20-Lect-Active%20Interm

20-Lect-Active%20Interm - 1 Chemical Reaction Engineering -...

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Unformatted text preview: 1 Chemical Reaction Engineering - Musgrave Active Intermediates & Non-elementary Rate Laws Lecture 20 - Page 1 Outline: 1. Bioreactions and bioreactors - Active intermediates and non Elementary rates laws (Fogler 7.1) Next Time: (Fogler 7.2-7.3) 1. Enzymatic Reaction Fundamentals (7.2) 2. Inhibition of Enzyme Reactions (7.3) Chemical Reaction Engineering - Musgrave Active Intermediates & Non-elementary Rate Laws Lecture 20 - Page 2 Not all reactions follow elementary rate laws The net rate r A involves both non-elementary and elementary rxns. r A = k 1 C A C B 1.5 2 k 2 C A 2 Power law, but non-integer exponent non-elementary Elementary Even rxns that appear simple may have complex pathways. For example , the rate law for: H 2 + Br 2 2 HBr is: r HBr = k 1 C H 2 C Br 2 1.5 C HBr + k 2 C Br 2 This is typical for rxns involving multiple elementary rxns and active intermediates For bioreactions and heterogeneous catalysis the rate laws are almost always non-elementary 2 Chemical Reaction Engineering - Musgrave Active Intermediates & Non-elementary Rate Laws Lecture 20 - Page 3 An intermediate reacts slow relative to its rate of formation. If long lived, it is called a trapped intermediate . An Active Intermediate is an intermediate product of a sequential rxn that is short lived - that is it quickly transforms into another species Reactant Active Intermediate E Product Reactant Intermediate E Product Slow enough that I accumulates Fast so I does not accumulate An active intermediate reacts fast relative to its rate of formation. Pseudo-Steady-State Hypothesis: The net rate of formation of the trapped intermediate is zero. r A * = r nA * = rxns n Chemical Reaction Engineering - Musgrave Active Intermediates & Non-elementary Rate Laws Lecture 20 - Page 4 Example: Consider the overall rxn: (CH 3 ) 2 N 2 C 2 H 6 + N 2 It is 1 st order at high pressures of azomethane (AZO) and 2 nd order at low P AZO . Why? If the rxn was composed of 3 elementary rxns: 2(CH 3 ) 2 N 2 (CH 3 ) 2 N 2 * + (CH 3 ) 2 N 2 k 1 * H 3 C N N CH 3 H 3 C N N CH 3 H 3 C N N CH 3 H 3 C N N CH 3 Collisions activate AZO (CH 3 ) 2 N 2 * C 2 H 6 + N 2 k 3 * H 3 C N N CH 3 N N H 3 C CH 3 Activated AZO decomposes (CH 3 ) 2 N 2 * + (CH 3 ) 2 N 2 2(CH 3 ) 2 N 2 k 2 H 3 C N N CH 3 H 3 C N N CH 3 H 3 C N N CH 3 * H 3 C N N CH 3 Collisions deactivate AZO 2 Chemical Reaction Engineering - Musgrave Active Intermediates & Non-elementary Rate Laws Lecture 20 - Page 3 An intermediate reacts slow relative to its rate of formation. If long lived, it is called a trapped intermediate . An Active Intermediate is an intermediate product of a sequential rxn that is short lived - that is it quickly transforms into another species Reactant Active Intermediate E Product Reactant Intermediate E Product Slow enough that I accumulates Fast so I does not accumulate An active intermediate reacts fast relative to its rate of formation....
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This note was uploaded on 04/25/2010 for the course CHEN 4330 taught by Professor Staff during the Spring '08 term at Colorado.

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20-Lect-Active%20Interm - 1 Chemical Reaction Engineering -...

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