MECHANISM OF T H E PHOTOCHEMICAL REACTION BETWEEN HYDROGEN AND CHLORINE. PART 111. T H E MEAN LIFE OF ACTIVITY I N ILLUMINATED CHLORINE* BY ABRAHAM LINCOLN MARSHALL
It was suggested in Part I of this series of papers1 that it might be possible to distinguish between the two types of mechanism which have been advanced to account for the photochemical reaction between hydrogen and chlorine by a re-investigation of the rate of decay of activity in illuminated chlorine, Chlorine atoms as postulated in the Kernst mechanism should have a relatively long life while excited chlorine molecules according to our present views might have a mean life period of the order of IO)-^ sec. Bodenstein and Taylor2 investigated this problem and were unable to detect any enhanced reactivity in chlorine 0.0006 j see. after illumination. They illuminated chlorine with light from a 2 5 candle power Osram lamp placed 30 cm. from the reaction system. The chlorine after illumination was passed thru a bent capillary into a stream of hydrogen. The capillary was blackened to prevent light from reaching the mixed gases. The capillary used by them was of 0.3 mm. radius, 2 0 mm. long with a volume of 0.0065 cc. Gohring3 has analysed the results of Taylor and showed them to be consistent with the Nernst mechanism. The chlorine which they used was electrolytic and is assumed to contain 0.1percent of oxygen. From this value Gohring calculates the concentration of atomic chlorine in the steady state to be 6.9(10)-13 mol-litre and the half life period as o.oo012 second. This would give rise in Taylor’s experiments to an undetectable amount of hydrogen chloride. Let E = radiation of frequency Y absorbed per second. V = volume of the vessel. Zll = number of impacts per second one chlorine atom makes with all the other chlorine atoms. z 1 2 = number of impacts per second one chlorine atom makes with oxygen molecules present in the gas. is1 = number of chlorine atoms-c.c. N2 = number of oxygen molecules-c.c. 1/2N1Z11 = total impacts between chlorine atoms per c.c.-second. Zll = kN1 where k = QTCT12.& and Q1 = velocity of chlorine atoms. c1 = diameter of chlorine atoms. z 1 2 = kiNz k = 2 U 2 B ml mz z T R T
yc.+
* Contribution from the Laboratory of Physical Chemistry, Princeton Univ. J. Phys. Chem., 29, 842 (1925). Z. Electrochem., 22, 202 (1916). a Z. Electrochem., 27, j I 6 (1921).
758
ABRAHAM LINCOLN MARSHALL
klNlNz = total impacts between atomic chlorine and oxygen. Gohring calculates I.~(IO)-~ of these impacts to be effective, then '
in the steady state-
E
2 - . -
hv
I = k Ni2 V
+
kiNiN2
I.S(IO)-'
For the moment let us neglect the deactivation caused by impacts between chlorine atoms; then in the steady state-
m, = h~VkiN22E X
I.5(10)-3
= concentration of C1 in steady state. The problem now is to calculate the concentration of C1 after the illumination is cut off.
1/1600
second
- -dN1 = 1.5(10)-~k1NiNz dt mi
In - = Ni
I . ~ ( I O ) - ~klN2t
67
kl = 2(2.55)2(10)-18 =
Nz =
1.2~(10)--'0 0.1
percent =
6(10)~~ 22,400 X 7,600
E = IO-^ g. ca1s.-sec. = 4 , 1 8 ( 1 0 ) ~ergs.-see. hv = 3 . 9 3 ( 1 0 ) - ~ergs. ~ for X =
v = 2 0 0 C.C. t
log N,
X 82.15(10)~ X 27r X 300
5oopp.
