Reaction of hydrogen with nitrous oxide

Following an initiating process consisting of reactions (xlii) and (iii), the chain propagating step (xliii) seems the natural one leading to the major observed products. It is exothermic 61.6 

Data on reaction (xliii) has also been obtained from studies with flames [167, 170, 171], shock tubes [290,291] and discharge-flow systems [186, 292], The data of Fenimore and Jones [167] from flame systems depend on a calibration of the H atom concentrations in the flames by means of reaction (—iDe), for which they assumed lOe = 10 exp (—12750/T). Substitution of yii d e from Table 46 reduces their 

Although reaction (xliii) appears to be the major reaction between H atoms and nitrous oxide, this does not exclude the occurrence of the alternative reaction (xliv). Indeed reaction (xliv) has been specifically suggested to explain (a) the formation of the nitric oxide responsible for the sensitizing effect of N2O on H2 + O2 explosions [268] (cf. Sect. 8.3), and (b) the formation of nitric oxide in H2+ N2O flames at 1500—2000 K [293] and shocked gases at 1900—2800 K [294]. More recent investigations of the thermal H2+N2O reaction have also shown that the mechanism is more complex than that suggested by Melville [287, 288]. 

Baldwin et al. [297] have recently re-examined the overall reaction at 813 and 873 K, and they confirm Melville s result that it is effectively zero order in H2 and approximately first order in N2O (more precisely, the order in N2O is 1.1 0.2 at 813 K and 1.2 0.2 at 873 K). The rates were effectively independent of vessel surface (B2O3, uncoated Pyrex or uncoated silica). A small influence of helium addition was noticed at 873 K. Thus, for pn 2 = PN20 = 25 torr, the addition of 75, 200 and 400 torr He produced a gradual increase in rate to the extent that the initial rate was almost doubled at the highest pressure. For Phj = Pn2 0 =10 torr, the addition of 480 torr increased the rate approximately five-fold. 

At 813 K comparison of the initial rates at 250 and 500 torr, respectively, for a mixture with H2/N2 0/He = 0.2/0.2/0.6, showed an order almost exactly unity in total pressure. At 873 K, similar mixtures at 125, 250 and 500 torr gave a log (rate) versus log (total pressure) plot with a gradient of 1.5, while for an equimolar mixture of H2 and N2O at total pressures of 25, 50 and 200 torr, the mean gradient was 2.0 0.4. Small amounts of NO are formed during the initial stages of the reaction, but its net rate of production decreases sharply as the reaction proceeds, and its concentration also passes through a maximum and decreases. A profile of [NO] versus time is shown in Fig. 51. 

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