The Photochemical Union of Hydrogen and Chlorine

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PHYSICS: L. HARRIS

P PRoC. N. A. S.

THE PHOTOCHEMICAL UNION OF HYDROGEN AND CHLORIN-E BY LouIs HARRIs* ZURICH, SWITZURLAND

Communicated December 9, 1927

The union of hydrogen and chlorine under the influence of blue light, with an excess of hydrogen, and in theS presence of small amounts of oxygen and water vapor, has been found to be expressed by the formula' = k (C12)2 (02) dt Under such conditions, Kornfeld and Miller2 were able to obtain a yield of about 104 molecules of hydrogen chloride per quantum of light. Bodenstein' had already estimated a yield of 106 molecules per quantum of light. The object of the present investigation was to determine the quantum yield and to obtain more facts concerning the kinetics of the reaction, using blue light, and under conditions affording a minimum concentration of oxygen. The reaction was carried out in an apparatus entirely of quartz, and conditions ensuring the absence of impurities, particularly stopcock grease, mercury vapor, etc. The usual stopcocks were replaced by solid chlorine capillary stopcocks described by Thon.3 However, it was necessary to modify the procedure recommended by Thon, because, whereas he was able to use pure chlorine in his capillary stopcocks, the present investigation necessitated the use of a mixture of hydrogen and chlorine in the capillary stopcocks. Chlorine was freed of oxygen, by washing a sample of fractionally distilled liquid chlorine, maintained at -78°C., with electrolytic, purified hydrogen, continuously for a period of six weeks. This chlorine was stored in a quartz gas-wash flask, connected to the rest of the quartz apparatus, and was used for all the experiments. The hydrogen-chlorine mixture, leaving the wash flask, passed in turn through a capillary stopcock, reaction vessel, capillary stopcock, liquid air trap, and, then, either to a Bunsen absorber or the vacuum line. The ratio of chlorine to hydrogen, for each investigation, was varied by changing the temperature of the alcohol-carbon-dioxide bath containing the wash flask. Pressures were measured with a Bodenstein quartz gage. The hydrogen pressure was determined by freezing out the chlorine and hydrogen chloride in the reaction chamber with liquid air. The temperature of the liquid air was measured with an oxygen thermometer. The reaction vessel was illuminated with light of wave-length greater than 4050 A. Light from a 500-watt "Nitra" lamp, maintained at a

d(HCI)

PHYSIC$:-.L HARRIS1

Void. VoL. 19281928 14,14,

ill

constant voltage, passed through a filter of ferrous sulphate, and one of copper ammonium sulphate, a lead glass plate and, finally, a ground glass plate, which served to illuminate the reaction vessel uniformly. The deflection of a very sensitive galvanometer, produced by the illumination of a thermopile, placed directly behind the reaction vessel, served as a measure of the energy. This system permitted the measurement of 0,432 ergs per square centimeter per second. However, the amount of energy absorbed by the chlorine (even with a pressure greater than one atmosphere) was so small that no difference in the deflection could be detected with the reaction vessel filled with chlorine or evacuated. It is only possible, therefore, to calculate a lower limit of the quantum yield. Since the area of illuminated surface was 110 cm. 2, the maximum energy absorbed would be 47,5 ergs per second. Assuming a mean effective wave-length of 4360 A, 1 erg = 2,2 X 1011 hv, and 47,5 ergs = 104,5 X 1011 hv. The most sensitive mixture (Cl2p = 334 mm.; H2p = 411 mm. at 0°C.) investigated, yielded 35. mm. Qf hydrogen chloride per minute in a vessel of 300 cc., or 6,27 X 1018.molecules of hydrogen chloride per second. This gives a minimum yield of 6 X 105 moleules of hydrogen chloride per quantum of light. The actual yield obtained, here, may well be ten times this value. Although this value is greater than heretofore reported, it is by no means to be considered as an upper limit of the quantum yield for this reaction. There action velocity constant, k', using the formula

d(HCl) dt

= k', remained constant for each run.

(Cl2)2 However, the value of k' for runs at different initial pressures of chlorine showed a definite trend-the higher the initial value of the chlorine pressure, the lower the value of k'. For example, the value of k' for an initial pressure of 150 mms. of chlorine was about 2,5 times as great as the k' for an initial pressure of 335 mms. of chlorine. This was, no doubt, due to the presence of some oxygen in the gas mixture-very likely of the order of 1/1000 of a per cent. Under the present experimental conditions, the oxygen pressure would increase with increasing chlorine pressure and the data obtained would seem to be in accordance with the Bodenstein-Dux equation,' above. Since the ratio of hydrogen to chlorine was always greater than unity, the formulae proposed by Cremer4 for the hydrogenchlorine reaction, cannot be applied, easily, to these measurements. Further work will, undoubtedly, lower the oxygen concentration and increase the quantum yield. Summarizing, a study was made of the photochemical union of hydrogen and chlorine under conditions affording a minimum concentration of

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oxygen. A yield greater than 6 X 10O molecules of hydrogen chloride per quanta of light was obtained. The reaction velocity measurements seem to be in accord with the Bodenstein-Dux equation: d(HCl) (Cl2)' dt (02) This work was performed in the physikalisch-chemischen Institut der Universitat Berlin, under the direction of Professor Max Bodenstein. The work was much facilitated due to the efforts of my predecessor, Dr. P. B. Ganguli, who assembled much of the apparatus and carried out some preliminary measurements. I wish to take this opportunity of thanking Professor Bodenstein for his kindness in placing the facilities of his laboratory at my disposal and for his ever helpful assistance. * NATioNAL R5EsARCH PftLOW. 1 Bodenstein u. Dux, Zeitschr. physik. Chem., 85, 297 (1913). 2

Komfeld u. Muller, Ibid., 117, 242 (1925).

8W. Thon, Ibid., 124, 327 (1926). ' E. Cremer, Ibid., 128, 285 (1927).