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12/4/161CHEM 230 F16 Topic 12 Kinetics1) Rate law•Determining from experimental data2) Collision Theory•k=Ae^(Ea/RT)3) Integrated rate laws•half-life•nuclear chemistry first order activity4) MechanismA Tale of 3 ReactionsH2(g)+ I2(g)→ 2 HI(g)H2(g)+ Br2(g)→ 2 HBr(g)H2(g)+ Cl2(g)→ 2 HCl(g)Careful experiments,carried out over manyyears, are consistent withthe simplest imaginablemechanism: a collisionbetween the tworeactant moleculesresults in arearrangement of thebonds:One might be tempted tosuppose that this wouldproceed in a similar way, . butexperiments reveal that themechanism of this reaction isfar more complex. Thereaction takes place in asuccession of steps, some ofwhich involve atomic H andBr.The mechanism of this reactionis different again. Although thefirst two reactions reachequilibrium in minutes to anhour or so at temperatures of300 to 600 K, a mixture ofhydrogen and chlorine will notreact at all in the dark, but ifyou shine a light on themixture, it goes off with a bangas the instantaneous reactionreleases heat and expands thegas explosively.Lower, S. Chem1 Virtual TextbookAccessed March 2012.Thermodynamics ≠ KineticsConsider a generic reaction:a A + b B⇌c C + d DThermodynamics: The reaction will progress until freeenergy is minimized (it reaches equilibrium.) Thecomposition at equilibrium is predictable from data intables using the free energy of the reaction, itsstoichiometry, and the temperature.(ΔHº, ΔSº, ΔGº, ΔEo)Thermodynamics cannot relate how fast!K=[C]c[D]d[A]a[B]bKinetics…Rate of ReactionsMeasure ratesas changes in concentrations orpressures over timeØStart with all reactants; no productsØFocus (mostly on) the forward reactionFind rate law(relationship between the rate and theconcentrations of reactants) rate = k*[A]xBreak into simple steps (mechanism) to understandrate of reactionCannot be predictedfrom stoichiometry, ΔHº or ΔGº ;experimental data must be collected
12/4/162Averill, B. Eldredge, P. General Chemistry Principles, Patterns, and Applications v 1.0, Flat World Education, Inc.2014. () Accessed Nov. 21,2014.AàBAverage RateΔ[B]Δt=[B]final−[B]initialtfinal−tiniitalReaction slows down;rate variesDtDtDB1DB2DB1>DB2Instantaneous RateTheinstantaneous rateis the rate at an exact point in time (thetangent line drawn to the graph at that point)Instantaneous rate depends on the time choseninstantaneous rate∝Δ[B]ΔtwhereΔt appoaches 0AàBInitial RateInitial rate: the rate of the reaction under the starting conditions,before the decrease in [reactants] and increase in [products]significantly affects the rateFormally, initial rate is the instantaneous rate whent= 0In practice, initial rate is average rate fromt= 0 tot= smallFind Rate Law (show relationship between rate and[reactants])rate=k[A]x[B]yFind x and y (orders) experimentally; theare not necessarily the stoichiometriccoefficientsCollect data