Modified Kinetics Lecture
Transcript of Modified Kinetics Lecture
-
7/26/2019 Modified Kinetics Lecture
1/68
CHEMICAL KINETICS
-
7/26/2019 Modified Kinetics Lecture
2/68
INTRODUCTION
Rate is a measure of how fast or slowa reaction occurs
Rate is the change in concentrationper unit time.
Units of rate are mol dm-3 s-1mol dm-3
min-1and mol dm-3 h-1
-
7/26/2019 Modified Kinetics Lecture
3/68
-
7/26/2019 Modified Kinetics Lecture
4/68
The Decomposition of Nitrogen Dioxide
-
7/26/2019 Modified Kinetics Lecture
5/68
Types of Rates
Initial Rates Rates measured at the beginning of the reaction,
which is dependent on the initial concentrations of
reactants. Instantaneous Rates
Rates measured at any point during the reaction.
Average Rates An overall rate measured over a period or timeinterval.
-
7/26/2019 Modified Kinetics Lecture
6/68
Instantaneous Rate
alue of the rateat a particular
time.
!an be obtained
by computing the
slope of a line
tangent to the
curve at that point.
-
7/26/2019 Modified Kinetics Lecture
7/68
REACTION RATE
/
The rate of a chemical reaction isde0ned as the change inconcentration of a reactant in a
particular time inter+al.
-
7/26/2019 Modified Kinetics Lecture
8/68
The Decomposition of Nitrogen Dioxide
2
-
7/26/2019 Modified Kinetics Lecture
9/68
Reaction Rates and "toichiometric Relationships
!onsider the reaction#
$N$%&'N%$ ( %$
Rate of disappearance of N$%&) Rate of formation of N%$)
Rate of formation of %$)
"toichiometric relationships of these rates
-
7/26/2019 Modified Kinetics Lecture
10/68
'n general
4or a reaction
15
-
7/26/2019 Modified Kinetics Lecture
11/68
Rate law
An e6pression which shows how the reaction rate is related to concentration ofreactants 7catalyst8 is called the rate law or rate e9uation.
4or a reaction
The rate law generally has the form
# rate constant, proportionality constant
temperature dependent
:alue and unit obtained from the rate law
and are small whole numbers called the order of reaction
Rate law is determined from e6perimental dataThe reaction rate with respect to A or ; is determined by +aryingconcentration of one reactant, keeping that of the other constant.
11
-
7/26/2019 Modified Kinetics Lecture
12/68
Rate law
$ompare 6periments 1 and ,?@ doubles, the initial rate doubles.
The date below is for the e6periment
1owe+er, for some elementary reactions thepowers in the rate law may correspond tocoeBcient in the chemical e9uation.
*(t (s(all+ the powers of o!e!trat"o! "!
the rate law are #"$ere!t from oe%"e!t.Thus for the reaction 78 abo+e, the rate is found
to be proportional to =>
-
7/26/2019 Modified Kinetics Lecture
17/68
%rder of a reaction
Also called rate order
't is the powersCe6ponents of the concentration terms in therate e9uation 7law8
$onsider the rate law
The reaction order with respect to a gi+en reactant speciese9uals the e6ponent of the concentration of that species inthe rate law, as determined e6perimentally.
and are reaction orders with respect to A and ; respecti+ely
The o+erall order of a reaction e9uals the sum of the ordersof the reactant species in the rate law
is the o+erall order
$an ha+e integral or half integral +alues
1/
-
7/26/2019 Modified Kinetics Lecture
18/68
%rder of a reaction
Reaction %bser+ed Rate
law
%rder
with
respect
to
Reactant
1
%rder
with
respect
to
Reactant
-
7/26/2019 Modified Kinetics Lecture
25/68
$omple6 Reaction
A comple6 reaction is that which occurs in two ormore steps.
*or example, the overall reaction#
$A ( + ! ( D
may involve the following elementary stepsin its mechanism#
"tep-# A ( +/
"tep$# ( A0/
"tep1# 0! ( D
%verall reaction# $A ( +! ( D/
and 0 are called reactive intermediates
-
7/26/2019 Modified Kinetics Lecture
26/68
-
7/26/2019 Modified Kinetics Lecture
27/68
!ultistep !echanisms
'n a multistep process, one of the steps will beslower than all others.
The o+erall reaction cannot occur faster than this
slowest, 't is the rate-determining step.
-
7/26/2019 Modified Kinetics Lecture
28/68
-
7/26/2019 Modified Kinetics Lecture
29/68
(low 'nitial (tep
A proposed mechanism for this reaction is
(tep 1# &%
-
7/26/2019 Modified Kinetics Lecture
30/68
4ast 'nitial (tep
The rate law for this reaction is found
7e6perimentally8 to be
;ecause termolecular 7 trimolecular8processes are rare, this rate lawsuggests a two-step mechanism.
