THE TEMPERATURE COEFFICIENT OF THE RATE OF COMBINATION O F HYDROGEN AND OXYGEN UNDER ALPHA RADIATION CHESTER H. SCHIFLETTI
AND
S. C. LIND2
School of Chemistry, University of iwinnesota, Minneapolis, Minnesota
Received October 9, 1988
Although this reaction has been extensively investigated under many conditions for various purposes (5), the temperature range of the alpharay reaction has never been extended above 130°C. nor below -80°C. The investigations of the thermal reaction by Hinshelwood and Thompson ( 2 ) and of the photochemical reaction by Kistiakowski (3), both a t higher temperatures, make it appear desirable to extend the temperature limits of the alpha-ray reaction in both directions. Owing to the nature of the reactants and to the absence of the reverse reaction, the decomposition of water vapor by alpha rays, the reaction lends itself admirably to such a purpose. The objects of the study are to contribute to the knowledge of the reaction mechanism, especially to the transition from a constant quantized reaction with no temperature coefficient a t lower temperature to a chain reaction with about the same temperature coefficient as the photochemical reaction at higher temperatures. Incidentally the sensitization or catalytic effect of ions of water vapor has been discovered and found to be 100 per cent efficient. GENERAL PROCEDURE
The manometer method of following the course of the reaction has been used in all except one experiment. The two methods of using radon, (1) by mixing directly with the reactants, and (2) by confining it in a thin alpha-ray bulb at the center of the reaction sphere, have been employed. Reaction spheres of Pyrex glass were used. Both static and circulating systems were employed with various conditions as to extent and temperature of the dead arm kept at lower temperature for condensation of water vapor, since it was not feasible to employ an internal drying agent at high temperature. Electrolytic gas was generated from barium hydroxide solution between platinum electrodes and stored in a vessel without stop1 2
Professor of Chemistry, Macalester College, St. Paul, Minnesota. Director of the School of Chemistry, University of Minnesota. 327
328
CHESTER H. SCHIFLETT AND S. C. LIND
cocks. Figure 1 shows the apparatus used in the circulation method, in which I is the reaction sphere containing an alpha-ray bulb at its center in the electrical furnace H at a controlled temperature, E is the all-glass magnetic circulating pump of simple design (8), and J is a heating coil to raise the temperature of a small platinum catalyst to produce initial saturation with water vapor. G is a liquid air trap sometimes used t o remove the water vapor from the circulating mixture.
FIG.1. APPARATUSUSED I N
THE
CIRCULATIOK METHOD
Radon purified by the method of Livingston (7) is introduced either into the alpha-ray bulb of figure 1, the stem of which is then sealed and closed as far as the bulb (by a technique to be described later), or is introduced into the tube shown in the breaker M of figure 2 , from which it is introduced into sphere G in the static method after breaking in vacuo. TREATMENT O F DATA
The quantity of radon is initially measured by its gamma radiation and its decay is subsequently followed by the table of Kolowrat. The gas pressures (at constant volume) have all been corrected to 0°C. The
HYDROGEN AND OXYGEN UNDER ALPHA RADIATION
329
kinetic constant is calculated in the usual way (reference 5 , p. 116) from the equation
(?)’
