Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

120 EXPERIMENTAL METHODS FOR FAST REACTIONSically, it is convenient to replace the spectroscopic flash tube with a high-intensitylight source and a monochromator. The spectrograph is then replaced by a photomultipliercoupled to an amplifier and an oscilloscope. Alternatively, the analyzinglight could be kept “white” until it has passed through the reaction vessel, and thephotomultiplier coupled with the spectrograph. Such an arrangement increases therisk of further photochemical reaction brought about by the analyzing light. Becauseit is often difficult to determine the concentration of an intermediate absolutelyat a given time, following any reaction which is not first order is rather difficult.From spectrograms taken before and after photolysis it is possible to estimatehow much net decomposition has taken place, but many systems involve “dark”reactions which restore the original reactants. Where this is not so, it is often foundthat, because the extinction coefficients of the intermediates are so high, their concentrationsmay be followed even when they are very small compared with thoseof the reactants. In such cases, the extent of permanent reaction has no significanteffect on the composition.It has been estimated’* that the temperature rise associated with the absorptionof the photolyzing light may be as high as several hundreds of degrees. This temperaturechange (and its accompanying pressure change) may affect the kinetic investigationdirectly, e.g., by increasing the rate coefficients, and it may also seriously interferewith the analysis by producing inhomogeneities in the reaction system. In solutionreactions the heating is never significant since the solvent acts as a “thermostat”.A similar result can often be achieved in gas reactions by adding a large amountof an inert gas.1.1.1 Applications offlash photolysis(1) An interesting application to gas-phase reactions was the study of the roleof tetraethyl lead as an antiknock19. Combustion of many hydrocarbons proceedsin two stages: the first is a precombustion step, and among the products are aldehydesand peroxides. The secondstageis the true combustion reaction. A mixture ofacetylene, oxygen and amyl nitrite (the latter is known to promote knocking; italso acts as a sensitizer) was used to show that at the ignition point a very rapidbuild-up of OH and other radicals occurs. When tetraethyl lead was added it wasfound that the time lapse between the two stages was increased, the appearanceof OH being correspondingly delayed, and the occurrence of detonation was greatlyreduced or even eliminated. The species apparently responsible for the antiknockaction was PbO. In the first place, it was formed between the two stages by thereaction of PbEt, with the oxygenated intermediates, which are known to lead toknocking. Secondly, it acted as a chain terminator by reacting preferentially withOH and the other propagating radicals to give atomic lead. From studies of therates of appearance and disappearance of the various absorbing species, it could be