NOTE ON XEGATIVE CATALYSIS BY J. A. 'CHRISTIANSEN
H. S. Taylor1 has recently proposed an interesting explanation of the phenomenon of negative catalysis in homogeneous systems based upon the supposition that the formation of molecular compounds with the negative catalyst prevents the molecules from reacting. Although this explanation is possible in some cases, it seems to me that it certainly cannot be PO in others. As I discussed this problem some two years ago in a Danish paper2 which has only been translated in part into German, I should be glad to take this opportunity of discussing the matter once more. In recent years our knowledge of bimolecular reactions has developed so far that we are able to form a somewhat definite picture of the mechanism t,hereof. This knowledge can be traced back to a paper by Arrhenius3. The more recent development is due to Kruger4, Goldschmidts, Marcelin6, W. C. McC. Lewis' and others. The first condition for a reaction to occur is a collision between the two molecules in question. However, when the reaction is not immeasurably fast, only a minute friction of the colliding pairs of molecules react, viz. those whose energy exceeds a certain value. On this assumption we are able approximately to estimate the reaction and calculation shows that approximately v=se
-Q
(1)
Here s denotes the gas-kinetic collision number and Q the mean difference between the energy of the reacting and the non-reacting pairs of molecules. The equation simply expresses that the number of molecules reacting is equal to the product of the number of colliding molecules and the probability for a pair of molecules to have the required energy. Equation ( I ) has been verified experimentally in several cases and there seems at present to be no other serious objection against the underlying hypothesis than that it seems to exclude the possibility of negative catalysis. If, namely, we admit to the reacting mixture a small quantity of a foreign gas, this gas cannot alter either the number of collisions or the energy distribution of the colliding molecules appreciably, i.e. neither of the two factors in Equation ( I ) . Of course the possibility exists that a certain fraction of one or both
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J. Phys. Chem. 27, 322 (1923). Reaktionskinetiske Studier, Diss. Copenhagen, (192I). Z. phys. Chem. 4,226 (1889;. GOttinger Nachrichten 1908, pp. 1-19. Physik. Z. 10, 206 (1909). Ann. phys. 3, 120 (1915). J. Chem. SOC.1915 and following years.
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J. A. CHRISTIANSEN
of the original gases can form non-reactive compounds with the foreign gas, but this fraction must be small if the ratio between the number of molecules of the latter and the former gas, respectively, is small. This remark is important because it seems that Taylor does not grant its correctness. He compares the case with that of a warden caring for one hundred lunatics. “The wardens would be powerless were all the lunatics simultaneously violent. Only a t intervals does an occasional lunatic become a candidate for a padded cell. The warden cares for him, the gentler ninety-nine do not require attention.” But this analogy is false because we cannot assume the molecules of the inhibiting gas to be intelligent and combine just with those molecules which they perceive being about to react. The possibility remains that the molecules of the inhibitor are able to prevent the elementary reaction by ternary encounters with the reacting pairs of molecules, e.g. by going off with part of the energy necessary for reaction. But just as was the case in the compound-formation theory, if the number of inhibiting molecules is small as compared with the number of the reacting ones, this effect cannot alter the reaction velocity appreciably. Thus it seems, that homogeneous negative catalysis should be impossible if the mechanism pictured above ie correct. However, it is not so because we have not yet taken into consideration the full consequences of our hypothesis. It is necessary also to take into account that the molecules of the reaction products just after the reaction possess an available energy greatly in excess of the mean energy at the temperature considered. Not only do they contain the energy (usually denoted as the critical energy) which was necessary for the reaction t o occur, but, as is well known, the reaction is very often accompanied by an evolution of heat which must appear in the resulting molecules from the reaction as kinetic energy or potential energy easily transformable into kinetic, Now these very “hot” molecules have sufficient energy to activate molecules of the reactants a t the first encounter, and when these react, the resultants in their turn again are able to act as a.ctivators and so on. Consequently, it is possible that the occurrence of one elementary reaction will give rise to a whole series of such reactions. That something like this sometimes will occur was suppoeed by M. Bodenstein’ as early as in 191.3,and he used this assumption (the hypothesis of “chain-reactions”) to explain the velocity of t.he photo-chemical formation of hydrogen chloride, Later on Bodenstein himself,2 Nernst3 and others discussed the kinetics of such reactions in more detain and finally the hypothesis was tested experimentally by Weigert4,who succeeded in proving it in a rather direct way, and consequently we must admit both on theoretical and experiZ. physik. Chem. 85, 346 (1913). Z. Elektrochem. 22, 63 (1916). a Die theoretischen und experimentellen Grundlagen des neuen Wjirmesatees, p. 134 (1918). 4 Weigert and Kellermann: Z. Elektrochem. 28, 456 (1922). I
