Oct., 1959
1753
THEKINETICSOF OXIDATIONOF HYDROGEN BROMIDE
THE KINETICS OF OXIDATION OF HBr* BY WILLISA. ROSSER,JR.,AND HENRYWISE Stanford Research Institute, Menlo Park, California Received March 98. 1960
The oxidation of gaseous HBr has been studied in the temperature range 700 to 800°K. When the concentration of HBr is much greater than that of 0 2 , the over-all reaction stoichiometry is 4HBr 0 2 = 2H20 2Br2,and the reaction is a homogeneous, gas-phase reaction. The rate of reaction varies with the concentration of each reactant as shown by the equation d(Brz)/dt = Pk(HBr)(02). The temperature variation of the specific reaction rate k may be summarized as k = 10l2J e-37J00’RT cc./mole sec. A reaction mechanism is proposed.
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I. Introduction In a study of the effect of bromine compounds on the oxidation of hydrocarbons and on hydrocarbon combustion, it was found that O2 and HBr react at moderate temperatures. By virtue of this reaction, a small percentage of HBr in a mixture of CH, and O2 eliminates the induction period which normally precedes the onset of observable reaction. Illustrative pressure-time curves are shown in Fig. 1. The oxidations of hydrocarbons other than CH4 have also been catalyzed by HBrS2 Consequently, a knowledge of the oxidation of HBr is required t o interpret observations made on systems containing both 0 2 and either HBr or substances from which HBr can be derived. 11. Experimental
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for quantitative measurement of the reaction rate. The reaction was studied a t temperatures from 700 to 800°K. and for reactant concentrations in the range 10-7 to mole/cc. Extra dr O2(> 99.6%) was used as su lied by the Matheson Co., fnc. Anhydrous HBr (> 99.8g), from the same source was used after impurities non-condensable a t the temperature of liquid nitrogen had been removed by vacuum distillation. The reactants were introduced separately into the reaction vessel from a small manifold attached to the gas storage vessels. (The concentration of one component, either 0 2 or HBr, was always much greater than that of the oth?:.) The minor component was introduced first and its initial pressure measured with the oil manometer attached to the apparatus. The oil manometer connection was then closed and the major component added. The total pressure and the pressure change were measured by means of the Bourdon gauge.
111. Experimental Results A. Reaction in a Quartz Vessel.-When the The various experiments were carried out using the apparatus schematically shown in Fig. 2. The essential elements concentration of HBr is much greater than that of of the system are: (a) a reaction vessel housed in a cylindri- 02,the experimental results are uncomplicated. cal resistance furnace, (b) a tungsten lamp, (0) a Beckman For extensive reaction the pressure change is oneDU monochromator to select light of the desired wave length half the pressure of Brz produced by the reaction, which then is detected by a photomultiplier tube, (d) a sili- i.e., 6p = (l/z)(p~rJ, The pressure measurecone-oil manometer for measuring low pressures, and (e) a Bourdon gauge for measuring higher pressures. The Bour- ments are only semi-quantitative since the predon gauge consisted of a Vycor spiral to which was attached a cision of measurement (0.2 mm.) is an appreciable small mirror. Movement of the mirror was detected by part of the total pressure change (1 to 2 mm.). means of a reflected light beam. The course of the reaction The ratio, Br2 produced/02 initially present, is was followed by photometric measurement of the concentraprecisely 2 ( *2%) for complete reaction. These tion of Br2 at a wave length of 4200 A. Two cylindrical reaction vessels of a comparable size were facts indicate that the over-all stoichiometry is used, one of quartz and the other of Pyrex. The dimensions given by of the quartz vessel (diameter = 4 cm., length = 60 cm.)
