Lecture 23. Tuesday, November 28. Reaction kinetics.

Lecture 23. Tuesday, November 28. Reaction kinetics. -...

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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY 1 John Kuriyan: University of California, Berkeley Chem C130/MCB 100A, Fall 2006, Lecture 23 Chemical Kinetics Definition of rates The rate of a chemical reaction is given by the rate of decrease in reactant concentration: For a reaction R P Reactant Product Rate = dt P d dt R d ] [ ] [ = (1) Positive rate [R] decreases, reaction proceeds in the forward direction. If we have a reaction: aA + bB cC + dD (2) Then the rate is defined as: Rate = dt D d d dt C d c dt B d b dt A d a ] [ 1 ] [ 1 ] [ 1 ] [ 1 = = = (3) Why do we define the rate this way? For the reaction: 2R P, the molar concentration of R drops twice as fast as the increase in molar concentration of P. Hence, for this case
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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY 2 = = dt R d dt P d ] [ 2 1 ] [ Rate (4) Order of the reaction In most cases (for uncatalyzed reactions and for many catalyzed reactions) the rate is proportional to powers of the concentration: For A + B products: rate α [A] x [B] y rate = k [A] [B] (5) rate constant The rate constant is a fundamental parameter of the reaction, and it depends on molecular properties as well as conditions of temperature, pressure, etc. and give the order of the reaction. The reaction is of order in [A] and order in [B]. The overall order of the reaction is ( + ). The order of a reaction is determined by the underlying reaction mechanism (i.e., the chemistry) rather than the stoichiometry. For example, consider the reaction A B (e.g., cis-trans isomerization)
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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY 3 The reaction might occur when two molecules of A collide, converting kinetic energy into the reaction: A + A B + B (6) |_____| collision In this case, the rate of the reaction is proportional to [A] 2 because of the probability of a collision between two molecules goes up as the square of the concentration. Likewise, if three molecules have to collide for the reaction to occur then: Rate α [A] 3 etc. (7) The order of the reaction can also be determined by the chemical mechanism, giving unexpected results if catalysts are involved. E.g. Iodide ion acts as a catalyst in the decomposition of H 2 O 2 . At first glance, the reaction is as follows: 2H 2 O 2 2H 2 O + O 2 (8) However, experimentally this reaction is first order in H 2 O 2 , not second order, when catalyzed by iodide. This is because of the nature of the underlying molecular steps:
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Restricted: For students enrolled in Chem130/MCB100A, UC Berkeley, Fall 2006 ONLY 4 Elementary steps in the reaction: Step 1: H 2 O 2 + I - ⎯→ 1 k H 2 O + IO - slow Step 2: H 2 O 2 + IO - 2 k H 2 O + O 2 + I - fast In this case it turns out that k 2 >> 1 , so the rate is determined by the first step only. Hence the rate of the overall reaction is: Rate = 1 [H 2 O 2 ][I - ] (9) Step 1 is known as a rate limiting step.
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Lecture 23. Tuesday, November 28. Reaction kinetics. -...

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