T H E OXIDATIOS OF HYDROGES SULPHIDE BY H. ATZSTIS TAYLOR AND E R S E S T hI. LITIXGSTOS’
The significance of reaction chains in explaining the kinetics of many exothermic reactions has been treated theoretically by Semenoff* especially under the conditions where the reaction becomes explosive. Two types of explosion are discussed. I n the first, termed thermal explosions, the rate of loss of heat by the system is less than its rate of production by the reaction whereby the temperature of the system continually increases, the reaction becoming explosive. In the second type, the reaction has definite chain characteristics becoming explosive when the chain length becomes infinite and frequently accompanied by chain branching due t o secondary activation of more than one reactant molecule. X property common to both types of explosion is the existence of a minimum critical pressure p above which an explosion will occur and below which no explosion occurs, n-hich is related to the temperature of the system by the equation log p = A/T
.
+B
where d and B are constants. For thermal explosions the constant A is directly related to the energy of activation E of the reaction, whilst for chain explosions A is a function only of the energy necessary to cause the branching of chains and otherwise independent of the true energy of activation of the reaction. The experimental confirmation of the above equation in an explosive reaction cannot therefore be judged as a sufficient criterion of either type of explosion. The proof of a chain mechanism or its absence must be added. The existence of these chains has been demonstrated in one or more of three ways, by studying the unusual kinetics of the reaction, by introducing some activematerialor “trigger” to start the chains, or by measuring the quantum yield in photochemical experiments. One significant feature of the unusual kinetics is frequently found in the effect of traces of foreign substances. Hinshelwood3 has pointed out that a chain mechanism is the logical explanation of the general phenomenon of ‘trace catalysis,’ and more specifically of intensive drying. It is well known4 that a number of reactions of hydrogen sulphide are markedly affected by the presence of small amounts of water vapor and also that its oxidation may under suitable conditions become explosive. It was Abstract from a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy a t Xew York University. Z. physik, 46, 109 (1927); 48, 571 (1928). a “Kinetics of Chemical Change,” pp. 167-187 (1929). ‘Randall and Bichowsky: J. Am. Chem. SOC., 40, 368 (1918); Taylor and Wesley: J. Phys. Chem., 31, 216 (1927).
OXIDATION OF HYDROGEN SULPHIDE
2677
deemed advisable therefore to investigate this latter reaction in the light of the foregoing remarks. The preliminary study of the kinetics of the oxidation reaction was made by a static method whereby the rate of pressure change of known mixtures of hydrogen sulphide and oxygen could be observed a t each of several temperatures. The reaction vessel, a pyrex bulb of 300 ccs. capacity was fitted by means of a mercury sealed ground joint to a capillary manometer and pump. The bulb was maintained a t constant temperature in an electrically heated furnace, the temperature being controlled by a rheostat in series with the furnace. A nitrogen-filled thermometer was used to indicate the temperature since the experimental conditions necessary were all in the neighborhood of 300' C. Hydrogen sulphide was prepared from pure calcium sulphide and stored over water; the oxygen being taken from cylinders of the compressed gas. I n the early experiments definite amounts of the two gases dried by passage over phosphorus pentoxide were let into the evacuated reaction vessel successively. Exceedingly erratic results followed, the reaction rate a t times being slow but frequently explosive. This condition was not improved by preliminary mixing of the two gases in an auxiliary vessel a t room temperature before admission to the reaction bulb. The cause of the lack of reproducibility was finally located in the different degrees of drying to which the gases were subjected. Gases which had been dried for long periods of time were found to react consistently a t an excessively slow rate. Instead therefore of drying the gases to a high degree, and the production of a uniformity of this is difficult, the gases were saturated with water vapor a t ZIOC,yielding mixtures which reacted at reproducible measurable rates. Even under these conditions it was necessary to clean the reaction vessel thoroughly with boiling nitric acid after each experiment. The smallest traces of reaction products seemed capable of inducing explosion. Analysis of the products from the slow reactions showed the presence, along with unchanged reactants, of sulphur, sulphur dioxide and sulphuric acid. The reaction is evidently not the simple one according t o the equation: 2H2S
+ 3 0 2 = ZHZO+ z S 0 2
This was further substantiated by the observed pressure decrease during the reaction which was always slightly greater than that demanded by the above equation. It should be noted that the manometer was heated sufficiently to maintain all the water, initially present and formed by reaction, in the vapor phase. The additional pressure decrease may be accounted for by subsequent reactions as: ZHZS SO2 = z H ~ O 3s nr to the formation of H2SOIor the solubility of the 802 in the liquid sulphur formed. I n view of this lack of a simple relation for the pressure change only a few typical results out of some hundreds obtained need be quoted to emphasize the main points observed.
