W17-Chem-260-L37_mechanism_1 (1).pdf - 3/31/17 Chemistry...

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3/31/171Chemistry 260Chemical PrinciplesLecture 37Fri Apr 7 2017(1) Elementary reactions occur by a direct collision of atoms, ions ormolecules and do not involve intermediates.(2) The rate of an elementary reaction is directly proportional to theproduct of concentrations of the reacting species, each raised to apower equal to its coefficient in the balanced equation of theelementary reaction.(3) Complex, or non-elementary, chemica***252325253l reactions proceed through aseries of elementary reactions.(4) There are 3 main classes of elementary reactions:BimolecuUnimolecull::arardN ONONON OkN OdtNOO+=+[][][][][ ] []223222Termolecular:(veryrare.)2dNOONOk NOOdtdIArIArIk IArdt+=++=Elementary reac0ons
3/31/172Reac0on mechanismsDefini0on:A detailed sequence of elementary reac0ons, andtheir rates, that yields the overall reac0on when combined.22222332()()()bimolecular()()()bimolecular()()()()Other mechanisms can be proposedExample A:. The right mechanism is usually determined byexperim(ent,))(NOgNOgNO gCO gNOgCONOgNOgCgNOgO gNO gCOg++++++3or based on calculations and computer simulations.The intermediate speciedisappears quickly after its creation. Its detection (invery small amounts) provides supporting evidence for the mecishanNOm.323334234( )unimolecularbimulecularbimolecularandare reaction intermediates. Twoatoms are created in the first step. OnExampe isconsumle B:e2ClgCHClHClCClClCHClHClCClClClCClCClClClCClCl+++++d in the secondstep, and the other is consumed in the third step.Reac0on kine0csMul0-step reac0onmechanismNumericalAnaly0cal(usually withapproxima0ons)Mechanism 7Mechanism 3Mechanism 2Mechanism 5Mechanism 6Mechanism 1Mechanism 4Rate law
3/31/17Reac0ons with a rate determining step[][][]1222222222222Reaction:Rate law:Mech22-(slaow)(fastnism):obskkNOFNO FdFkNOFdtNOFNO FFNOFNO F+=+⎯⎯→++⎯⎯→The rate determining stepWhy the mechanism is consistent with the rate law?22:
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