Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

8 FLAMES 167which is often, though not invariably, held at a comparatively high temperature.The other component is injected into the main stream and as the gases emerge fromthe thermostat they pass a detector. In the cases when the reaction is quenched,it is assumed that this process also occurs instantaneously-an assumption whichis difficult to uphold rigorously. The distance between the injection point and thedetector may be varied, and for each measurement the steady state is set up. If oneof the reactants is in the atomic state, it is often made from the molecular speciesby passing it through a tube across which a radio-frequency discharge is appliedbetween two external electrodes. The uncertainties in defining the reaction time andtemperature tend to be minimized by using long reaction tubes and low flow rates.An example of a gaseous process whose kinetics have been measured by the flowmethod is the decay of nitrogen atoms in the presence of oxygen molecules'02.A stream of nitrogen atoms, generated from gaseous molecular nitrogen with thehelp of rf power, was passed along a quartz reaction tube of about 150 cm lengthand 2 cm diameter situated inside a furnace held between about 150" and 450" C.There were four inlet jets approximately equally spaced inside the furnace for admittinggas to the reaction system. The light emitted by the emergent gas was observedwith a photomultiplier placed with its axis normal to the tube, and the pressurenear the observation point was measured with a manometer. The oxygen wasadmitted to the atomic N stream through the fist jet and then at successive downstreamjets nitric oxide was added. The latter was to determine the concentrationof N atoms in the gas arriving at the particular jet since active nitrogen can betitrated with NO, the end-point being a sharp transition from a blue NO-emissionto a green continuum (the air afterglow). The end-point corresponds to the completeremoval of N atoms by the very rapid reactionN+NO N2+0The primary reaction, whose kinetics were followed in this system, wasN+02 + NO+OIt was found to be first order in both N atom and oxygen molecule, and the secondorderrate coefficient of 8.3 x lo9 exp(-7100/RT) 1. mole-'. sec-' was evaluated.8. FlamesA flame which is commonly used in the laboratory for kinetic measurements(such as that from a modified Bunsen burner) is essentially a steady-state systemwith the combustion zone fixed. The processes involved in flames are radical chainreactions in which the three normai stages (initiation, propagation and termination)References pp. 176-179