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 RatesA +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 LawsA + 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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