Page - 5 - in Advanced Chemical Kinetics

products but it does depend on the temperature), and ρ = 1,…,m are the reaction numbers, while thestoichiometric coefficientsαρi,βρiare thenonnegative integers. The reactions in which the reactants are in different phases and their rates are affected by surface areas are called heterogeneous reactions, i.e., the reaction betweengases and liquids, solids and liquids, etc. As in the case of gas solid catalytic reactions, reactants at elementary stepswillbegasphasecomponentorsurface intermediate.Thus,Eq. (1)cannowbewrittenas X i αρiAiþ X j αρjXj���! ���kþi k�i X i βρiBiþ X j βρjYj, (2) here againAi and Bi are the reactant and products in the gas phase andXj andYj are the surface intermediate. In a more typical form, it can be written as αAþPjαρjXj���! ���kþi k�i βB þPjβρjYj. Withanassumption thatαandβareeitherzeroorone, it implies thatonlyonemolecule inan elementaryreaction fromthegasphase reactsorzeroatall. Theconcentrationof the involvedspecies canbemeasuredas (single stepreaction); The reaction rates measured on either side are kþi a�xð Þαρi (forward rate of reaction) and k�i xð Þβρi (backward rate of reaction) and the product formation is the difference between the rateof forwardandbackwardreactions, i.e., dx dt ¼ kþi a�xð Þαρi�k�i xð Þβρi: (3) Theequilibriumisadynamicalprocess,andwhenasystemgoestoanequilibrium,theleft-hand sidewillbecomeequal to theright-handside, i.e., dxdt¼0 kþi a�xð Þαρi�k�i xð Þβρi¼0, (4) while the conversions of products to reactants and reactants to products are still going on, although there isnonet change in thenumberof reactantandproductmolecules. According to Le-Chatelier’s principle, if a system at equilibrium state is disturbed by an external force, then thesystemtries tooffset the forceandattainsanewposition. Thesystembecomescomplexwhenareactionundergoesmore thanonepathwayswhenmore thanoneproductsare formedfromthesamereactantsordifferent reactantsproduce thesame products. Such typesof reactionsarecalledparallel reactionsor side reactions, i.e., At initial space: a 0 time t>0: (a�x) x Complex Reactions and Dynamics http://dx.doi.org/10.5772/intechopen.70502 5