XCV.—The thermal decomposition of nitrogen pentoxide

1925 ◽  
Vol 127 (0) ◽  
pp. 657-671 ◽  
Author(s):  
Herbert S. Hirst
Keyword(s):  
Thermal Decomposition ◽  
Nitrogen Pentoxide

scholarly journals The thermal decomposition of nitrogen pentoxide at low pressures

1925 ◽  
Vol 109 (751) ◽  
pp. 526-540 ◽  
Keyword(s):  
Thermal Decomposition ◽  
The Other ◽  
Nitrogen Tetroxide ◽  
Nitrogen Pentoxide ◽  
Other Hand ◽  
Rate Of Reaction ◽  
Reaction Proceeds ◽  
Products Of Reaction ◽  
Glass Surfaces ◽  
Low Pressures

Unimolecular reactions possess a unique interest in that, as Perrin (‘Ann. Physique,’ vol. 11, p. 5, 1919) first pointed out, for the occurrence of such, some type of interaction between radiation and matter must take place. Although such reactions appear to be extremely rare, many physical processes such as evaporation, ionisation in gases at high temperatures and radio-active decay, proceed at rates conforming to a unimolecular law; true chemical reactions which are definitely unimolecular and not pseudo-unimolecular in character are, on the other hand, stated by many ( e. g ., Lowry, ‘Trans. Farad. Soc.,’ vol. 17, p. 596 (1922) ) to be non-existent. In order to substantiate this statement, it is clearly necessary to prove the more complex character of any reaction which satisfies the usual criteria of unimolecular change. The thermal decomposition of gaseous nitrogen pentoxide apparently fulfils these conditions, for Daniels and Johnston (‘J. Am. C. S.,’ vol. 43, p. 53 (1921)) showed that the reaction proceeded according to a unimolecular law over wide ranges of variation of pressure, and Lueck ( ibid ., vol. 44, p. 757 (1922)) obtained practically identical unimolecular constants for the decomposition in solution in carbon tetrachloride and chloroform. On the other hand, Daniels, Wulf and Karrer ( ibid ., vol. 44, p. 2402 (1922) ) suspected the reaction to be autocatalytic, owing to the apparent retardation of the reaction velocity in the presence of ozone, but the experiments of one of us (Hirst, ‘J. C. S.,’ vol. 127, p. 657 (1925), and of White and Tolman (‘J. Am. C. S.’ vol. 47, p. 1,240 (1925)) proved this to be erroneous. In addition, it has been shown that the reaction proceeds uniformly according to the unimolecular law even in the presence of extensive glass surfaces, or of gases which may be either indifferent, such as argon and nitrogen, or the products of reaction, such as nitrogen tetroxide or dioxide or oxygen. The rate of reaction may be expressed in the form - d C/ dt = 4·98 × 10 13 e -24.700/RT . C. Attempts have been made to interpret the experimental results on the hypothesis that the reaction is in reality bimolecular, and only apparently unimolecular in character; but owing to the abnormally large value of the energy of activation, namely, 24,700 calories per gram. molecule, the number of molecules which could be activated per second by inelastic collision, calculated according to the kinetic theory, falls far short of the observed reaction rate, being, in fact, some 10 5 times smaller.

A Study of the Thermal Decomposition of Nitrogen Pentoxide.

1927 ◽  
Vol 31 (10) ◽  
pp. 1572-1580 ◽  
Author(s):  
F. O. Rice ◽  
Dorothy M. Getz
Keyword(s):  
Thermal Decomposition ◽  
Nitrogen Pentoxide

scholarly journals A comparison between unimolecularand gaseous reactions. The thermal decomposition of gaseous acetaldehyde

1926 ◽  
Vol 111 (758) ◽  
pp. 380-385 ◽  
Keyword(s):  
Thermal Decomposition ◽  
Bimolecular Reaction ◽  
The Other ◽  
Unimolecular Reaction ◽  
Fundamental Characteristic ◽  
Chemical Similarity ◽  
Gaseous State ◽  
Nitrogen Pentoxide ◽  
Perfect Correlation ◽  
Rate Of Reaction

