Lecture 8- Chemical Kinetics

Lecture 8- Chemical Kinetics - Midterm Exam 1 Friday 1:10...

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Midterm Exam 1 Friday, Jan 28, 2011 1:10 – 2:00 pm 198 Young (last name begins with “A” - “L”) 176 Everson (last name begins with “M” - “Z”) Exam covers Chapters 2.6 – 2.9, all of 9, and 10.1 – 10.3 Closed Book and Closed Notes No electronic devices except calculator Practice Midterm on course website At least one assigned homework problem will appear on the exam Review session Thur, Jan 27 th at 7pm in Rm ?.
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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 10 10 = = s M s m V N V N Z B A AB RT E B B A 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 RT Ea AB B A e Z V N V N k Rate / × = = RT Ea Ae k / = Collision Theory T k d V N V N Z B B A 8 2 =
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Collision Theory vs Orientation RT E AB A e Z rate = 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 AB A e PZ rate = P = Probability or steric factor A + B Product
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Steric Factor vs. “Cone of Attack” rate = PZ AB 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 ‡.
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Lecture 8- Chemical Kinetics - Midterm Exam 1 Friday 1:10...

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