Chemical Kinetics
Study of Reaction Rates
Reaction Rate


[
] = concentration in M

Rate = [A]t2 – [A]t1 = D[A]
t2 – t1
Dt
Changes can be ______ or _______
Reaction Rate
We will always work with rate as a positive
quantity.
 Concentrations of reactants______
____________, the rate expression will include a
negative sign in order to keep the rate positive
 Instantaneous Rate – _______________
_________________________________



will be the slope of a line tangent to the
at that point
curve
____ and ___________ of the reaction (which
species is being studied) affect the ____
Introduction to Rate Laws

Chemical reactions are _____________. *

After a period of time, enough _______ accumulate so
that the ____________________________.



At this point the concentration of the reactants
depends on the difference in the rates of the forward
and reverse reactions.
This tends to complicate matters, so we will ______
_______________________, before the products
have had time to build up to significant levels.
If we choose conditions where the reverse reaction can
be ________, the _______________________
________________________________________.
Rate Law
Rate = k [reactant]n
k = rate constant, determined ______________
n = ____ of the reactant, determined experimentally, can
be an integer, including zero or a fraction
[ ] = law only depends on concentration of _______
Rate Law Key Points:




[_______] do not appear b/c conditions are
set where reverse rxn is __________
n,
k,
We study rate laws in order to be able to
_________________________________
________________________________.
Types of Rate Laws
Differential rate law


rate depends on ________________
aka __________
Integrated rate law

Shows how the concentration of species in the
rxn depend on _________
Either type provides the information to
determine the individual steps of the rxn.
Determining the form of the Rate Law
Form of rate law
 Determine the power to which each
reactant concentration must be raised
in the rate law
 Order of a particular reactant must be
obtained by observing how the reaction
rate depends on the concentration of
that reactant
Method of Initial Rates



Initial rate – ________________ rate determined
just after the reaction _______ (before initial
[reactants] have changed much)
Several __________ are needed w/different
[________], each with a initial rate, then this info is
compared to determine the form
Sample Exercise 12.1 pg. 570


We will determine the order of each
reactant by comparing initial
concentrations and initial rates from
several experiments.
Compare experiments where the
reactant your studying is changing but
the others are constant.
Integrated Rate Law


Reactions involving a single ________ all
have the same rate law form:
Integrated – concentration of a reactant
as a function of _______
First Order Rate Laws

Doubling the _______, doubles the ______
2 N2O5  4 NO2 + O2
Rate = k [N2O5]
When integrated with time the law becomes:
ln[N2O5] = -kt + ln[N2O5]0
[N2O5] @t
time
[N2O5] @ t=0
First Order Rate Laws


First Order Rate Law:
Rate = k [A]
Integrated First Order Rate Law:
ln [A] = -kt + ln [A]0
If [A]0 and k are known, the [A] at any time can be
calculated
Uses the form ___________, straight line graph



y=
x=
m=
b=
First order always gives a _________________, by
plotting ____________
Can also be expressed as a ratio of [A] and [A]0
ln [A]0 = kt
[A]
First Order Rate Law
Half Life of a Reactant

First Order Rxn Half Life Law*:
t1/2 = .693
k
Second-Order and Zero Order Rate Laws
These are similar to the first order. Each has rate
law, integrated rate law, a plot for a straight line, a
relationship of the slope to k and a half-life
equation. In each law if you know k and [A]0 you can
calculate the [A] at any time.
A few special things Second Order – doubling the conc of A, quadruples
the rate, tripling increases 9x
Zero Order – rate is constant, not affected by
concentration
Get your handout!!!
Integrated Rate Laws
with more than one reactant


If the concentration of one reactant is much
smaller than the others, the amounts of the
larger concentrations will not change very
much and can be assumed to be constant.
The change of the concentration over time of
the reactant with the small concentration can
used to determine the order.
Rate = k [A]n ([B]m[C]p) – very large
Rate = k’[A]n
Reaction Mechanisms


___________________, which only tells
reactants, products and stoichiometry
NO2(g) + CO(g)  NO(g) + CO2(g)
This reaction occurs by elementary steps, a
reaction whose rate law can be written
from its molecularity.
NO2(g) + NO2(g)  NO3(g) + NO(g)
slow
NO3(g) + CO(g)  NO2(g) + CO2(g) fast
These are the two elementary steps by which
the above reaction occurs.
Intermediate – a species that is neither a reactant or product, but is
formed and consumed during the reaction.
Molecularity




Unimolecular – ____ molecule
Bimolecular – _____ molecules
Termolecular – ______ molecules (very rare,
_________________________________)

The rate law for an elementary step follows
directly from the molecularity of that step.
Reaction Mechanism – series of elementary steps
that must:
1) Sum of the elementary steps must give the overall balance
equation
2) The mechanism must agree w/ the experimentally determined
rate law.
Rate–Determining Step: slow step (highest Ea)


A reaction is only as fast as its slowest step
The slow step will be indicated to you.
A mechanism can never be proven absolutely, it is possibly
correct.
Material Not in Your Text

Sometimes the rate expression
obtained by the process involves a
reactive intermediate ([intermediate] is to
small to determine experimentally)

The intermediate must be eliminated
from the rate expression.
Model of Chemical Kinetics
Chemical rxns speed up when
___________________, all rate constants
show an exponential increase w/ absolute
temperature.
Collision Model
1)
2)

3)