The reaction of active nitrogen with acetylene. I

Abstract

The reaction between active nitrogen and acetylene was studied in a low-pressure flow reactor using a leak to a mass spectrometer as the analytical device and varying reaction time from a few to 45 ms. In mixtures in which the initial ratio of acetylene to atomic nitrogen is larger than unity a comparatively slow reaction takes place whose main products are cyanogen, hydrogen and cyanoacetylene. The reaction is accelerated by rising pressure and temperature. The proposed rate determining steps are C 2 H 2 + N K1 ⇆ K-1 C 2 H 2 N*, C 2 H 2 N*+ N 2 → K2 C 2 H 2 N+ N 2 with K 1 = 1·1X 10 -12(+-0·12) exp (-2000(±400)/ RT ) cm 3 molecule -1 s -1 and k -1 = 4x10 -s T e s -1 , if k 2 is taken to be the collisional frequency. The radical C 2 H 2 N is found to react competitively with nitrogen atoms and with acetylene to form the end products. At low (≤ 0·5) initial acetylene to atomio nitrogen ratios a much faster reaction occurs which is accompanied by chemiluminescence of CH and CN radicals and whose main products are hydrogen cyanide, cyanogen and hydrogen. Simultaneously, a rapid recombination of nitrogen atoms takes place, as many as 20 atoms reacting per molecule of acetylene consumed. This rapid reaction has a negative temperature coefficient. It is suggested that this is a chain reaction started by metastable nitrogen molecules ( A 3 Σ + u ) nitrogen atoms which react with acetylene to form hydrogen cyanide and cyanide radicals. The latter cause the recombination of nitrogen atoms by the chain reactions CN + N + N 2 → NCN + N 2 NCN+N → N* 2 + CN, with 3 x 10-30 <k 6 < 2 x 10 -29 cm 6 molecule -2 s -1 . These chains are broken by recombination of cyanide radicals and by the reactions of CN and NCN radicals with acetylene. Cyanogen also causes a catalytic recombination of nitrogen atoms. Ammonia added to acetylene-active nitrogen mixtures suppresses the rapid reaction but has no effect on the slow reaction in richer mixtures.