MISCELLANEOUS NOTES ON GAS KINETICS BY S. C. LIND
The object of the present paper is to describe some experiments made several years ago which have remained unpublished on account, either of their uncompleted condition, or of the negative character of the results in one case. The recent trend of chemical kinetics appears to lend some interest to them either as suggestive of profitable directions of further research or possibly to spare others unnecessary experimental duplication. 1. Photo-sensitivity of a Hydrogen-Bromine Mixture at Higher Temperatures Reaction tubes of the Victor Meyer type, about 8 crns long, 2 crns in diameter, of z 5 cma volume, containing atmosphere (N. T.P.) each of Hz and Br2 were exposed at 250°C in a colorless vapor bath t o the light from a 5 ampere - I I O volt carbon arc lamp. The reaction tube and arc lamp were both in the vertical position, I O crns apart. It was found under these conditions that hydrogen and bromine combine at a rate I z times that of the dark thermal reaction' at the same temperature. Kastle and Beatty2 had previously shown in a qualitive way that at 196' C, Hz and Br2 combine in sunlight much more rapidly, than in the dark, and that even in diffused day-light the increase in rate was quite marked. The character of the experiments of Kastle and Beatty would hardly justify their use for quantitative calculation, and the promised fuller report by them did not appear. Consequently, the writer is prompted to use his results at 250' C for some consideration in connection with the Nernsta theory of the excessive reaction between Hz and Clz from the standpoint, of the Einstein' photochemical equivalence law. Nernst postulated that the wave lengths capable of absorption by Clz gas would liberate C1 atoms, which would take part in a cyclical reaction with Hz through the two partial steps: C1+ Hz= HC1+ H 2 5 ooo cals. and H+Clz=HCl+Cl+ 19000cals. Nernst assumed that on account of the positive heat of reaction of both steps this cycle would be spontaneously repeated a large number of times, thus accounting for the observed excess of a million fold over the requirements of the Einstein photochemical law. Nernst further stated that the well known photo-chemical inactivity of Hz+Brz is due to the fact that one step in the H2+Br2 cycle has a negative heat of reaction and therefore does not take place spontaneously, namely: Hz+Br=HBr+H- 15,000 cals.
+
3
Bodenstein and Iind: Z. phys. Chem. 57, 168(1907). Kastle and Beatty: Am. Chem. J. 20, 159 (1898) Nernst: Z. Elektrochem., 24,335 (1918) Einstein: A-n. Physik, (4) 37, 832;38,881 (1912)
56
S. C. LIND
Since, however, the H2+Br2 reaction does become light-sensitive a t higher temperatures, as shown by Kastle and Beatty and by the present paper, one of three conclusions seems necessary: (I) The heat of reaction of the step H2+Br may change so as to become at 250’. A thermodynamic calculation based on the data of Lewis and Randall‘ for free energy and entropies based on the third law indicates that this can not be the case. This leaves open only the other two possibilities2: ( 2 ) that the Nernst “multicyclical” hypothesis is not correct or (3) that the H2-Br2reaction under the conditions of the experiment does not exceed the requirements of the Einstein law, and therefore no multi-cyclical hypothesis is required. A rough calculation based on the experiment reported above for H2+Br2 a t 2 5 0 ° , indicates that the rate of reaction observed will not exceed the photo-chemical equivalence, no matter what assumptions are made as to absorption efficiency and other influencing factors, and hence the Nernst hypothesis is not violated by the H2+Br2 reaction. The fact, however, that the Nernst principle is not violated in the Br reaction does not, by any means, constitute a proof that it is true in the H2+C12 reaction*. I n fact, it‘would be difficult to reconcile it with an observation attributed by Kastle and Beatty (loc. cit.) to Amato, that H2 and C12 a t - 12’ C may be exposed for hours to direct sunlight without combination taking place. It is entirely improbable that the heat of reaction of either of the partial reactions involved in the interaction of H2and C12 can change from a high positive to a negative value in the temperature interval of + 2 0 ° to - 12’ C. A thermodynamic calculation similar to the one used for the H2+Br2 reaction does not indicate much change in velocity in this interval. This leaves then only the choices either that the absorption diminishes so that there is no primary atomisation at - 12’, or the Nernst mechanism or Amato’s observation is incorrect. It seems for several reasons that the latter is the case. I n the first place, we do not in general find photochemical reactions to have high temperature coefficients. The use of the Bunsen and Roscoe actinometer involving an aqueous system has not permitted of wide variation of temperature, but the data available4 do not show values that would explain Amato’s result. Bodenstein and DuxS worked in a non-aqueous system and reported that within their limited range, temperature had little or no influence. It therefore appears that the observations of Amato at - 12’ must have been due to the presence of O2or some other inhibitor. However, in view of its important bearing on the Nernst hypothesis, the temperature co-efficient of the H2+C12 reaction ought to be made the subject of further investigation.
