MECHANISM O F T H E PHOTOCHEMICAL REACTION BETWEEN HYDROGEN AND CHLORIXE. 11* BY ABRAHAM LINCOLN MARSHALL
I n a previous communication1 it has been shown that the a m m n t of reaction induced in a mixture of hydrogen and chlorine by a given number of hydrogen atoms is strongly dependant on the total pressure of the system, increasing rapidly with increased pressure. In this connection it seemed advisable to reinvestigate the photochemical hydrogen chlorine reaction over a wide pressure range. This paper will present the results of a preliminary study of the problem. The results are of interest not only in this particular connection but as applied to “chain reactions” in general. It has been definitely established that the number of molecules of hydrogen chloride formed per quantum of light absorbed increases rapidly with increased pressure. The range of pressures investigated was from 0.001cm., to 6 cm. and the quantum
FIG. I
yield in this range increased from about 2 0 to over 2 5 , 0 0 0 molecules. At present very little can be said concerning its dependence on the relative pressures of hydrogen and chlorine but this subject will be dealt with in a subsequent communication. Experimental The method used was similar in many details to that already described. It consisted in drawing a mixture of hydrogen and chlorine thru a quartz reaction vessel where it was illuminated by a quartz mercury arc. The whole apparatus (Fig. I ) was swept out by a stream of chlorine for two weeks before use in order to ensure complete absence of impurities from the walls of the vessel which would inhibit the reaction. Liquid chlorine was collected in C after being twice redistilled at liquid air tempera tures, the middle fraction having been collected each tjme. The amount of chlorine passing thru the system in a given time was determined by the size of the capillary E and the temperature of the liquid chlorine jn C. The capillary D when cooled by liquid air was used to stop the flow of chlorine.
* Contribution from the Laboratory J. Phys. Chem. 29, 842 (1925).
of Physical Chemistry, Princeton University.
I454
ABRAHAM LINCOLN MARSHALL
Carefully purified hydrogen, dried over potassium hydroxide, was admitted at E in amounts regulated by this stopcock. After illumination the gases were passed thru liquid air traps at B to remove chlorine and hydrogen chloride. The pressure of the residual hydrogen was then measured on a McLeod gauge, the gas then passing thru the stopcock F to a mercury condensation pump. It was possible to regulate roughly the rate of flow of the gases by control of the stopcock F and this method was used to obtain the higher pressures employed. Three check experiments were usually made at each pressure. The apparatus was then filled with hydrogen from D and the gases caught in the liquid air traps, blown out in turn into a solution of potassium iodide in order to determine their composition. At the time these experiments were performed, I had no method for measuring the pressure in vessel A during the course of the reacti m . I n order to calculate the chlorine pressure in the reaction chamber and from it the number of quanta absorbed, it has been assumed that the hydrogen pressure measured on the guage was the same as that of the partial pressure of hydrogen in A and that the chlorine pressure could be calculated from this by multiplying it by the ratio of the number of cubic centimeters of chlorine leaving the chamber to that of the number of cubic centimeters of hydrogen.
Calculation of Light Absorption In order to calculate the number of quanta absorbed in any experiment, it was necessary to know the amount of energy incident on the system and the average extinction coefficient of chlorine for this energy. Coblentz, Long and Kahlerl give a value for the energy radiated from a 2 2 0 volt Cooper-Hewitt quartz mercury arc a t a distance of 40 cm. measured perpendicularly from the center of the lamp axis. This lamp was consuming 400 watts energy and a current of 3 amps. The value given by them is 0.00 I 7 g. cals. per square centimeter per second in the wave length range o .to 1 . 4 ~ and 66y0of this energy is in the range o to 0 . 4 5 ~ . I n the same paper they also state that the decrease in energy with distance from the lamp follows the inverse square law for distances greater t h m 36 cm. Harrison and Forbes2 give measurements on the distributicn of energy in the spectrum of a quartz mercury arc over the range 2300AO to 14,000A0 and its variation with current and applied voltage in the lamp circuit. From Fig. 7 p. 16 of their paper one obtains the following data for the relative intensities of the spectral lines for a lamp consuming 3 amps. current and 2 5 watts per em. power. From these figures one obtains the sa,me gross distribution as given by Coblentz, The lamp used in this investigation consisted of a IIO volt D.C. Hanovia quartz mercury arc and burned on 3 amps and 180 watts. The total Bureau of Standards Scientific Papers, No. 330.
* J. Optical Soc. 10, I
(1925).