+
+
2678
H. AUSTIN TAYLOR AND ERNEST M. LIVINGSTON
The general course of the reaction is typified by the data in Table I.
TABLE I 2HZS:3O2 Time in nuns.
Initial Pressure 386 mms. Pressure Change in mms.
Time in mins.
Temp.
2 0 1 . 8 ~C.
Pressure Change in rnms.
3
0.j
I80
33.0
5 6 9
2.5
2 83
41.5
4.0
37 3 7 80 985 1053
48.0 61.j 66.0 66.j
6.0
25
11.0
55
17.0
The reaction shows a small induction period and a t this temperature is seen to be still incomplete after eighteen hours. The effect of a change of pressure will be considered later. The data quoted in Table I1 mere obtained with approximately the same initial pressure of g a m using however an increased reaction surface, the bulb being packed ivith pieces of pyrev tubing the surface of Tyhich had been etched.
TABLE I1 zH2S:302with increased surface
Pressure 387 min.
Time in mins.
Pre:sure change in mms.
Time in rnins.
3 4
2.0
7
j. O
9
Temp.
202OC.
Pressure change in mms.
8.0 11.5
The rate of reaction therefore is almost doubled showing some heterogeneity. The increase of surface however must have been considerably more than two and in all probability the greater part of the total reaction is homogeneous. If the reaction is a homogeneous chain reaction an increase of surface would be expected to decrease the rate due t o a shortened chain length. The observed increase in rate may therefore be complex, a decrease being more than balanced by an increase due to heterogeneity. Sorrish and Rideall in a study of the rate of formation of hydrogen sulphide from hydrogen and sulphur found the reaction to be partly homogeneous and partly heterogeneous. Oxygen was found to act catalytically for both reactions, the effect being greater for the heterogeneous reaction. Since the surface acts catalytically in the formation of hydrogen sulphide it must also catalyse its decomposition particularly since in this case an excess of oxygen is present. It is possible therefore that the primary reaction is a decomposition of hydrogen sulphide followed by subsequent oxidation. Judging from the J. Chem. Soc., 123, 696, 1690, 3202 (1923).
2679
OXIDATION OF KYDROGES SULPHIDE
susceptibility of the whole reaction to traces of reaction product especially water, this latter oxidation may well be of a chain type. It consequently follows that the energy of activation measured for these slow rates should be most significant of the primary reaction. The data quoted in Table I11 in conjunction with those previously given allow a calculation of this energy to be made. TABLE
2H2S:3 0 2
111
Initial Pressure 400 mms.
Temp. 209.3' C.
Time in mins.
Pressure Change
Time in mins.