NOTE ON NEGATIVE CATALYSIS
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mental reasons that such chain-reactions are in fact possihle. We must add that there is no reaeon to be seen why they should not occur also when the reaction is a thermal dark reacti0n.l We are now able to understand the phenomenon of negative catalysis. The velocity of the chain-reaction is obviously equal to the number of chains starting per second multiplied by the number of links in each chain. We should expect that the former number could be evaluated by means of Equation ( I ) and consequently we cannot expect this factor to depend on the presence of small quantities of foreign molecules to any considerable degree. But the latter number obviously depends on the probability of the breaking of a chain and this probability can be altered considerably by the admixture of small quantities of a foreign gas, the molecules of which are able to take up the energy from the “hot” molecules of the reaction products or to react with them in some way or another. A somewhat detailed discussion of the kinetics of a chain-reaction, viz. the formation of hydrogen chloride is to be found in a paper by R. Gohring2 and also in my Danish paper. I n these two papers it is shown that the oxygen according to Bodenstein’s experiments and the above theory acts as an inhibitor by removing the active hydrogen molecules (or atoms?) resulting from the elementary reaction, and not by inactivating the active chlorine molecules (atoms?). The theory enables us not only to understand the occurrence of negative catalysis but by reversing the argument we are able to discover instances of chain-reactions. If we find homogeneous reactions which are inhibited by minute quantities of foreign substances, it is often possible to conclude that chain-reactions occur in the mixture. As typi‘cal instances we might mention, besides the hydrogen chlorine reaction, the oxidation of phosphorus at ordinary temperature and the oxidations of different organic substances (e.g. acrolein) studied by Moureu and ITufraisse.8 Although apparently the last-mentioned reactions are not homogeneous, a more detailed analysis probably will show that they are in fact so. Also they both have the characteristics of chain-reactions to such a degree that it seems impossible to explain their peculiarities by the hypothesis that the inhibitor covers the surfaces of the oxygen acceptors. Finally it must be remarked that the above view does not necessarily cover the case of the dissociation of oxalic acid dissolved in concentrated sulphuric acid. I n my Danish dissertation I proposed for reactions which are going on in a system in thermal equilibrium the name homothermal reactions, while reactions which are brought about by radiation (e.g. from the sun) or molecules (e.g. from a glowing wire not in thermal equilibrium with the reacting mixture could be named heterothermal reactions. * 8.Elektrochem. 27, 5 1 1 (1921). Compt. rend. 174, 258 (1922);176,624,797 (1923). As will be seen from the above the hypothesis set forth by these authors concerning the mechanism of this reaction is not sufficient t o explain the inhibitory action of so small quantities of hydroquinone, but it becomes so if the occurrence of chain-reactions also is taken into account.
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J. A. CHRISTIANSEN
Contrary to what was the case in the above reactions we have here in the system a component (HzS04)which is reversibly dissociable with formation of two or several substances in small concentrations, one of these being the inhibitor, in this cape, water. Consequently if we assume the velocity of the reaction (C0OH)z --t C02+CO+HzO to be determined by the concentration of SO3,i.e. if the primary reaction takes place between (COOH)2and SOj we understand at once that a small increase in the concentration of water can give rise to decrease of the velocity on account of the displacement of the equilibrium
HzS04
+HzO+S03
That this assumption has some reality in it we see from the remark in Bredig’sl paper that addition of 1% SOs t o the 100% HzS04 increases the velocity to such a degree that it becomes unmeasurably fast. On the other hand it seems to me that the mechanism of the inhibition pictured by Taylor, viz. that the water forms a comound with the sulphuric acid or (and) with the oxalic acid cannot possibly be the true one. However complete the compound-formation between the acid and the water be, water in small amounts can never appreciably alter the concentration of the substance ( (COOH)2and HzS04) which according to his picture determines the velocity. Generally speaking it seems that the reaction-mechanisms in the known cases of negative catalysis are so different that they can hardly be considered from one single viewpoint but they must be divided into several classes, one of which includes the important cases of chain-reactions mentioned above. Chemical TJaborntory, Uniuersity OJ Copenhagen. Bredig and Lichty: Z. Elektrochem. 12,450 (1906).
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