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53 Cards in this Set
- Front
- Back
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What is chemical kinetics the study of?
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a. the rates of chemical reactions
b. mechanisms of chem reaxns |
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what are chemical reactions?
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series of elementary steps that convert reactants to products.
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Factors that affect reaction rates?
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1. concentration of reactants plays important role, increase concentrations, increase reactions.
2. Physical state of reatants 3. temperature (increase temperature increase reaction) 4. catalyst(increases reaction rate without being consumed. |
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what are the physical states of reactants?
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1. homogeneous-all reactants are in same phase
2. nature of homogeneus phase (gas or liquid) 3. heterogeneus- reactants in different phases a. gas-solid b. liquid solid solid particle is important because it tells the surface area. |
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Rate of reaction formula?
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if for reactants?
-change in concentration/ change in time for products positive! |
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if the stoichiometry is different?
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use fractions to cancel the coefficients out!
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rate law?
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relation between reactant concentrations (what's dissappearing) and reaction rate
must be determined experimentally can not get rate law directly from balanced overall reaction will be used to determine the reaction mechanism (how fast things dissappear and mechanism) |
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Formula for rate law?
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Rate=kD^mB^n
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m and n?
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generally small positive whole numbers
can be zero, fractions, negative not necessarily equal to a and b |
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k?
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rate constant, relates the rate to concentration
depends on reaction, temperature, presence of catalyst |
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order of reaction?
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if m or n=1 first order
2= second order 0= zero order |
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units for rate constant?
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depend on rate law!
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rate units?
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MOLARITY/seconds
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units for [A] and [B]?
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mol/liters
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how do you determine rate law?
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using initial rate of a reaction- rate when reactants are just mixed.
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Initial rates approach?
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*Run series of expirements with different initial reactant concentrations
* Measure rates when reactants are just mixed *Use initial rates to determine m and n in rate |
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First order reaction?
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plot of ln(a)t vs. t will be linear for first order!
m=slope=-k b= intercept=ln[A]0 all radioactive nuclear decays are first order. |
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Solution to rate law for first order reaction?
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ln[a]t=ln[a]0-kt
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half life ?
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time required for 1/2 of the reactant to be consumed!
t1/2= ln2/k |
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first order for gases?
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(PA)t/(PA)0=e^-kt
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second order reactions?
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rate or reaction= k[a]^2
plot of 1/at is linear for second order, slope of graph is k. half life= 1/k[a]0 each successive half life is twice as longa s the one before it! different from first order. |
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zero order reactions?
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rate of reaction=k
plot of at is linear for zero order with slope=-k half life- [A]0/2k each successive half life is half as long as the one be4 it! proceeds at constant rate until all of the reactant is consumed. |
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As we increase temperature?
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the reaction rate increases because the rate constant increases.
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Arrhenius Equation?
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k=ae^-Ea/RT
t=temperature r=gas constant ea=activation energy, minimum energy molecules must have to react a=frequency factor lnk=lnA-Ea/RT graph of lnk vs. i/t is a straight line. ea and a are independent of t! |
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frequency factor?
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collision frequency of reactants
probability of finding reactant sin favorable orientation during collision |
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at temperature 1 and 2? the Arrhenius equation.
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ln(k2/k1)=-Ea/R(1/t2-1/t1)
lnk=lnA-Ea/Rt a and k have the same units |
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Why are concentrations multiplied in the rate law?
Rate law= k[A]^m [B]^n |
mlecules must collide to react
number of collisions(collision frequency) is determined by the product of the concentrations. |
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What is the role of temperature in the rate law?
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most collisions do not give a reaction because colliding molecules don't have enough energy.
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What is the energy of colliding molecules?
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kinetic energy and activation energy!
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kinetic energy of collidint molecules?
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kinetic energy of colliding molecules is converted to potential enerygy that stretches and bends bonds.
if enough ke is converted -bonds of reactants can break and -products can form. |
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activation energy?
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minimum energy molecules must have to react!
energy barrier that must be surmounted before products can form! |
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What is the relation between collision energy and t?
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total fraction of molecules with CE is greater than Ea is area under curves.
at given t, a smaller activation energy has a larger fraction! -for a given reacton, the activation energy (independent of T) is constant -for low T, only a small fraction of melecules have collision energy is greater than activation energy, slow reaction rate! as t increases, fraction of molecules both CE is greater than Ea increases, no reaction rate increases Fraction of molecules with CE is greater than Ea is f=e^-Ea/Rt Same temperature dependence as ARRhenius equation k=Ae^-Ea/Rt |
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What is a reaction energy diagram?
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shows progress of reactions from reactants and products!
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Ea (fwd)
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activation energy for the forward reaction!
A2+B2=2AB exo thermic because Ea(rev)>Ea(fwd) |
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Ea (rev)
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activation energy for the reverse reaction
2AB=A2+B2 endo thermic because Ea(rev)>Ea(fwd) k(rev)<k(fwd) the larger the volume of Ea, the smaller the values of k. |
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For the reverse reaction DELTA H?
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Ea(rev)-Ea(fwd)>0! positive and endothermic!
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for the forward reaction
DELTA H? |
Ea(fwd)-Ea(rev)<0 negative and exothermic
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In Arrhenius equation what is A?
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frequency factor=pruduct= pZ
Z=collision frequency p=orientation probability factor. (are molecules lined up properly to react?) p=closer to 1, reaction is likely ! |
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What is the transition state theory?
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when molecules react:
-short lived transition state(activated complex) is formed -reac tants- combined to give activated complex |
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What is the activated complex?
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intermediate between reactants and products!
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What does every reaction do?
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goes through it's own transition state and activated complex!
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Reaction mechanism?
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series of elementary steps (reactions)by which an overall reaction occurs.
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What does a valid mechanism must have?
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a. be physically reasonable
b. account for the experimental rate law! c. add to give the balanced overall reaction |
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What are elementary steps?
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produce new molecules or significantly change molecular energy and geometry.
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What are some mechanism rules?
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1. for elementary steps rate laws are determined from reactant coefficients.
2. molecularity of elementary steps: a. unimolecular b. bimolecular c. termolecular 3. some elementary steps are reversible and reach equilibrium (forward=reverse) 4. intermediates produced in one step and consumed in another. can not be in experimental rate naw! 5. rate determining step= slowest elementary step, determines the overall rate! |
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unimolecular?
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single molecule dissociates.
much more probable than a termolecular step! |
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bimolecular?
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2 molecules react
steps 1 and 2 are bimolecular much more probably than a termolecular step! |
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termolecular?
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3 molecules react!
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mechanisms with fast, reversible step?
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add intermediate but the intermediate can not be in final rate law!
use the fast reaction to get rid of the intermediate and to plug it back into the rate! |
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What is a catalyst?
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1. accelarates rate of chemical reaction
2. recovered unchanged after teh reaction has occured 3. changes mechanism of reaction 4. lowers activation energy |
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what is a homogeneous catalysis?
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catalyst in same phase (liquid or gas) as reactants
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what is a heterogeneous catalysis?
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catalyst is in different phase from reactants.
ex. reactants are gasses, catalyst is solid. |
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What is the catalytic converter in automobile engine?
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al2o3 with catalyst embedded
catalysts: CuO, Cr2O3, help reduce pollutants main process: to converto hydrocarbons into auto engine. in the complete combustion also gives some co catalytc converter to change co to co2 |
