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TAMU CHEM 102 - class11-2

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Class 9.1 Introduction toChemical KineticsCHEM 102HT. HughbanksChemical Kinetics Reaction rates– “How fast?” Reaction mechanisms– “How?” Answers to these questions depend onthe path taken from reactants toproducts.Reaction RatesA +B  C + D Follow progress by measuring any oneconcentration:Rates of change related by coefficients frombalanced equation. d[A]dt, d[B]dt, d[C]dt, d[D]dt2 NO2  2 NO + O2concentrationtimeNOO2NO2 rate =12d[NO2]dt=12d[NO]dt=d[O2]dtFactors That Influence Rates Identity & form of reactants, products– H2 + I2 vs. H2 + Br2– solution vs. gas phase, etc. Concentrations of various species– usually reactants– sometimes products, other species Temperature– usually, faster at higher T– strong dependence CatalystsConcentration Effects: Rate LawsA + B ProductsEmpirically, usually find thatRate = k [A]n[B]mn = “order of reaction with respect to A”m = “order of reaction with respect to B”n + m= “overall order of reaction”k = rate constant = k (T)Example: rate of a redox reactionReaction Orders Order of a reaction can NOT be found bylooking at a balanced equation!A + B ProductsRate = k [A]n[B]m In general: & n,  & m are not necessarily equal Reaction order can only be discovered inexperimentsExamples2 N2O5 4NO2 + O2rate = k [N2O5] BUT2 NO2 2NO + O2rate = k [NO2]2 CAN’T predict these from equations!More ExamplesH2 + I2  2HIrate = k [H2][I2] rate = k [H2][Br2]1/21 +  k [HBr][Br2]-1 BUTH2 + Br2  2HBrFinding rate laws, rate constants “Method of Initial Rates”– combine known amounts of reactants– determine rate by measuring change insome concentration over a “short” time– repeat with different initial concentrations– find experimental rate lawProblemA + 2B  productsExpt. [A]0[B]0 Initial Rate 1 0.10 0.10 0.0032 2 0.10 0.20 0.0032 3 0.20 0.30 0.0128 find rate law & rate constant, k (concentrations in M, rates in M/min)Rates & MechanismsExperiments  Rate LawRate Law  Mechanism (?) MECHANISM: “The detailed molecularprocesses by which a chemical reactionproceeds.” A series of “elementary steps”which combine to give an observed netreaction.Rate laws & mechanisms Start with overall reaction Guess some mechanism(s) Derive corresponding rate laws Compare with experiments Repeat as needed We need to relate rates of individualsteps to the overall, observable rate laws.A reaction profilereactantsproductsintermediateA + B  C  D + A + BCD + EEnergy“Reaction Coordinate”1st step is rate-determiningElementary Steps ELEMENTARY STEP: A chemicalequation or reaction that describes aprocess as it occurs at the molecular level.A single reaction event which occurs inone simple atomic or molecular collision. Most reactions do not occur in a singleelementary step.Reactions vs. Elementary Steps Normal chemical eqs. tell us the overallstoichiometry of a reaction.2 C8H18 + 25 O2  16 CO2 + 18 H2O Eq. for an elementary step looks just like a“normal” eq., but actually describes asimple molecular event. NO2 + NO2  N2O4Reactions vs. Elementary Steps Not always easy to tell an elementarystep from a (slightly) more complicatedreaction2 NO2  N2O42 NO2  2 NO + O2 The first one is an elementary step, thesecond is not. You can’t really tell thisfrom the equations.Types of Elementary Reactions Unimolecular decomposition: one moleculefalls apart: A  Product(s) Bimolecular reaction: two reactant moleculescollide: A + B  Product(s) Termolecular reaction: three reactantmolecules: A + B + C  Product(s)(such steps rare in gas-phase and soln. rxns.)NO examples of more complex elementaryreactions are known.Rates of Elementary StepsFor an elementary step, the rate law can bewritten from the equation: A  Product(s) rate = k [A] A + B  Product(s) rate = k [A][B] 2A  Product(s) rate = k [A]2 A + B + C  Product(s) rate = k [A][B][C](not for gas phase reactions)Can ONLY write the rate expression for anelementary step!Rate Determining Steps If a single step in a reaction mechanism ismuch slower than the other steps, then the rateof the slow step is crucial in determiningoverall rate. The rate determining step (RDS) can bethought of as a “bottleneck” in the formation ofproducts. Steps that follow the RDS havenegligible effect on the overall rate of


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