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correspond to a surface to volume ratio of 1 cm.-l. The Pyrex vessel, of similar dimensions, was partially filled with borosilicate-glass beads to give an over-all surface to volume ratio of about 5 cm.-l. The monitoring light beam passed 1 to 2 mm. above the bed of beads which occupied the lower third of the reaction vessel. I n order to obtain temperature uniformity within the reaction vessel, the furnace was heated by four resistance windings, each independently regulated by hand adjustment of a variable voltage supply; and the reaction vessel was housed within a heavy-walled aluminum liner. The temperature of the reaction vessel was measured by six eternal chromel-alumel thermocouples distributed uniformly along the length of the reaction vessel. By adjusting the voltage applied to each of the four heater windings, one could easily limit the temperature range indicated by these six thermocouples to less than two degrees. The temperature within the reaction vessel was considered to be the average of the temperatures indicated by the external thermocouples. The rate of reaction was measured by observing the rate of production of Brz as a function of time, of reactant concentrations and of temperature. The pressure change associated with the reaction was also observed; the total pressure change is quite small ( 1 to 2 mm.) and is not suitable (1) This research waa aupported by the Engineer Reaearoh and Development Laboratories, U. 8. Army, Corps of Engineers, under contract DA-44-009-ENG-2863; and by the Division of Physical Sciences, Stanford Research Institute, Menlo Park, California. (2) F. F. Rust and W . E. Vaughan, Ind. Eng. Che’hem., 41, 2695 (1849).
4HBr
+ 02
e
2H20
+ 2Brz
(1)
The rate of production of Br2 was found to be proportional to the product of the concentrations of HBr and 02. For presentation of the data, however, it is convenient to convert the differential rate expression
e) 2k(HBr)(02) dt =
to an integral form. The integral expression corresponding to eq. 1 and 2 has the form -log [I
- (Br2)/2(O2)i] = (log e)2k(HBr)it
(3)
where the subscript “i” refers to initial concentrations. As shown in Fig. 3, as well as by eq. 3, the left-hand side of eq. 3 is proportional to time. The rate constant k, derived by means of eq. 3 from the slope of curves like that in Fig. 3, varies with temperature but not with the initial concentrations of the reactants, as long as these concentrations satisfy the condition (HBr)>>(02). When the initial concentrations of the reactants satisfy the condition (02)>>(HBr), the experimental results involve features that differ from those associated with the inverse condition
WILLISA. ROSSER, JR.,AND HENRYWISE
1754 60.
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50
5:
I
I
I
I
1
served Brz has been produced by reactions which lead to the stoichiometry of eq. 4
I
I
1- N O AODiTlVE B 4 9 mm "81 t 10 5 m"' HBI
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2HBr = Hz
40-
-
y o 30-
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P m m L PRESSURE OF o2 : 1 5 3 n m P4RTlAL PRESSURE OF CHI : l58mrn TEMPERLTURE 465. C ~
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T l M E -$e
l/2 PBr.. Accordingly it may be concluded that some Hz is produced by the reaction between O2 and HBr when the concentrations of the reactants satisfy the condition (02) >>(HBr). Even for the condition (O2)>>(HBr), eq. 2 describes the initial rate of production of Brz although not the rate of production of Br2 for extensive reaction. To illustrate this behavior, the rate expression is converted to the integral form -log [l - 2(Br2)/(HBr)i] = (4log e)k(O&
TUBE W R N I C E
Fig. Z.-Schematic
Vol. 63
(5)
As shown in Fig. 4,the left hand-side of eq. 5 does not increase linearly with time for extensive reaction. The initial slope of the curve shown in Fig. 4 may still be used to derive a value for the specific reaction rate k
IHBII,~551XldBMOLES/cr
T
0
: 732.
100
K
I 200
I 300 TlME
Fig. 3.-The
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I
I
400
SO0
I 600
I
700
1.5
rate of production of bromine when (HBr)
>> (02).
