Page - 80 - in Advanced Chemical Kinetics

2.Competitionkinetics Fordetailedinvestigationofcompetingreactions, it isnecessarytohaveagoodknowledgeabout the rate constant data that will used to proposewhich reaction is predominant. For instance, the Fricke dosimeter contains threemain active species (350molm�3H+, 1molm�3 Fe2+ and 0.25 mol m�3 O2) that have very high rate constants with eaq � and to find out the reaction mechanism involved in thedosimetry, it is necessary to investigatewhich solute(s)willmainly reactwitheaq �. Thereactionsandtheir correspondingrate constantsaregivenas [1–3]: eaq�þHþ!H k1¼2:3�1010M�1s�1 (1) eaq�þFe2þ!Feþ k2¼1:6�108M�1s�1 (2) eaq�þO2!O2� k3¼1:9�1010M�1s�1 (3) Theextentofreactionisproportional totheproductk[solute] foreachofthethreesolutes, that is, eaq�þHþ:eaq�þFe2þ:eaq�þO2¼ k1Hþ½ � : k2 Fe2þ : k3O2½ �¼5�104 :1:30 (4) Alternatively, it canalsobe concluded that 99.94%of thehydratedelectrons reactingwith the threesoluteswill reactwithH+, soundersuchconditions thereactionofeaq �withFe2+andO2 will be ignored.Therefore, it is compulsory tohaveanwidecollectionof rate constantdata to applykinetics fora radiation inducedchemical reaction. In Fricke dosimeter, hydrogen ions are considered as strong scavengers of hydrated elec- trons [4–6]. Theeffectivenessof a chemical scavengerdependsupon theproduct k[scavenger] thatmusthaveahighervalue than k[substrate]. For example, tert-butanol isused to scavenge hydroxyl radicalsandbyusingconcentrationof1molm�3 tert-butanol ithas k[tert-butanol] = 6.0�105 s�1.Therefore, tert-butanolwouldbeanefficient scavenger forhydroxyl radical (can scavenger over 99%of the hydroxyl radical), if k[solute] for the reaction of hydroxyl radical with the solute is less than 6.0� 105 s�1 [7]. Similarly, oxygen is used to scavenge hydrated electronsandhydrogenatoms fromaqueousmedia [5, 8, 9] and forboth the radical (eaq �and ●H) in air-saturatedmedia, k[O2] = 5�106 s�1, so that oxygen canbe expected to interfere in the radiolysis of aqueousmedia if k[solute] for the reactionsofhydrogenatomsandhydrated electronswith thesoluteareof thesameorderas,or less than,5�106 s�1. Incaseofradiolysisoforganicspecies, theirproductsalso itselfoftenactasscavengersandit is commonly found that the product yield is not in direct relationwith the absorbed dose. To estimate the possible reasons of such effects, competition kinetics can be employed in an effective way if the radiolysis mechanism is known and the necessary rate constants are available. For example, cyclohexene is produced when cyclohexane is irradiated and both cyclohexene and cyclohexane have appreciable rate constants with hydrogen atoms, one of the radical specie produced during gamma radiolysis of aqueous media. The reactions are summarizedbelowas: Advanced Chemical Kinetics80