14.4 Reaction Mechanism

Transcription

1 14.4 Reaction Mechanism Steps of a Reaction Fred Omega Garces Chemistry 201 Miramar College 1 Reaction Mechanism

2 The Ozone Layer Ozone is most important in the stratosphere, at this level in the atmosphere, ozone absorbs UV radiation 100 Km Mesosphere Mesosphere Stratosphere Troposphere 50 Km Stratosphere 10 Km Ozone Layer Mt. Everest Troposphere 2 Reaction Mechanism

3 Mechanism of Ozone; Chapman Cycle Chapman cycle shows that O 3 exist at steady state. It is constant in the stratosphere. hν 242 nm or less O 2 O 2 2 O O O 2 O 3 O nm or less hν O 3 lives for ~ s before it dissociates. Ozone removal step: O 3 + O g 2O 2 Slow ozone removal step O 3 Reaction Mechanism

4 Reaction Coordinate for ozone destruction A Key reaction in the upper atmosphere is O 3 (g) + O (g) 2O 2 (g) 19 kj The E a (fwd) is 19 kj, and the DH rxn as written is -392 kj. O kj O 3 + O E act (rev) = 411 kj A reaction energy diagram for this reaction with the calculate E a(rev) is shown kj 2O 2 Reaction Progress 4 Reaction Mechanism

5 Path to Destruction: Ozone 1. Water Vapors: H 2 O OH + H H + O 3 OH + O 2 OH + O H + O 2 Net: O + O 3 2O 2 2. N 2, Dinitrogen : N 2 + O 2 2NO NO + O 3 NO 2 + O 2 NO 2 + O NO + O 2 Net: O + O 3 2O 2 3. CFCs CCl 2 F 2 CClF 2 + Cl Chlorofluorocarbons Cl + O 3 ClO + O 2 ClO + O Cl + O 2 Net: O + O 3 2O 2 \ 10,000 O 3 will breakdown to O 2 for every Cl 5 Reaction Mechanism

6 Energy (kj) Influence by CFC: Ozone Comparison of activation energies in the uncatalyzed decompositions of ozone. The destruction of ozone can be catalyzed by Cl atoms which leads to an alternative pathway with lower activation energy, and therefore a faster reaction. Progress of reaction 6 Reaction Mechanism

7 Reaction Mechanism The mechanism of a reaction is the sequence of steps (at the molecular level) that shows how reactant chemicals combine to form the final products. Elementary Steps Sequence of steps which describes an actual molecular event. Stoichiometry The overall stoichiometric reaction is the sum of the elementary steps. Scientist want to learn about mechanisms because an understanding of the mechanism (how bonds break and form) may lead to conditions to improve reaction product yield, (or prevent side products formation. i.e, depletion of ozone.) 7 Reaction Mechanism

8 Ozone: Revisited Chapman s Cycle O 3 O 2 + O O + O 3 2O 2 2O 3 3O 2 Elementary Steps give rise to Rate Law Since elementary steps describes a molecular collision, the rate law for an elementary step (unlike the overall reaction) can be written from the Stoichiometry. Consider an elementary step ia + jb Product (slow step) rate = k [A] i [B] j The rate of the reaction is directly proportional to concentrations of the colliding species. 8 Reaction Mechanism

9 Elementary Step: Rate Law Consider the following proposed mechanism for the conversion of NO 2 to N 2 O 5. What is the rate law. Step1 NO 2 + O 3 NO 3 + O 2 (slow) Step2 NO 3 + NO 2 N 2 O 5 (fast) rate = k 1 [NO 2 ] 1 [O 3 ] 1 In a series of steps, the slowest step determines the overall rate. In the mechanism for a chemical reaction, the slowest step is the rate-determining step. 9 Reaction Mechanism

10 Elementary Steps: Order of reaction Elem. Step Rate Law Order Molecularity 1 A Product Rate = k[a] 1st order unimolecular 2 2A Product Rate = k[a] 2 2nd order bimolecular A + B Prod. Rate = k[a][b] 3 3A Product Rate = k[a] 3 3rd order Termolecular 2A + B Product Rate = k[a] 2 [B] A + B + C Prod Rate = k[a][b][c] * Termolecular mechanism (3-elementary step) is very rare. Scientist who propose such a mechanism must make careful measurements. 10 Reaction Mechanism

11 Multiple Elementary Steps Most reactions involve more than one elementary step. Rate-Limiting - When one step is much slower than any other, the overall rate is determined by the slowest Rate-determining step. Reaction is only as fast as the slowest elementary step Analogy: Leaving class after an exam. On a single lane highway, speed of traffic is only as fast as creepy crawler 12- cars ahead. 11 Reaction Mechanism

