ON T H E NATURE OF ACTIVATED MOLECULES L. J. TVALDBAUER AND I. J. PATTON
The radiation hypothesis of chemical reaction, as held by Trautz, Perrin, Lewis, and others, is that the increase in internal energy which a molecule must receive before it is capable of reacting (Le., the critical increment E) is communicated to it by infra-red radiation present in the system, the addition of energy being made in terms of quanta. I n accordance with this hypothesis, chemical action depends upon the absorption of a nearly monochromatic radiation of frequency v, given by the relation E, = Khv, where E, denotes the critical increment per gram-molecule. There should, therefore, be found a line of this frequency, or a band including it, in the absorption spectrum. 4 o m , while this sometimes occurs, there is a very large number of cases in which there is no such correlation whatever.’ Furthermore, since an increase in the velocity of the reaction with temperature must, on this hypothesis, be due to increased radiation density U, dv, the temperature coefficient of the velocity constant should be the same as that of the radiation density. Trautz and Lewis2have calculated the rate of bimolecular reactions from the frequency of collision between active molecules and obtained a relation agreeing with experiment. But this agreement in no way proves their hypothesis, for, in plain language, their result means that the velocity of reaction depends on the number of collisions between molecules which, by whate x r mechanism, are already capable of reacting; a statement that is almost axiomatic. They do assume “heats of activation,” Q.. and Qb per grammolecule of substances X and B respectively; but since in their equations only the sum of these two appears, it will be seen that we have again simply our old friend, “heat of reaction,” which may be considered quite apart from any imagined mechanism. This lack of any necessity to assume a radiation mechanism in the case of bimolecular reactions, has been pointed out by Dushman3 who, however, consideis some such assumption necessary in the case of monomolecular reactions. Since, however, outside of radioactive disintegrations, the very existence of monomolecular reactions is extremely problematical, it is doubtful whether a radiation hypothesis is applicable even there. True monomolecular reactions are to be expected between gases, If anywhere; yet every case investigated had been found t o be a catalyzed reaction, induced either by the walls of the vessel, by impurities in the reactants, or by the reaction products themselves. Furthermore, the fact that the only reactions yet Langmuir: J. Am. Chem. SOC.,42,2090(1920). J.Am. Chem. Soc.,43,39;(1921). 3 Taylor: “Treatise on Physical Chemistry,” 2 , 1036 (1924).
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L. J. WALDBAUER AND I. J. PATTON
recognized as truly monomolecular (namely, disintegration of radioactive elements) are known to continue unaltered, in the dark, and at temperatures approaching the absolute zero, seems to indicate very clearly that the absorption of energy quanta plays no part’,and that the explanation must be sought in the interaction of electrons and protons in the crowded and complex nucleus. It appears to the authors that a very simple explanation of the existence of active molecules can be obtained without recourse to any hypothesis of radiation. I n the Bohr atom, the electrons move in elliptical orbits, with the nucleus a t one focus. Their velocity is greatest near the nucleus and least in t,he outer portion of the ellipse. The valence electrons are, of course, furthest from the nucleus, and therefore less firmly held, on the average, throughout their journey; but at the outer limit’s of their orbits, they are a t their greatest distance Erom the nucleus, and a t their slowest speed. Collisions between molecules in this state will be almost certain to cause reaction, and such molecules can be called “activated.” This does not exclude the absorption of radiat’ion quanta as an auxiliary influence. I t s effect will be merely to move the valence electron into an outer orbit, further removed from the attraction of the nucleus, and consequently to produce a somewhat higher degree of activation. The increase of velocity caused by rise of temperature will be due only in small measure, if at all, to the absorption of low frequency radiation, but will depend almost entirely on the rapid increase in the number of collisions and in the force of each collision, due to the increased velocity of the molecules. h very simple calculation on the basis of the gas laws will show, for instance, that in the case of hydrogen, when the temperature is raised IO’, from 40’ to j O ” C . , the kinetic energy is increased by 6 3 . j m (m = mass of H2molecule), and the momentum is increased by 0.97 ni. I t will be seen that factor is sufficient in itself to account for the approximate doubling of the reaction rate on a 10’rise in temperature. Unfortunately, like the radiation hypothesis, this explanation of the activated molecule is not at present susceptible to experimental or mathematical demonstration, since the question of the probability of an electron or electrons being in that section of their orbits where they are most likely to react involves the yet unsolved problem of the motion of “n” bodies. .@ ru? du Capitaine Ferber,
Paris ( X X ? ) ,France.
