Unit 3 Flashcards
Chemical kinetics
describe how systems undergoing a chemical reaction change with time
Reaction rate
Rates of reactions can be determined by monitoring the change in concentration
of either reactants or products as a function of time. D[A] vs Dt
๐
๐๐ก๐ = ๐ถโ๐๐๐๐ ๐๐ ๐๐๐๐๐๐๐ก๐๐๐ก๐๐๐ over
๐ถโ๐๐๐๐ ๐๐ ๐ก๐๐๐
Rate Law
an equation that shows how the reaction rate
depends on the concentration of each reactant.
ate = k[A]^m[B]^n
[A], [B] โ conc. in M (or P)
k โ rate constant
m โ reaction order in A
n โ reaction order in B
Reaction orders (thus rate laws) must be determined experimentally
Units of rate: M/s โ[stuff] per secondโ
Units of k: โฆdepend on the rate law expression
Integrated rate laws
are mathematical functions that give concentrations through time
The half-life
is the time it takes to react 50% of the reactants
The Arrhenius equation
describes how the rate constant changes with temperature
Reaction mechanisms
connect microscopic molecular processes to the overall rate
- Kinetics can reveal how a reaction occurs
Experimental determination of rate law
- Method of Initial Rates:
The order of reactions (m, n) and the rate constant (k) are determined experimentally
Experimental strategy -
- change initial [B] while holding initial [A] constant and
measure the new initial reaction rate each time;
- then change initial [A] while holding initial [B] constant and
measure the new initial reaction rate each time;
- solve a system of rate law equations to determine the order
Graph method of rate law
- Graphical Method/ Integrated Rate Law
Experimental strategy:
- monitor the course of a reaction over time
- plot data
- shape of plotted line reveals order of reaction
zeroth order
straight line, decreasing slope.
RECOGNIZING Z EROTH ORDER REACTIONS
Plot the experimental [A] vs. time
If the graph is linear, the reaction is zero-order.
the rate constant k = (โ slope)
zeroth order half life
Half-life: the time required for reactant concentโn
to reach half of its original value
- dependent on the (initial) concentration
- gets shorter over the course of the reaction
(each successive half-life is half as long!)
First order
not linear, take the ln of it to know if the rxn is first order.
RECOGNIZING FIRST ORDER REACTIONS
Plot the experimental ln [A] vs. time. If the graph is linear, the reaction is first-order.
ยฎ the rate constant k = (โ slope)
First order half life
t 1/2 is independent of the initial concentration
each successive half-life is an equal period of time โ ๐ก1/2 is characteristic
Second order
Even more exponential than first order, take the reciprocal to get the slope.
RECOGNIZING A SECOND ORDER REACTION
Plot the experimental 1/[A] vs. time. If the graph is linear, the reaction is second-order.
ยฎ the rate constant k = slope
Second order half life
t 1/2 is dependent on the initial concentration
t 1/2 gets longer over the course of the reaction (each successive t 1/2 doubles in length)
REACTION MECHANISM
the step-by-step sequence of elementary reactions by
which reactants become product
- each step usually involves
only a small amount of
bond breaking/making.
elementary reaction
describes an individual molecular event
overall reaction
describes the reaction stoichiometry
the slowest step is called the rate limiting step
REACTION INTERMEDIATE
a species that is formed in one step of a
reaction mechanism and consumed in a later step
Molecularity
A classification of an elementary reaction based on the number
of molecules (or atoms) on the reactant side of the chemical equation
UNIMOLECULAR REACTION:
an elementary reaction that
involves a single reactant
molecule.
BIMOLECULAR REACTION
an elementary reaction that
results from an energetic collision
of two reactant molecules
TERMOLECULAR REACTION
an elementary reaction that
involves three atoms or molecules.
RARE !
Guess and Check to find mechanism
- Kinetics Experiment
- Propose a mechanism
- Compare experimental and predicted, they must be equal.
Collision theory
Orientation factor (๐) (a.k.a. steric factor)
Reactants must collide with the โcorrectโ orientation
* 0 < ๐ โค 1
* ๐ for simple molecules is 0.001โ1
* ๐ can be < 10-5 for large molecules
* ๐=1 if the reaction is not sensitive to orientation
Collision theory with rate constant
Collision theory: for a bimolecular reaction to take place, reactants A and B must
collide with proper orientation, and an energy greater than the activation energy Ea .
๐ = ๐๐๐
๐ = fraction with correct orientation
Is it collision frequency (Z)?
f = fraction with
โsufficientโ energy
๐ = collision frequency
k depends strongly on temperature
- one (or more!) of Z, p, f must depend on T
Z and f both do
TRANSITION STATE
the unstable group of
atoms which represent the highest
energy species along the pathway from
reactants to products.
ACTIVATION ENERGY
the minimum energy
required for a successful reaction
Arrhenius equation
Ea is the activation energy
R is the ideal gas constant
๐ = ๐๐๐ ๐ = ๐ ,-! //0
A (= Z p) is the frequency factor
(a.k.a. โpre-exponential factorโ)
higher T ยฎ smaller Ea /RT
ยฎ much smaller ex
ยฎ much larger reciprocal
ยฎ much larger k
ยฎ much increased rate
Catalyst
A substance that provides an alternative reaction mechanism that is of
lower energy than the uncatalyzed mechanism ยฎ faster reaction!
- involved in the rate-determining step of the new pathway
ยฎ often appears in the rate law of the catalyzed reaction
- reacts early in the mechanism; regenerated later in the mechanism
ยฎ not consumed in the reaction; does not show up
in the overall reaction
- catalyzed reaction has the same endo/exothermicity as the uncatalyzed
reaction
Enzymes
are catalysts of biological organisms, typically
protein molecules with large molecular weights
substrate enzyme complex
