1) Exponents in rate law do not depend on 1) Exponents in rate law do not depend on stoichiometricstoichiometric coefficients in chemical reactions.coefficients in chemical reactions.2)What is the detailed way in which the reactants are converted2)What is the detailed way in which the reactants are convertedinto products? This is not described by the chemical into products? This is not described by the chemical equation, which just accounts for mass balance.equation, which just accounts for mass balance.3) Rate at which reaction proceeds and equilibrium is3) Rate at which reaction proceeds and equilibrium isachieved, depends on the achieved, depends on the MechanismMechanism by which by whichreactants form products.reactants form products.MechanismMechanism ConceptConceptElementary Reactions:Elementary Reactions: these are hypothetical these are hypothetical constructs, or our guess about how reactants are converted constructs, or our guess about how reactants are converted to products.to products.The The Mechanism Mechanism is a is a setset of of Elementary ReactionsElementary Reactions!!HH22ClCl22H HClClHClHCl+suppose reaction actually takes place during a collision ofsuppose reaction actually takes place during a collision ofHH22 with Cl with Cl22 (this is the (this is the binary collision picturebinary collision picture):):The above is a The above is a bimolecularbimolecular elementary reaction. elementary reaction. A A unimolecularunimolecular elementary reaction might be elementary reaction might beHOHO22 ÆÆ H + O H + O22HOHO22 just dissociates without any other influence. just dissociates without any other influence.Rate Laws for Elementary Reactions:Rate Laws for Elementary Reactions:1) A 1) A ÆÆ Fragments, depends only on A (No collisions) Fragments, depends only on A (No collisions)2) A+A 2) A+A ÆÆ Products, depends only on A, A collision Products, depends only on A, A collisionElementary reaction is one place where Elementary reaction is one place where stoichiometry stoichiometry and and rate rate areare related. However never know when you have an related. However never know when you have an elementary reaction. Must guess and then verify with experiment. elementary reaction. Must guess and then verify with experiment. Elementary reactions are hypothetical constructs!Elementary reactions are hypothetical constructs!A + B A + B ÆÆ Products. Products.UnimolecularUnimolecular Decompositions DecompositionsAn example of An example of MechanismsMechanisms, , SteadySteady StateState ApproximationApproximation,,and and ElementaryElementary ReactionsReactionsA A ÆÆ Fragments FragmentspyrazinepyrazineObserved Experimentally toObserved Experimentally tobe first order in be first order in pyrazinepyrazine..LindemannLindemann [Lord [Lord CherwellCherwell] suggested the following mechanism:] suggested the following mechanism:Assume, however, that after A* is produced by a collision it hangs Assume, however, that after A* is produced by a collision it hangs around for some time before decomposing. This time lag betweenaround for some time before decomposing. This time lag betweenactivation and reaction may be thought of as the time necessary to activation and reaction may be thought of as the time necessary to transfer energy among the internal (transfer energy among the internal (vibrationalvibrational) coordinates.) coordinates.-dAdt= k[A]or C - B or C - B ÆÆ C + B with C + B withIf AIf A** exists for a reasonable time it could suffer a collision and drop exists for a reasonable time it could suffer a collision and drop down to a lower energy where it cannot decompose. down to a lower energy where it cannot decompose.EE11 , E , E22 , E , E33 < < EEminmin; ; E*E*≥≥ EEminmin = energy necessary for decomposition = energy necessary for decompositionAgain EAgain E44 , E , E55 , E , E66 < < EEminminCollision betweenCollision betweenA(EA(E11) and A(E) and A(E22) ) creates creates ““activatedactivated””A*(E*)A*(E*)Competition betweenCompetition betweenreaction of reaction of A*(E*)A*(E*) to toform products and form products and collisional collisional cooling of cooling of A*(E*)A*(E*) to to produce produce unreactiveunreactiveA(EA(E55) and A(E) and A(E66))(k(k11, k, k-1-1, and k, and k22 are kinetic rate constants) are kinetic rate constants)Mechanism Elementary Steps:Mechanism Elementary Steps:kk22 is decomposition step assumed irreversible. is decomposition step assumed irreversible.A + Ak1æ Æ æ æ A*+ AA*+ Ak-1æ Æ æ æ A + AA* k2æ Æ æ æ PdPdt= k2[A*]DonDon’’t know what [At know what [A**] is, however the number of [A] is, however the number of [A**] must be small] must be small or the reaction would go to completion very quickly. A* is a or the reaction would go to completion very quickly. A* is a ““bottleneckbottleneck”” for the reaction since product is only formed via A*. for the reaction since product is only formed via A*.Step 1Step 1Step 2Step 2Step 3Step 3d[Ad[A**]/]/dtdt = 0 = k = 0 = k11[A][A]22-k-k-1-1[A[A**][A]-k][A]-k22[A[A**] ] Steady state approach allows us to solve for concentrationSteady state approach allows us to solve for concentrationof of unknownunknown species [A species [A**] in terms of ] in terms of knownknown [A] concentration. [A] concentration.Solve for [A*] ÆÆThe The Steady StateSteady StateApproximation Approximation Fundamental result of Fundamental result of Lindemann Lindemann ““UnimolecularUnimolecularReaction MechanismReaction Mechanism””Note, multistep mechanism leads to complex rate expression!At this point, it looks like At this point, it looks like MrMr. . Lindemann Lindemann will have to handwill have to handin his Theoristsin his Theorists’’ Club ID card since his scheme seems to Club ID card since his scheme seems to predict a second order kinetic dependence on [A]predict a second order kinetic dependence on [A]22!!This says the rate of decomposition of AThis says the rate of decomposition of A** is much is much fasterfasterthan the rate of deactivation.than the rate of deactivation.Thus, Thus, dPdP//dtdt @@ k k11[A][A]22 }}2nd order in [A]2nd order in [A] Two Limiting CasesTwo Limiting CasesI)I)Rate =dPdt= k2[A*] =k2k1[A]2k2+ k-1[A]Mechanism Elementary Steps:Mechanism Elementary Steps:““Physical InterpretationPhysical Interpretation”” of this particular limit: of this particular limit:A + Ak1æ Æ æ æ A*+ AA*+ Ak-1æ