Lecture 4: Mechanisms and Catalysts
Lecture 4: Mechanisms and Catalysts Chem 31B
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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) + 2g) → 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 2F2 Which of the following mechanisms is consistent with this rate law? a.) 2NO2 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 F2each with a coefficient of 1
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