G. Ronald Brown and C. A. Winklerl McGill University Montreal. P.Q.. Canada H3C 3 ~ 1
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The Kinetics of Nitrogen Atom Recombination An uhgrradue physical chemistry experiment
Onlv mis~it~nossihle for students to follow nrocesses - ~ -, -~ - e -l d - o~ in the gas phase by visually observable changes in the system. The nronounced chanees in color intensitv with temoerature of t h i NZoa +2N02 e&ihrium may ac&t, in pa; a t least, for its no~ularitvas an undermaduate experiment ( I ) . Favorable reception hy studen& has also been reported by Thrush ( 2 )for a kinetic studv of oxygen (03P) atom recombination in which photometric m%urements are used to follow the rate of change in the intensity of the "air afterglow" that results when small amounts of NO are added to an 0 atom stream. The present paper describes a similar, but conceptually and technically simpler study of the kinetics of the recombination of nitrogen (N4S) atoms in which concentration-time relations are determined directly by utilizing visual observations of emissions to make gas phase titrations of N atoms with NO ( 2 4 ) . The data so ohtained allow quite simple treatment to provide the desired kinetic information. lntroducllon Nitrogen atoms may he produced by subjecting Nz to either an eleetrodeless microwave discharge or to a condensed dc discharge hetween electrodes. In an appropriate flow system, the atoms can be transferred from the discharge into a reaction vessel where, at pressurest 2 Tarr (270 Pa), the rate of N atom recombination is largely determined by the rate of the termolecular, homogeneous reaction (3) ~~~
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N+N+M%N~+M (1) The third body, M, may he N o r Nz. Reaction (1) is a n elementary process for which the rate equation is
-d[Nl=2k1 [NI2[M]
(2) dt The fraction of NZdissociated is small, and to good approximation
where P is the pressure in the reaction vessel and T is the temperature. The rate equation then becomes -~
N+O+M-NO+M+hu(hlue)
~
(5)
Therefore.. as .lNOl. is increased the color of the emission changea from yellow, due to reaction (1). to violet, and then to blue. When NO is present in excess, i.e., 1x1< IN01, the N atoms are completel~consumed,and the && the" cot&ns 0 atoms and unreacted NO. This condition is indicated by the yellowish-green emission, known as the "air afterglow"that results from the reaction (6) O+NO+M-NOn+M+hu(green) At the eauivalence point both N atoms and NO are completelv consumld and reaitions (I),(5),and (6) cannot occur. Conseauentlv, the absence of emissions corresponds to the ''endpoi&' for the titration. The rate of reaction may he ohtained by titrating the N atoms at several positions ajOtIRa reaction vessel in which the gas is flowing at a known linear velocity. Apparatus The apparatus (Fig. 1) is similar in its essential features to that described for the study of the recombination of 0 atoms (2). Construction of a conventional fast flow system for the study requires good quality stopcocks. Nitrogen atoms may be produced in Np,takendirectly from a cylinder without purification, by e so-called condensed discharge between electrodes mounted in a Pyrex tube approximately 50 cm long and 25 mm 0.d. The hollow aluminum electrodes may be machined from rod or made from sheet, and are attached to tungsten wires ( 2 1 mm diameter) that are sealed through theends afthe tube (Fig. 1,inset). A satisfactory eledrieal circuit to produce the discharge is schematically represented in Figure 2. Alternatively,a microwave unit (e.g. Raytheon microwave generator, Model PGM-IO,85 WRF at 2450 MHz) mav he used to vroduee an electrodeless discharge in a Vycor tube; 11Am ad., joined to the remainder of the apparatus by graded seal~.~The N atom concentration is determined at each of several jets in the reaction vessel (- 26 mm ad.) with NO, the flow rate of which is controlled by a needle valve ( N 2 )and determined by measuring the rate of pressure drop in a known volume using a differential manometer (D)that contains silicone oil (eg. Dow Corning 105 Diffusion Pump Oil). The total volume, VF, of the flask, F, and the connecting tubing should he about 300 ml. If desired, simple jets
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The rate of recombination of N atoms, a t a given pressure, may he ohtained as the slope of a plot of [N] against reaction time. The [N] can he determined by a gas phase titration of N atoms with NO, which reacts rapidly with N atoms according to the reaction N+NO-Np+O
(4)
Reaction (4) is arrompanied by visually ohsewahle emissions that change color as the ratioof NO to N is varied ( 4 ) . When [N] > [ ~ athe, NO is rapidly consumed, and the gas contains unreacted N atoms toeether with 0 atoms oroduced by reaction (4). This results the reaction Additional information concerning this experiment may he ohtained from the authors on request. Although the power supply for a dc discharge may be constructed from materials that cost less than a microwave unit, the N atom production from it generally fluctuatesmore than that from the microwave discharge.
Figure 1. Schematicdiagram of me apparatus; A, to microwave unit; 8,N2flask I);C, to Ne cylinder; D, differential oil manometer: E, to vacuum pump; F, calibrated NO volume; G, to MacLeod gauge; H, stopcock: J 1 4 4 , Inlet ieh; M. mercury manometer; N1. NZ, needle valves; P, manostat: and S, NO storage vessel.