12 Rate Determining Step from Rate Law Consider: NO 2 + CO g NO + CO 2 Mechanism: (1) NO 2 (g) + NO 2 (g) NO 3 g) + NO (g) (2) NO 3 (g) + CO (g) NO 2 + CO 2 (g) Rate = k[no 2 ] 2 Net: NO 2 (g) + CO (g) CO 2 (g ) + NO (g) Which is Rate limiting step (1) or (2)? RDS is the step that determines the rate law. When scientists propose a mechanism, they can only say that it is consistent with the experimental data. There may be other mechanisms that are consistent with experimental data as well. If experiments are done in the future to disprove the mechanism, then the proposed mechanism must be revised. 12 Reaction Mechanism

13 RDS and Rate Law: Example Consider the reaction : NO (g) + O 3 NO 2 + O 2 Two mechanisms (elementary steps) are proposed: Mechanism 1 NO + O 3 NO 2 + O 2 Proposed rate law: Rate = k [NO] [O 3 ] Mechanism 2 O 3 O 2 + O (slow) NO + O NO 2 (fast) Proposed rate law: Rate = K [O 3 ] What are the Rate Laws? When a potential mechanism is proposed, 2 factors must be considered - Rate limiting step must be consistent with observed rate law. Sum of all the steps must yield the observed stoichiometry. 13 Reaction Mechanism

14 Complicated Reaction Mechanism Reaction mechanism in which slow step (rate determining step) involves an intermediate. Consider: A B Mechanism: A D int (fast) NET: RATE = k[int] int B A B (slow) -but the rate law cannot be written in terms of an intermediate (catalyst and reactant okay, but not intermediate). -It must be expressed in terms of stable species How is the Rate Law modified? 14 Reaction Mechanism

15 Modification of Rate Law RATE = k [int] Written in terms of reactants- k eq = [int] [A] Rate = k [int] [int] = k eq [A] Rate = k k eq [A] = K' [A] The rate law is now expressed in terms of the reactant. 15 Reaction Mechanism

16 Rate Laws from Mult. Steps Mechanism. Consider the reaction below, what is the rate law based on the two proposed mechanism: 2 NO 2 (g) + O 3 N 2 O 5 + O 2 Mechanism (1) Mechanism (2) NO 2 + NO 2 D N 2 O 4 (fast) NO 2 + O 3 D NO 3 + O 2 (slow) N 2 O 4 + O 3 N 2 O 5 + O 2 (slow) NO 3 + NO 2 N 2 O 5 (fast) Rate = k [N 2 O 4 ] [O 3 ] [N k eq = 2 O 4 ] [NO 2 ] [NO 2 ] [N 2 O 4 ] = k eq [NO 2 ] 2 Rate = k k eq [NO 2 ] 2 [O 3 ] Rate = k[no 2 ] [O 3 ] bimolecular Rate is based on slowest elem. step Rate = k[no 2 ] 2 [O 3 ] Termolecular 16 Reaction Mechanism

17 Rate Laws from Mult. Steps Mechanism. The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed b a two-step mechanism: H 2 O 2 (aq) + I - (aq) H 2 O (l) + IO - (aq) (slow) H 2 O 2 (aq) + IO - (aq) H 2 O (l) + I - (aq) + O 2 (g) (fast) a) Rate Law: Rate Law = k [H 2 O 2 ] [I - ] b) Overall reaction: 2 H 2 O 2 (aq) H 2 O (l) + O 2 (g) c) Intermediate: IO - (aq) Catalyst: I - (aq) 17 Reaction Mechanism

18 Enzyme Catalysis Reaction Consider oxidation of ethanol to aldehyde: CH 3 CH 2 OH (l) ADH CH 3 CHO + R-H 2 ADH - Alcohol dehydrogenase Mechanism E + S D ES ES E + P (slow) E + S E + P Rate = k [ES] K eq = [ES] K eq [E] [S] = [ES] [E] [S] Rate = K K eq [E] [S] = K [E] [S] 18 Reaction Mechanism

19 In Class Exercise Phosgene (Cl 2 CO) a poison gas used in WW1, is formed in the following reaction. Mechanism: i) H + + Cl 2 D Cl + HCl fast, reversible ii) HCl + CO D ClCO + H + fast, reversible iii) ClCO + Cl Cl 2 CO slow i) What is the overall reaction? ii) What is the intermediates and the catalyst for the reaction? iii) What is the rate law from this mechanism? iv)what is the order of the reaction with respect to each reactant? v) What is the overall molecularity? vi) How would doubling the concentration of CO affect the reaction? 20 Reaction Mechanism

20 Summary The dynamics of the series of steps of a chemical change is what kinetics tries to explain. Variation in reaction rate are observed through concentration and temperature changes, which operate on the molecular level through the energy of particle collision. Kinetics allows us to speculate about the molecular pathway of a reaction. Modern industry and biochemistry depend on its principles. However, speed and yield are very different aspects of a reaction. Speed is in the kinetic domain, yield is in the equilibrium domain. 21 Reaction Mechanism