Lecture37A - Lecture 37 1 Collisions, Molecular Beams and...

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Unformatted text preview: Lecture 37 1 Collisions, Molecular Beams and Scattering (30.5-10) Consider a binary (bimolecular) elementary reaction: ( 29 ( 29 ( 29 ( 29 A g B g C g D g + + We know from gas kinetic theory (lecture 30) that the kinetic energy of the reactants can be expressed two ways. Fig 30.5 2 2 2 2 1 1 2 2 1 1 2 2 where , react A A B B cm r A B A B A B KE m u m u Mu u m m M m m m m = + = + = + = + As we move in the center of mass frame, our reactants and products are confined to a plane. Furthermore, irregardless of the reaction products: 2 2 1 1 ' ' 2 2 prod cm r KE Mu u = + The internal energy can include rotation, vibration and electronic energy. Lecture 37 2 2 r For the reactants, we can usually characterize from gas kinetic theory for a thermalized sample: ( 29 2 2 3 so 3 1 1 3 as expected 2 2 2 B B r B k T u m k T E u k T = = = = Internal degrees of freedom in our reactants will also be thermally populated ,int , , , and similarly for B. A A rot A vib A elec E E E E = + + The products C,D may not (and most likely will not) be thermalized immediately after the reaction takes place. Especially for a strongly exothermic bimolecular reaction. There is also a substantial spread in relative velocities for a thermalized gas sample. Only a fraction have relative energy sufficient to overcome the activation energy barrier. How great is the velocity spread? Lecture 37 3 Consider a gas sample of Argon at 298K....
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Lecture37A - Lecture 37 1 Collisions, Molecular Beams and...

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