Pressure Change
2
0
7
6 9 I4
IO
4 5
9
I3 15
The calculation however will only be approximate, for since the initial rate of pressure change is approximately linear, the temperature coefficient is given by the ratio of the pressure changes after a given short interval of time. Taking nine minutes as a convenient period the pressure ratio is 13/6 giving an energy of actiration of 4;,000 calories. Since the reaction is complex being partly heterogeneous, this value is undoubtedly lower than the true value for the homogeneous reaction. Norrish and Rideal find a value of jz,400 calories for the energy of activation of the homogeneous formation of hydrogen sulphide. Since the heat of formation is 2,730 calories the energy of activation of the decomposition would be j j,130 calories. Bearing in mind that the 47,000 calories obtained above is a minimum value, not only because the reaction is probably heterogeneous but also since some oxidation has proceeded, and Xorrish and Rideal find still lower energies for the direct union of oxygen and sulphur, the difference between the 4j,ooo calories obtained herein and that calculated for the pure homogeneous decomposition namely j 5,000 calories would not, appear too great to detract from the plausibility of the assumption that the primary reaction may be a decomposition of hydrogen sulphide. Attempts to justify ihis assumption by determining the order of the reaction met with difficukies owing to the tendency of the system to explode. To find the order by conventional methods it would be necessary to determine the relative rates of reaction for different gas mixtures at different total pressures. As will be seen later the pressure necessary for the reaction to proceed a t a conveniently measurable rate with one mixture caused explosion with a different mixture, or alternatively a rate of reaction too slow to measure. A complete treatment would necessitate therefore, the study of a series of very slow rates a t pressures so low that no mixture would explode. It might be suggested that the reaction rates could be increased by an increase in the temperature of the system. Succeeding work will show that the possibilities here are also considerably restricted, since the pressure below which the reaction is not explosive for a given mixture is itself an exponential
2680
H. AUSTIN TAYLOR AND E R S E S T M. LIVINGSTOX
function of the temperature. To work a t pressures and temperatures only just below the lowest' explosion limits of various mixtures would require an extremely uniform temperature throughout the reaction vessel and a temperature cont'rol excessively sensitive. The determination however, of the critical limits for explosion and their dependence on pressure, temperature and composition is found to furnish added evidence on the nature of the reaction. The apparatus for these measurements was essentially the same as that used in the earlier work. Mixtures of known composition were made up in a storage vessel and saturat,ed with water vapor at 2 1°C. The reaction vessel was evacuated by means of a mercury vapor pump backed by a hyvac oil pump, the high vacuum being found necessary to remove completely the reaction products after explosion. Frequent' cleaning with nitric acid was also still found necessary. The furnace being brought to constant temperature a definite quantity of mixture, measured on the manometer, is let into the reaction bulb. The occurrence of an explosion is plainly visible or at the smaller percentages of hydrogen sulphide can be observed from the oscillations of the manometer. By repetition of this process the minimum pressure, for each temperature and for each mixture, above which explosion occurs and below which no explosion takes place can be determined. Among the products after explosion, hydrogen sulphide, oxygen, sulphur dioxide, water, sulphur trioxide and sulphur were identified in varying quantities depending on the composition of the mixture used. A period of induction of from one half to two minutes was noted in each case of explosion. Seither a change in the dimensions of the reaction vessel nor in the extent of surface by introducing powdered pyrex had any effect on the dope of the pressure-temperature curve. The critical limits observed are given in Table IV.
TABLE ISPer cent HzS
"K
Minimum Pressure in mms.
Per cent
13,s
5
540.8
'53
40
10
523.5
138
40
IO
545.2 506.3 523 ' 5 539.7 559.1 505.2 531.9 545 ' 2 566.3
40
50
30 30 30
523.5
566.3
97 162 I I8 94 74 160 107 91 67 128 97 ' 5 73
40
501.7
213
I5
15 15 15 20
20
20 20
545.2
50 50 50 50 60 60 60 60
"K
Minimum Pressure in mms.
523.5 559.1 595'2 523.5 543.2
566.3 595.2
566.9 595.2 624.7 659.9
I22
88 61
Increased Surface 50 50
566.3 595.2 624. I
158 119 82
2681
OXIDATIOS OF HTDROGEN SULPHIDE
The accompanying Fig equation
shows that these results satisfy the Semenoff
I
log p = A/T
+B
the logarithms of the minimum pressures for each gas mixture plotted against the reciprocals of the absolute temperatures yielding straight lines which are parallel, having a slope A = 1735. Fig. 2 shows the curve obtained when the minimum pressures are plotted against composition, the temperature being constant. The form of the curve is typical of such explosive reactions as shown by Semenoff and his co-workers, there being a particular gas mixture which has the lowest minimum pressure for explosion a t a given temperature.
