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6
DECOMPOSITION OF GASEOUS SULPHUR DIOXIDE
oxygen molecules lies between 6000 cals. and 12,000 cals. Adding this to the increment for S., namely 21,500 cals., it follows that the total increment for (S20) lies between 27,500 and 33,500 cals. Applying the heat- quantum relation to the stoichiometric equation given above we obtain :--
Whence,
or
2 x 82,000 2 Eso
or
(27,500) (33,500)
2Eso, lies between 197,500 and 191,500 Eso, lies between 98,700 and 95,700 cals.
a value which agrees satisfactorily with the critical increment, 96,700 cals., calculated from the photochemical decomposition. A good deal of arbitrariness, however, surrounds the choice of the value for the critical increment of (low, partial) activation of the oxygen molecule. Taking the value 96,700 cals., obtained from the photochemical conditions, as correct, it would follow that the sum of the critical increments of gaseous (S,+202) Is 29,500 cals. This relatively low value, together with the great evolution of heat accompanying the union of sulphur and oxygen, indicates that sulphur and oxygen should react easily and that the sulphur dioxide mole- cule should be thermally very stable; the latter conclusion is in agreement with the conclusions of von Wartenberg.1
Summary.
1. The decomposition of sulphur dioxide gas under the action of light radiated from a uviol mercury vapour lamp has been investigated, and the resulting photostationary state, characteristic of a given set of conditions, determined, using a number of different light filters.
2. It is shown that the wave-length chiefly responsible for such decom. position of the gas is 313. It is noteworthy that the shorter wave-lengths, also present in the above source of radiation, contribute very little to the decomposition. The wave-length 313 lies within the first absorption band of sulphur dioxide (at a pressure of 600 mm.), but does not corre- spond with the head of the band, already known to occur at 296 1μ. The actual wave-length producing maximum decomposition depends on the nature of the source of radiation, ¿e., upon the intensity distribution of the emission spectrum.
3. Of the radiation emitted from the uviol lamp the longest wave-length capable of decomposing sulphur dioxide is the 313μμ line itself. The conclusion is drawn that any wave-length within the absorption band of the substance is capable of bringing about decomposition provided the intensity is sufficiently great.
4. The bearing of the photochemical decomposition of sulphur dioxide upon the critical increment of the thermal union of oxygen and sulphur vapour is discussed.
It is concluded that the molecule S, as distinct from
the atom is involved in this process.
Muspratt Laboratory of Physical and Electro-Chemistry, University of Liverpool.
Zeitsch, anorg. Chem., 56, 320, 1908.
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