Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

166 EXPERIMENTAL METHODS FOR FAST REACTIONSones will not be seen. The apparatus used was of the conventional stopped-flowtype'" using two 50 ml reagent syringes driven via a magnetic slip-clutch by avariable-speed motor. The syringes were connected to the mixing chamber by 3-waytaps which were used to evacuate the system (for the removal of air-bubbles) andto admit the reactant solutions. The stopping syringe activated a trigger circuitwhich had the dual purpose of triggering the monitoring oscilloscope and stoppingthe drive motor. One reactant solution contained an organic halide, RX, and acobalt(I1) solution, while the other contained sodium cyanide (of at least 5 timesthe concentration of the cobalt) and an equivalent amount of sodium hydroxide tosuppress hydrolysis. These solutions were de-oxygenated since the first product ofthe reaction is air-sensitive. The first product, Co"(CN)Z-, was formed within themixing time, of the order of 1 msec, ahd it was the reaction of this, unstable, compoundwith the organic halide, RX, whose kinetics were of interest.5 CN-+Co*' = Co(CN);-(very fast; occurred within the mixing time)2 CO(CN):-+RX = CO(CN)~R~-+CO(CN),X~-(fast; kinetics of this reaction followed)The latter reaction was followed by observing the increase in light absorption dueto the product Co(CN),X3-. A two-step mechanism was proposed for this process;the rate-determining production of R- radicalsCo(CN)Z-+RX -+ CO(CN)~X~- +R-being followed byCo(CN)Z-+R- -+ Co(CN),R3-.7.2 FLOW METHODS IN THE GAS PHASEMention has already been made of one of the inherent difficulties of followingthe kinetics of fast gas reactions. The absence of large concentrations of inert "thirdbodies" means that temperature equilibration in gases tends to be comparativelyslow. This is quite a serious problem in flow systems since it is difficult to counteractany thermal effects due to the heat of reaction. In addition any change in pressurecaused by a possible mole number change and the pressure drop associatedwith the flow make it difficult to define the reaction conditions accurately. Despitethis, kinetic data obtained using gas-phase flow systems have been reported"'.The general experimental arrangement is,similar to that shown in Fig. 21. Onereaction component is passed along a tube inside a thermostatted compartment