PI
nat log -
=
const. =
PP Eo (e-% - e - h )
FIQ.2
(Y)
From the values of -
for each interval the number of molecules of hy-
drogen and oxygen reacting per ion pair is calculated and reported as - M / N , where iV is the total number of ion pairs from O,, H2, or H,O except in cases where ions of water vapor may be neglected. Comparison of yields under different conditions is therefore obtained by direct com-
330
CHESTER H. SCHIFLETT AND S. C. LIND
parison of corresponding M / N values. It is assumed that the amount of ionization by alpha particles is unaffected by temperature a t any temperature employed. EFFECT O F WATER VAPOR
Duane and Scheuer (1) found that water vapor is not decomposed by alpha rays. On the other hand it is normally ionized and hence may possibly contribute to the forward reaction just as Lind and Bardwell (6) later found many foreign gas ions do in various reactions including the effects of ions of nitrogen, oxygen, and neon in water synthesis. Such an effect of water vapor in water synthesis would be “autocatalytic,” increasing with increase of water vapor produced in the reaction itself. Such an effect has hitherto escaped observation because a t ordinary temperature most of the water formed condenses out. An effect of HzO+ (or possibly HzO-) ions would be expected to become relatively more important a t low pressures of electrolytic gas saturated with water vapor. But in the reactions at low pressure in the investigations of recoil atom effect (4)an internal drying agent was used so as to prevent any influence of water vapor on the recoil effect. In the present experiments at higher temperatures, however, the water vapor concentration becomes sufficiently great to contribute materially to the ionization and also, as will be shown, to the reaction rate. On going up in temperature a marked increase in - M / N is observed, which may be due either to a true temperature effect or to the catalytic effect of water vapor ions or to both. The effect of water ions must be known or eliminated in order to evaluate the true temperature coefficient. Both methods have been used, to remove water vapor by the circulation method or to calculate the additional effect in the presence of water vapor and subtract it. Both lead to the same net result a t a given temperature. Moreover the agreement confirms the 100 per cent efficiency of water vapor ions, which was assumed in making the subtractive correction. Another accelerating effect, that of low pressure, which was first thought to exist, proved to be spurious upon more complete removal of water vapor and was due to the relatively enhanced effect of the latter at low pressures of reactants. The practically complete elimination of water vapor (not extreme drying) leaves then only the true yield - M / N which remains constant a t 4.53 from -180°C. to ordinary temperature and then rises steadily to 20
* Throughout this paper the results are expressed in terms of - M / N , Le., the number of molecules of hydrogen and of oxygen reacting per ion pair produced both in hydrogen and in oxygen ( - M H % + 02,”~% + 0 % ) . In terms of water molecules formed the value is 213 X 4.5 = 3. This is lower than the earlier value 4,because the values of the average intensity of ionization obtained from the Mund equation are higher and hence N is higher and M I N is lower.
.
HYDROGEN AND OXYGEN UNDER ALPHA RADIATION
331
a t 400°C.; between 400°C. and 500°C. the values were not reproducible owing to the thermal reaction; above 500°C. the reaction becomes explosive. EXPERIMENTS AND CALCULATJONS I N STATIC SYSTEM
In the following tables are reported the observations and calculations of experiments under various conditions of temperature and of water removal. The experiments in static systems above 100°C. were found quite useless in attempting to find the true temperature coefficient of reaction rate, because the slow rate of diffusion of water vapor from the reaction sphere out to room temperature produces a higher partial pressure of water vapor in the reaction sphere than is allowed for by simply applying the water vapor correction for the outside (lower) temperature. The application of cooling baths outside is ineffective in speedy removal of water vapor, since it is the rate of diffusion outward, not the rate of condensation after arriving there, which is the controlling factor. The apparent yield in water synthesis per ion pair continues to rise even a t a constant higher temperature, which may be due either to an actual increase of rate due to accumulation of more water vapor (which seems improbable, beyond a certain limit, over the long periods of time that prevail toward the end of the reaction) or to the increasing importance of water vapor relative to the diminishing pressure of electrolytic gas, since both are ionized in proportion to their amounts present. Table 1 will illustrate such a false rise of - M / N st 275°C. due to this cause. Po = the pressure of reactants corrected to G'C., after subtracting aqueous tension a t the outside temperature. P,, = the pressure of reactants at 0°C. increased by the ion equivalence4 of the water vapor pressure. As already explained, the large rise in - M / N is spurious and caused by accumulating water vapor above equilibrium with the outside temperature, but owing to the uncertainty of its amount, correction cannot be made until the water concentration is known or eliminated. One method of procedure which was adopted with fair success was to make a rate measurement at each temperature so as to obtain a value of M / N before enough water vapor had been produced or the reactants sufficiently diminished to influence the value of M / N . Such measurements were made at 25", 75", 125", 160", 199°C. The details are not reported but the results are given in figure 3 and agree with the results where water vapor is rigidly removed. A second method consisted in employing an alpha-ray bulb (with comI t is P,, which is substituted in the general kinetic equation to obtain the velocity constant
(y)' from which M I N is calculated in the usual way.