= 1/1600 sec. =
log
3 . 9 3 ( 1 0 ) - ~ ~X I . ~ ( I O ) -X~
200
x
4.18(10)4 X 1.25(10)-~0 X 2.68(10)~6 X I.j(Io)+ 2
-
1.25(10)-~0
2.33 X 1600 = 10.318 1.36 = 8.96 N1 = ~ . I ( I o ) atoms * C1-c.c.
-
= 2.68(10)16
X 2.68(10)~~
759
HYDROGEN AND CHLORINE
If each atom is responsible for the formation of IO^ molecules of HC1 NHCl= ~ . I ( I o )molecules '~ = 0.0034 percent HC1 which is an undetectable amount under the experimental conditions of Bodenstein and Taylor. Warburgl has also treated this problem neglecting the effect of oxygen and concludes that one should expect a considerable amount of hydrogen chloride to be formed. Assuming we are dealing with pure chlorine the concentration
of atomic chlorine in the steady state is given by the equation
N1 =
- ldN -
I:E -
= kN12 dt and the concentration K, of atomic chlorine at time t after illumination is given by l/N1 - l/N1= kt
the rate of decay of activity is
k
= Q ~ T 1/T= u ~ ~ 14,500
N1 =
v'
x
4*18(10)4
K
22 Eo.--44(10)-~~2/= 7.j5(10)-"
7
= 4 1 . 4 1 (10)z4 = I . I ~ ( I O ) ' ~
3.93(10)-~~ X zook I / N ~= 4.7(I0)-I4 8.4(10)-la Nl = I . I ( I O ) ~ ~atoms of chlorine It will be observed that in this calculation practically none of the atomic chlorine has combined in 1/1600 second, and this on the assumption that everj impact of chlorine atoms with one another leads to combination. From analogy with the observed results on the combination of atomic hydrogen and atomic bromine, one would only expect about one in every thousand impacts to be fruitful. If the analysis given by Gohring and Warburg is correct it should be possible to obtain very considerable amounts of hydrogen chloride in an experiment of this nature provided pure chlorine free from inhibitors for the reaction is used and is subjected to intense illumination,
+
Experimental A diagram of the apparatus used is given in Fig. I. The whole set up between the capillary H and the liquid air trap C" was so arranged that it could be surrounded by a furnace and heated to 3o0°-4000C. While the apparatus was being heated a stream of chlorine gas from an electrolytic generator was passed thru the apparatus. On several occasions the capillary H was cooled in liquid air to block it with solid chlorine and the whole set up was evacuated with a diffusion pump. This treatment was continued for several days to ensure the complete removal from the system of all impurities which were volatile or were capable of reacting with chlorine, In the subsequent experiments inhibitors of the type described by Chapman and Burgess2 due to albuminous material on the walls of the vessel were absent. This procedure is an improvement over that previously used since it greatly Z. Electrochem., 27, 139 (1921). Chem. SOC.,89, 1399 (1906). %J.
760
ABRAHAM LINCOLN MARSHALL
shortens the time necessary to clean up the apparatus and makes it more probable that a proper cleansing has been accomplished. Only two stopcocks were used in the entire apparatus and these never came in contact with chlorine and were protected by liquid air traps from the rest of the system so that impurities from the grease used for lubrication might not diffuse into the carefully purified reaction system. The capillary B thru which the chlorine gas was forced after illumination was made of a special ruby glass supplied by the Corning Company, which sealed directly to G702P glass of which the whole set-up was constructed. The capillary was constricted to the required diameter by a process of trial and error; the two capillaries used in this investigation were about 8 inm. long 9 LL
FIG.I
and had volumes respectively of 0.00018 and 0.00034 cc. These volumes were determined by weighing the amount of mercury required to fill the capillary. The part A consisted of a quartz to glass graded seal with a quartz window at the end. The distance from the window to the capillary opening was 5 cm. An Hanovia quartz mercury arc was used as a source of illumination and was contained in a black box with a diaphragm attached with an opening the size of the window at A. The end of A was placed against the opening in the diaphragm, and the arc was 4 cm. from A. It consumed 3 amps. at 60 volts. The whole apparatus on the hydrogen side of the capillary was painted black after the preliminary baking out process and the experiments were carried out in a darkened room free from all stray light. The chlorine used was taken from a cylinder of electrolytic chlorine. It was washed with water and passed over heated glass powder to decompose any oxides of chlorine present. While collecting the chlorine in the container D the whole apparatus was evacuated the capillary B having been shunted by a glass by-pass which was later sealed off. The chlorine was condensed in liquid air any oxygen present being removed by the pump. After filling D the apparatus was sealed off at the capillary H from the source of chlorine; container E was cooled in liquid air and D with a COz-ether mixture and chlorine distilled into E in a high vacuum. Due to the poor heat exchange consequent on the high vacuum conditions the chlorine in D remained solid thruout the distillation. The first fraction of the distillate was collected. A similar dis-