I
b
id
7
1
so
FIG.I
In this case the mixture contains approximately 18 per cent of hydrogen sulphide. Such a result is found for purely thermal explosions as well as for those resulting from chain reactions. I n order t o differentiate between the two types of explosion as was mentioned previously additional evidence for chains or their absence is necessary. I n this case the extreme sensitivity of the reaction to traces of product especially water would suggest the possibility of a chain mechanism. The effect of an increased surface as a test for chain characteristics has been shown to be vitiated since the reaction is somewhat heterogeneous. The possibility of a measurement of the quantum yield of the photochemical reaction was considered. A mixture of hydrogen sulphide and oxygen in a quartz flask was exposed for several hours a t room temperature to the full radiation from a quartz mercury vapor lamp. The only change observed in the system was a deposit of sulphur on the flask, the pressure remaining unaltered. The absence of the higher frequency radiation due to air absorption apparently prevented oxidation of the hydrogen which must have been formed along with the sulphur. It seems significant however that the reaction which did occur was the hydrogen sulphide decomposition. Semenoff has shown that the addition of some active material or “trigger” which would start chains could be used as a criterion for their presence.
2682
H. AUSTIN TATLOR A S D ERNEST M. LIVINGSTON
Ozone w s used by him in the sulphur oxidation. The addition of ozone in this case could not therefore give a conclusive result,since the chains if observed might be due solely to the sulphur from a primary decomposition of the hydrogen sulphide. The evidence adduced so far therefore is not at all conclusive and there remains only the interpretation of the slope of the log p - I / T curve. I n the case of a purely thermal explosion this slope as has been stated should be directly proportional to the energy of activation of the reaction. Semenoff has shown this relation to be E = 9.9 .i whence if A. is 1 7 3 j , E should be 17,180 calories, a value which is widely
different from the observed energy of activation. I t would appear then that the explosion cannot be purely of the thermal t,j-pe. The Jralue of A for a chain reaction explosion however does not depend on E as such but is a function of the energy necessary to cause the branching of chains, the proportionality factor depending on some power of the pressure according as the chains are ended on the surface or in the gas phase. Thus if L- is the energy necessary to cause the branched chains
h
=
r/nR
where R is the gas constant and n is the power of the pressure referred to, which in oxidation reactions will probably lie between o and 2. Hence Lwill have a value of the order of 2 0 0 0 to 3000 calories, a value which could not possibly be the whole energy of activation for the reaction and must hence be an additional quantity requisite to maintain chains which have already started. The actual possibilities for the source of chains are many. Evidence has been presented from the earlier work that the primary reaction may involve
OXIDATION O F HTDROGES SULPHIDE
2683
a decomposition of the hydrogen sulphide. The induction period in the esplosions might be accounted for on that basis and the photochemical decomposition shows it to be a relatively simple reaction. The system would thus contain hydrogen, sulphur and oxygen. Hydrogen and oxygen as Hinshelwood’ has shown combine according to a chain mechanism, Pease2 having definitely identified hydrogen peroxide as an intermediate product. Pease bases the explosiveness of t,he reaction on the “possibility that a freshly formed peroxide molecule is subjected to dissociation yielding atomic oxygen.” The more recent explanation of Alyea3 attains the same end of both straight and branched chains by means of hydroxyl groups. The presence of atomic oxygen in a system containing free sulphur, Semenoff has shown, leads to sulphur oxidation by a chain mechanism. There are therefore numerous possibilities for both straight and branched chains and the conclusion that the hydrogen sulphide oxidation proceeds by a chain mechanism seems inevitable.
Summary The slow reaction between hydrogen sulphide and oxygen studied by a I. static method is shown to be partly homogeneous and partly heterogeneous, the former predominating. The critical explosion pressures for various gas mixtures a t different 2. temperatures have been measured and shown to agree with the Semenoff theory. 3. Evidence is presented to show that the oxidation must proceed by a mechanism involving both straight and branched chains. S i c h o l s Chemical Laboratory, S e w York Uniaerszty, .Yew I’ork, S.Y .
‘Proc. Roy. SOC., IlOA, 591 (1928); 122A, 610 (1929). Chem. SOC.,52, 5106 (1930). J. Am. Chem. Soc., 53, 1324 (1931).
* J. Am.
