L7-10_notesCh13_17_07

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

Info iconThis preview shows pages 1–8. Sign up to view the full content.

View Full Document Right Arrow Icon
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
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 2
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
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Femtochemistry How to measure reaction kinetics of fast reactions Femtosecond = 10 -15 s Picsosecond = 10 -12 s Nobel Prize 1999: Prof. Ahmed Zewail, Caltech
Background image of page 4
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
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
A Pump-Probe Experiment The pump (excitation) laser pulse is fired at time t = 0 and imparts energy h ν 1 to a molecule, such as ICN. The molecule is excited to a state ICN* from which it dissociates after a short while in a unimolecular reaction. The probe (detection) laser pulse is set at an energy h ν 2 that is absorbed only by the product CN. It is fired at variable delays after the pump laser. ICN and I do not absorb the probe laser radiation. ICN* t>0 I + CN ICN + h ν 1 t=0 h ν 2 [CN] t t=0 1 st order reaction 100 200 300 fs pump laser fires probe laser
Background image of page 6
How do you control time delays that are so short? By changing the path length of the light. Use the relationship between the speed of light, c , time, t, and distance, x : x = c t; t is the delay time for probe laser to fire after pump laser. c = 3x10 8 m s -1 t = 100 fs = 10 -13 s after pump x = 3x10 8 m s -1 x 10 -13 s = 3x10 -5 m = 0.03 mm If you want to fire the probe laser pulse in increments of 100 fs, you need to lengthen the path length of the light each time by 0.03 mm.
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 8
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 39

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

This preview shows document pages 1 - 8. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online