The decomposition of nitrogen pentoxide

Nitrogen pentoxide can be made by dehydration of anhydrous nitric acid with phosphorus pentoxide and sublimation from the mixture at about 40°. Resublimation with ozone and more phosphorus pentoxide gives pure white crystals which rattle around in the reaction chamber after slight evacuation. Direct oxidation with ozone of nitrogen dioxide prepared by heating lead nitrate is a somewhat better way of making the material. 

The purified nitrogen pentoxide is sealed off in apparatus like that of Fig. 10, submerged completely in a thermostat, and pressure readings are taken at frequent intervals over the whole course of the reaction. 

The concentration of nitrogen pentoxide at any time is determined from the total pressure, but the calculation is complicated by the fact that one of the decomposition products, NO2, undergoes association, and this equilibrium, 2N02 = N204, shifts continuously as the reaction progresses and the products accumulate. It is convenient to use a graphical method of calculation. 

The degree of dissociation a of N2( 4 is obtained from its dissociation constant K, and values of Pnnai are plotted against various values of pi so that the proper initial pressure can be picked off on a curve from any final pressure. 

From a similar graph4 it is possible to calculate the partial pressure of nitrogen pentoxide, n2o6 at any time knowing the total pressure and the initial pressure. The pressure increase, P total — pi, 4 Details are given in J. Am. Chem. Soc., 43, 53 (1921). 

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In the case of relatively simple reaction mechanisms, the net or overall effect of the elementary reactions can be determined by adding them together. For example, the stoichiometric equation for the decomposition of nitrogen pentoxide is... 

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. ... 

In 1925 the only known gaseous unimolecular reaction was the decomposition of nitrogen pentoxide. The result of the decomposition is expressed by the equation... 

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. 

Mills and Johnston313 found that the decomposition of nitrogen pentoxide, both by itself and in the presence of nitric oxide, could be adequately described by a mechanism that elaborated slightly on the proposal of Ogg... 

The unit of the velocity constant k is sec-1. Many reactions follow first order kinetics or pseudo-first order kinetics over certain ranges of experimental conditions. Examples are the cracking of butane, many pyrolysis reactions, the decomposition of nitrogen pentoxide (N205), and the radioactive disintegration of unstable nuclei. Instead of the velocity constant, a quantity referred to as the half-life t1/2 is often used. The half-life is the time required for the concentration of the reactant to drop to one-half of its initial value. Substitution of the appropriate numerical values into Equation 3-33 gives... 

These formulas are nicely illustrated in Table I and in Figs. 1, 2 and 3 with data taken from the next following chapter on the decomposition of nitrogen pentoxide at 45°. 

This calculation is illustrated in Fig. 5 using data on the decomposition of nitrogen pentoxide (page 64). The data of Table II are obtained by interpolation and extrapolation with this formula. 

Fig. 14.—Pressure-time curves showing the decomposition of nitrogen pentoxide at various temperatures.

The Decomposition of Nitrogen Pentoxide in tiie Presence of the Solid Phase at 25°... 

We have discussed the influence of temperature on the decomposition of nitrogen pentoxide and we next consider the influence... 

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. 

The simplest reaction which has been studied directly in the gas phase and in solution is the decomposition of nitrogen pentoxide.11 It is not a chain reaction and it is free from wall effects. The gas phase reaction seems to be free from complications and it has been checked in many laboratories. It is an excellent unimolec-ular reaction, the decomposition rate being exactly proportional to the concentration. This proportionality constant is nearly the same from 0.05 mm. to 1,000 mm. in the gas phase and up to an osmotic pressure of fifty atmospheres in solution, and the energy of activation is practically the same in the gas phase and in a group of chemically inactive solvents. 

The effect of radiation of 3,660 A on the decomposition of nitrogen pentoxide, and dioxide is shown in Table II. 

In general, the reaction of decomposition of N205, which is seemingly of the first order (according to Smith and Daniels [99], Johnston and his co-workers [100-102]), is in fact the resultant of reactions (a) and (b) and of the second order reactions (c) and (d). Indeed, Johnston [100] found the decomposition of nitrogen pentoxide in the presence of nitrogen dioxide to be a second order reaction. 

Regardless of the particular mechanism, if the activation process is accomplished through the absorption of single or multiple frequencies, then a marked increase in the radiation density of these frequencies should cause a change in reaction velocity. The decomposition of ozone and nitrous oxide have been studied from this point of view and are reported here. Since the inception of this work, it has been found elsewhere3 that the oxidation of alcohol vapor and the decomposition of nitrogen pentoxide and hydrochloric acid were not affected by infra-red radiation. The present work confirms these results for two additional reactions. 

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... 

The reaction mechanism for the decomposition of nitrogen pentoxide is complex, as described in Sec. 2-2. However, a satisfactory rate equation can be developed by considering the two reactions... 

Now let us determine graphically the order and rate constant of the decomposition of nitrogen pentoxide in carbon tetrachloride (CCI4) solvent at 45°C ... 

On completion of his graduate training he became an instructor in the Chemistry Department of the University of Wisconsin, 1927-1928, and research associate in the following year. There, in association with Professor Farrington Daniels, he made his first major contribution to reaction kinetics—and that an experimental one—the demonstration that in liquid solvents, as in the gaseous phase, the decomposition of nitrogen pentoxide was a unimolecular reaction. How puzzling unimolecular processes were at that time can be fully appreciated only by those who then learned of their... 

In the decomposition of nitrogen pentoxide the situation w as even more inexplicable until it w as suggested that the reaction proceeds in steps and that the first-order rate constant must not be identified with a single elementary reaction (Ogg, J. Chem. Phys. 1947,15, 337). 

A great many reactions follow first-order kinetics or pseudo first-order kinetics over certain ranges of experimental conditions. Among these are many pyrolysis reactions, the cracking of butane, the decomposition of nitrogen pentoxide (N2O5), and radioactive decay processes. 

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