HYDROGEN AND CHLORINE
761
tillation was made from E to 8’. The chlorine collected in F was kept in an ice bath which gave a chlorine pressure of 3.7 atmos. This pressure was used to drive the chlorine at a high rate of flow thru the capillary B after illumination. Electrolytic hydrogen was led in thru the liquid air trap C’ and the mixture of hydrogen and chlorine passed out at M into a solution of potassium iodide. The chlorine as prepared above was exceedingly dry and it became necessary to moisten it. This was accomplished by first collecting some mater in the container G, which was cooled in liquid air, before the chlorine mas collected. The water was made by passing hydrogen containing some oxygen over a piece of heated quartz tubing K which was sealed into the apparatus with graded seals. The combustion was carried out at low pressures to avoid an explosion. The protection given by the liquid air traps was necessary to prevent any contamination from entering the system after baking out; the traps were in the furnace in the preliminary heat treatment. During the experiments the water was kept at room temperature.
Results The first series of experiments was made with dry chlorine. Four experiments were made with chlorine flowing at a rate of 0.5 j cc.-sec. corresponding to a time of 0.00033 see. for passage thru the capillary. The iodine released by the chlorine was titrated with N/4 thiosulfate and methyl red was used as an indicator for the acid titration. All the solutions used were carefully neutralised before an experiment so that a drop of N / I O acid or alkali was sufficient to produce a marked change in the indicator. In these experiments the solution remained neutral and no trace of acid was found. The time of passage thru the capillary was decreased to o.ooozo sec. by increasing the chlorine temperature to 2ooC. Still no acid mas detected. It was then decided to met the chlorine before illumination. A series of experiments with moist chlorine passing thru the capillary into hydrogen 0.00030 sec. after illumination produced no detectable amount of hydrogen chloride, These results with pure chlorine were entirely unexpected in the light of the previous discussion and might lead one to conclude that the Kernst mechanism, on which that analysis was based, was incorrect. The effect of the glass walls has however not been considered and it is quite possible that these may be catalysing the recombination of the chlorine atoms. It will apparently be necessary t o resort t o some other method of experimentation t o settle this point. The author has suggested a method of attack1 involving a quantitative study of the rate of reaction at low pressures and being a continuation of previous work2. If the mechanism involves chlorine atoms it Trans. Am. Electrochem. Soc., Spring Meeting (1926). J. Phys. Chem., 29, 1453 (1926).
762
ABRAHAM LINCOLN MARSHALL
should be possible to calculate the rate of reaction as a function of the pressure from a knowledge of the quanta absorbed and the rate of diffusion of the atoms to the surface of the container where they are removed from the system.
Summary The experiments of Bodenstein and Taylor on the mean life of reactivity in illuminated chlorine have been repeated using an improved technique. Pure chlorine was used and passed into hydrogen 3 ( 1 0 ) ~sec. after intense illumination by a quartz mercury arc. No reactivity was detectable. This result was directly contrary to a mathematical analysis based on the Nernst atomic mechanism which predicted a large reaction. Only two possibilities seem to arise : either this mechanism is incorrect and it is necessary to assume a mechanism involving excited chlorine molecules, or the walls of the capillary remove the chlorine atoms by causing their recombination. Further experimentation is necessary to reach a decision and a possible method of attack is pointed out. Princeton, N . J.
