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Lecture 4: Mechanisms and Catalysts

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Lecture 4: Mechanisms and Catalysts Chem 31B

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Notes from Lecture 4: Mechanisms and Catalysts on January 11, 2016
Principles of Chemistry II
Jennifer Poehlmann
Class Notes
Chemistry, Rate Laws, Rates, reactions, reaction rate, integrated rate law, chem 31B, Half Life, first order, second order, zero order
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This 2 page Class Notes was uploaded by it's lit notes on Monday January 11, 2016. The Class Notes belongs to Chem 31B at Stanford University taught by Jennifer Poehlmann in Fall 2015. Since its upload, it has received 19 views. For similar materials see Principles of Chemistry II in Chemistry at Stanford University.


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Date Created: 01/11/16
  Monday  January 11, 2016  1  Important Information:  ­Midterm #1 Wednesday, January 20  Lecture 4: Mechanisms and Catalysts  Collision Theory  ● molecules generally collide before a reaction can occur  ● the more molecules → the more collisions (more opportunities for the reaction to happen)  → this is why concentration is related to rates  ● however, the collisions must have the correct orientations and energy in order for the  reaction to be successful  ● increasing the temperature gives the reactant molecules sufficient energy to react in a  forward reaction ​nd a reverse reaction  ● at equilibrium, the rate forward = the rate reversed    Temperature and Reaction Rate  ­Ea/RT  ● k = Ae​ ○ E​  = activation energy (stays constant)  a​ ○ R = 8.314 J/molK  ○ T = Kelvin  ○ <KE> = 3/2RT  ○ RT = the average energy of the molecules compared to E​ a  ○ A = frequency factor → the frequency of attempts and success rate of collisions  ● larger activation energy means smaller k which means slower rate  ● larger temperature means bigger k which means faster rate    Clicker Question #1  Which reaction would you expect to have the smallest orientation factor?  a.) H(g) + I(g) → HI(g)  b.) H2​(g) + 2​g) → 2HI  c.) HCl (g) + HCl (g) → H​ 2​(g) + C2​(g)  d.) they would all be the same as long as they’re at the same temperature  e.) you need more information  solution: if A is small, k is small, so rate is small → looking for the rxn with the slowest rate  ­in equation A, both reactants are simple and spherical → easiest reaction with a fast rate  ­in equation C, the reactants are not simple and take different shapes → hardest reaction to make,  which means a slow reaction rate    Arrhenius Plots  ● the Arrhenius Equation can be algebraically solved to give the following form:    Monday  January 11, 2016  2  ○ ln(k) = ­Ea  ( )+ lnA  R T ○ ln( k1) = Ea(( 1 −( 1 )  k2 R T2 T1 Reaction Mechanisms  ● a reaction might occur in multiple steps, therefore multiple collisions  ● mechanism­​  the set of collisions (elementary steps) that it takes to carry out the reaction  ● rate­limiting step­​ the slowest step in a reaction that determines the rate law  ● if we know the set of elementary steps in a reaction, then we can write a rate law based  on the stoichiometry of the rate limiting step  ● if we know the rate law, we can propose a possible mechanism, based on how many and  which molecules are in the rate limiting steps  ● we can never assume the overall reaction is a single elementary step without data  ● intermediate species­​  something that is produced but then used up in the reaction  mechanism    Clicker Question #2  The following rate law has been experimentally determined for the reaction of:  2NO​ 2​ F2​→ 2NO​ 2​  Rate = k(NO​ 2​F2​  Which of the following mechanisms is consistent with this rate law?  a.) 2NO​2​ F2​→ 2NO​ 2​ b.) NO​ + F​ → NO​ F + F (fast)  2​ 2​ 2​      N2​+ F → NO​ 2​  (slow)  c.) NO​ + F​ → NO​ F + F (slow)  2​ 2​ 2​      N2​+ F → NO​ 2​ (fast)  d.) 2​→ 2F (slow)       2NO + 2F → 2NO​ F (fast)  2​ 2​ solution looking for the mechanism in which the slow step has N2 ​d F2​each with a  coefficient of 1 


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