Nitrogen pentoxide at low pressures

The most satisfactory explanation is that the rate is increased beyond the maximum rate of activation by collision through the operation of a chain mechanism. The observations of Sprenger on the peculiar behaviour of nitrogen pentoxide at low pressures suggest strongly that chains are propagated. Moreover, if the rate of the azoisopropane reaction at the lowest pressures should prove to be greater than can be accounted for on the basis of the simple collision mechanism, a chain mechanism can be assumed without difficulty since the reaction is quite markedly exothermic. 

Fig. 16.—Influence of pressure on the specific reaction rate k for the decomposition of nitrogen pentoxide at low pressures.

THE THERMAL DECOMPOSITION OF NITROGEN PENTOXIDE AT LOW PRESSURES By j. H. Hodges and E. F. Linhorst Department of Chemistry, University of Michigan Communicated December 13, 1930... 

Further tests of the influence of infra-red radiation on the reaction velocity have been made by Rice, Urey, and Washburne,f who exposed a molecular beam of nitrogen pentoxide to black body radiation of high temperature with a negative result, and by Kassel, J who also found that radiation of wave-length less than 5 n does not increase the reaction rate even at low pressures, where its effect relative to that of collisions might have been expected to be greater than at atmospheric pressure. 

The only example of all the unimolecular reactions known where such a difficulty has actually arisen in an acute form is the decomposition of nitrogen pentoxide. It appears that at low pressures nitrogen pentoxide reacts at a rate which is considerably greater than the maximum possible rate of activation by collision, however great a value of n be assumed. There is a limit to the maximum rate theoretically possible, since, when n is increased beyond a certain point, the increase in the term E — EArrhenius + n- )RT produces a decrease in the calculated rate which more than compensates for the increase due to the term (E/RT)1l2n 1 multiplying the exponential term. 

The question immediately arises as to how far this independence of collision will continue as the pressure of nitrogen pentoxide is indefinitely decreased. The matter is of great importance for checking theories of chemical kinetics and accordingly many other investigators undertook the study of the decomposition of nitrogen pentoxide at very low pressures. 

Summary.—The mechanism of the activation process in gaseous systems has been investigated from the point of view of (1) activation by radiation (2) activation by collision. An increase in the radiation density of possible activating frequencies has resulted in no increased reaction velocity. The study of the bimolecular decomposition of nitrous oxide at low pressures has led to the conclusion that the reaction is entirely heterogeneous at these pressures. A study of the unimolecular decomposition of nitrogen pentoxide between pressures of 7io mm. Hg and 2 X 10 3 mm. Hg shows no alteration in the rate of reaction such as was found by Hirst and Rideal but follows exactly the rate determined by Daniels and Johnson at high pressures. No diminution of the reaction velocity as might be ex-expected from Lindemann s theory was observed. 

The only method utilized commercially is vapor-phase nitration of propane, although methane (70), ethane, and butane also can be nitrated quite readily. The data in Table 5 show the typical distribution of nitroparaffins obtained from the nitration of propane with nitric acid at different temperatures (71). Nitrogen dioxide can be used for nitration, but its low boiling point (21°C) limits its effectiveness, except at increased pressure. Nitrogen pentoxide is a powerful nitrating agent for alkanes however, it is expensive and often gives polynitrated products. 

In a purely photochemical reaction the absorption of radiant energy is plainly responsible for the activation. This suggested the possibility that thermal reactions are also due to activation by the thermal radiation which is present at every temperature. The argument was very forcibly presented by Perrin who showed that if the specific rate of a imimolecular gas reaction remains constant, with indefinite diminution in pressure, activation must be by radiation since the number of opportunities for activation by collision also diminishes without limit. In fact, the decomposition of nitrogen pentoxide, the first gas reaction shown to be unquestionably unimolecular, was found to have a specific reaction rate constant over a wide range of pressure, and apparently increasing at very low pressures. ... 

The technique and the reaction system developed for the foregoing reaction was then applied to the study of a unimolecular reaction. The decomposition of nitrogen pentoxide has been found to remain unimolecular over enormous variations in pressure.12 But what is of most concern to the immediate problem is that it seems to remahji unaffected at extremely low pressures. Hunt and Daniels13 found that the decomposition rate was unchanged even when the partial pressure of nitrogen pentoxide... 

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