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Volume 54, Number 3, March 1977 1 185
protruding into the reaction vessel may replace the annular jets depicted in the tigure. Avacuum pumpis requiredofsufficient capacity ie.g. a Welch 14OZBJto g w e a linear velucity of about 1-2 mts in the reaction vessel. The connecting tuhing between the reaction vessel and pump should be of minimum lm@h and at least 15 mm o.d. The pump ~ h o u l dhe protected by a trap cuolcd with liquid nrtrugen and b d a s t volume df about 5 1may bekerpased between the pump and the reaction vessel to minimize fluctuations in pressure in the system.
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Procedure3 T o prepare the apparatus for student use, the 10-1storage flask must be filled with NO (Matheson) which need not be rigorously purified (5).However, i t is desirable t o condense the gas using liquid nitrogen, pump off residual noncondensable gases, and f m d y distill the NO into the storage bulb from a trap immersed in aDry Ice-acetone mixture (-78'C) which retains traces of N203 and NZOC If the experiment is t o be completed in a 3-hr laboratory period, the student must be provided with the flow rates of Nz a t various pressures in the reaction vessel, since these arerequired for calculation of the linear velocities and thus the reaction times. Alternatively, the student may be asked t o determine these flow rates. This may be accomplished by any appropriate method, as, for example, by a soapbubble flow meter attached t o the outlet of the manostat,P. The Ng flow is then measwed when some of the NI-. passes through - the reaction vesel, with ngiben setting of the needle valve NI.after which it is determined when the h,w through thereaction vesselisstopped but the tuml flow Nl entering at C is left unchanged. The difference represents the Np flow in the reaction vessel a t a particular pressure. In this way, the relation between pressure in the reaction vessel and flow rate of Nz may be established over a range of pressures. If a suitable flow meter is not available, a n alternative method may be used. The flask El (-5 1) and the connectine tubine from sto~coekH a . of whi& must bdknown. to needle valve N I ., th; total volume.. V... is filled to atmmpheric pressure uith Nn with stopcock H open to the manustat. The needle valrc .Vl is adjusted untrl the pressure in the reaction vessel is about 5 Torr. StopeockH is then c l d , and the rate of decrease of pressure in Vn (and correspondingly in the reaction vessel) is determined with manometer M. With the same adjustment of the needle valve, the relation between the pressure in the reaction vessel.. Pa. ... and then in the flask and connectine tubine. -. Pa. -. is then determined for various constant values of P e , maintained by careful adjustment of stopcock H. The flow rote a t any particular value of PRmay then he obtained by finding its corresponding value of PB, and taking the slope a t this Pn of the tangent to the plot of Pn against time. With this slope, d P ~ l d tthe , flow rate of N2 in the reaction vessel Since the relation between is aiven bv f = dnldt = VndPalRTdt. pr;ssure ih the reaction vessel and the flow rate in i t is independent of the manner in which the pressure is adjusted, atmmpheric pressure may be maintained in the flask and the pressure in the reaction vessel regulated with the needle valve. The system is stabilized and reproducibility of the data is improved if the pressure in the reaction vessel is adjusted t o t h e desired value (between 2 and 5 Torr), and the discharge is activated at least 1hr prior to commencement of the experiment.'Presumably this procedure reduces the effect of impurities adsorbed on the wall of the reaction vessel. To titrate the N atoms st any given jet, the stopcocks are opened between it and the NO starage vessel, S , and the flow of NO into the reaction vessel adjusted with the needle valve N2 until i t is equal to the N atom flow rate at that point, as indicated by the absence of light emission in the reaction vessel downstream from the jet The NO flow rate, so adjusted, is measured by closing the stopcock that separates the arms of the differential manometer and measuring the time reauired for a oresswe droo of 20-40 mm of oil in the volume. ~.Vc.6 . This procedure isrep.arrd io; each of thr otherpts in the reaction vessel. to obtain data from u hirh may t w calculated the rate uf decrease in IN1 UP to recombination between theaucceraivejets, i.e, with rear~
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Figure 2. Sdmmaticdiagam of a mndensed dc power supply forUw discharge; C, condenser 1 wf, 5000 VDC: D, to discharge tube: L, line voltage: R, resistors. abovt 10 Ml,i 0 0 mA: T rectifierW s . 866.4;V, vsMc, hlamp; X, him voltage transfwmw.e.g. Hammond X788. f=-- P dV (7) RT dt where dVIdt is the volume flow rate of the gas. For a reaction vessel of cross section A = r r z d V -- A dl = ~ dt dt
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When the titration8 at one pressure hare been completed, the flow of N, may be adjusted tu givr a different pressure in the reaction veaael, and NO titrations again made after allowine about 1/2 hr to establish steady conditions in the system. This p&dure may he repeated a t a number of different pressures to establish the order of the recombination reaction in respect to the concentration of the third body.
Calculations T o calculate the reaction times, "plug" flow is aasumed: i.e., it is assumed that the linear flow velocity, u, of the gas in the reartion vessel rs independent of its rad~allocation. The flow rate of N1 is 186 / Journal of Chemical Education
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2
"
(8)
where dlldt is equal t o the linear velocity, o, of the gas in the reaction vessel. Thus
"=- f R T
(9)
"w2P
and the reaction time d * d r2P t=-= (10) u fRT where d is a measured distance along the reaction vessel from the point corresponding to t = 0, e.g., from jet JI t o another jet downstream. The tlow rate uf NO,fuo. which is equal to the flow rate
