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# 5 Kinetics_Book Notes.docx

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University of Minnesota Twin Cities

Chemistry

CHEM 1061

Hyunjoo Im

Fall

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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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