12 Reaction Mechanisms v2

12 Reaction Mechanisms v2 - A Relevant First-Order Reaction...

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1 1 A Relevant First-Order Reaction Cis – trans isomerization in cooking oils. Many restaurants are converting to cooking oils that are all cis because it is healthier. But at high temperatures the molecules isomerize to produce an equilibrium mixture of both the cis and trans isomers. So the cooking oil has to be replaced. If the rate constant for the isomerization is 8 x 10 -3 /hr at 250 o C (480 o F), how long will it take for 25% of the oil to isomerize? (A) 36 hrs (B) 87 hrs (C) 173 hrs (D) 347 hrs t 1/2 = ln(2)/k kt A A o t ] [ ] [ ln 2 Today’s To Do Predict the rate law for a given reaction mechanism.
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2 3 Reaction Mechanisms A reaction mechanism is a series of elementary reactions or steps. Elementary reactions are either a unimolecular (single molecule) or a bimolecular (two molecules) reaction. Two situations: First step is slow First step is fast 4 Rate Laws for Elementary Reactions Unimolecular: first order O 3 O 2 + O Rate = k [O 3 ] Bimolecular: second order NO + O NO 2 Rate = k [NO][O]
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3 5 Example of a Reaction Mechanism 2I - + H 2 O 2 + 2H 3 O + I 2 + 4H 2 O All in aqueous solution (aq) omitted. Proposed mechanism: a series of elementary steps! 1) H 2 O 2 + I - OH - + HOI (SLOW) 2) HOI + I - OH - + I 2 (FAST) 3) OH - + H 3 O + 2H 2 O (FAST) 4) OH - + H 3 O + 2H 2 O (FAST) For any reaction several mechanisms can be written. In order to be a valid possibility, the steps Must sum to the overall reaction equation. Must predict the experimentally observed rate law.
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This note was uploaded on 03/29/2010 for the course CHE 132 taught by Professor Hanson during the Spring '08 term at SUNY Stony Brook.

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12 Reaction Mechanisms v2 - A Relevant First-Order Reaction...

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