L26 - Activation Energy Barrier Temperature determines the...

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1 Lecture 26 Get Over It: Reaction rates Key Idea: Substances such as food and wood do not just burst in flames. Rates of chemical reactions may vary over many orders of magnitude ~ 1 s ~ 10 years ~ 10 4 to 10 6 years Molecular Workbench Simulation: + Legos: + A B AB Lego Demo Results 0 10 20 30 40 50 60 0 50 100 150 200 Time Number of Legos AB A Forward Rate [A][B] k fwd [A][B] = Reverse Rate [AB] k rev [A][B] = k’s are called “rate constants” . .. Rate= k [A] m [B] n is called the rate “law” or “expression” What factors affect rates of chemical reactions? 1. Concentrations of reactants 2. Mixing and dispersion of reactants 3. Temperature 4. Catalysts Cl + NOCl NO + Cl 2 Why is TEMPERATURE a factor? Effective collision of reactants Ineffective collision Temperature Kinetic energy Collision frequency Exothermic reaction “Reaction Coordinate” Æ (e.g., time) CH 4 + 2 O 2 CO 2 + 2 H 2 O C + 4 H + 4 O (From Wed, Lecture 25)
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2 H 3 CNC H 3 CCN Why is TEMPERATURE a factor? Why is TEMPERATURE a factor? Potential Energy Diagram H rxn Transition state E a ,
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Unformatted text preview: Activation Energy Barrier Temperature determines the fraction ( f ) of all molecules with E ≥ E a f = e − Ea / RT Arrhenius equation k = A e − Ea / RT frequency factor exponential factor Rate = k [H 3 CNC] = A e Ea/RT [H 3 CNC] Catalysts decrease the activation energy barrier 2 NO N 2 + O 2 Pt Catalysts decrease the activation energy barrier The catalytic converter in your car 2 CO + O 2 CO 2 2 NO N 2 + O 2 Pt catalyst Pt Pt 3 Enzymatic catalysis http://www.indiana.edu/~oso/animations/An6.html Wrap-Up Points: * Rates of reactions depend on the concentration of reactants and the temperature. * Temperature determines the fraction of reactants capable of overcoming E a , the activation energy barrier. As the temperature increases, a larger fraction of molecules have E > E a . * Catalysts increase rates of reactions by lowering their activation energy barriers....
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This note was uploaded on 09/28/2009 for the course CHEM 1A taught by Professor Nitsche during the Fall '08 term at Berkeley.

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L26 - Activation Energy Barrier Temperature determines the...

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