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72 terms

Ch. 15: Chemical Kinetics

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To be useful, reactions must occur at a
reasonable rate
Chemical kinetics concerns
reaction rates
Reaction mechanism
series of steps by which a reaction takes place
To track concentration vs. time, the _________ _________ _________ is determined by a spectrometer when the two reactants mix in a chamber
volumetric flow rate
Reaction Rate
change in concentration of a reactant or product per unit time
Δ[A]/Δt =
average reaction rate
It is customary to work with __________ reaction rates
positive
The instantaneous reaction rate at any time can be found by computing the ________ of the ________ ________.
slope of the tangent line
Stoichiometry determines
relative rates
The reaction rate changes with
time
To avoid complications, we study the rate of reaction under conditions where the ________ __________ makes only a negligible contribution.
reverse reaction
The reaction rate depends only on the concentrations of the
reactants
Rate=k[A]^n is called a _______ ________ in which k is the __________ constant and n is the ______ of the reactant, both which are determined by __________.
rate law; rate; order; experiment
The rate constant can be affected by changes in
temperature
Differential rate law (or just rate law) expresses how the ______ depends on __________.
rate; concentration
Integrated rate law expresses how _______ depend on _______.
concentrations; time
We can work backwards from the ______ _______ to find the steps by which the reaction occurs.
rate law
If rate/concentration=1, then the reaction is
first order
The overall reaction order is
the sum of the orders of the reactants
For a first order reaction, plotting _____ vs. time will give a straight line.
ln[A]
In a first order reaction, the slope of the line=
-k
The time required for a reactant to reach half of its original concentration
half-life
Does half-life for a first order reaction depend on concentration?
no
t 1/2=
0.693/k
Does half-life for the second order reaction depend on concentration?
yes
t 1/2 (second order) =
1/ k[A]o
For a second order reaction each successive half-life is _______ the preceding one.
double
For a zero-order reaction, the rate is
constant
Zero-order reactions are most often encountered when a substance such as a __________ ___________ or an ____________ is required for the reaction to occur.
metal surface; enzyme
In a pseudo-first order rate law, the concentration of one reactant is _______ compared with the concentrations of all the others.
small
intermediate
a species that is neither a reactant nor a product but that is formed during the reaction sequence
A reaction whose rate can be written from its molecularity
Elementary step
Molecularity
the number of species that must collide to produce the reaction indicated by that step
The rate-determining step is the _______ step.
slowest
What is the steady-state approximation?
The assumption that the concentration of any intermediate remains constant as the reaction proceeds
d[I]/dt=
0
Rate laws obtained by assuming steady-state conditions are often___________.
complicated
To use steady-state approximation, first write the proposed _____________, construct a steady-state expression for each ______________ in which the sum of the rate laws that produce I are then set equal to the sum of the rate laws that consume I, solve for [I1], [I2],....construct an overall _______ ________ for the reaction, and substitute the concentrations of the intermediates to obtain an overall rate law that contains only reactant and/or __________ concentrations.
mechanism; intermediate; rate law; product
Chemical reactions ________ ______ when the temperature is increased.
speed up
The collision model explains that molecules must collide to _______.
react
Only a small fraction of __________ produces a reaction because not all reach the threshold energy.
collisions
Activation energy
The energy that must be overcome to produce a chemical reaction
Activated complex/transition state
The arrangement of atoms found at the top of the potential energy "hill"
ΔE has no effect on the
rate of the reaction
The rate depends on the size of the
activation energy, Ea
Only collisions with energy greater than the ________ ___________ are able to react.
activation energy
The fraction of effective collisions increases ____________ with temperature.
exponentially
Number of collisions with at least activation energy=
(total number of collisions)e^(-Ea/RT)
In order for a collision to lead to a reaction, the collision must involve enough _________ to produce the reaction and the relative ___________ of the reactants must allow formation of any new bonds necessary to produce products.
energy; orientations
zpe^(-Ea/RT) =
k
steric factor
the fraction of collisions with effective orientations (p)
In the Arrhenius equation, A is the
pre-exponential factor or frequency factor
most rate constants obey the ___________ ___________ to a good approximation, indicating that the collision model is physically reasonable.
Arrhenius equation
You can speed up a reaction by raising its
temperature
What do enzymes do?
They increase the rates of reactions even though the body temperature remains constant.
A substance that speeds up a reaction without being consumed itself is called a
catalyst
Enzymes are __________ catalysts.
biological
A catalyst allows the reaction to occur along a pathway with a lower _________ ___________, making a much larger fraction of collisions effective at a given temperature.
activation energy
A homogenous catalyst is one that is present in the ____ ________ as the reacting molecules.
same phase
A heterogeneous catalyst is one that exists in a _______ ________ than that of the reacting molecule, usually as a solid.
different phase
__________ refers to the collection of one substance on the surface of another substance.
Adsorption
___________ refers to the penetration of one substance into another.
Absorption
Heterogeneous catalysis involves four steps:
1) Adsorption and activation of the reactants 2) Migration of the adsorbed reactants on the surface 3) Reaction among the adsorbed substances 4) Escape, or desorption, of the products.
The most effective catalytic materials are transition _______ ________ and ________ metals such as palladium and platinum.
metal oxides; noble
_________ quickly destroys much of a converter's catalytic efficiency.
Lead
NO forms ozone in the ________ atmosphere, which is harmful, and destroys ozone in the ________ atmosphere.
lower; upper
A _________ is a species with an unpaired electron.
radical
As a rule of thumb, when there is an increase of 10 degrees Celsius, the reaction rate
doubles
Photochemistry
the interaction of radiation with molecules which produces a chemical change
If A+B--> products, and [A]>>B, then [A] remains about _______. [B] will then exhibit "_______-_____ ______" kinetics and ln[B] vs t will be linear.
constant; pseudo-first order
OH+CH4 --> CH3 + H2O, -d[OH]/dt=
k[OH][CH4], must make [CH4]>>[OH] in order to make [OH] "pseudo-first order
k'=
k[A