332
CHESTER H. SCHIFLETT AND S. C. LIND
TABLE 1 Rate of combination at 275°C. with incomplete removal of water vapor Volume of reaction sphere = 10.41 cm3. Diameter of reaction sphere = 2.708 cm. Initial radon = 0.1059 curie. Static system with radon mixed with 2H 3. 0 2 ELAPSED TIME
&t
-1M __
RECOIL ATOM COR-
Po
N
RECTION
days
0 0
1 1 2 2 3 3 3 4 4 5 5 5 5 5
hours
0 22 6.58 20,42 4.25 20.92 4.83 10.25 18.25 4.92 20,42 2.92 8.58 21.83 22.83 23.83
1.0000(
0.8478 0.7955 0.7216 0,6758 0.5968 0,5620 0.5396 0.5082 0,4689 0.4174 0.3975 0.3814 0.3447 0.3421 0.3396
367.8 312.7 289,5 248.2 215.3 168.8 142.4 128,8 112.0 93.1 62.7 58. I 46.5 15.9 11.1 7.9
400.8 345,7 322.5 281.2 248.3 201.8 176.4 161.8 145.0 126.1 100.2 91.1 79.5 48.9 44.1 40.9
FIU.3
-
-
-
1.119 1.137 1.155 1.180 1.218 1.268 1.311 1,351 1,407 1.520 1.664 1.797 2,334 4,019 5,710
9.2 12.5 17.5 25.7 24.8 38.1 38.6 32.5 34.4 42.2 45.2 79.9 125.1 375.1 284.4
3.7 4.9 6.8 9.8 9.3 13.9 13.5 11.2 11.7 13.7 14.0 23.8 33.6 87.5 61.9
TEMPERATURE OF REACTION
degrees C.
25 25 285 275 275 275 275 275 275 275 275 275 275 275 275
HYDROGES AKD OXYGEN UXDER ALPHA RADIATION
333
pletely closed stem) mounted at the center of the reaction sphere. This permitted surrounding the entire sphere with liquid air or carbon dioxide mixture so as t o freeze the water vapor on the walls as ice. This method is naturally limited to the low temperature measurements. When the bath is removed the water vapor produced at higher temperature causes an enhancement5in M", as shown in table 2. A special experiment was made to examine the value of M/N a t - 78°C. which was apparently higher than that either at 25" or - 183°C. The results mere: at 25"C., 4.4, 4.65; a t -78"C., 5.1, 4.95, 4.61; a t 25"C., 4.03. The TABLE 2 Rate of synthesis at low temperatures in absence of water vapor compared to that in i t s presence at higher temperatures Volume of reaction sphere = 14 cc. Radon in alpha-ray bulb = 0.100 curie
days
1 2 3 4 5 6 7 8 9 10
hours
-
19.0 19.5 20.5 19.5 20.0 19.5 20.5 19.5 20.5 20.5 19.0
1.0000 0,8672 0,7216 0.5982 0.5034 0.4190 0.3512 0.2912 0,2450 0.2039 0.1703 0.1439
446.0 415.0 371.7 340.4 318,7 304.6 294.0 285,8 276.7 241.9 151.8 141.6
482.0 451.0 371.7* 340.4* 318.7* 340.6 294,0* 285.8* 312.7 277.9 187,8 177.6
5.00 7.58 7.23 6.95 4.81 5.22 4.72 6.22 35.0 48.1 21.2
degrees C.
4.42 6.56 6.25 6.00 4.52 4.08 5.38 30.3 41.6 18.4
25
-78 -78 -78
- 183 - 183 25 353 380 290
* At these temperatures water froze on the sides of the vessel so that no water vapor is present in the path of the alpha particles. results for - 78°C. are lower than in table 2 but still somewhat higher than those for 25"C., for which no explanation has been offered. EXPERIMENTS WITH CIRCULATING SYSTEM
It became evident that for the prompt and complete removal of water vapor the reactant gases should be circulated through an outside liquid air trap. This was accomplished by means of an all-glass pump and an alpharay bulb to prevent circulation of the radon. The results given in table 3 demonstrate the constancy of M/N a t 175°C. under these conditions and That these enhanced values are not due alone to the higher temperature is evident when they are compared with those of corresponding temperatures in the absence of water vapor, as shown in table 4.
