L5_kinetics115b08

L5_kinetics115b08 - EXAM 1: JANUARY 31, 3:30-4:30 P.M. SGM...

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L5: Next: Multi-step Reactions: Approximations in Rate Equations. 1. Reactions with a fast equilibrium step and a slow reaction step. 2. Steady state approximation EXAM 1: JANUARY 31, 3:30-4:30 P.M. SGM 101
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Rate expressions from mechanisms: Example 2 (fast equilibrium) Cl 2 2 Cl k 1 fast R f =k 1 [Cl 2 ] k -1 fast R b =k -1 [Cl] 2 Cl + O 3 ClO + O 2 k 2 slow R=k 2 [Cl][O 3 ] Overall: Cl 2 + 2O 3 2ClO + 2O 2 Rate: R = k 2 [Cl] [O 3 ] R f =R b k 1 [Cl 2 ] =k -1 [Cl] 2 2 2 1 1 1 k k k k = 11 22 32 2 1 1 1 2 [ ] [] [ ] k kk k RO C l O C l == ⎛⎞ ⎜⎟ ⎝⎠ 1 1 2 1 2 2 1 k C C l k l = Note: a non-integer order in Cl! (1/2)
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Reaction simulator link: http://michele.usc.edu/java/kinetics/simtwo.html Unimolecular Decay, 100 particles (ideal, 1<->1, absolute Fast equilibrium) Regime A: Red <-> Green , k 1 =0.015, k -1 =0.015 Regime B: rate limiting step with fast equilibrium (100/20) 1. Red <-> Blue K 1 =K -1 =0.025 2. Blue -> Green K 2 =0.0025 Notice that Red and Blue go down together Change rates to get to Regime C: Steady state 1. Red -> Blue K 1 =0.0025 2. Blue -> Red K -1 = 0.025 2. Blue -> Green K 2 =0.025 Notice fixed and small concentration of intermediate You can log on and try other combinations to test the validity of the assumptions of the two approximations.
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Rate expressions from mechanisms: Example 3:
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L5_kinetics115b08 - EXAM 1: JANUARY 31, 3:30-4:30 P.M. SGM...

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