Ch1b09Lecture19

Ch1b09Lecture19 - 1 Lecture 19 - February 23, 2009 Summary...

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Summary of last lecture: •more kinetics Preview of coming attractions: •the last bit of kinetics •organic reaction mechanisms: displacement reactions Lecture 19 - February 23, 2009 1
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Kinetics of fast reactions For second order reactions with rates limited by the bimolecular collision frequency, the upper limit to k ~ 10 10 - 10 11 M -1 sec -1 Important examples of this class of reactions are proton transfer processes: H + + A - HA. In his Nobel Prize Lecture (1967), Manfred Eigen noted that he was challenged by the statement in his physical chemistry text “The rate of true neutralization reactions has proven immeasurably fast” and he subsequently developed “relaxation methods” to measure these rates. 2
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Background context: the problem Consider the forward rate of the neutralization reaction H + + A - HA As an elementary step, the rate law is d HA ( ) dt = k H + ( ) A " ( ) if (H + ) 0 = (A - ) 0 , the integrated rate equation becomes " d H + ( ) dt = k H + ( ) A " ( ) = k H + ( ) 2 which may be solved to give: H + ( ) = H + ( ) 0 1 + kt H + ( ) 0 with the initial half-life t 1 2 = 1 k H + ( ) 0 3
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t 1 2 = 1 k H + ( ) 0 when (H+) 0 = 1 M, t 1/2 = 10 -10 s when (H+) 0 = 10 -4 M, t 1/2 = 10 -6 s when (H+) 0 = 10 -7 M, t 1/2 = 10 -3 s with and k ~ 10 10 M -1 s -1 while the reaction rate is too fast at high concentrations, the use of dilute solutions changes the time scale to more experimentally accessible values. However, even 10 -3 s is difficult to achieve in studies where the reactants are mixed and the approach to equilibrium is monitored. 4
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Eigen developed “relaxation methods” where the equilibrium is perturbed by either a temperature or a pressure jump (T-jump and P-jump), and the relaxation of the system to equilibrium monitored. K must change with T;
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Ch1b09Lecture19 - 1 Lecture 19 - February 23, 2009 Summary...

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