JOHNT. HERRON
2736
Mass Spectrometric Study of the Reaction of Nitrogen Atoms with Ethylene
by John T. Herron Physical Chemistry Division, Mass Spectrometry Section, National Bureau of Standards, Washington, D . C. 3033.4 (Received March 10, 1966)
The reaction of nitrogen atoms with ethylene has been studied using a mass spectrometer to follow the partial pressures of reactants and products. From the experimental results the initial steps in the reaction are postulated to be
+ C2H4 +HCN + CH3 N + CH3 +HCN + 2H
N
H
+ C2H4
+C2H.5
+ C2H5 +2CH3 N + C2Hs +NH + C2H4 H
(1) (2)
(3)
(4) (5)
N+NH +N2+H (6) Addition of H atoms greatly increases the yield of HCN and effectively removes the discrepancy between the nitric oxide and the ethylene titration techniques for the measurement of nitrogen atom concentrations. An upper limit to the rate of reaction 1 a t 320°K. is . ~ set.-'. found to be ICl I 7 X lo9 ~ mmole-'
Introduction Because of its rapidity and apparent simplicity, the reaction of ethylene with active nitrogen has been extensively used as a means of measuring the nitrogen atom concentration.' Since about 9501, of the nitrogen in the condensable products is in the form of hydrogen cyanide, the amount of hydrogen cyanide recovered under conditions of complete consumption of nitrogen atoms is taken to be equivalent to the quantity of nitrogen atoms originally present. The generally accepted mechanism of the reaction is
+ C2H4 4HCN + CH3 CH3 + N HCN + 2H (or Hz) N
--f
(1) (2)
However, as was pointed out in an earlier mass spectrometric study of the reaction, the ratio of nitrogen atoms consumed to ethylene consumed is much too large to be accounted for by this simple me~hanism.~ In view of these earlier resdts this reaction has been reinvestigated in considerably greater detail in order to define unambiguously the stoichiometry of the reaction and hence to understand better the mechanism. The Journal of Physical Chemistry
Experimental The apparatus and general procedure have been described pre~iously.~I n brief, prepurified grade nitrogen gas, partially dissociated into atoms by means of a 2450-Mc./sec. electrodeless discharge, was flowed into a 24.5-mm. i d . tubular reactor. A second reactant could be added through a movable central inlet tube which could be located between 1 and 15 cm. from the sampling orifice of a mass spectrometer. Total pressure was varied from about 1to 3 torr.5 The partial pressures of stable reactants and products were followed using an ionizing energy of 44 e.v. while the relative partial pressures of nitrogen and hydrogen atoms were followed using an ionizing energy of 24 e.v. (1) Extensive references are given by H. G . V. Evans, G. R. Freeman, and C. A. Winkler [Can. J . C h . ,34, 1271 (195611 and by A. N. Wright, R. L. Nelson, and C. A. Winkler [&id., 40, 1082 (1962)l. (2) Since these reactions are carried out in a great excess of Nz, any NZproduced in the reaction is undetectable. (3) J. T. Herron, J . Chem. Phys., 33, 1275 (1960). (4) F.S. Klein and J. T. Herron, aid., 41, 1285 (1964). (5) A torr is defined as 3 / m of a standard atmosphere.
2737
MASSSPECTROMETRIC STUDY OF REACTION OF NITROGEN ATOMSWITH ETHYLENE
The relative nitrogen atom partial pressure was put on an absolute basis by means of the nitric oxide titration technique.6 Although this titration technique has been subject to criticism,' the recent e.s.r. measurements of Westenberg and deHaas8 and the present work (see below) would seem to settle the question in its favor. No attempt has been made to put the hydrogen atom partial pressure on an absolute basis. Instead, it has been assumed that hydrogen and nitrogen atoms have the same mass spectrometric sensitivity. Thus the partial pressures of hydrogen atoms given below could be in error by a factor of 2 or more.
