J. F. Ogilvie
Memorial University of Newfoundland St. John's, Newfoundland, Canada
I1
The Hydrogen-Oxygen Second Explosion Limit A
In the study of chemical kinetics, the hydrogen oxygen reaction has a special position. Not only because of its practical importance, this reaction maintains this position also because of its complicated nature, including the effects of an induction period, inert diiuents, surfaces, and competitive and consecutive reactions. Notwithstanding these formal features, the existence of multiple explosion limits separated by a region of slow reaction can be a fascinating topic in physical chemistry courses. Despite these commendable attributes, the hydrogen-oxygen reaction and its explosion limits are not even mentioned in otherwise quite reputable physical chemistry textbooks-a great pity, because the existence of chemical explosion phenomena far transcends this particular reaction which is certainly intrinsically important. Of course a laboratory experiment would make the accessibility of these explosion limit conditions much more meaningful than a mere lecture recitation of the properties of these systems. Fortunately a simple experiment, which has been devised some years ago, is available which enables an admirable demonstration of a t least two aspects of these phenomena at only moderate expense for installation and operation. Although this experiment was basically devised by Professor R. R. Baldwin of the University of Hull, U.K., about a quarter century ago, i t is still, with minor variation, actively exploited in c o m b ~ ~ t i 0research, n such as the influence of additives on the explosion limits (1). As an undergraduate experiment this has been used for several years in the University of Cambridge, and also in other universities.
chemistry experiment rate until the third explosion limit CD is reached. The exact position of the peninsula is somewhat dependent on the geometry of the reaction vessel, as also at the greater pressure end 0.f the range of the first explosion limit (near B). The second limit is sensitive neither to the diameter of the reaction vessel nor to the nature of its surface, but the addition of inert gas makes the mixture less explosive. These observations indicate that a chain center is now being destroyed in the gas phase. As for the third explosion limit, the exothermicity of the chemical reactions is retained within the gas leading to accelerated reactions and eventually to explosion. I n this case the explosion boundary is of the expected form-logarithm of ignition pressure inversely proportional to temperature-for the case of a critical concentration such that heat production exceeds heat release to the surroundings by convection, conduction, and radiation. I n this experiment the object is to study the second explosion limit. Mixtures are prepared of such composition that a t the temperature of the reaction vessel they are within the non-explosive region a t some point E. The pressure is then reduced until explosion occurs a t F. In this way a section of BC is determined for mixtures of hydrogen and air. One may account for the phenomena described above by assuming that the reaction involves radicals which in certain regions participate in a branching chain reaction. The most likely initiation process is
Theoretical (2)
The velocity of most gas phase reactions increases with pressure, but the hydrogen-oxygen reaction shows remarkable behavior. The reaction proceeds a t measurable rates between 700" and 875'K. Below this range, the r i t e is extremely small, whereas above, all mixtures explode. Within the range, the rate depends on pressure as illustrated in Figure 1. The first explosion limit, curve A B in Figure 1, depends a t a particular temperature on the surface-to-volume ratio of the reaction vessel, the nature of the surface, and the partial pressure of inert gases. These observations indicate the occurrence of a chain reaction, the chain carriers being destroyed on the surface. Any mixture within the peninsula ABC is explosive. At greater pressures, a second explosion limit is reached a t which the mixture becomes non-explosive. Above BC there is little detectable reaction; but as the pressure is increased, particularly a t the end of the temperature range near C, the reaction proceeds a t an increasing 342
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hgure 1. Graph of explosion limits for hydrogen-oxygen rnidures. The curves are approximately accurate for a porticulm reaction vessel.
H*
+ 0, + HIO + 0
---
AH
6 kJ mole-'
The following processes are suggested to be important in the mechanism for the region of slow reaction 0 Ha OH H A H 8 kJ mole-' (1) H OH
+ + OQ + HI
---
OH H20
+ +0
+H
AH Ah'
69 kJ male-' -62 kJ mole-'
(2)
-
The factor 2 appears because both the hydroxyl radical and oxygen atom react by reactions (3) and (1) to produce hydrogen atoms. I n this experiment air is used instead of pure oxygen; M can then be either hydrogen or air. The efficiencies of nitrogen and oxygen are very similar' (6); in any case the ratio of partial pressures of nitrogen and oxygen does not change appreciably under the experimental conditions. Therefore the expanded form of kp[M]is =
+ HOI
+
k,(HJPm, kJ*nP.x,
+
Thus, kr'H"PHz Ica'"' Psi, = constant, i.e., 2ke. A plot of Pn2 against Psin these being the partial pressures at the explosion boundarg, should yield a straight line of gradient -k4'ai=)/kpcH'), The activation energy for the branching reaction may be obtainea by observing the variation of the second limit with temperature a t constant composition of the initial gas mixture. According to the condition 2kl = kr[M], with [MI proportional to the total pressure P, at the explosion boundary, one can write in which E, and E4are the Arrhenius activation energies for the processes (2) and (4) respectively. A plot of log P, against reciprocal temperature yields the overall activation energy E, = Ez - E4. The activation energy of reaction (4) can he taken to be approximately zero; in fact, this reaction has a negative
+
H1O
+ OH
+ H02 -. Hs02 + H
AH AH
--
-225 kJ mole-> 63 kJ mole-I
(5) (6)
become important, again leading to an explosive region above CD.
