Ch. 13: Chemical Kinetics Flashcards
the ____ of a chemical reaction is a measure of how fast the reaction occurs
rate
fast rate=….?
a large fraction of the molecules reacting to form products in a given period of time
slow rate=….?
a small fraction of the molecules reacting to form products in a given period of time
How are measurements of rate usually expressed?
change in some quantity per unit time
the reaction rate is defined as the _____ of the change in concentration of a reactant divided by the change in time
negative
why is the reaction rate negative in terms of the reactants?
because reactant concentrations decrease as the reaction proceeds
when rate is defined is it defined in respect to the products or the reactants? why?
products because then the reaction rate is naturally a positive number
the reactant concentration ____ with time because…?
decreases
reactants are consumed in a reaction
the product concentration _____ with time because…?
increases
products are formed in a reaction
what is the equation for average reaction rate when looking at the reactants?
- (stoichiometric coefficient) [(change in concentration) / (change in time)]
as the reactants transform to products, their concentrations _____ and the reaction rate _____
decrease
decreases
for most reactions, the rate depends on the ____ of the reactants
concentration
how can you find the instantaneous rate of the reaction?
the slope of a line tangent to to the curve at that point
what is the most common way to study reaction kinetics?
spectroscopy
what common three ways are reaction rates measured in a vessel?
- spectroscopy
- pressure measurement
- polarimetry
name three way in which aliquots (samples) that are periodically withdrawn from the reaction vessel and analyzed can be used to determine the progress of the reaction.
- gas chromatography
- mass spectrometry
- titration and other wet chemical techniques
how can the removal of aliquots be used to determine the reaction rate?
by taking these samples at regular time intervals and determining the relative amounts of reactants and product as a function of time
define rate law
a relationship between the rate of the reaction and the concentration of the reactants
what is the rate law?
rate= k[A]^n
what is the k in rate law?
k is the constant of proportionality called the rate constant
what is the n in rate law?
n is the reaction order. it is usually an integer and it reflects how the rate depends on the concentration
what is the order when the rate is independent of the concentration?
zero order / n=0
what is the order if the rate is directly proportional to the concentration of A?
first order / n=1
what is the order if the rate is proportional to the square of the concentration of A?
second order / n=2
what conditions are necessary for the rate of a reaction to be constant (aka for n to equal zero)?
the amount of reactant actually available for reaction must be unaffected by the changes in the overall quantity of reactant
How is reaction order determined?
experimentally
what is a common way to determine reaction order?
method of initial rates
how does the method of initial rates work?
the initial rate is measured by running the reaction several times with different initial reactant concentrations to determine the effect of the concentration on the rate.
do the zero, first and second order all have the same units?
no
what is the rate law for more than one reactant?
Rate= k [A]^m [B]^n
what is the overall order of a rate law for two reactants?
the sum of the order for A and the order B
define overall order
the sum of the orders of all the reactants in a chemical reaction (all the n’s)
define integrated rate law
a relationship between the concentrations of the chemical reactants in a chemical reaction and time
what is the integrated order if the rate is directly proportional to the concentration?
first order
what type of graph do the zero, first and second order integrated rate law form?
a linear graph
define half life (t 1/2)
the time required for the concentration of a reactant to fall to one-half its initial value
define lifetime (𝓣)
the time for a reactant to decrease to 1/e of the original concentration
what does the lifetime of an chemical represent?
the average life expectancy of the chemical entity
what is the equation for decay time for first order reactions?
t= ln(n) / k
what are t and n for half-lifes?
t= t 1/2
n=2
what are t and n for lifetimes?
t=𝓣
n=e
what is the equation for lifetime in a first order reaction?
𝓣=ln(e)/k
or
𝓣=1/k
for first order reactions are lifetimes and half-lifes are independent of the initial concentration of the chemical?
yes
for a second order reaction is the half-life dependent of the concentration?
yes
the time for the concentration to reach half is twice as long as the first time due to the depleted concentration
how does the decreasing concentration of a second order reaction affect the half -life of the chemical?
as the concentration decreases the time it takes for the concentration to reach half the initial increases
how does the decreasing concentration of a zero order reaction affect the half -life of the chemical?
as the concentration decreases the time it takes for the concentration to reach half the initial decreases
what two things does the rate law relate?
the rate of the reaction to the concentration of the reactants
what two things does the integrated rate law relate?
the concentration of the reactants to time
half-life, lifetime. and decay time of a first order reaction are ______ of/on the initial concentration.
independent
the half-lives and decay times of zero-order and second-order reactions _____ of/on the initial concentration.
depend
define activation energy (Ea)
an energy barrier in a chemical reaction that must be overcome for the reactants to be converted into products.
