Nitrogen pentoxide, decomposition

Applying this result to the then-unresolved mechanism for nitrogen pentoxide decomposition, Busse and Daniels suggested the mechanism to be... 

Unpiotonated hydioxylamine is oxidized rapidly by ozone, / = 2.1 X 10 (39). The reaction of ozone with the lower oxides of nitrogen (NO and NO2) is also rapid and quantitative the end product is nitrogen pentoxide, which is also a catalyst for the decomposition of ozone (45). Nitrous oxide, however, reacts slowly (k < 10 ) (39). Nitrogen-containing anions, eg, nitrite and cyanide, also ate oxidized by ozone (39). Nitrite is oxidized to nitrate (fc = 3.7 X 10 and cyanide is oxidized rapidly to cyanate (fc = 2.6 X 10 (46) and 10 -10 (39)). Cyanate, however, is oxidized slowly. 

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... 

Experimental studies of the catalytic decomposition of ozone in the presence of nitrogen pentoxide follow a rate expression of the form... 

Rate constants for the first-order decomposition of nitrogen pentoxide (N2O5) at various temperatures are as follows (Alberty and Silbey, 1992, p. 635) ... 

NITROGEN PENTOXIDE 3.4.1 Thermal decomposition of N205 (a) Gas phase decomposition of N2Os... 

The thermal decomposition of nitrogen pentoxide proceeds according to the stoichiometric equation... 

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. ... 

There are many examples of first-order reactions dissociation from a complex, decompositions, isomerizations, etc. The decomposition of gaseous nitrogen pentoxide (2N2O5 4NO2 + O2) was determined to be first order ( d[N205]/dt = k[N205j) as is the release of product from an enzyme-product complex (EP E -t P). In a single-substrate, enzyme-catalyzed reaction in which the substrate concentration is much less than the Michaelis constant (i.e., [S] K ) the reaction is said to be first-order since the Michaelis-Menten equation reduces to... 

The question arises in an even more fundamental form when we consider such changes as the slow decomposition of nitrogen pentoxide, and we have to seek the reason why the molecules do not decompose all at once or not at all. Chemical transformations taking place in finite time thus... 

Where the rate of reaction depends upon the presence of an accidental catalyst, measurements are characterized by great lack of reproducibility. Many homogeneous reactions, on the other hand, have quite definite and reproducible rates. For example, the measurements of Bodenstein and of Kistiakowsky on the rate of decomposition of hydrogen iodide agree excellently, as do those of numerous investigators of the rate of decomposition of nitrogen pentoxide. 

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

Nitrogen peroxide has no influence on the thermal decomposition of nitrogen pentoxide, but an almost instantaneous reaction occurs between nitric oxide and the pentoxide ... 

According to Schumacher and Sprenger f an oxide N03 is produced when nitrogen pentoxide reacts with ozone, but this does not appear to play any part in the thermal decomposition. 

In the earlier experiments on the decomposition the nitrogen pentoxide had never been free from some nitrogen dioxide, since in the sealing off of the apparatus a certain amount of decomposition had been inevitable. Daniels,... 

Wulf, and Karrer attempted to repeat some of the previous work in the presence of ozone, which ensures the complete absence of lower oxides since it oxidizes them instantaneously to the pentoxide. In the decomposition of the pentoxide, therefore, there should be a period of induction during which the decomposition products are re-oxidized by the ozone. When all the ozone is used up brown fumes should appear and the decomposition of the pentoxide should go forward in the normal way. It appeared, however, that when the lower oxides were removed in this way the decomposition did not take place at all. This led to the conclusion that the reaction was not, after all, a simple unimolecular decomposition but a change depending on collisions between molecules of nitrogen pentoxide and molecules of nitrogen dioxide. 

Rice and Getz J made a very complete study of the matter from this point of view. At 65° C. they found k = 0-286, compared with the value 0-292 of Daniels and Johnston. In order to test the possibility that the reaction might depend on catalysis by dust, they compared the velocity constants for filtered and unfiltered nitrogen pentoxide and for gas which had been passed through an electrical dust precipitator. In some experiments the gas was dried with phosphorus pentoxide, in others not. In some it was prepared by the dehydration of nitric acid with phosphorus pentoxide and in others by the action of chlorine on silver nitrate. Nitric acid was found to have no catalytic effect on the decomposition. Some of the principal results are summarized below, all data referring to 65° C. 

Busse and Daniels have, moreover, found that hydrogen, carbon monoxide, bromine, and chlorine are without influence on the reaction. Certain organic vapours, which are themselves attacked by nitrogen pentoxide, bring about rapid decomposition. Hirst found that argon exerted no influence, while Hunt and Daniels showed that the presence of a large excess of nitrogen did not alter the rate of reaction at all. 

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. 

Decomposition of Nitrogen Pentoxide. The principal facts about this reaction have already been stated. The observation of Sprenger that below a certain pressure the reaction ceases entirely indicates a chain mechanism, but its nature is not clear. 

The very low value of Ashmore and Burnett is difficult to explain. It is easy to demonstrate that the discrepancy is not resolved by assuming the N03 intermediate in nitrogen dioxide decomposition is the pernitrite radical, in contradistinction to the symmetric nitrate radical. Their calculation of k5 depended on an experimentally obtained value for k 5 and an equilibrium constant K5- 5 calculated from thermodynamic properties for N03 measured by Schott and Davidson and Ray and Ogg. These results, obtained in a nitrogen pentoxide system, pertain to the nitrate radical, not the pernitrite radical. Guillory and Johnston176 reported an equilibrium constant based on estimated... 

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... 

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