J . Phys. Chem. 1984,88, 666-668
666
permittivity liquids, like aqueous systems. The method requires a reference liquid with known dielectric parameters which do not differ widely from those of the unknown sample. The measurements are self-contained in the sense that separate calibration measurements at spot frequencies to determine an accurate time origin are not needed. The proposed measuring sequence and averaging procedure showed a time referencing accuracy of 0.03 ps which made possible an extension of the frequency range to 14 GHz. For liquids of lower permittivity, the measurement accuracy and the frequency range can be expected to be even larger. For high-permittivity liquids showing nonnegligible dc conductivity the single-reflection method is unsuitable.' For such systems the total transmission or total reflection methods should be applied, where again the use of known reference liquid improves accuracy.
Some disagreement between experimental and theoretical curves can be observed around 200 MHz. It is believed that this is due to an error obtained on truncating the pulses, which show a slightly rising slope even after 5 ns. This illustrates that, even after extensive signal averaging to eliminate noise errors, systematic errors may remain if steady initial and final levels have not been reached within the observational time window. The dielectric parameters correspond very well with those of S ~ g g e t t . 'The ~ interpretation would be to attribute T~ = 21 ps to relaxation in the bulk water, shifted from the free water value of 14.9 ps, while T~ is attributed to relaxation in the modified water of the hydrated glucose complex.
Conclusion It has been demonstrated that with a dual-channel TDS system the single-reflection method can be directly used to study high-
Thermal Decomposition of Hydrogen Cyanide behind Incident Shock Waves Attila Szekely,* Ronald K. Hanson, and Craig T. Bowman High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering, Stanford University, Stanford, California 94305 (Received: October 26, 1983)
The thermal decomposition of hydrogen cyanide was studied over the temperature range 2700-3600 K by using a shock tube technique. HCN-Ar mixtures (12-200 ppmv HCN) were heated by incident shock waves and CN absorption at 388 nm (B2Z+, v = 0 X2Z+,v = 0 electronic transition) was used to follow the progress of the decomposition. The low-pressure rate coefficient for the reaction HCN Ar H CN + Ar (1) was closely fit by the Arrhenius expression k , = 10'6.0*0.15 exp[-(54650 & 111O)/v cm3/mol s. The present results are discussed and compared with previously published determinations.
-
+
-
Introduction The low-pressure rate coefficient for the decomposition of hydrogen cyanide HCN
+ Ar
+
H
+ C N + Ar
(1)
has been studied by several over the combined temperature range 2200-5030 K. Roth and Just' used atomic resonance absorption spectroscopy (ARAS) to monitor the buildup of H atoms behind reflected shock waves in the temperature range 22OC-2700 K. Their data were best fit by the expression
k , = 5.7
X
loi6exp(-58910/T) cm3/mol s
Tabayashi et aI.* studied the decomposition of H C N behind incident shock waves over the temperature range 2600-3600 K, using C N broad-band absorption around 388 nm. Their reported rate expression
k l = 1.26 X 10l6 exp(-50170/T) cm3/mol s is a factor of 6 above the results of Roth and Just in the overlapping temperature range. More recently, we used C N emission to study reaction 1 over the temperature range 3570-5030 K behind incident shock waves.3 The reported rate coefficient
k l = 4.07
X
lOI4 exp(-44740/T) cm3/mol s
agrees reasonably well with extrapolations of the results of Roth and Just based on weak-exchange process theory4vs but lies below (1) P. Roth and Th. Just, Ber. Bumenges. Phys. Chem., 80, 171 (1976).
(2) K. Tabayashi, T. Fueno, K. Takasa, D. Kajimoto, and K. Okada, Bull. Chem. SOC.Jpn., 50, 1854 (1977). (3) A. Szekely, R. K. Hanson, and C. T. Bowman, 'Shock Tubes and Waves", Proceedings of the 13th International Symposium on Shock Tubes and Waves, State University of New York Press, Albany, NY, 1982, p 617.
0022-3654/84/2088-0666$01.50/0
+
TABLE I: Kinetic Mechanism Employed in Modeling the HCN-AI System
reaction
+ A r + H + CN + Ar 2 H, + C N + H + HCN 3 CN + HCN+C,N, + H 4 C2N2+ Ar + 2 CN + Ar 1 HCN
5 H,
+ Ar+2
H
+ Ar
rate coefficient, cm3/mol s
ref
1.00 X 1 O I 6 exp(-54650/T)
this hork
7.5 x 1013 1.0 X l O I 3 6.47 X l o i 6 exp(-50040/T) 2.20 X 1014 exp(-48310/T)
6 7 8 9
the value reported by Tabayashi et al. by a factor of about 7 at 3600 K. In the present work, we have studied reaction 1 over the temperature range 2700-3600 K (nearly coincident with the temperature range covered by Tabayashi et al.) using CN broad-band absorption spectroscopy, and attempted to resolve the discrepancy between the results of ref 2 and the combined results of ref 1 and 3. Since this reaction is one of a few where rate data are available over such a wide temperature range (almost 3000 K), an accurate determination of kl should enable useful tests of unimolecular reaction rate theories.
Experimental Section The experiments were conducted behind incident shock waves in a 15.2-cm i.d. stainless-steel pressure-driven shock tube, which is described in more detail in ref 3. Mixtures of HCN in Ar (Airco Industrial Gases, 0.75% in Ar, with the following analyzed impurities:
