C. ~\~AVROY~\NNIS~ AND C. A. WINKLER

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1 THE REACTION OF NITROGEN ATOMS WITH OXYGEN ATOMS IN THE ABSENCE OF OXYGEN MOLECULES1 C. ~\~AVROY~\NNIS~ AND C. A. WINKLER ABSTRACT The reaction has been studied in a fast-flow system by introducing nitric oxide in the gas stream with excess active nitrogen. The nitrogen atom consumption was determined by titrating active nitrogen with nitric oxide at different positions along the reaction tube. The rate constant is found to be kl = 1.83 ( f 0.2)X1015 cc2 sec-i at pressures of 3, 3.5, and 4 mm, and with an unheated reaction tube. The homogeneous and surface decay of nitrogen atoms involved in the above system were studied using the nitric oside titration method, and the rate constants were found to be kl = 1.04f 0.17X10LG cc2 mole-? sec-i, and kr = 2.5f 0.2 set-' (? = 7.5f 0.FX10-5), respectively, over the range of pressures from 0.5 to 4 mm with an unheated reaction tube. INTRODUCTION The enzission spectrum of the blue nitric oxide afterglow has been studied recently by Kaplan and his associates (1). Fronz the sensitivity of its vibrational intensity distribution to the addition of heliunz, it has been postulated that the spectrum arises from the three-body recombination of a ground-state oxygen atom and a ground-state nitrogen atom, by way of some intermediate state, into the excited upper states of the beta, gamma, and infrared bands, N(4S) + 0(3P) + He + NO (intermediate) + Iie + NO(A2Z, B2H) + He. The lifetime of the intermediate, relative to the duration of the collision, is not known. Kaufman and I<elso (2) have obtained support for the above mechanism. In the present paper, the kinetic characteristics of the reaction between nitrogen atoms and oxygen atoms have been studied, using a fast-flowing system. EXPERI MENTAL Active nitrogen was produced by a tnicrowave generator. A quartz tube of 12-mm i.d., and 10 cnl long, was used as a discharge tube, and the reaction vessel consisted of a pyrex tube of 20-mm i.d., and 30 cm long. Nitrogen (>99.9yo pure) was passed into the discharge tube through a liquid-air trap to remove water, while purified nitric oxide was passed into the reaction tube through s~nall jets, located at four different points along the reaction tube. Oxygen atoms were produced by addition of nitric oxide at the first jet, to an excess of active nitrogen, at pressures of 3, 3.5, and 4 mm and with an unheated reaction tube. The nitrogen atom consumption, after the first jet, was determined by titrating active nitrogen with nitric oxide (2, 3) at subsequent jet positions along the reaction tube. The same procedure was used to study the homogeneous and surface decay of nitrogen atoms, that is, the disappearance of nitrogen atoms along the reaction tube was determined by titration with nitric oxide at the different jet positions. 'Manz~scripl received April 11, Contribution from the Upper Atlizosphere Chemistry Research Group, AhGill Uniuersity, with financial assistance front Contract A F19(604)4104, the Geophysics Researclt Directorate, Air Force Research Division, from /he Defence Research Board of Canada, and from the National Researclt Cozincil of Canada. 2Holder of n Colizinco Fellowship, , and a National Research Cozi~zcil Studentship, Can. J. Chem. Vol. 39 (1961)

2 1602 CAN.4DI.4.N JOURNAL OF CHEMISTRY. VOL RESULTS AND DISCUSS101 The following reactions may be involved: k, N+O+M -NO+i\iI The reaction N + wall - k5 O+O+M-02+M 0 + wall - k4 4x2 + wall k wal!. NO+O+M -+NOZ+M was not taken into account, since NO? would react rapidly with oxygen atoms to produce NO (4) and, moreover, reaction [2] is very fast (5) in conlparison with reaction [7]. Since the systein was free froill 0 2, except for small anlounts fornled by reactions [5] and [GI, it was assumed that there would be negligible influence froill the reactions O+On+M '03$M I:81 0 $ [91 N+O? ' NO+0. I101 The disappearance of nitrogen atoms in reactioils [I] to [4] is given by and - - (N)li k44t log k kg 1 gm k', - I - I h.64 1 ) The rate constant for reaction [5] was assumed to be kg = l.gxiolg cc2 sec-i, with 7 = 1.65X10-5 (6) which, for the conditions used, correspoilds to a value of ks = sec-i. Several values of ks and k4 have been reported; the value of kr in particular presuinably depends on the condition of the wall of the reaction vessel used. Reinvestigation of reactions [3] and [4], with the proceclure described above, gave the results shown in Fig. 1, on the assunlptioll that only homogeneous decay of nitrogen atoms occurred, i.e. that the rate constant, over the pressure range 0.5 to 4 mm, is given The data indicate that the value of k~ decreases as the pressure is increased up to about 2.5 mm, then remains essentially constant with further increase of pressure. Hence, the

3 M.4VROYAXXIS AND WINKLER: REACTION OF N AND 0 ATOMS 1603 I I I I 0 I PRESSURE - MM surface decay of nitrogen atoms is practically negligible at pressures above about 2.5 mm. A typical plot of l/((n)l(m)) against reaction time at 2.5 mm pressure yields a straight line (Fig. 2). SECONDS (X 10') To separate the homogeneous and surface decay of nitrogen atoms, plots were made of In (N) against time. Approximately straight lines were obtained, the slopes of which depended, of course, on (M). The average slopes of these lines are plotted against (M) in Fig. 3. For pressures of 2.5 mnl and above, an extrapolation of the line passes through zero, indicating the small effect of the surface decay at these pressures. For pressures lower than 2.5 mm, an extrapolation gives an intersection corresponding to k4 = 2.5Zt0.2 sec-l. The recombination coefficient is given (7) by y = (2rkr)/C, where r is the radius of the reaction vessel and C is the root mean square atomic velocity, and is found to be y = (7.5Zt0.6) X10+. The value of y, and that of k3 (Table I), are in good agreement

4 CAhTADIAN JOURNAL OF CIIEMISTRY. VOL TABLE I Variation of k3 (from equation [Ill) as a function of pressure, reaction time, and initial reactant concentrations with unheated reaction tube Total pressure (M) (mole/cc) (N) rl (mole/cc) (N) r? (n~ole/cc) t (sec) ka (cc2 mo!e-? sec-i) (nlm) x 107 x lo9 x 10" x 102 x lo-l Average Average Average 0.90 NOTE: Average ka = 1.04 (+0.17) X 10'8 cc? mole-2 nec-1 (this value is lor the disappearanceof lmo 2! atoms). 11 refers to zero time.

5 MAVROYANSIS.AND WINKLER: REACTION OF N AND 0 ATOMS 1605 with those of other investigators (8, 9, 10, 11) but not with those previously reported from this laboratory (12, 13). Since the reaction of nitrogen atoms with oxygen atoms was studied at pressures of 3, 3.5, and 4 mm, the effect of reaction [4] is very small, and has been neglected. The integrated equation for kl, with k'~ = 0, is given by (N) 11 log ---log k - k " (N ) 1- a (l+k3(ivi) (N),,At) (0) $1 (l-ct~at) The results are shown in Table 11, with an average value of kl = 1.83(f 0.2)X1015 cc2 in01e-~ sec-i. TABLE I1 Rate constants for the reaction of nitrogen atoms with oxygen atoms at various pressures with unheated reaction tube Total pressure (N)t, (rnole/cc) (0) r (mole/cc) (N) 1, t (sec) kl (ccz mole-2 sec-1) (mill) x lo9 x lo9 log (N)rz x102 x lo-l6 Nora: A verage k1 = 1.83 (f 0.2) X10'5 cc? mole-? sec-1. COhIPXRISON WITH THEORY Using the hard sphere collision model, and assuming the reaction scheme to be the stationary concentration of the binary con~plex NO* may be taken approximately as (NO)* = ZN. 07NO(N) (O), where ZN.O is the collisio~l frequency of N and 0, and T~~ is the mean lifetime of the complex NO*.

6 1606 CANADIAN JOURNAL OF CHEMISTRY. VOL The over-all rate is d (products) - kl(n) (0)(M) = k2'(~) (NO*) dt fro111 which, by substituting k2' = PZNo*.Me-E?lRT, the experilnental specific rate constant may be written as k,,, = PZN~:~.MTN~~-~~/~~; TNO is the reciprocal of a ui~imolecular rate constant, and it should be about 10-l3 sec, if we neglect activatioi1 energy. If there is any activation energy, it may be absorbed in the exponei1tial term, thus, Z,,, = ZNO*,MZN.O~NO. The collision frequencies call be calculated, assumiilg collision diameters of 3.5, 3.75, 2.95, and 1.8X10-S cm for NO, (h4) = N2, N, and 0 respectively. For T = 300" K, Z,,, = 1.65 X 101j cc2 sec-i. The calculated value of Z,,, is in good agreeineilt with that given experimentally by kl = 1.83 (f 0.2) X 101j cc2 mole-2 sec-l. Wigiler (14) has developed an expression which gives an upper limit for the rate of associati011 reactions, by determining the probability of decrease of the relative energy of the two atoms below zero, under the influence of a third body. For the atomic reaction Al + Az + A3 -+ A1A2 + A3 the rate coilstai1t is given by where k' is the rate constant, k the Boltzinann constant; Vo is the relative potential energy of the atoills A1 and A2 for infinite separatioil of A3; m,, nz,, m2 are the reduced inass and the illasses of A1 and A?; gl?, gl, and g2 are the statistical weight factors of AIA2, Al, and A?; ql? is the separatioil of the atoins Al and A?; a13 and az3 are the sum of the collisioi1 radii of AIA3, and AzA3 respectively. The term 2'i will cause a small fall-off in rate with illcreasing temperature. For Vo, the I-Iulburt-I-Iirschfelder (15) poteiltial energy curve was used, give11 by the equation V(r) = 6.609[(1- e-z) x~l x)eC2"- 1] ev, where x = (r-re)/re, V(m) = 0, and re = A, which, according to Vailderslice (16), fits better than a Morse curve for the X211 state of NO. The adopted radii were 0.89 (17), 1.49, and A (derived froill viscocity data) for 0, N, and N2 respectively. The ratio of the statistical weights inay be taken as taking illto account the separation of the levels. The final result, for T = 300" K, is k' = 3.4X1016 cc2 sec-l. The agreement between the observed and calculated value inight be taken as satisfactory, since equation [13] gives an upper limit for the reaction rate, when only atoms are involved.

7 ~~ -- MAVROYANNIS AND WINKLER: REACTION OF N AND 0 ATOMS REFERENCES I. C. A. BARTH, W. J. SCHADE, and J. I~APLAN. J. Chem. Phys. 30, 347 (1959). 2. F. KAUPMAN and J. R. KELSO. J. Chem. Phys. 27, 1209 (1957). 3. G. B. KISTIAICOIVSKY and G. G. VOLPI. J. Chem. Phys. 27, 1141 (1957). 4. H. W. FORD and N. EN DO XI^. J. Chem. Phys. 27, 1156 (1957). 5. G. B. KISTIAICOWSICY and G. C. VOLPI. 1. Chem. Phvs (1958). fi: 1. E. MORGAN. L. ELIAS. and H. I. SCHI~P. 1. Phvs.'~hem. 33. b30 (i960). d , ~ ~, --- \---~, 71 K. E. SCHULE~ and I<. J. LAIDLER. J. ~hern: ~hys. 17,-1212 (1949). 8. G. G. R~IXNELLA, R. R. REAVES, and P. HARTECIC. J. Chem. Phys. 29, 608 (1958). 9. J. HERRON, J. L. FRANKLIN, P. BRADT, and V. H. DIBELER. J. Chem. Phys. 29, 230 (1958). 10. T. WENTINIC, JR., J. 0. SULLIVAN, and I<. L. WRAY. J. Chem. Phys. 29, 231 (1958) HERRON. S. L. FRANKLIN. P. BRXDT. and V. H. DIBELER. 1. Chem. Phvs (1959). \, 12. k. KELLY and C. A. WINIC~R. Can. J. Chern. 37, 62 (1959)." 13. R. BACK, W. DUTTON, and C. A. WINICLER. Can. J. Chem. 37, 2059 (1959). 14. E. ~VIGNER. J. Chem. Phys. 5, 720 (1937); Trans. Faraday Soc. 34, 29 (1938); J. Chem. Phys. 7, X91 \----/. 15. H. HULRUI~T and J. 0. HIRSCHFELDER. J. Chem. Phys. 9, 61 (1941). 16. J. T. VANDERSLICE, E. A. ~~IIXSON, and W. G. ~IAISCH. J. Chem. Phys. 31, 738 (1959). 17. LANDOLT-BORKSTEINS TABLES. 1st Erganzungsband. p. 69.

THE REACTION OF ACTIVE NITROGEN WITH PROPANE' Can. J. Chem. Downloaded from by on 04/05/18

THE REACTION OF ACTIVE NITROGEN WITH PROPANE' ABSTRACT 'The reaction of nitrogen atoms with propane has been found to produce hydrogen cyanide as the main product, together with smaller amounts of acetylene,