Nitrous oxide, decomposition reaction with hydrogen

Rice, Fryling, and Weselowski (J. Amer. Chem. Soc., 1924, 46, 2405) make all reaction rates proportional to the concentration of what they call residual molecules, which have to be formed endothermically from one of the reactants. The proportion of these increases with temperature and accounts for the increase in reaction rate. Something of this kind may be true in special cases, for example, in the formation of HBr the residual molecule would be the bromine atom. But this resolution into atoms is only the limiting case of ordinary activation, and it is difficult indeed to see what the residual molecule could be, or what tautomeric change could occur in the simple decomposition of hydrogen iodide or nitrous oxide. 

It is probable that the actual mechanism of the decomposition is unimolecular, but, the observed order of the reaction being zero, we cannot tell whether the molecules decompose singly or by interaction with their neighbours. The catalytic decomposition of hydrogen iodide on the surface of platinum can actually be shown to be unimolecular. The heat of activation in this instance is even lower (14,000 cals.), but is again subject to the same uncertainty as the values for the unimolecular reactions of nitrous oxide. 

Vanpee (62) has studied the explosion of formaldehyde-oxygen mixtures. The dependence of explosion pressure upon temperature, vessel diameter, and the addition of inert diluent is in agreement with the thermal theory and consistent with studies of the slow reactions (54, 61). Typical of other reactions which have been reported to exhibit thermal explosion limits are the decomposition of nitrous oxide (75), the reaction between nitrous oxide and hydrogen (38), and cyanogen-air (32). 

Translationally hot species may also have enhanced reaction rates. Reuben and Linnett have proposed that such hot oxygen atoms are formed in the thermal decomposition of nitrous oxide, and that these react at an accelerated rate with further nitrous oxide. Williams and Ogg have shown that hydrogen atoms with 42 kcal.mole" of translational energy react at an enhanced rate with hydrogen iodide, and have demonstrated a similar effect in the reactivity of hot methyl radicals. 

The intermediates in the photolysis of cyclohexane which might react with nitrous oxide to form N2 are hydrogen atoms, excited C6Hi0, and excited C6Hi2. Hydrogen atoms can be eliminated on the basis that (H) is only 0.14 (25) and that the rate of reaction of H atoms with nitrous oxide is too slow (5). The cyclohexene which is formed in the photolysis, Reaction 3, could initially have as much as 7 e.v. excess energy and could conceivably sensitize the decomposition of nitrous oxide. Such a reaction would produce N2 but would not affect the yield of hydrogen. Since < (H2) is reduced by nitrous oxide, excited cyclohexene cannot be the main source of N2. 

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