334
CHESTER H. SCHIFLETT AND S. C. LIND
are to be contrasted with those of table 1 where water vapor was allowed to accumulate. TABLE 3 Synthesis of water at iY5"C. in absence of water vapor Volume of reaction sphere = 12.150 cc. Volume of system outside oven = 8.104 cc. E O = 0,100 curie, in alpha-ray bulb. Circulation with all-glass pump
days
-
hours
2 2 2 3 4 4 5 6 7
18.75 16.25 17.67 23.25 20.75 13.25 24.67 21.25 2? 33 22.00
-
-M
PO
T I M E ELAPSED
1. 0000 0.8686 0.6176 0.6111 0.5860 0.4987 0.4407 0.4045 0.3467 0.2873 0.2405
159.6 134.8 110.6 109.4 106.2 95.1 87.1 81.9 74.9 67.6 65.1
x 192.6 167.8 143.6 109.4* 106.2% 128.1 120.1 114.9 107.9 100.6 95.1
_.
10.48 6.21 11.97 12.65 11.13 12.21 10.81 11.86 12.02
-
TEYPERA-
N
ATURE
degrees C.
4.5 8.7 9.2 8.0 9.1 7.4 8.6 8.9
25 25
-
175 175 175 175 175 175 173
TABLE 4 Synthesis of water at various temperatures in absence of water vapor Volume of sphere = 14 cc. E O= 0.100 curie in alpha-ray bulb. Circulation by all-glass ~pump TIME ELAPSED
1 2 2 3 3 4 4 5 6
20 17.5 6.25 17.75 4.00 20.00 19.25 19.75 18.75 18.5
@-At
1.0000 0.8608 0.7326 0.6657 0.6106 0.5655 0.5016 0.4213 0.4196 0.3532 0.2956 0.2469
PW
595.7 557.1 510.8 461.6 446.9 434.1 184,l 148.7 445.9 393.4 326.7 92.7
543.7 502.9 464.5 414.1 400.4 374.3 162.7 133.7 398.0 353.5 292.1 82.1
(ku)' -
x
-"' x
-
-
6.15 17.2 6.1 15.0 130.6 24.4
4.5
I i 1
TENPERATURE
-
degrees C .
13.2 4.65 11.45 99.8 18.6
300 25 300 500 350
13.6 25.14 198.4
358 404 500
(1)
17.8 33.15 260,4
(1) More reaction gas added.
Circulation by thermal convection was also tried, but was found to give results less satisfactory than the foregoing. It only remained then t o apply the circulation method at various temperatures. In each case the alpha-
335
HYDROGEN AND OXYGEN UNDER ALPHA RADIATION
TABLE 5 Synthesis at higher temperature in absence of water vapor Volume of sphere = 14 cc. E O = 0.105 curie in alpha-ray bulb. Circulation by means of all-glass pump TIME ELAPEED
1
e-ht
-IM
pc,
-
pe4
. dags
1
2 3 4 5 5 6 7 7 8
hours
-
17.75 19.0 14.25 17.75 15.50 21.75 22.00 19.25 16.25 16.5 16.5
,
1.0000 0.8751 0.7243 0.6269 0.5100 0.4333 0.3453 0.3447 0.2939 0.2511 0.2506 0.2093
288.4 276.8 254.1 220.6 207.2 142.6 136.6 226,2 208.0 195.3 286.0 119.0
315.4 301.8 279.1 245.6 232.2 167.6 161.6 251.2 233,O 220.3 311 .O 144.0
N
-
TEMPERATURE
degrees
C.
-
4.6 13.4 4.6 40.2 4.2
400 25 480 25
14.0 12.3
284 375
175.0
525
(1) More reaction gas added.