Results and Discussion Figure 1 shows the variation in partial pressure of reactants and products a t a fixed reaction time as a function of the initial ethylene partial pressure. The limiting yield of hydrogen cyanide is seen to be considerably less than the initial nitrogen atom partial pressure. Of greater interest is the fact that the ratio of nitrogen atoms consumed to hydrogen cyanide produced is a function of the initial ethylene partial pressure. This is shown in Figure 5 (curve A). The simple mechanism, consisting of reactions 1 and 2, is inadequate to explain these observations. In the earlier mass spectrometric study of the reaction, the following mechanism was employedg
N
+ CzH,
+ [C2H,N]
+ [CdLN]
N
+CzH4
[CZH~N] +HCN CH3
+ Nz
+ CHi
+ i\i --+ HCN + 2H (or H2)
This mechanism predicts that the ratio of nitrogen atoms consumed to hydrogen cyanide produced should be a function of the initial atom concentration. However, Verbeke and Winkler' measured the ratio of nitric oxide consumed in the nitric oxide reaction to hydrogen cyanide produced in the ethylene reaction over about a tenfold range in initial atom concentration, and found the ratio to be constant. Assuming that the nitric oxide consumed is a measure of the nitrogen atom, concentration, their results would rule out the above mechanism. These observations have been confirmed in the present work. Figure 2 gives the ratio AN/HCN as a function of time for two different initial nitrogen atom partial pressures. The curves are seen to be identical within experimental uncertainty. Since nitrogen atom abstraction reactions from either ethylene or methyl radicals are not energetically feasible, a mechanism involving hydrogen atoms Offers
30
0
Ir)
P v)
L 20 2 n
a
0 4
10
3 V
9 0
IO
0
20 30 INITIAL C2H4,10-3 torr.
50
40
Figure 1. Partial pressure of reactants and products as a function of initial ethylene partial pressure: total pressure, 2.0 torr; reaction time, 0.05 sec.
I 5.0
t
2.0 I
1
I
I 10
I
I
I
I
I
I
I
1
I
I
I
I 40
I
20
30 TIME, io-*sec
I
I 50
Figure 2. Ratio of nitrogen atoms consumed to hydrogen cyanide produced as a function of time for two different initial nitrogen atom partial pressures and the same initial ethylene partial pressure: open circles, (N)o = 42.4 X torr; shaded circles, (N)o = 22.3 X torr; (CaH& = 10.7 X 10-3 torr; total pressure, 2.6 torr.
an alternative explanation for the experimental observations. That hydrogen atoms are present at a sufficiently high partial pressure to play some part in the reaction can be seen from Figure 3. To account for these observations, the following reaction mechanism is proposed
+ CzH4 +HCN + CHs N + CH, +HCN + 2H
N
H
+ C2H4
(32%
(1)
(3)
..-,
B. Kistiakowslcy and G. G. volpi, J. them. Phys., 2,, 1141 (1957). (7) G.J. Verbeke and C. A. Winkler, J . Phys. Chem., 64,319 (1960). (8) A. A. Westenberg and N. deHaas, J. C h . Phw., 40, 3087 (1964). (9) This type of mechanism has been discussed by W. Forst, H. G. Evans,and c. A. Winkler, J . Phvs. c h m . , 61,320 (1957). (6)
Yolunts 60,Number 8 August 1066
JOHNT. HERRON
2738
+ CzH5
H
N
+ CzH5
--j
+ NH Nz + H
(5)
1
1
(4)
4CzH4
+ NH
N
2CH3
I
1
30
L
b
n
(6)
followed by the usual atom- and radical-recombination reactions. This list of reactions is not meant to be inclusive. It does not account for the formation of nitrogen-containing products other than hydrogen cyanide or molecular nitrogen, and it takes no account of radicaldisproportionation reactions, or of radicaladdition reactions to ethylene.1°
-0
ui
5 20 n a
E z a O
u)
5 10 ir 0 a
LL W
0 10
0
r
I
I
I
20 30 INITIAL ~ ~ ~ ~ , 1 0 - 3 t o r r
40
A N
I\
30
-
Figure 4. Partial pressure of reactants and products as a function of initial ethylene partial pressure: 2% hydrogen added to the incoming nitrogen gas; total pressure, 2.0 torr; reaction time, 0.05 sec.
,
0
B 1.0
I
0
%
I
I r)
IOH 0
a I
I O
2.0 TIME.
IO-^
3.0 sec
4P
5.0
Figure 3. Hydrogen and nitrogen atom partial pressure aa a function of time a t (CzH&/(N)o ratios of: A, 0.066; B, 0.32; and C, 1.0. The data in curve C were taken with a liquid nitrogen cooled trap on the low-pressure nitrogen line ahead of the discharge zone.
