948
BHUNOREITZNER
the point (2.84, 0.0470) has an intercept I = 0.057 f 0.005 and a slope - K = -(0.0035 f 0.0020). It should be emphasized that the points for all moderators, with the possible exception of C2HJ3r, appear to approximate the same line. The upward trend exhibited by the C2H6Brdata could be due, in part, to an incorrect choice of the value of the apparent diameter of the compound. The solid curves of Figs. 1 and 2 were calculated using eq. 1 and the above values of K and I . It is seen that the curves for Xe and Kr moderation are identical. This is due to the fact that, whereas the Xe-Br cross-section is larger than that of the KrBr, per collision, because of the similarity in atomic weights, krypton is capable of removing, on the average, more energy from BrS0than is xenon. It
Vol. 65
should also be noted that this kinetic theory results in curves for the molecular additives (Fig. 2) which are in reasonable agreement with the data. This suggests, as do the data of Fig. 3, that the molecular additives serve mainly to remove B F excess kinetic energy. If moderation by the molecular additives were via a process other than kinetic-energy removal, it would only be under the most fortuitous circumstances that their moderation data would be described by the kinetictheory curves. I n summary, the data of Fig. 3 would tend to support the conclusion stated earlier that the reaction of Bf1° with methane occurs principally, if not completely, as a result of the y-recoil kinetic energy acquired by B?O.
THE INFLUENCE OF WATER ON THE THERMAL DECOMPOSITION OF a-LEAD AZIDE BY BRUNOREITZNER Explosives Rearch Section, Pictatinny Arsenal, Dover, New Jersey Received October 81, 1960
The thermal decomposition of a-Pb(N& is considerably affected by water vapor. At water vapor pressure below 8.9 mm. ( t = 240") the induction period of the autocatalytic reaction Pb(N& Pb 3Nz increases with increasing water vapor pressure. Above 8.9 mm. the autocatalytic reaction is com letely suppressed, and a hydrolysis reaction takes lace which yields a basic lead azide and HNs. Hydrazoic acid is partialyy decomposed to give NZand N& which reacts wit%undecomposed HN3 to give NH4N3. The increase in the induction period and the suppression of the autocatalytic reaction are explained as being due to the poisoning action of the water vapor on the autocatalytic lead nuclei. An explanation for the long induction periods found with alkali and alkaline earth azides is proposed. --f
+
Introduction between the different reaction mechanisms under The thermal decomposition of a-lead azide under vacuum and in air. vacuum was intensively studied during the past 30 Experimental Procedures years by Garner and his sch001,~-~ and more reThermal decompositions carried out under nitrogen cently by Griffiths and Groocock.6 The decom- in the apparatus shown in were Fig. 1, in which the volume of position curves (fractional decomposition, a, vs. nitrogen released during decomposition was measured, time) obtained by these authors were sigmoid indi- The reaction vessels were charged with 50 mg. of pure cating an autocatalytic reaction. The induction lead azide (cationic impurities less than 0.1%; particle size approximately 7 p ) , flushed with pure nitrogen while the times derived from these curves mere relatively outlet tubes were immersed in the displacement liquid and short compared with the duration of the main sealed. The total volume of the sealed reaction vessels reaction. The final solid decomposition product was and the outlet tubes was approximately 0.4 cc. The vessels lead. Recently, Todds described the thermal de- were completely immersed in the metal bath. Thermal equilibrium was reached after 3 minutes. The measured compo&tion of d e a d azide in air a t 240'. He volume of nitrogen was corrected to standard conditions. found tetragonal lead oxide as the final solid de- The water vapor pressure over the lead azide samples was composition product and two intermediates which controlled by the displacement liquids (H2S04 and HsP04 he identified as basic lead azides. Stammler, Abel of various concentrations, dioctyl phthalate, and dilute and Kaufman7 studied the decomposition in air by NaOH and AgNOs solutions). The reaction temperatures a thermogravimetric method. The decomposition were 200,240 and 250' (deviation h0.5"). curves were characterized by steps a t about 11.8Results 12.4 and 16.0-16.8'% loss in weight. Decomposition Tests at 200" with 0.01 N AgNOa solution and beyond the last step was practically negligible a t concd. H2S04 as displacement liquids showed a temperatures of 200' and below. fractional decomposition (a) of only about 0.003 The object of the present work is to find a link after 96 hours. In both cases X-ray diffractograms (1) W. E. Garner and A. 8. Gomm, J. Chem. Soc., 2123 (1931). of the decomposition products were still those of (2) W.E.Garner, A. S. Gomm and H. R. Hailes, ibid., 1393 (1933). a-lead azide. Silver azide was found in the AgN03 (3) W. E. Garner, "Chemistry of the Solid State," London 1955, solution. Decomposition a t 250" with concenCh. 7 and 9. trated HzS04as the displacement liquid yielded a (4) W. E. Garner, Proc. Roy. Soe. (London), A246, 203 (1958). ( 5 ) P. J. F. Griffiths and J. hl. Groocock, J. Chem. Soc., 673, 3380 curve similar to those shown in Fig. 2. The in(1957). duction period was 1 hour 10 minutes, followed by (6) G. Todd, Chemiatry and Induatry. 1005 (1958). an acceleratory period. The sample exploded (7) M.Stanimler, J. E. Abol and J. V. R. Kaufman, Nature. 186, 456 (1960). after 1 hour 26 minutes, when a had reached a value (Y-
June, 1961
INFLUENCE OF
WATER
ON
THERMAL DECOMPOSITION O F LEADAZIDE
over the displacement liquids, two mechanisms could be observed. I n the case of low water pressures, the decomposition curves (Fig. 2) are similar to those obtained under vacuum, except that the induction times are considerably increased with increasing water vapor pressures. At the beginning of the reaction a small amount of gas is evolved. The hygroscopic displacement liquids are then drawn back into the outlet tubes, and finally the acceleratory reaction starts. The maximum reaction rates (da/dt),, in the straight portion of the curves, together with the aqueous tensions PH*O (25'), the densities of the displacement liquids and the induction times ti (arbitrarily defined as the intersection of the time axis with the tangent to the steepest part of the curves) are shown in the legend of Fig. 2. Curve 6 was obtained using first a dilute phosphoric acid as the displacement liquid and then replacing it after 360 hours by concentrated
THERMOMETER
m
TEMP CONTROLLER
TO B
D,SPLACEMENT LIQUID
Fig. 1.-Slow
949
decomposition apparatus.
I3 HOURS 30 MIN
181 HOURS
)c >: 06
4
;:
U
JI 0.4
* c
I
370 TiYE,HOURS.
Fig. 2.-Slow
decomposition of a-Pb(N& under relatively low water vapor pressures ( t = 240').
Curve IV?
Displacement liquid)
1 2 3 4 5
H2SOa Hap04 Hd'O4 HzS04 DioctyI phthalate (HsP04and \€&SO, (after 360 hr.)
6
P (25')
(dcm.9
1.831 1.695 1.580 1.513
i15ii
1.831
of 0.34. The reaction rate d a/dt in the final stage min.-'. preceding explosion was 5.6 X All subsequent experiments were carried out a t 240'. Depending upon the water vapor pressure
pirro (25') (mm.)
(drr/dt)max X 103 (min. -1)
11