I455
HYDROGEN AND CHLORINE
TABLE I
x
Galvanometer deflection
I8
11290A0 10148 5790
34 I04
4359
62
4078
48 98 6
Total 156
3663 3350 3132 2967 2536 2300
58
8 I9 I1
__
Total 310 energy radiated would thus be slightly less than 50% of that quoted by Coblentz for a Cooper-Hewitt lamp of twice the length. This amounts to 0.0008 g. cals., cm.,-2sec.-1in the range 0 - 1 . 4 ~at 40 cm. from the arc. In these experiments the area of the vessel was 19.6 cm.2and the lamp was 76 cm. from the middle of the reaction chamber which wis I j cm. long. Assumed the inverse square law the total energy incident on the reaction vessel per hour amounted to.. 0.0008 X ($)'X
19.6X3600
= 15.6 g. cals., hr.-l or 65.7 (1o)'ergs hr.-I
The distribution of energy in absolute units amongst the various wave lengths js given approximately in Table 11. The value of E given in the column three js the extinction coefficient of chlorine given by von Halban and Sie-
x 5 7 90.4" 43 59 4078 3663 3350 3130 2967 2536
TABLE I1 Energy in Ergs x 10-7
.
E
14.8
0. I
7.5 6.9 14.0' 0.85 8.3 I . 14
1.64 3.99
2.7
27.17 65.5 21
I7 1
I456
ABRAHAM LINCOLN NARSHBLL
dentopf‘. From these values one can calculate the amount of energy of each wave length absorbed by chlorine by means of the formula €
d h
I
Jo
Cd
J
= - log -
which is the mathematical expression for Beer’s law. C is the concentration jn mols per liter, d the thickness of the absorbing layer in cm., J, the incident energy and J the energy transmitted. Table I11 gives the calculated values for the energy absorbed by chlorine at a pressure of 0.024 cm. under the above conditions.
TABLE I11
x
J, - J (10)~ ergs
4078
0.7
3663 33 50 3130 2967
15.3 2.8 8.3
”
1.0
”
” ”
The total energy zbsorbed is then 28.1 ( 1 0 ) ~ergs and the only raoge of any importance is 3ooc-4000 A”. The t3tal energy incident in this range is 31.2 (IO)’ ergs, Over this range one can now calculate an average extinction coefficient for chlorine which ammnts to e = 16.8. The average wave length of absorbed light is 350oAO which gives a value of h v = 5.6 (10)-12 ergs. Hence the to tal number of quanta incident 3n the reaction vessel in the wave length range 2967 -4o78A” per hour is 31.2 (IO)’ 5.6 (10)-12
= 5.6 ( 1 0 ) ~ quanta ~
From the method used in calculating the average chlorine pressure in the reaction vessel, it can easily be seen that the only results for which any great degree of accuracy can be claimed are those in which the hydrogen pressure greatly exceeds that of the chlorine which in turn i s large compared to that of the hydrogen chloride. The results obtained are presented in the three following tables. Ths first column gives the time of the experiment in minutes and the second and third the amount of chlorine passing thru unchanged and the amount of hydrogen chloride formed. These are expressed in C.C. of thiosulphate and alkali used. The fourth column gives the amount of hydrogen used in terms of the current flowing thru the hydrogen generator. The remaining columns are selfexplanatory. In the experiments given in Table IVthe arc was 4.2 cm. from the reaction vessel. For the experiments a t higher pressures, it was necessary to move the arc further away from the vessel. Table V gives the results obtained with the arc 5 5 cm. for the vessel. Z. physik. Chsm. 103, 7 (1922).
HYDROGEN AND CHLORINE
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HYDROGEN AND CHLORINE
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ABRAHAM LINCOLN MARSHALL
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HYDROGEN AND CHLORINE
1461
In Table VI the results are given for the variation in quantum yield with intensity of incident light a t constant total pressure. The intensity was varied by altering the distance of the arc from the reaction system, the range covered being from 39 to 190 cm. Coblentzl has shown that overpthis range the inverse square law holds in calculating light intensjty from the mercury arc. From the results given it can be seen that for a twenty-fold change in light intensity the quantum yield is unchanged. This is in direct contradiction t o the results of Baly and Barker2 who observed a sixty-fold increase in reaction velocity for a twenty-fold increase in light intensity for mixtures of constant composition. M. C. C. Chapman3 also fails to check Baly’s result over a six-fold increase in light intensity.
Summary I , The photochemical hydrogen-chlorine reaction has been investigated by a dynamic method over the pressure range 0.001-6.0 em. and the quantum yield found to increase from about 2 0 to over 2 j,000 molecules.
2. At a total pressure of 5.9 cm. it has been found that the quantum yield is independent of light intensity when the latter is increased by twenty times.
Research Laboratory, General Electric Company, Schenectady, N . Y .
Bureau of Standards Scientific Paper KO.330. J. Chem. Soc. 119, 653 (1921). J. Chem. Soc. 125, 1521 (1924).
