Seite - 6 - in Advanced Chemical Kinetics

!n1 kþ1 C a�xð Þn1 A a ↗t ↘t !n2 kþ2 D a�xð Þn2 Here a is the initial concentrationof thespeciesAandafter sometime t>0, itdispersed intoC andD. Similarly, sometimes ithappens thatdifferent chemical speciesgive thesameproducts !n1 kþ1 C a�xð Þn1 A a ↗t ↘t !n2 kþ2 D a�xð Þn2 !n1 kþ1 C b�xð Þn1 B b ↗t ↘t !n2 kþ2 D b�xð Þn2 or a systemis reversible atdifferent stages. Inall these cases,weneed to followall theirpaths toget thedetailedmechanisms, n1 kþ1 C a�xð Þn1 A a ↙t ↖t n2 kþ2 D a�xð Þn2 The rate of reaction (Wρ(c)) is proportional to thenumber of collisions per unit timebetween the reactants but onlya small fractionof the total is effective, i.e., not every collisionbetween the reactantsgives the result. Theremaybea fewreasonsbehind its ineffectiveness [1], i.e.: The reactantmoleculesmayattain insufficient energy (<EAct, i.e., activation energy, J/mol) at different stagesduring the reaction. Themoleculesmaynotgetalignedproperlyororientateduringthecollision (dependingupon thegeometryof theparticlesandkindof reaction that is takingplace), etc. IfZ is theeffectivecollision inwhichmoleculeshaveenergy≥EAct, then ZE¼Z0e EAct RT , (5) and eEAct/RTgives the fractionof collisionswithenergy≥EAct. In a complex chemical reaction, the reactant molecules (intermediates, complex Θi) pass through different transition states due to their bond breaking and energy redistribution fac- tors.Here thespeciesstays foraveryshortperiod,usuallycalledtransitionperiodofactivated complex(where thehiddenreactionsbetweenthechemical speciesarestillgoingonveryfast). Advanced Chemical Kinetics6