CHEM-5151_S05_Kinetics_extras

CHEM-5151_S05_Kinetics_extras - Extra Slides for Kinetics...

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1 Extra Slides for Kinetics Lectures Atmospheric Chemistry 5151 Why do we care about gas phase reactions? • Atmosphere is primarily gas phase medium – Pollution emissions often in gas phase • Consider the lonely OH molecule here in Boulder… – Number of photons seen per second • Assume 1360 W m -2 , all radiation is 500 nm, and OH molecule has cross section of 0.5 nm 2 • Answer ~ 2000 photons per second – Number of heterogeneous collisions per second • Assume ~ 10 5 particles cm -3 with average diameter of 500 nm • ~ 50 collisions per second • We will derive this later – Number of gas phase collisions • Use 630 Torr, 298 K • ~ 8 x 10 9 collisions per second!! • We will derive this later Extra
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2 Before we start… • Where do molecules hide all of their energy? – Translations – just moving Æ related to temperature – Rotations – spinning, right round, like a record baby, right round – Vibrations – within molecule, atoms changing inter-atomic distances – Electronic – electrons within molecule occupying different orbitals, for lack of a better analogy, think of electrons as little planets orbiting the sun (nucleus) and orbits are different distances from center • Good concepts to keep in mind Extra • Example 1: OH + CH 4 Æ H 2 O + CH 3 H f (kcal mol -1 )S f (cal K -1 mol -1 ) – OH 8.89 43.91 –CH 4 -17.80 44.55 –H 2 O -57.798 45.133 3 35.05 46.36 H rxn = -13.8 kcal mol -1 , S rxn = 3.0 cal K -1 mol -1 , & G rxn = -14.7 kcal mol -1 for 298 K • Example 2: N 2 O 5 Æ NO 2 + NO 3 H f (kcal mol -1 f (cal K -1 mol -1 ) –N 2 O 5 3.18 85.01 –NO 2 8.17 57.40 3 17.6 61.76 H rxn = +22.6 kcal mol -1 , S rxn = 34.15 cal K -1 mol -1 , & G rxn = +12.4 kcal mol -1 for 298 K G = 0 at 662 K; change from spontaneous to non-spontaneous reaction • Note: we have not included change in H f ’s with respect to temperature – Often these are small – for initial order-of-magnitude type guesses, do not worry about “Practical Thermo” Extra
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3 What are chances of truly termolecular collision? • How long is collision vs how long is time between collisions? – Collisions last ~ 10 -13 s to 10 -12 s • For argument’s sake, say 10 -12 s – Time between collisions ~ 10 -9 s • Remember, we already said ~ 8 x 10 9 collisions s -1 – Means collision duration / total time ~ 0.001 • Chance of 3 molecules colliding simultaneously ~ 0.001 2 = 10 -6 • To have truly termolecular collision, fighting factor of a million in probability – Much more likely if process occurs as a series of bimolecular collisions Extra Rate Laws • Rate of reaction: change of concentration with respect to time • Example: OH + CH 4 Æ H 2 O + CH 3 • How do you express the rate of this reaction?
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This note was uploaded on 03/03/2012 for the course CHEM 5151 taught by Professor Staff during the Spring '08 term at Colorado.

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CHEM-5151_S05_Kinetics_extras - Extra Slides for Kinetics...

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