define frequency factor (A)
the number of times that reactants approach the activation energy per unit time
define activated complex
a high-energy intermediate state between reactant and product
define transition state
a high-energy intermediate state between reactant and product
the higher the activation energy, the _____ the reaction rate (at a given temperature)
slower
define exponential factor
a number between zero and one that represents the fraction of molecules that have enough energy to make it over the energy barrier on a given approach
what does the exponential factor depend on?
temperature (T) and the activation energy (Ea)
what is the exponential factor?
e^-Ea/RT
a low activation energy and a high temperature make the negative exponent _____, so that the exponential factor approaches ____
small
one
if the activation energy is zero, then the exponent is ____, and the exponential factor is _____
zero
exactly one
a large activation energy and a low temperature make the negative exponent _____, so that the exponential factor is ____
large
very small
if the temperature approaches 0 K, the exponent approaches an indefinitely _____ number, and the exponential factor approaches ____
large
zero
how does a small change in temperature effect the number of molecules having enough energy to surmount the activation barrier?
it greatly effects the number of molecules that have enough energy to surmount the activation barrier
the exponential factor increases with ____ temperature
increasing
a large activation energy results in a _____ exponential factor
small
what is the arrhenius equation in the form y=mx+b?
ln k= -(Ea/R)(1/T)+ln A
define arrhenius equation
an equation which relates the rate constant of a reaction to the temperature, the activation energy, and the frequency factor
define collision model
a model of chemical reactions in which a reaction occurs after a sufficiently energetic collision between two reactant molecules
define orientation factor
in the collision model, the fraction of sufficiently energetic collisions in which the reactants are in the correct orientation to react
define collision frequency
the number of collisions that occur per unit time
what letter represents the orientation factor?
p
what letter represents the collision frequency
z
what is the arrhenius equation in terms of collision model?
k=pze ^(-Ea/RT)
if p=0.16 how many successful collisions are there out of 100?
16
why might to molecules colliding with significant energy not react?
the must collide in such a way that allows the necessary bond to occur
what is the approximate orientation factor of individual atoms and why?
approx. 1, because atoms are spherically symmetric and thus any orientation can lead to the formation of products
what does a orientation factor of more than one mean?
that collisions do not have to occur for chemicals to react
more reactions than collisions
what is the process dubbed the harpoon mechanism
the transfer of an electron without a collison
the atom basically harpoons a passing molecule with an electron and reels it in through the coulumbic
do most reactions occur in a single step?
no
chemical equations are used to represent a chemical reaction and it usually represents the _____ equation, not a series of individual steps by which the reaction occurs
overall
what does the overall chemical equation show?
the substances present at the beginning of the reaction and the substances formed by the reaction
define reaction mechanism
a series of individual chemical steps by which an overall chemical reaction occurs
define elementary step
an individual step in a reaction mechanism
can elementary steps be broken down into simpler steps?
no
define reaction intermediates
species that are formed in one step of a reaction mechanism and consumed in another
for a valid reaction mechanism the individual steps in the mechanism ____ to the overall reaction
add
elementary steps are characterized by their _____
molecularity
define molecularity
the number of reactant particles involved in and elementary step
what are the most common molecularities?
unimolecular and bimolecular
what does unimolecular step look like?
A —> products
what does a bimolecular step look like?
A + A —> products
or
A + B —> products
define unimolecular
describes a reaction that involves only one particle that goes on to form products
define bimolecular
an elementary step in a reaction that involves two particles, either the same species or different, that collide and go on to form products
define termolecular
an elementary step of a reaction in which three particles collide and go on to form products
how common are termolecular steps? why so?
very rare because the probability of three particles simultaneously colliding is small
can rate law be deduced from the overall reaction?
no
can rate law be determined from an elementary step?
yes
since we know that an elementary step occurs through the collision of the reactant particles, the rate law is proportional to…?
the concentration of those particles
define rate-determining step
the step in a reaction mechanism that occurs much more slowly than any of the other steps
the rate determing step determines what?
the rate law for the overall reaction
for a purposed reaction mechanism to be valid, what two conditions must be met?
- the elementary steps in the mechanism must sum to the overall reaction
- the rate law predicted by the mechanism must be consistent with the experimentally observed rate law
is a valid mechanism a proven mechanism?
no we can only say that the given mechanism is consistent with the kinetic observations and therefore possible
define steady rate approximation
the assumption that the rate of formation of an intermediate is equal to its rate of consumption
what is the first step in using the steady rate approximation mechanism?
determining a preliminary expression for the rate of the reaction
how does the steady-state approximation allow for the intermediate concentration of a chemical to be used in determined the rate of the reaction?
it assumes that after a short period of time the intermediate will have reached a stead state in which the rate of production of the intermediate is the same as the rate of consumption of the intermediate
how can you use steady-state approximation if one of the chemicals is an intermediate and therefore its concentration is unknown to figure out the reaction rate?
by determining the equations for the rate for production and consumption of the intermediate and then using substitution and basic algebra to figure out the reaction rate
what is RDS?
rate determining step