+
Lewis and Randall: ‘LThermodynamics’’pp. 464 and 607 (1923) possibility which does not particularly concern the Nernst hypothesis. The absorption of bromine may increase with temperature so that the necessary atomization first takes place at a temperature above the ordinary. This would furnish an additional reason for the absence of interaction between bromine and hydrogen a t ordinary temperature and would throw no doubt upon the Nernst hypothesis, meaning simply that no bromine atoms would be present and that no cyclical reaction would take place even if they were. See note a t end of this paper. M. Padoa: Gazz. chim. ital. 511, 193 (1921) Bodenstein and Dux. Z. phys. Chem. 85,306 (191.3) 1
* There is another
MISCELLANEOUS NOTES ON GAS KINETICS
57
2. Activation of a H2+C12 Mixture for Thermal Reaction In the course of some experiments with electrolytically generated Hz+ Clz mixture it was observed that the thermal reactivity increases progressively to a remarkable degree with the continued generation from HCI solution. This is quite similar to the well known increase of photo-chemical activity of such mixture, and suggests that the inhibitors for the thermal and the photochemical reactions may be identical in nature, as well as the reaction mechanism in both cases.
Attempt to arrest the Propagation of an Explosion in a Gas Mixture by an Electrical Field The following experiment was made about I 2 years ago under the impression that flame propagation is perhaps maintained by electron emission in the flame front and that if the electrons could be removed from the field of action by the imposition of an electrical field, the flame might be arrested. The advances made since that time in the theory of flame propagation as well as in electronics, would perhaps, not justify the experiment to-day, but it will nevertheless be briefly described in hopes that the negative result may have some interest in other connections. A straight glass tube about I meter long and 1.5 cms internal diameter, closed with ordinary glass connections and stop-cocks a t both ends, was cut in the middle so that a brass section about I O crns long, of the same bore could be inserted. This was accomplished by means of a close-fitting glass sleeve over the outside of the joints, made gas-tight by Khotinski cement. The tube was mounted horizontally and an ordinary thin steel knitting needle was used as a central axial electrode through the brass section. The needle projected out into the glass tubes a t both ends where it was centered and electrically insulated by resting on thin glass legs. To one end of the needle was fastened a platinum wire which was sealed through the glass tube t o connect with one pole of a Wimshurst machine, the other pole being connected to the brass section of the explosion tube. A platinum spark gap was placed at one end of the tube to ignite the gases. Electrolytic 2H2S-02 mixture, generated from NaOH solution was introduced into the tube and brought to any desired pressure by means of a manometer. The pressure was first ascertained a t which an explosion flame would just pass through the tube without a field. The explosion mixture was then introduced a t a pressure somewhat above this critical value, the field was applied, the mixture sparked, and by observing in a dark room, one could easily see whether the flame traveled the entire length of the tube or stopped on reaching the brass section where the field was applied. The first experiments served to indicate that the flame could be arrested a t pressures as much as 2 5 mms above the critical explosive pressure. The results, however, were illusory owing to two factors: 3.
(I) Moisture on the walls from previous combustion.
58
S. C. LIND
(2) Thin sparks or silent discharge in the brass section previous to the explosion caused premature impoverishment of the mixture by substitution of water vapor. It was finally concluded that the flame is not arrested by an electrical field. However, the source of voltage was not satisfactory. If the experiment were repeated a t all, it should be with a more steady source of potential. The only two observations that appear of possible interest were: (I) That one can easily detect the difference between the bright yellow Na flame in H2 and O2 generated from NaOH or of the pale violet K flame in Hz and O2 from KOH solution, even when the Hz+02 mixture has been washed through HzSO4solution, through several inches of Closely packed cotton, and also allowed to stand in a gasometer for several weeks before use. ( 2 ) Even below the explosive pressure, a Hz+Oz mixture will be caused to combine rapidly by continued sparking. Since the energy supply to maintain sparking is much smaller than that to maintain incandescent glow to a wire or wire-net system it is possible that a sparking system might be used instead of a glow system to keep explosive gases from accumulating in switchboxes,’ city sewer systems or even in certain parts of mine workings. I am indebted to Dr. D. C. Bardwell for his kind assistance with the calculations involved.
Washington,D. C. July 1923.
Note at the time of Proof Inspection I n a more recent paper (“Sitzb.preuss.Akad. May 3 , 1923, pp. 110-II~), Nernst and Noddack express strong doubt as to the general applicability of thermodynamic treatment of photochemical partial reactions, which doubt is shared by the writer. They nevertheless state that the chain or multicyclical theory probably remains the most likely one for the hydrogen-chlorine reaction. At the same time, they suggest the possibility that it may be the Clz molecule which is activated, and which reacts directly with Hz. The HCI formed retains an excess of energy for a short time which it is able to impart afresh t o other C12 molecules. This is the theory originally put forward by Bodenstein (Z.Elektrochern.22,53-61(1916)), which would have a more general applicability than the atomic chain theory, as was pointed out by the writer (“Chemical Effects of Alpha Particles and Electrons”, p. 141). The probability that the energy of activation can be imparted in this way has received indirect but strong support in the recent results of Cario and Franck (Z.Physik. 11, 161-6 (1922) ). Washington, November 1923. 1H. C. P. Weber: Chem. Met. Eng. 27,942 (1922)
