Lecture 6 - Chemical Kinetics

Lecture 6 - Chemical Kinetics - Reaction Mechanism NO2(g) +...

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Reaction Mechanism NO 2 (g) + CO(g) NO(g) + CO 2 (g) Overall rxn rate = k[NO 2 ] 2 Step (1) NO 2 (g) + NO 2 (g) NO 3 (g) + NO(g) rate = k 1 [NO 2 ] 2 Step (2) NO 3 (g) + CO(g) NO 2 (g) + CO 2 (g) rate = k 2 [NO 3 ][CO] NO 2 (g) + CO(g) NO(g) + CO 2 (g) rate = k[NO 2 ] 2 slow fast A I P slow fast
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[] [ ] [ ] ( ) ( ) [ ] EQ t k k EQ A e A A t A + = + 1 1 0 ) ( [ ] EQ A [ ] EQ B Kinetics of Reversible Elementary Step AB k 1 k -1 [] [] B k A k dt A d 1 1 + = [ ] ( ) ( ) t k k EQ e B t B 1 1 1 ) ( + = B k A k dt B d 1 1 = k 1 k -1 exchange rate = k 1 + k -1 [ ] 1 1 = = k k A B K EQ EQ eq [A] 0 –[A] relaxation time = 1 1 1 + k k
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AB k 1 k -1 Initial conditions: [A] = [A] 0 and [B] = 0 When reaction proceeds halfway to equilibrium [ ] [ ] ( ) [ ] [ ] ( ) EQ EQ A A A A = 0 5 . 0 , [][] { } [] [] {} [ ] [ ] ( ) 2 1 5 . 0 0 0 0 = = EQ EQ EQ EQ A A A A A A A A { } () t k k A A A A EQ EQ 1 1 0 ln + = 2 / 1 1 1 2 1 ln t k k + = 2 / 1 1 1 2 ln t k k + + = 1 1 2 / 1 2 ln + = k k t . [ ] [ ] t k k A A A A EQ EQ ) ( ln 1 1 0 + = Half-life (t 1/2 ) of Reversible First-order Reaction [A] EQ t 1/2 [A] 0 –[A] EQ [B] EQ
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Example 28-5: cis-2-butene trans-2-butene What is k 1 and k -1 if t 1/2 = 474 sec and K c = 0.406? K c = k 1 /k -1 = 0.406 k -1 = k 1 /K c = k 1 /0.406 474 2 ln 1 1 2 / 1 = + = k k t AB k 1 k -1 [ ] [] 1 1 = = k k A B K EQ EQ c cis trans 474 2 ln 2 ln 2 / 1 1 1 = = + t k k 474 2 ln 406 . 0 1 1 = + k k 474 2 ln 406 . 0 1 1 1 = + k 1 4 1 1 10 2 . 4 406 . 0 1 1 474 2 ln × = + = s k 1 3 4 1 1 10 04 . 1 406 . 0 10 2 . 4 × = × = = s K k k eq k 1 k -1 s
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Pseudo First-order Kinetics A+B C k 1 k -1 [] [] [] [] [] C k B k C k B A k dt B d 1 ' 1 1 1 + = + = [] [] C k B k dt C d 1 ' 1 + = () EQ t k k EQ B e B B t B + = + 1 ' 1 0 ) ( [A] 0 >> [B] 0 and [C] 0 = 0 [ ] EQ B [ ] EQ C [B](t) [C](t) k 1 ’=k 1 [A] ( ) ( ) t k k EQ e C
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This note was uploaded on 04/06/2011 for the course CHEM 107B taught by Professor Jamesames during the Winter '09 term at UC Davis.

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Lecture 6 - Chemical Kinetics - Reaction Mechanism NO2(g) +...

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