We have recently observed that the thermal decomposition of acetone in the gaseous state is a homogeneous, unimolecular reaction. As in the decom­position of nitrogen pentoxide, the number of molecules which react in unit time is very many times greater than the number which could receive the necessary energy by collision with other molecules. With the object of seeing whether this fundamental characteristic of unimolecular reactions would be confirmed in yet another instance, we have examined the thermal decomposi­tion of gaseous acetaldehyde, which at 500° C. decomposes smoothly into methane and carbon monoxide. The chemical similarity between this and the decomposition of acetone made it seem probable that acetaldehyde might also decompose in a unimolecular manner. But although the new reaction proved to be homogeneous, it was bimolecular. We therefore have the opportunity of making a comparison between the molecular statistics of two chemically similar reactions taking place in the same region of temperature, one of which definitely depends on molecular collisions, while the other appears to be independent of them. The contrast between the two supports strongly our previous theoretical conclusions, because, while in the unimolecular reaction the number of molecules reacting bears no relation whatever to the number which could be activated by collision, in the bimolecular reaction there is almost perfect correlation between the number of collisions suffered by activated molecules and the observed rate of reaction.

scholarly journals A homogeneous unimolecular reaction-the thermal decomposition of acetone in the gaseous state

1926 ◽  
Vol 111 (757) ◽  
pp. 245-257 ◽  
Keyword(s):  
Thermal Decomposition ◽  
Absolute Temperature ◽  
Unimolecular Reaction ◽  
Theoretical Discussion ◽  
Theoretical Interest ◽  
Gaseous State ◽  
Nitrogen Pentoxide ◽  
Different Temperatures ◽  
Acetone Vapour ◽  
Special Instance

Unimolecular reactions in gases have a rather exceptional theoretical interest, because they involve, apparently, the spontaneous change of isolated molecules. One example only has been satisfactorily investigated, the thermal decomposi­tion of nitrogen pentoxide, and an examination of the molecular statistics of this reaction leaves a certain mystery about the method by which the mole­cules are caused to decompose. It is unwise to infer too much from what might be a quite special instance; hence we have during the last few years searched for other examples. We now find that the thermal decomposition of acetone vapour satisfies the experimental criteria of a homogeneous, unimolecular reaction. Moreover, the peculiarities of the nitrogen pentoxide decomposition are here reproduced, although the absolute temperature at which the acetone decomposition can be studied is approximately three times as high as that at which nitrogen pentoxide reacts with conveniently measureable speed. Since then this analogy exists between the two reactions, taking place at such different temperatures, we may begin to draw theoretical con­clusions with more confidence. The theoretical discussion is reserved for the last section, after the experimental work has been described, because the conclusions depend very much upon the actual numerical results.

Applications of Photoelectron Microscopy

1976 ◽  
Vol 34 ◽  
pp. 506-507
Author(s):  
William J. Baxter
Keyword(s):  
Electron Microscopy ◽  
Thermal Decomposition ◽  
Chemical Composition ◽  
Ultraviolet Radiation ◽  
Thin Layer ◽  
Edge Effects ◽  
Electron Optics ◽  
Surface Layers ◽  
Crystalline Orientation ◽  
Fluorescent Screen

In this form of electron microscopy, photoelectrons emitted from a metal by ultraviolet radiation are accelerated and imaged onto a fluorescent screen by conventional electron optics. image contrast is determined by spatial variations in the intensity of the photoemission. The dominant source of contrast is due to changes in the photoelectric work function, between surfaces of different crystalline orientation, or different chemical composition. Topographical variations produce a relatively weak contrast due to shadowing and edge effects.Since the photoelectrons originate from the surface layers (e.g. ∼5-10 nm for metals), photoelectron microscopy is surface sensitive. Thus to see the microstructure of a metal the thin layer (∼3 nm) of surface oxide must be removed, either by ion bombardment or by thermal decomposition in the vacuum of the microscope.

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