Lecture+5+-+Chemical+Kinetics

Lecture+5+-+Chemical+Kinetics - Collision Theory N A N B Ea...

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Arrhenius prefactor (A) Fraction of molecules having E > E a (Diffusion-limited rate constant when E a ~ 0) 1 1 11 10 = s M k diffusion Arrhenius prefactor (A) = 1 1 11 1 3 16 11 10 10 = = s M s m V N V N Z B A RT E B AB B A RT E B A A a e T k d V N V N e Z V N V N rate k = = = μ π 8 2 11 RT Ea B A e Z V N V N k Rate / 11 × = = RT Ea Ae k / = Collision Theory T k d V N V N Z B AB B A 8 2 11 =
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Activation Energy (E a ) A + B C A B E a Maxwell Distribution Fraction of exp(-E a /RT) <KE> = 3RT/2 E a E > E a molecules KE > E a
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Collision Theory vs Orientation RT E A e Z rate = 11 Only accurate for reactions involving atoms or symmetric molecules. For larger molecules, calculated rates are too high: Not every collision with sufficient energy results in product formation. Proper Orientation of collision also matters. RT E A e PZ rate = 11 P = Probability or steric factor A + B Product
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Steric Factor vs. “Cone of Attack” rate = PZ 11 e -(Ea/RT) Steric factor HI + Cl --> HCl + I Fraction KE > E a
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Transition State Theory A+B AB P [ ] [] B A AB K c =
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Thermodynamics of Transition State Theory K RT G ln = Δ o RT G e K o Δ = Δ = Gº (activated complex) - (reactants) Δ A + B AB C + D k rate = k[A][B] rate = ν [AB ] = ν [A][B]K k = ν K ν How do we determine ν ? ] ][ [ ] [ B A AB K = Express K in terms of Δ G : RT G e k o Δ Eyring eqn
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RT G e K o Δ = k = ν K Eyring Eqn: Statistical Mechanics provides: h T k B = ν M = molarity m = molecularity of reaction Statistical Frequency Factor ( ν ) = ~ 10 12 s -1 10 11 s -1 Arrhenius prefactor ν rate = k[A][B] = ν [AB ] = ν [A][B]K A + B AB C + D k ) ( 1 1 Δ = s M e h T k k m RT G B o AB vs
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Rate Constant vs Δ H and Δ S Transition State Theory : ) ( 1 m RT G B M e h T k k Δ = o Substitute Δ G º‡ = Δ H º‡ -T Δ S º‡ Δ H º‡ is strength of bonds broken in reactants minus bonds formed in ‡. Δ S º‡ is disorder of transition state (‡) minus disorder of reactants.
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This note was uploaded on 08/10/2011 for the course CHE 107B taught by Professor Ames during the Spring '11 term at UC Davis.

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Lecture+5+-+Chemical+Kinetics - Collision Theory N A N B Ea...

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