FURNACE
FLASK MERCURY MAWMETERS
-
+
thus control the radical concentration so the rate again becomes slow. M is any third molecule which can take part in the reaction. There is now direct experimental evidence for the existence of the hydroperoxyl radical HOZ, perhaps best by vibrational absorption spectra (3, 4). This relatively unreactive radical, it is assumed, is able to disuse to the walls where it is destroyed. Thus a t the second limit, the rate of chain branching is equal to the rate of the chainbreakingprocess eqn. (4), i.e.
kJM]
H, Hs
(3)
Below the first l i t AB, the production of atoms and diatomic radicals in balanced by their removal a t the surface of the reaction vessel. At the first limit the diffusion of hydrogen atoms to the w d s is hindered; the concentration of radicals increases and explosion ensues. The oxygen atom and hydmxyl radical react much more rapidly than the hydrogen atom, so their diffusion to the w d is not usually a process which affects explosion limits. At the second limit the gas phase removal of chain centers becomes important. The termolecular collisions -197 kJ mole-' (4) H + 01 M H01 M AH
+
temperature coefficient (Ed = -5.4 kJ mole-') which has been determined by a. different technique (6). At still greater pressures, diffusion of HOz to the wall is hindered and the inefficient radical regeneration reactions
Figure 2. Diagrommotic representation of the apparatus suitable for study of the hydrogen-air explosion limits in undergroduata loboiatorier.
Experimental
1)epicrcd in Figure 2, the apparatus con.is~-.bxsirnlly uf a Wactim v r w l , enclwed withiu a ftrrnarc, and a ..mnll g1uc.i wruunr mnnifold havmg pn,vhiorc for hydrogen' 111111 air inIet.i, manometers for pressure measurement, and connection to a mechanical vilcuum pump. The reaction vessel, of volume about 0.5 1, is farmed from tubing of borosilicrute gless or fused silica, of 40mm diameter and 0.4-mlength. One end is closed rounded, and the other end is drawn down and attached to a standard taper mound cone to connect to the vacuum manifold. This vessel is n ~ w n t c drentrally on two psirs of stubby fret inside a . i l i a tube of leugtll 11.5 in and im.w dinmeter ht lenjt 53 mm. On rhir outer tube between Layers of abestos paper is wound a helical mil of niohrome wire Wgauge, 1.2-mm diameter). The nichrome wire is wound so that the resistance between the center of the coil, at the midpoint of the silica. tube, and eaoh end is about 20 ohms. To prevent temperature gradients along the reaction vessel, the heating wire should be wound more closely near the ends of the support tube than near the middle. Larger diameter niohrome wire is spot-welded to the center of the coil and to each end to lead outside the furnace enclosure. About 60 V, reduced from the ac mains supply by either a. variable auto-transformer or more simply a heavy-duty series resistor, is applied between the center of the mil and each end. The furnace tube is mounted on a reinforced asbestos cradle inside sn asbestos-board box of length 0.57 m and of square cross-section 100 mm. This box has a removable lid for assembly purposes and an aperture in each end, one covered by a borosilicate window for visual inspection (and also assembly purposes), and the other smaller opening to pass the small tube to oonnect the reaction vessel to the vacuum line. This asbestos box is mounted on asbestos supports inside a sheet metal box, and the space between the two boxes is loosely filled with asbestos powder for insulation. The temperature of the furnace is conveniently maintained by a Dohrmann Series 2300 indicating temperature controller for the range 0-1000°F (Cole-Farmer Ltd., 7425 N. Oak Perk Ave., Chicago, Ill. 60648, U.S.A. (or sirnilas device)). The mirror scale of the meter of this controller ca,n be read + l 0 K on the Celsius scale, -20 to +540°C. The platinum resistance ther-
-
~
~
CAUTION: Althoughauly very small quantities of hydrogen gas are used in each trial of this experiment, precautions, with regard to the safe disposal of unreacted hydrogen, the possible absorption of hydrogen by oil in mechanical pump, and the pumping of hydrogen and sir together in quantity through the m e c h a ~ c apump, l are advisable. Volume 48, Number 5, May 1971
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343
