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chapter 15

Chemical kinetics study of the rates of chemical reactions
chemical kinetics studies how to determine a reaction rate experimentally and how factors such as temp. and concentration of reactants influence reaction rates. also studies the pathway taken by atoms/molecules as the reaction proceeds
Rate of reaction change in concentration/change in time
rate of reaction-given this reaction: N2O5->2NO2+1/2O2 (rate of rxn can be written several ways based on decrease of N2O5 w time/the increase in NO2 w time or the increase in 02 w time -<|[N2O5]/<|t -- for reactant bc concentration will be decreasing +1/2(<|[NO2]/<|t] +2<|[O2]/<|t -(t) for products bc concentration will be increasing
the rate of decomposition of N2O5 is to 1/2 the rate of formation of NO2 and twice the rate of formation. rate expressions
Factors that affect the rate of a rxn: reactant concentration temp presence of catalyst surface area (if a solid is reacting)
reactant concentration As RC^ the rate of the rxn ^ in most situations
Temp when temp ^ reactions usually speed up
presence of a catalyst a catalyst is a substance that increases the rate of rxn w being consumed in the overall (net) rxn. a catalyst is usually written over the arrow in an equation
surface area bc reactions occur at the surface of a solid, rxn rates ^ w an ^ in surface area
effects of concentration on reaction rates by evaluating how the rxn rate is affected when the concentrations of the reactants are varied, the effect of concentration can be determined (temp must be kept constant)
ex. A+B->C rate of disappearance of a: .30m/l*min if we double the [] of A and calculate the rate of disappearance & found it be .60m/l*min, we could say that the rate of rxn is directly proportional to the [] of reactant A
ex. A+B->C rate of disappearance of a: .30m/l*min if we halved the [] of A and the rate changed to .15m/l*min is the rxn rate still directly proportional to [] of reactant A?
some reaction rates can be dependent of [] or the rxn rate may be dependent on the reactant [] raised to some power
if rxn contains more than 1 reactants the rxn rate may depend on the concentrations of each of them or only one of them *note:rxn rate can also depend on catalyst concen or even product []
relationship bt reactant [] and rxn rate expressed by an equation called a rate equation or rate law
in rate law there is proportionally constant bt rate & concentration called rate constant (K) the rate constant is specific w each temp and varies with temp
reaction orders: rate=K[A][B]2 K[A]/rate-directly proportional rate/[B]2-exponentially proportional 2x rate=k[2A][B]2 4x rate=k[4A][2b]2 (powers to which concentrations are raised are called reaction orders)
reaction orders: rxn rates must be compared to compare rates/make ratio (rate of doubled conc/rate of original=rate change factor) now use following equation to solve for rxn order for the specific reactant X(exponent)=log of rate change factor/log of conc change facto
reaction orders: a rxn is the exponent to which [] of substance is raised in rate law
concentration time equations we sue concentrations time equations to determine how long it would take for a pre determined amount of reactant remaining after a certain amount of time
for first reorder reactions c.t.equation is ln[R]t/[R]o=-kt or log[R]t/[R]o=-kt/2.303
for second order reactions c.t.equation= 1/[R]t - 1/[R]o=Rt
Half life of a rxn as a reaction procceeds, the concentration of a reactant decreases bc it is being consumed
half life of a rxn the half life-the slower the rxn/h.l. is used mostly w 1st order reactions.
half life (t1/2) can be calculated using this equation: t1/2=.693 for 1st order reactions. half life is independent of concentrations.
collision theory for this rxn: NO+Cl->NOCl+Cl -a 10 degrees rise in temp causes the rate of rxn to triple. this shows tha rate of rxn is very dependent on temp.
collision theory states that for rxn to occur, reactant molecules or particles must collide with an energy greater than some minimum value & must collide w proper orientation
activation energy (Ea) the minimum energy of collision required for 2 molecules or particles to react is called
in collision theory, the rate constant (k) is considered a product of 3 factors k=zfp
k=zfp z=collision frequency f=fraction of collisions w energy greater than activation energy p=fraction of collisions that are properly oriented
concentration affects ___ more Z
temperature affects ____ more f
catalysts lower ____ so rxns can go faster Ea
we cannot control _____ p
transition state theory explains a rxn in terms of an activated complex. a.c. is a transition state. unstable grouping of atoms
Reaction with a large ____ run _____ Ea/slow
Reactions with a small ____ run ____ Ea/fast
the effects of catalysts on reaction rate catalysts used to speed up a rxn/not consumed in rxn function-provide diff pathway to Ea for rxn
catalysts allow rxn to proceed at higher (take less time) and at a lower (lower temp, lower energy costs)
enzymes catalysts-biological cell contains thousands of diff enzymes
reaction mechanisms elementary reactions give us reaction mechanism
sum of elementary rxns give us overall net reactions
molecularity -how elementary rxns are classified -number of molecules or particles on the reactant side of elementary rxn
rate law for an elementary can be predicted by looking at rxn rate law is prop to product of conc of each reactant
remember this predicted rate law must be compared to experimental rate law/may or may not be true
elementary reactions/reaction order 1st order-unimolecular 2nd order-bimolecular
Created by: dixxfranks



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