The most important feature of the mechanism is the competition between hydrogen and nitrogen atoms for ethyl radicals in steps 4 and 5, respectively. This competition roughly determines whether the nitrogen atom will be detected as hydrogen cyanide (reaction 4 followed by 2) or remain undetected as molecular nitrogen (reaction 5 followed by 6 ) . An increase in the hydrogen atom partial pressure would favor reaction 4 relative to reaction 5 and hence bring about a decrease in the AN/HCN ratio. To introduce hydrogen atoms into the reactor, molecular hydrogen was added to the incoming nitrogen gas ahead of the discharge zone. The molecular hydrogen was partially dissociated into hydrogen atoms,
The Journal of Physical Chemistry
10
I
20 30 INITIAL C~H,, 10-3torr
.1
40
50
Figure 5. Ratio of nitrogen a t o m consumed to hydrogen cyanide produced as a function of the initial ethylene partial pressure; curve A from. the data of Figure 1 and curve B from the data of Figure 4.
which entered the reactor along with the nitrogen atoms. The effect of adding about 2% molecular hydrogen is shown in Figure 4. Under these conditions the ratio AN/HCN, shown as curve B in Figure 5 approachesunity.l1Pl2 These data indicate that the argument as to whether the nitrogen atom concentration is correctly deter(10) The reaction of nitrogen atoms with molecular hydrogen or with hydrogen cyanide i s immeaaurably slow under the conditions of these experiments. The same ia true for the reaction of hydrogen atoms with hydrogen cyanide. In studying the reaction of nitrogen atoms with ethylensd, a peak was obsemed in the maea spectrum a t m/e 44 which might be due to CDsCN. Its intensity was about 5% of that of hydrogen cyanide. (11) Ideally, this experiment should be carried out by premixing ethylene with partially dissociated hydrogen and in turn reacting this with active nitrogen. This would eliminate any question as to whether or not NH compounds, formed when a mixture of NZand HZ are passed through a discharge, play any part in the reaction. (12) When CZD4 WIM reacted under these conditions, some CIDSH was formed. This would appear to confirm the occurrenm of reaction 5. However, the possibility of CiDsH being formed by meam of an exchange reaction cannot be ruled out.
MASSSPECTROMETRIC STUDY OF REACTION OF NITROGEN ATOMS WITH ETHYLENE
mined by the maximum amount of nitric oxide consumed NO reaction, or the maximum amount of in the N hydrogen cyanide produced in the N C2H4 reaction is based on an erroneous assumption as to the mechanism of the ethylene reaction. The addition of hydrogen atoms brings the two methods into substantial agreement.l 3 As can be seen from Figure 3 there is a residual hydrogen atom partial pressure a t zero ethylene flow due to impurities in the nitrogen. If the nitrogen is further purified by trapping out condensable impurities before passing the gas through the discharge, the hydrogen atom partial pressure is reduced almost to zero (curve C of Figure 3). At the same time, however, the nitrogen atom partial pressure drops to about one-third of its original value. The reasons for l5 the latter phenomenon is somewhat uncertainl4# and need not concern us here. The significant point is that the addition of ethylene to purified active nitrogen leads to the same marked increase in the hydrogen atom partial pressure as in the case of the untreated active nitrogen (Figure 3). Thus, there is no reason to expect that the over-all mechanism differs in the two cases.
+
+
2739
The f i s t term in this expression can be readily found. The numerator is the ratio of the partial pressure of ethylene a t time zero to the partial pressure of ethylene a t time t. The integral in the denominator is graphically evaluated from measurement of the nitrogen atom partial pressure as a function of time. The second term cannot be readily evaluated. However, it is observed that the first term becomes smaller as the ratio (C~H~)O/(N)O is reduced. A plot of the first term, designated as kl', os. (C2H&/(N)o is shown in Figure 6. Extrapolation of these data to (CzH,)o/(N)o = 0 gives a value of kl' which can be identified with k1 on the assumption that, as ( C ~ H ~ ) O / (approaches N)~ zero, the second term becomes negligible with respect to the first.
The Rate of the Reaction The question as to the rate of reaction 1 is dependent on our knowledge of the over-all mechanism. Since previously reported values for kl are based on the assumption that reactions 1 and 2 adequately describe the over-all reaction, it must be concluded that these values are incorrect. If the mechanism given in the previous section is correct, then kl can only be obtained by first making some simplifying assumptions. These are that a t low initial ethylene partial pressures, the mechanism can be represented by eq. 1-6, and that a steady-state treatment in terms of C2H5 is valid under these conditions. The latter assumption appears t b be justified since the partial pressure of C2H6, as indicated by measurements made a t reduced electron energies, was vanishingly small. Solution of the relevant equations leads to the expression
Since normally there is a residual hydrogen atom flow a t zero ethylene flow, the second term is never zero, and kl is only any upper limit. The value so obtained a t 320°K. is kl 5 7 X loQ mole-' see.-'. Some previously reported values for kl are: kl (470400°K.) = 5.8 X 1O1O mole-' s e ~ . - l , ~ kl (310°K.) = 9.6 X 1O'O cm.a mole-' sec.-',le and kl (295°K.) = 1.0 X 1O1O cm.3 mole-' sec.-l.l' The range of values is not unexpected in view of the different experimental techniques used and the different reaction mechanisms assumed.
where the subscripts refer to zero time and time t.