mometer used in the control circuit and the thermocouple for temperature measurement are compactly housed in a probe which can with care be situated on top of the reaction vessel within the furnace tube. Small appendages of glass fixed on the reaction vessel facilitate this placement. If more accurate tear sensitive temperature indication is required, s. suitable additional thermocouple may be inserted into the furnace enclosure and connected to a. millivolt potentiometer. The vacuum manifold, generally of 6-mm bore borosilicate tubing, has a. liquid-nitrogen cooled trap to protect the vacuum pump (which need be neither lmge nor efficient for this experiment,). there are a, ball.llest flask of volume 111r2 .. Then -~~ . - 1., a b ~ pto i permir rapid rxhauitiol>of the s).strm, nu nir inlet, and a hydroger, inlet wlliell fuurrioni boTh 8 5 nixlwmerer and buhbler to prurerr ar,ai!ist e x w i d hydrogel) preiiurc. Close t b the tap leading to the reaction vessel are two mercury columns for manometricpurposes. The procedure of the experiment consists simply of admitting a certain Dressure of hydrogen into the reaction vessel, raoidls admittinglir to atmosphericpressure, and waiting a reprodicibie period of 30 sec for mixing. At this stage the pressure of gas in the reaction vessel is first rapidly reduced by expansion into the evacuated line and ballast flask, then more slowly reduced by pumping out. Passage across the explosion boundary is denoted by a "kick" of the mercury column in the double manometer. The Dressure reading just before the kick is recorded. The experiment consists o f two parts, determination of the activation energy of the branching reaction (2), and determination of the relative efficienciesof hvdroeen and air as third bodies. For the former ~ ~ - -set ~ -of - trials a consta& oressure eauivalent to 200 mm He ia i-..~-, a nnd d t,he is varied s t 1O0K intervals between ~ temneratnre ~ - ~ ~ -. ~ 725 and 785'K. For the latter purpose, a. constant temperature of 785'K is employed, and the hydrogen pressure is varied between the equivalents of 125 and 450 mm Hg in shout six increments. Before taking any mettsurements when starting the experiment an any day, some trial runs, with 100 mm Hg pressure of Hz in the reaction vessel and sir to atmosphere, should be conducted to condition the surface of the reaction vessel. These trial. can alao serve ns pmvricr for ndjt.iring the cxhnusring procedure u, prrrnit optimum ubwvatiou of the tnnnomcrer "kick". ~
~
~~
L~~
~
~~
~
~
Analysis of Results
To determine the activation energy, the plot of logarithm of pressure a t the explosion boundary versus reciprocal temperature is used. The gradient of the best straight l i e is proportional to the activation energy, as earlier mentioned. The value thus derived pertains essentially to reaction (2). As this reaction is endothermic, there must be a minimum magnitude of the activation energy equal to the endothermicity. I n fact many experimental studies have demonstrated
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Journal of Chemical Education
that the apparent activation energy is close to the endothermicity as the reverse reaction probably has zero energy of activation (6). The numerical values of the activation energy derived from this experiment are susceptible to error due to temperature inaccuracy or gradients, and pressure gradients down the vacuum line from the reaction vessel. The shock wave accompanying explosion takes a finite time to travel from the reaction vessel to the manometers. However, an important cause of discrepancy between experimental and true activation energy is the over-simplification of the mechanism as presented above, particularly as concerns the effectof the surface. Reference (1)and references therein should be consulted for further details. For the relative effect of third bodies in reaction (4), the experiment seems to produce good results. Hydrogen is known ( 6 ) to be about 2.5 times as effective as either nitrogen or oxygen in removing the exothermicity of reaction (4). A major source of discrepancy might arise from the presence of water vapor which is about ten times as efficient as hydrogen (5). As water is being slowly produced during the mixing period, this period should be kept nearly constant, of duration about 30 sec, during the runs. Finally the production of the faint glow or flash that often accompanies explosion deserves comment. The color is usually reported to he yellow to red, depending on partial pressure of oxygen and, especially, nature of the surface. Although difficulty may sometimes be encountered in observing the flash, it is worthwhile to make an effort as the light emission provides a further element of fascination in this complicated reaction mechanism. Litemture Cited (1) BaLowrN, R . R.. H o ~ ~ mD. a , E., *No WA=TEB.R. Soc.. 66, 189 (1970).
W., Tran8. Fa7adov
. AND vow E m s , G., "Combustion. Flsmea. and Explosions of (2) L ~ w r sB., G a a d (2nd ed.) Aaademic Press. New York. 1961, pages 4-64. 131 M ~ r m a m D. . E.. *an Jncox. M. E.. . I Chem. . Phus. 38, 2627 (1963); .. 40, 605 '(1964). (4) O a ~ ~ v r m J.,F.. Bpedrochim. Ada, 23A, 737 (1967). (5) Tnamn, B. A,, "Progress in Reaction Kinetics." Vol. 3, Pergamon, Oxford, 1065, pages 74-88.
