Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

INTRODUCTION 113defect-proton in water associated with the latter’s strongly hydrogen-bonded structure.Interestingly, the rate coefficient for this reaction is larger in ice than in liquidwater’. Other fast reactions in water are often between ions, which are heavilysolvated. The nature of the solvent is very important in determining the rate coefficientof a liquid-phase reaction and it is in many ways somewhat unfortunate thatthe common solvent, water, is apparently one of the most complex. This situationis not likely to reduce the number of kinetic studies performed with aqueous solutionsbut rather to act as a spur for further investigations into its detailed natureand properties. An advantage of having a large excess of solvent molecules surroundingthe reacting particles is that there is usually no difficulty in dissipating the heatof reaction; in gas-phase reactions this is often a serious problem.The problems which must be overcome in any method for following fast reactionsmay be grouped under two headings. In the first place, the reactions must be initiatedhomogeneously and in a time which is short compared with the reactionhalf-life. In traditional kinetic investigations the reaction will generally be startedby mixing the two reactive components. For reactions which are complete in lessthan a few milliseconds this technique is not appropriate because the mixing timecannot be reduced much below this value. Secondly, the course of the reactionmust be followed by recording some varying parameter of the system at knowntimes after initiation of the reaction. Most of the techniques described here avoidthe problem of mixing. Rather than dividing the methods according to whether theymay be used for gas or solution reactions, it is convenient to group them accordingto the way in which they attempt to deal with these two problems. The first fewmethods use what has been termed the “perturbation” method: the equilibratedsystem is subjected to a sudden perturbation and its rapid readjustment is followeddirectly on a necessarily very short time scale. The second group uses the “competition”method : the system is disturbed by some physical process (often periodicin nature) which competes with the chemical reaction. Information about the ratesof the chemical processes can be deduced if the physical process is quantitativelycharacterized. Unfortunately, however, not all the methods lend themselves to quitesuch a rigid demarcation. The dividing line may be drawn at about Section 3 (Chemicalrelaxation methods) although others (cf. Section 7-Flow methods) includevariants from both categories.Flash photolysis is one of the few methods for studying the kinetics of fast reactionswhich have been applied with equal success for both reactions in the gas andin the liquid phase. It was originally developed by Norrish and Porter for followinggas reactions3, and was first used for solution reactions4 in 1954; it is applicableto reactions which are initiated by light. The reactants are subjected to a light flashof very high intensity in a region of the spectrum where at least one of the speciesis photosensitive. The type of excitation produced by the absorbed light differs fromcase to case, but essentially the method involves following the fate of the excitedspecies by means of subsequent light absorption or emission.References pp. 176-1 79