(13) It should be emphasized that these ohmations on the &ect of added hydrogen on the AN/HCN ratio do not involve the nitric oxide titration reaction. The latter waa used solely to relate the measured ion current to partial pressure and was carried out only in pure nitrogen. (14) R. A. Young, R. L. Sharpless, and R. Stringham, J. C h . Phus., 40, 117 (1964). (15) J. T.Herron, J . Res. NatE. Bur. Sa.,A69, 287 (1965). (16) E. R. V. Milton and H. B. Dunford, J. C h . Phys., 34, 51 (1961). (17) E.M.Levy and C. A. Winkler, Can. J . C h m . , 40, 686 (1962).
0. I
0.2 0.3 0.4 INITIAL C , q / INITIAL N
0.5
0.6
Figure 6. Apparent rate constant of reaction 1 as a function of the ratio of initial ethylene partial pressure to initial nitrogen atom partial pressure: total pressure 0, 0.82 torr; 0, 1.63 torr; 0, 2.47 torr.
Volume 69, Number 8 August 1966
I. R. MILLER
2740
The activation energy for reaction 1 has been re'~ ported to be less thanabout 1k ~ a l . / m o l e . ~ ~However, in view of the chain mechanism postulated above, it is
possible that it could be greater since reaction 1 could be masked by the much faster atomic hydrogen reaction.
Effect of Adsorbed Polyelectrolytes on the Polarographic Currents. I
by I. R. Miller Polymer Department, W e i m n n Institute of Science, Rehovoth, Israel
(Received March 10, 1966)
The diffusion current of an ionic depolarizer through an adsorbed polyelectrolyte layer of equal charge has been calculated. It was assumed that the diffusion coefficient of the depolarizer is considerably smaller in the surface layer than in the solution and that its distribution coefficient between the layer and the aqueous phase is given by the Donnan equilibrium. The experimentally established square root time dependence up to saturation level of the polyelectrolyte surface coverage was applied. Two extreme cases were considered, lateral interaction and no lateral interaction at partly covered surfaces. In the first case, the instantaneous current was assumed to be proportional to the surface coverage. In the second case, the distribution coefficient of the depolarizer and its diffusion coefficient in the surface layer were considered to be linearly dependent on the surface occupancy of the polyelectrolyte. The theory was checked with respect to experimental data obtained on positively charged polyvinylpyridine with Cd2+ as depolarizer.
Introduction The effect of surface-active agents on reversible diffusion currents has not been treated quantitatively although it has been studied experimentally by several a ~ t h o r s . l - ~There were, however, attempts to elucidate quantitatively the influence of surface-active substances on the electrode reactions and thus on the irreversible polarographic currents. Weber, Koutecky, and Koryta6 assumed the over-all rate constant keff to be linearly dependent on the coverage; Kuta and Webera further took into account the variation in potential by the adsorbed surface-active substance, which they assumed to be proportional to the coverage. They obtained for the instantaneous current i, at any mercury drop age t the expression
i,
- e) +
= nFqt*'aCo[ok,(l
x
exp(--x/m - l)(anF/RT)A@ ( 1 ) The Journal of Physical Chemistry
In this case, ok, and 112, are the rate constants for the free and covered surface, respectively, n is the number of electrons involved in the reaction, F is Faradays, p is a constant depending on the flow velocity of mercury, z is the charge of the depolarizer, a is the transfer coefficient, and A$ is the potential difference between the completely covered and the uncovered surfaces. The purposes of the present work is to elucidate the effect of adsorbed charged polyelectrolytes which partly or fully cover the surface on the diffusion current. (1) P. Delahay and I. Trachtenberg, J. Am. Chem. SOC.,80, 2094 (1958). (2) C.Tanford, ibid., 74, 211 (1952). (3) J. Kuta and I. Smoler, 2.Elektrochem, 64, 285 (1960). (4) C. N. Reilly and W. Stumm, Progr. Polaro., 1 , 81 (1962). (5) J. Weber, J. Koutecky, and J. Koryta, 2. Elektrochem., 6 3 , 583 (1959). (6) J. Kuta and J. Weber, Electrochim. Acta, 9, 541 (1964).
