REACTIONS OF HALOGEN COMPOUNDS WITH NITRIC OXIDE AND CARBON MONOXIDE

Reactions of halogen compounds with nitric oxide and carbon monoxide 

The gas-phase reaction of nitric oxide with chlorine has received a great deal of attention. Very often it has been studied concurrently with its reverse, the decomposition of nitrosyl chloride, viz. 

In the early work on these reactions, some inconsistencies appear which have been accounted for chiefly by the recent work of Ashmore et al. However, the homogeneous third-order reaction (I) seems to be very well established and an important part of the mechanism. 

Trautz and Wachenheim have studied the formation of NOCl from NO and CI2 by following the change in total pressure as a function of time. They suggest that reactions (I) and (2) are not adequate to account for their observations since CI2 enhances the rate coefficient of the reaction. Welinsky and Taylor criticize this interpretation because of the manner in which Trautz and Wachenheim handled their data. Upon recalculating the rates in a more reliable manner it is shown that within experimental error no effect of chlorine on the rate coefficient is observed. This appears to be consistent with the data of Welinsky and Taylor, Waddington and Tolman, Kiss , and Krauss and Saracini . In every case the production of NOCl (1) follows the third-order rate equation. All of these workers followed the reaction via the change in total pressure with time. 

However, there remains a serious problem in interpreting this work. Plots of both log and log 2 versus IjT show considerable curvature corresponding to an apparent increase in activation energy with temperature. Part of this increase, but by no means all of it, can be accounted for using transition state theory and allowing for excitation of vibrational modes. 

---