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Remedial Kinetics and Mechanism

A + B  C. conc. time. Remedial Kinetics and Mechanism. to determine / describe / predict the course of reactions. ___________ : identify reactants, products, intermediates identify ___________reaction steps

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Remedial Kinetics and Mechanism

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  1. A + B C conc. time Remedial Kinetics and Mechanism to determine / describe / predict the course of reactions ___________: identify reactants, products, intermediatesidentify ___________reaction steps ___________: measure reaction ratesrate constantsexplore effects of T, P, [H+], …relate rate constants to molecular propertiesdevelop theories  make predictions Rate data (growth of products, decay of reactants) are the basic experimental input to reaction kinetics. The shapes of kinetic data plots depend on rate constants and concentrations. Their functional form  ___________ Modified by Jed Macosko

  2. In general, a chemical equation is written For an infinitesimal advancement dx the concentration of each reactant/product changes by Empirical Chemical Kinetics __________dependence of reactant and product concentrations The ___________of a reaction (the advancement) By definition, Reaction rates usually depend on reactant ___________, order in B total order = x + y rate constant In elementary reaction steps the orders are always integral, but they may be fractional in multi-step reactions. The ___________is the number of molecules in a reaction step. Modified by Jed Macosko

  3. A B mixing chamber detector Measurement of Reaction Rates Sampling of reaction mixture, followed by chemical analysis, chromatography, spectroscopy Quenching of whole reaction or sample, followed by … Matrix isolation is a particular quenching technique used to study otherwise short-lived species. Flow The time between mixing and measurement is varied by changing the ___________or the ___________. • Real time analysis involves measurement of a quantity that varies throughout the reaction. This could be: • a general property of the systempressure, volume, conductance, optical rotation, … • a molecular property: usually by spectroscopy Accurate initiation is necessary for real time experiments. Modified by Jed Macosko

  4. [A] 0 t ln[A] [A] t 0 t 0 [A] t 0 [A]-1 t 0 Simple Rate Laws Zero order: !st order: __________ 2nd order: Modified by Jed Macosko

  5. Second-order Kinetics: [A]  [B] or more usefully, Modified by Jed Macosko

  6. a(t) a(t) t t log r slope = x 0 log a Determination of Order 1 __________Methods Test the data against an appropriate integral rate law.e.g.ln[A]vst or 1/[A] vst. __________Methods Plot the rate directly: Determine initial slopes or tangents from a single curvefor different initial concs. and plot versus __________: Modified by Jed Macosko

  7. __________Method Zero-order: the reactant is used up in two half-lives. 1st-order: the half-life is constant in time: 2nd order: the half-life __________with time In general, a(t) 1st-order 2nd order a(t) t½ 2t½ t½ 2t½ Determination of Order 2 __________Methodtogether with one of the other methods All except one reactant is added in large excess, so that their concentrations do not vary significantly. Then the other reactant has __________order kinetics: Modified by Jed Macosko

  8. [A] [A]-1 t 0 t 0 Data Analysis “__________” methods of data analysis are often useful to explore the order of reactions, or to display the results (e.g. a semi-log plot to demonstrate exponential decay). However, these methods should be __________for quantitative data analysis, since errors (and thus weighting) can be distorted. e.g. Modern data analysis uses computer methods for direct curve fitting, e.g. by __________ minimization. A good fit has 2/(no. of degs. freedom) = __________. Modified by Jed Macosko

  9. ln k 0 1/T Temperature Dependence The __________“law” is an empirical description of the T dependence of the rate constant:  The pre-exponential factor is often interpreted as a collision rate. __________theory predictsT½dependence forA. The exponential factor describes the fraction of collisions with sufficient energy for reaction, as predicted by the __________distribution: n(E) Ea E The __________energy is defined by Curvature in the Arrhenius plot is often attributed to tunneling, but other reasons include complex reactions, and T dependence of the pre-exponential factor, which arises naturally in most theories. Modified by Jed Macosko

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