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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 – CH 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 ) S f (cal K -1 mol -1 ) – N 2 O 5 3.18 85.01 – NO 2 8.17 57.40 – NO 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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