L7-10_notesCh13_17_07

L7-10_notesCh13_17_07 - L7 Today The molecular perspective...

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What is necessary for a rxn to occur? - molecules must collide; - molecules must collide in a favorable configuration: e.g., Br + HI -> BrH + I (OK) Br + IH -> not so good .... The reaction Br + HI link - molecules must have enough energy to break old bonds before new bonds can be formed: Br + HI -> [Br ....... H ........ I] # -> BrH + I E Reaction coordinate Activated complex or transition state L7: Today: The molecular perspective
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2BrNO --> 2NO + Br 2 Molecules must collide Molecules should possess E >E a Where E a is the activation energy How can the molecule surmount the high potential energy of transition state? insert link: activation energy and kinetic energy; 14.6, sections 2 and 4 http://www2.wwnorton.com/college/chemistry/gilbert/tutorials/ch14.htm
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A + B C + D E a2 E a1 Δ H Use Δ H or Δ E depending whether P or V are constant (see chapter on thermochemistry) Reaction progress also is called reaction coordinate: the pathway between reactants and products. Reaction intermediates will appear as small minima on top of the barrier between reactants and products. Definitions when drawing energy curves or reaction profile for A + B C + D L8: Observing Reaction Intermediates
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Femtochemistry How to measure reaction kinetics of fast reactions Femtosecond = 10 -15 s Picsosecond = 10 -12 s Nobel Prize 1999: Prof. Ahmed Zewail, Caltech
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Use lasers to "clock the reaction: Detection (probe) laser beams is delayed by a period of 50-5,000 femtosecond with respect to excitation (pump) laser beam Each laser light pulse lasts about 100 fs Minimize collisions by working in high vacuum and beams of molecules
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