4
THE PHOTOCHEMICAL DECOMPOSITION OF
amounted to 32 per cent. From this result it can be inferred that one or more of the wave-lengths 313, 302, and 297μu are effective in bringing about the decomposition. From the relative intensities of the three lines it becomes very probable that the 313μ line is mainly responsible for the decomposition. To test this a filter was sought for, which would leave the 313 line as the shortest line being transmitted to the reaction vessel, and which, at the same time, would not grdly diminish the intensity of this line. The filter referred to in the next case was found to fulfil this function.
Experiment 8. Vessel Wall + Filter of p-Toluidine between two Uviol Sheaths-A solution of p-toluidine containing 0107 gram in 20 C.C. alcohol was prepared according to Krüss.1 Krüss states that this filter when used in a thickness of o8 mm. absorbs all light between 308μμ and 274.
This was verified spectrographically. An experiment with this filter showed decomposition of sulphur dioxide to the extent 3*3 per cent.
This demonstrates that the line 313μμ is capable of de- composing the sulphur dioxide, and further that this line alone accounts for more than 94 per cent. of the total decomposition effected by the uviol lamp. In contrast with this it may be recalled that the next longer line in the uviol spectrum, namely 334μm, causes no decomposition at all (cf. expt. 3).
Garrett has shown that sulphur dioxide gas has a first absorption band commencing at 318-2μu with its head at 296'1μp and a second band commencing at 232 4μμ approximately and continuing beyond the limit of the ordinary plate (210). Thus the line 313 lies just within the first absorption band and on the longer wave-length side of the band head. The above observations tend therefore to confirm the view that any wave-length within the limits of a band can activate or decompose the substance.
Experiment 9. Vessel Wall + B-Naphthol Filter within Two Uviol Sheaths. The fact that the lines in the region 302μμ to 274μμ have no appreciable effect is probably due to their weak intensity. Thus in Cohen's work, when the 253μu was present as a strong line, it was very efficacious, but in the present work, where it is reduced to a weak line, it appears to have practically no effect. The argument that very weak lines have no appreciable influence photochemically was confirmed by the use of a B-naphthol filter. An alcoholic solution of B-naphthol (containing o'144 gram in 50 c.c. alcohol), when used as a filter, absorbs the 313 line to some extent and transmits it as a weak line. It also transmits the 302 and 297up as weak lines. Using this filter no de- composition at all is observed.
The Thermal Decomposition of Sulphur Dioxide.
In applying quantum considerations to the energetics of a chemical process (occurring under thermal conditions), it is necessary to calculate in terms of the head of the band which is responsible for the decomposition or activation of the given molecular species. The results obtained in the photochemical experiments described above indicate that in the case of SO, the band involved is that having its head at 296′1μp, and con- sequently the critical increment Eso, of this substance reckoned per gram- molecule is 96,700 cals.
1Zeitsch. Physikal. Chem., 51, 257, 1905. 2 Phil. Mag. [vi.], 31, 505, 1916.
*Cf. Krüss, loc. cit.
GASEOUS SULPHUR DIOXIDE
On the quantum theory of thermal chemical change, the heat of reaction is related (exactly, in the case of unimolecular processes, approxi- mately, in the other cases) to the critical increments of reactants and resultants thus-
heat evolved,
―
(critical increment of resultants) -
(critical increment of reactants).
This expression has already been applied by W. C. M. Lewis1 to the case of the thermal formation and decomposition of sulphur dioxide. The heat evolved in the formation of one gram-molecule of SO, from the gaseous components, sulphur and oxygen, is 82,000 cals. The critical increment of one gram-atom of gaseous sulphur and one gram-molecule of oxygen was calculated to be of the order 103,000 cals., whence the critical increment of the SO, molecule is 185,000 cals. per gram-molecule, which corresponds to a wave-length 153μm. We have seen, however, that SO, can be decomposed by a much longer wave-length (296·1μμ) so that the critical increment calculated in the above manner is apparently far too great. In this calculation it was assumed that atomic sulphur was actually produced by the dissociation of SO, and that consequently for the formation of SO, it was necessary to use the data representing the absorption of energy necessary to produce one gram-atom of sulphur from the S, molecules which chiefly compose the vapour of sulphur over the region 200° to 500°. This energy term is itself of the order of 50,000 to 70,000 cals, and it is evident that the excessive value obtained in the previous calculation for the critical increment of SO, is due in the main to the introduction of this quantity.2
In view of the photochemical decomposition occurring at a wave-length as long as 2961μ it seems necessary to assume that in the formation of SO, from gaseous sulphur and oxygen the atom of sulphur is not involved, but that instead we have to do with the simplest molecular form S2. This indicates that in the photochemical process we are dealing with the activa- tion of a SO, molecule rendering it capable of reacting with a second mole- cule rather than with a direct dissociation of an individual SO, molecule.
The gaseous S, molecule has a critical increment in the sense that over the temperature range mentioned the vapour consists of S. molecules. From the data available Lewis has calculated that the energy 21,500 cals. approximately must be absorbed for the production of one gram-molecule of S from Ss. Further the oxygen molecule is known to possess two absorp- tion bands in the short infra-red region, viz. at 3'2μ and 4'8μ,3 of which the latter is the more marked. The critical increment corresponding to a wave- length 4.8μ is 6000 cals. in round numbers, per gram-molecule.
It is not clear whether one or both of the oxygen molecules has to be activated in order to react with S-since the stoichiometric equation, S2+ 202 = 28O,, probably includes two bimolecular processes.
We can only say therefore that the energy required for the partial activation of the
1 Trans. Chem. Soc., 115, 182, 1919. "It may be pointed out perhaps that the calculation of Lewis is correct in principle for the dissociation of SO, into atomic sulphur and molecular oxygen. A revision of the numerical values employed would lead to a value somewhat lower than 185,000 cals., but still much greater than the value inferred from the photochemical result.) A wave-length corresponding to the "second or "further absorption band of SO, would be likely to decompose SO, into atomic sulphur and oxygen, but the photo- chemical result appears to indicate that the actual mechanism is one which follows a path of lower increments, for reactants as well as for resultants.
11
Cf. Coblentz, Carnegie Inst. Washington Pub., No. 35, 1905.
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