FIG.4 ray bulb and reaction sphere are calibrated for ionization by means of reaction rate of electrolytic mixture at 25°C. The detailed results of two of these experiments a t various temperatures are given in tables 4 and 5. One experiment was carried out in a closed static system consisting of a
336
CHESTER H. SCHIFLETT AND S. C. LIND
sphere and short connecting tube all enclosed at the reaction temperature 300°C. so as to test the complete correction for a known amount of water vapor. Assuming 100 per cent efficiency of the ions of water vapor gave the value M / N = 14, in good agreement with those of the circulating method, thus proving the r61e of water vapor ions and their 100 per cent efficiency in promoting synthesis. In figure 4 are plotted the results of a number of experiments a t temperatures ranging from - 183°C. to 400°C. From this curve are calculated the temperature coefficients which are shown in table 6 along with photochemical results for purposes of comparison. TABLE 6 Comparison of radiochemical and photochemical temperature coeficients
I
TEMPERATURE COEFFICIENT8
Photochemical TEMPERATURE INTERVAL
~
Radiochemical
degrees C.
183 t o 100 t o 200 t o 300 t o 400 t o 400 t o
25 200 300 400 500 490
1.00 1.039 1.043 1.047 1.047 -
I
1.045 1.07 1.07
-
1.13
-
1 ~
1.12 1.08
1.09
-
1.07
DISCUSSION
The most outstanding feature of the temperature coefficient of this reaction is the complete absence of any influence of temperature from -185°C. to 25"C., a range of more than 200°C. On account of the lack of influence of temperature on the primary step, ionization, it might be supposed that any reaction resulting immediately and quantitatively from ionization should show no temperature coefficient. The fact that it does not in the lower temperature range furnishes evidence of two types of reaction, the stably quantized type, and at higher temperature, a chain mechanism of some kind. For convenience one may assume for the low temperature range the ion "cluster" type such as has already been proposed for this and other reactions. At higher temperatures one would hardly expect the clusters to increase in size or efficiency of reaction, and it seems more probable to assume a breaking down of the cluster and a transition to some type of chain mechanism which ultimately leads to explosion. It is not improbable that the cluster mechanism persists and overlaps the chain mechanism, which gradually becomes predominant toward higher temperatures.
HYDROGEN AND OXYGEN UNDER ALPHA RADIATION
337
The complete efficiency of ions of water vapor in promoting the reaction is interesting, especially in view of the low efficiency (14 per cent) found by Rosenblum (9) of carbon dioxide ions in promoting the oxidation of carbon monoxide. In other cases however, there was ample analogy to support 100 per cent efficiency of foreign gas ions, also in this reaction (loc. cit.). SUMMARY
The combination of hydrogen and oxygen under alpha radiation was studied a t temperatures varying from -185°C. to +500"C., with radon mixed with the reacting gases and with radon in an alpha-ray bulb, with and without circulation of the gases. The results obtained for the reaction a t 25°C. agree with those previously obtained, viz., three molecules of water formed per ion pair (calculations being based on higher values of intensity of ionization than formerly). The reaction is found to proceed with approximately this same ion pair yield at - 185°C. and at -78°C. A positive temperature effect sets in above room temperature. The temperature coefficient is found to vary between 1.02 and 1.05. These values are compared with those found for the photochemical reaction. The method of applying the correction for reaction due to ionic catalyst (water vapor) present in the system is considered. It is found that the ions of water vapor possess the same efficiency in producing reaction as the ions of the reacting gases themselves. REFERENCES (1) (2) (3) (4) (5)
DUANEASD SCHEUER: Le radium 10, 33 (1913). HIXSHELWOOD AND THOMPSON: Proc. Roy. Soc. London 118A, 170 (1927). KISTIAKOWSKI, G.: Proc. Nat. Acad. Sci. 16, 194 (1929). LIND: J. Am. Chem. SOC.41, 531 (1919). LIND, S. C.: Chemical Effects of Alpha Particles and Electrons, 2nd edition. The Chemical Catalog Co., Kew York (1928). (6) LIXDA N D BARDWELL: J. Am. Chem. SOC.48, 1575 (1926). (7) LIVINGSTON, R. S.: Rev. Sci. Instruments 4, 15 (1933). (8) PORTER, BARDWELL, AND LIND: Ind. Eng. Chem. 18, 1086 (1926). (9) ROSENBLTX: Proc. Nat. Acad. sci. 18, 374 (1932).
