THE RATE OF DECOMPOSITION OF NITROGEN PENTOXIDE AT MODERATELY LOW PRESSURE

Abstract

Introduction. - At moderately high pressures, the decomposition of gaseous nitrogen pentoxide was shown by the original work of Daniels and Johnston [1] to be homogeneous and of the first order and this has been completely confirmed by the work of subsequent investigators [2]. At low pressures, however, there has been a striking lack of agreement as to the rate of decomposition of this substance. Hirst and Rideal [3], Hibben [4], and Loomis and Smith [5] have all tried to follow the rate of the decomposition at low pressures by freezing out the nitrogen oxides during the course of the reaction and measuring the pressure of the oxygen which had been formed. Working at initial pressures in the range 0.035-1.450 mm. of mercury Hirst and Rideal report that the specific rate of decomposition becomes greater at low pressures, the increase in rate being appreciable at 0.25 mm. and several fold at their lowest pressures. Hibben, on the other hand, working in the pressure range 0.03 to 0.18 mm. finds throughout the same specific rate of decomposition as at high pressures. Loomis and Smith, however, conclude that the method used in all three sets of experiments is unreliable, since, in the first place, they find that appreciable amounts of oxygen can be occluded and carried down with the condensed oxides of nitrogen, and in the second place, find that nitrogen pentoxide is appreciably adsorbed on the surface of pyrex glass. More recently Sprenger [6] has attempted to follow the rate of decomposition by pressure measurements made with a quartz fibre gauge. Working in the range 0.01 to 0.05 mm. pressure, he comes to the extraordinary conclusion that the nitrogen pentoxide decomposes at approximately its high-pressure rate when it is first introduced into the reaction flask, and later, with a considerable fraction of the original nitrogen pentoxide still remaining, it ceases to decompose at all. Finally, Rice, Urey and Washburne [7] report that preliminary measure ments made by Miss E. Wilson show that the specific rate of decomposition of nitrogen pentoxide falls below the high-pressure rate even at pressures of several millimeters. Since the low-pressure rate of homogeneous gas reactions is of great theoretical interest, especially in connection with the theory of activation by collision as developed by Rice and Ramsperger [8] and by Kassel [9], further work on the low-pressure rate of decomposition of nitrogen pentoxide seems desirable in view of the almost complete lack of agreement in the results described above.