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Chem107:
chapter 15
| Question | Answer |
|---|---|
| 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