] and [HCl].
-
+
-
+
The Journal of Physical Chemistv, Vol. 81, No. 7, 1977
