Rate of Dissociation of N2O4 by Ultrasonic Absorption Measurements

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Abstract
Sound‐absorption measurements as a function of the sound frequency in the range of 78 to 394 kc/sec have been made in gaseous mixtures of nitrogen tetroxide, nitrogen dioxide, and a third inert gas. Inert gases studied were nitrogen, argon, and carbon dioxide. The absorption coefficient per wavelength varies with frequency in the typical bell‐shape manner associated with a relaxation mechanism. The frequency of maximum absorption increases with increasing concentration of N2O4, is nearly independent of the total pressure, and varies only slightly with temperature in the range 25° to 40°C. The sound absorption is ascribed to the perturbation of the equilibrium reaction N2O4=2NO2. The (equilibrium) rate of dissociation of N2O4 is calculated using an extension of the theory developed by Freedman. It is concluded that the dissociation of N2O4 proceeds according to a Lindemann unimolecular mechanism; specific rate constants for the limiting low‐concentration and high‐concentration regions at 25°C are 4.5×106 liter mole—1 sec—1 and 1.7×105 sec—1, respectively. The relative efficiencies of the various molecules for energizing N2O4 are: N2O4, 1.0; NO2, 1.0; N2, 0.50; Ar, 0.30; CO2, 1.0. Measured absolute values of the absorption coefficients are in good agreement with calculated values. The effects of viscosity, heat conduction, and vibrational heat capacity relaxation on the observed magnitude of the absorption are considered negligible.