Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

5 ‘C HEM1 C AL’ METHODS 317The technique has been used by Melville and his colleagues in studies of a numberof reactions: the reaction of atomic hydrogen with hydrazine2”, and olefins’”and the photolysis of ammonia’22. Atomic hydrogen was formed by mercurysensitisedphotolysis of molecular hydrogen. Farkas and Sachsse had also appliedthe 0-p-H’ conversion previously to determine the atomic hydrogen concentrationin mercury-sensitised photochemical systems” 3, and PatatzZ4 and SachsseZZSused the conversion to detect H atoms formed in the pyrolysis of ethane. An assumptionis made in the latter work that catalysis of the para-hydrogen conversionby methyl radicals is negligible compared with the effect due to atomic hydrogen.It is not clear how far such an assumption is justified since the conversion is, infact, catalysed by all free radicals, including “stable” ones such as nitric oxide andeven molecular oxygen. Indeed, the method has been used to determine radicalconcentrations, as in the photochemical experiments of West’26. The method isobviously most valuable where the nature of the reaction intermediates is alreadyestablished, and a measure of their concentration is required. A second restrictionexists in that if reaction (23) is not to affect the concentration of intermediate (asituation only possible for atomic hydrogen) then it must proceed more rapidlythan the fastest step of the chemical reaction under investigation.Isotope exchange reactions can be used in the same way as the ortho-parahydrogen interconversion, and the results may be interpreted more readily. Theobvious reaction to compare with the ortho-para exchange isH+D, + HD+D (24)and in systems containing molecular hydrogen, the processD+H, + HD+H (25)also leads to the formation of HD. The rate coefficients for these processes arekno~n’~’*’’”, so that measurement of the rate at which HD appears (mass spectrometrically,for example) can be used to calculate the atomic hydrogen concentration.If, however, the experimental conditions are such that H,, D, and HDrapidly reach their equilibrium concentrations, then the method becomes unsuitable.Fenimore and Jones229 experienced this difficulty in flame studies: at allpoints within the flame the HD concentrations were at their equilibrium values.These workers found that the reactioncould be used instead of (24) to measure atomic hydrogen concentrations. At flametemperatures (26) proceeds much more slowly than (24): Fenimore and Jones usea value of k26 = lo’, exp(-25,000/RT) l.mole-’.sec-’ for reasons which theydiscuss’ ’ 9.References pp. 336-342