5 Kinetics_Book Notes.docx

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University of Minnesota Twin Cities
CHEM 1061
Hyunjoo Im

Book Notes 13.1) Chemical Kinetics – A preview  Chemical kinetics: study of rates of chem reactions (rxns), factors that affect these rates, and sequences of molecular steps (rxn mechanisms)  Catalyst: substance that speeds up rxn but not changed/used up in rxn o Not a spectator, like spectator ions  Goal of chem kinetics: want to measure & predict chem rxn rates  Variables affecting rxn rates: o Concentrations of reactants: concentration ↑ rxn rates generally ↑ o Temperature: Temperature ↑ rxn rate ↑ o Surface area (for rxns that occur on surface rather in solution (sol’n)): SA ↑ rxn rate ↑ o Catalysis: Catalysts speed up rxn, inhibitors slow them down 13.2) The Meaning of Reaction Rates  Rate of a rxn measured in concentration (M = mol/L) per unit of time; change in molarity/change in time; always positive  General Rate of reaction: has same value regardless of which reactant/product you study (since each reactant & product has their own related rates, not the same) o Found by dividing rate of disappearance of reactant or formation of product by stoichiometric coefficient for that reactant/product in balanced equation. o For rxn aA + bB → cC + dD, general rate of rxn = -1/a (Δ[A]/ Δt) = -1/b (Δ[B]/ Δt) = 1/c (Δ[C]/ Δt) = 1/d (Δ[D]/ Δt)  Average Rate of Reaction: previous methods up there show average rate, b/c rxn starts fast and then slows down. Always varying. 13.3) Measuring Rxn Rates  Concentration of reactant ↑ faster the reaction goes  Instantaneous rates of rxn: check amt of stuff left in very short time intervals and calculate rate Δt approaches 0…derivatives!  Initial rate of rxn: instantaneous at beginning of rxn  Basically reactant rates are negative and product rates are positive 13.4) The Rate Law of a Chem Rxn  Rate law: relates rxn rate to concentration (conc) of reactants m n  Rate law for disappearance of reactants in generic equation (↑): Rate = k[A] [B] o [A] and [B] = molarities of reactants at particular time o Exponents = small positive integers (0,1,2) but may be negative or occasionally nonintegeral; *determined by experiment, NOT from stoichiometric coefficients, may be same sometimes, but that’s just a coincidence. o #s for exponents establish order of a reaction. 1 = first order, 2 = second order, overall order of a rxn = sum of exponents = m + n + …. o k (proportionality constant) = rate constant; depends on 1) particular rxn 2) temperature, and 3) presence of a catalyst; units of k depend on values of exponents m, n … -1 -1  First order rxn: unit for k = s rxn rate unit = Ms  Hypothetical zero order rxn: A → B, rate law is Rate = k[A] = K -1 o rate constant has same units as rate of rxn, Ms  For second order overall rxn, units of k = M s so that rxn rate always has units Ms -1 (1 – overall order) -1  unit of k for rxn of any overall order = M s  Difference btwn rate of rxn and rate constant of rxn o Rate gives change in conc w/ time whereas rc = proportionality constant that relates rate of rxn to conc of reactants o Rate remains constant throughout rxn and has same value regardless of initial conc of reactants. Except for zero order rxns, rate of rxn varies as concs vary. o Rate of rxn and rc of rxn have same numerical values and units only in zero order rxns. When not zero order, numerically equal only when conc of all reactants = 1M, but units remain different  Method of initial rates: determine initial rate for diff initial conc of reactants to establish exponents to determine rate law o Involves many experiments where initial conc of some reactants held constant & others varied in convenient multiples o Initial rate or rxn, instantaneous rate, and average rate all = equal @ very start of rxn & know conc of reactants w/ certainty @ beg. of rxn; if tendancy for reverse rxn to occur, that effect = very minimal @ very start of rxn  Effects of initial rate caused by doubling conc of 1 reactant while other conc = constant o Zero order in the reactant initial rate = unchanged o First……doubles o Second……quadruples o Third…….eightfold 13.5) First-Order Reactions  First-order rxns: single reactant yields products: A →
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