c
1
0.20
I Y
1 0
100
200
300
400
so0
600
TIME - m e
Fig. 4.-The
rate of production of bromine when (HBr)
>(OS). I n particular, the observed pressure drop for extensive reaction is considerably less than '/2 the pressure of Br2 produced. The results of several determinations may be summarized by -6p = (0.2 to 0 . 3 ) p ~ , , . These observations indicate that a portion of the ob-
Values of k derived in this manner agree with the values derived from eq. 3 (see Table I for comparison). The failure of eq. 2 and 5 to describe the experimental results for prolonged reaction i s attributed to the presence of Hz in the system. A reaction between H2 and Brz, known to occur a t these temperatures, will consume the product Br2 and regenerate the reactant HBr. Consequently, the net rate of disappearance of Br2 will be less than it would be if Hzwere not present. The temperature variation of k is presented in Fig. 5. The line shown is intended to fit the data for the condition (HBr)>>(02) because these data were determined with greater precision than those for the opposite condition, (HBr) > 02 VESSEL RBI < < 02 PACKED P Y R E X V E S S E L
0 QUARTZ
A
47 49 53 49 44
ns, > >
01
-
Av. = 48.4 2c 2.9" 3.46 1.99
x x
10-7 10-7
5.43 5.45
x x
104 10"
50.8 51.3
Av. = 51.1 I
Standard deviation.
I30
135
*('XI-'
for which (HBr)(02) HBr + O2+HOz + Br (8)
Fig. 5.-The
I40 X IO'
I45
I50
specific reaction rate as a function of temperature.
According to the mechanism outlined above, the specific reaction rate k refers to the initiation reaction, eq. 8. The Arrhenius expression for k (eq. 7) then implies a steric factor of about and an activation energy of 38 kcal. for reaction 8. This activation energy requires that the heat HBr +HZO1+ Br HOz (9) of formation of the H 0 2 radical be no greater than Br + Br -+ Brz (10) about 2.5 kcal. This maximum value, AHf = HzOz +2 0 H (11) 2.5 kcal., compares reasonably well with the energy OH HBr +HzO + Br (12) When (02)>>(HBr), it is necessary to include of dissociation D(H-02) = 47 f 2 kcal.6 or AHf = reactions 13 and 14 both of which compete with 4.5 f 2 kcal. The reaction between O2and HBr is undoubtedly HOz +H On (13) responsible for the absence of an observable inHOz +destruction a t walls (14) duction period during the sensitized reaction bereaction 9. The hydrogen resulting from reaction tween CH4 and O2(see Fig. l). The reaction pair 13 or possibly from reaction 14 will react with HBr + Os +HOe + Br (8) Brs to give the features associated with the conHBr Br + CHa +CH, (15) >>(HBr). dition (02) The decomposition of H20zhas been recently is formally equivalent to the reaction studiedas4and it is known that above 700°K. the CH4 02 +CHs HOz (16) decompositionis primarily homogeneous and begins and apparently provides radicals a t a rate much as written in eq. 11. grea.ter than does the initiation reaction associated (3) P. A. Giguere and I. D. Lin, Can. J . Chem., 85, 288 (1957). with the unfiensitized reaction. (4) C. N. Satterfield and T. W. Stein, THISJOURNAL,61, 537
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(5) S. N. Foner and R. L. Hudson, J . Chem. Phys., 28, 1364 (1955).
(1957).
RADIATION CHEMISTRY OF POLYVINYL CHLORIDE BY A. A. MILLER Research Laboratory, General Electric Company, Schenectady, New YO& Received March 98,I069
The irrdation, with high-energy electrons, of the polyvinyl chloride structure, -CHaCH( C1)- was studied. Radiation 'elds for HCl were determined as a function of temperature. PostArradiation changes in vacuo and in air, were measured ry electron paramagnetic resonance and by visual observation of color. It is concluded that the major primary chemical processes are the formation of -CH&HC1. and -CH=CHHCl. A free-radical chain dehydrochlorination, involving the unstable -CHCH( Cl>- radical, appears to explain the development of conjugated unsaturation. The radiation yield for HC1 increases with temperature from a minimum value of G = 5.6 a t -90' and below, to G = 23 a t 70'.
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Introduction has already been reported by Chapirol and, alThis paper discusses Some aspects of the rad& though similar over-all results were observed in the tion chemistry and related free-radical chemistry of Present work, the interpretation is somwhat difpolyvinyl chloride (pvc),-CH2CH(C1)-. A study ferent. Much more research has been done on the of the chemical effectsof y-radiation in this polymer (1) A. Chspiro, J . Chim. P ~ U U .88,896 , (1966).