35
-
7/26/2019 Modified Kinetics Lecture
31/68
4ast 'nitial (tep
A proposed mechanism is
Step 1 is an equilibrium-
it includes the forward and reverse reactions.
31
-
7/26/2019 Modified Kinetics Lecture
32/68
4ast 'nitial (tep
The rate of the o+erall reaction
depends upon the rate of the slowstep.
The rate law for that step would be
;ut how can we 0nd =&%;r
-
7/26/2019 Modified Kinetics Lecture
33/68
4ast 'nitial (tep
&%;r
-
7/26/2019 Modified Kinetics Lecture
34/68
4ast 'nitial (tep
;ecause Ratef Rater ,
k1=&%@ =;r
-
7/26/2019 Modified Kinetics Lecture
35/68
4ast 'nitial (tep
(ubstituting this e6pression for
=&%;r
-
7/26/2019 Modified Kinetics Lecture
36/68
Mea!"!0 of the Rate Express"o!
Example1 It has been determined e6perimentally thatthe se9uence of steps for a chemical reaction is as
follows#A ? ; $ 7slow8
$ ? ; J 7fast8
Hhat is the rate e9uation and the o+erall chemical
e9uationGSol(t"o!# The o+erall chemical e9uation will beobtained by adding the abo+e e9uations i.e.
A ? ; $ 7slow8
$ ? ; J 7fast8A 2 3* D 4slow5
(ince the rate of reaction is controlled by the slowstep, the rate e9uation will be
Rate k=A@=;@
-
7/26/2019 Modified Kinetics Lecture
37/68
MOLECULARIT/ 6ERSUS ORDER OF REACTION
The term molecularity is not the same asorder of reaction.
The total number of molecules or atoms
which take part in a reaction asrepresented by the chemical e9uation isknown as the mole(lar"t+ of reat"o!.
The sum of the powers to which theconcentrations are raised in the rate lawis known as the or#er of reat"o!.
-
7/26/2019 Modified Kinetics Lecture
38/68
DIFFERENCES *ET)EEN ORDER ANDMOLECULARIT/
-
7/26/2019 Modified Kinetics Lecture
39/68
DETERMINATION OF ORDER OF REACTION
The rate e9uation of a reaction with a multi-stepmechanism cannot, in general, be deduced from the
stoichiometric coeBcients of the o+erall reactionK
it must be determined e6perimentally.
The e9uation may in+ol+e fractional e6ponentialcoeBcients, or it may depend on the concentrationof an intermediate species.
The rate e9uation is a diDerential e9uation, and itcan be integrated to obtain an integrated ratee9uation that links concentrations of reactants orproducts with time.
-
7/26/2019 Modified Kinetics Lecture
40/68
DETERMINATION OF ORDER OF REACTION
D"$ere!t"al Rate Law 7Rate law8 - e6presses howthe rate depends on concentration.
I!te0rate# rate law - e6presses how the
concentration depends on time.
He can work backwards from the rate law to infer
the steps by which their reaction occurs.
!ost chemical reactions do not take place in a single
step but result from a series of se9uential steps.To understand a chemical reaction, we must learn
what these steps are.
-
7/26/2019 Modified Kinetics Lecture
41/68
CONCE7T CHECK
LU(T'% The following tabulated datarefer to the hypothetical reaction
A ?
-
7/26/2019 Modified Kinetics Lecture
42/68
6ample# The following tabulated data refer to
the hypothetical reaction
A ?
-
7/26/2019 Modified Kinetics Lecture
43/68
The general e6pression for a rate law is
4rom 1 M < changed by a factor of
Ji+iding < by 1
Thus
'n a similar fashion
Hill yield
Thus the rate law is
3
-
7/26/2019 Modified Kinetics Lecture
44/68
7b8 To calculate the rate constant, ,substitute any of the three sets ofdata into the rate law e6pression
-
7/26/2019 Modified Kinetics Lecture
45/68
*or the reaction# "$
%2$,3a45
( 1I, 3a45
6$"%'$, 3a45
(I1, 3a45
The data below were obtained. 3a5 Determine the order of the reaction w.r.t. each reactant. 7rite the rate law
for the above reaction.
3b5 !alculate the rate constant, k, and give its appropriate units.
3c5 !alculate the reaction rate when each reactant concentration is 8.$8 M6pt.N
=(
-
7/26/2019 Modified Kinetics Lecture
46/68
"olution# The rate lawhas the form Rate ) k9"$%2$:x9I:y,
herexandyare rate orders.
3a5 !alculation of rate order,x# we nee to use the e4uations with varying concentration of 9"$%2$:
Dividing gives
similarly
This reaction is first order w.r.t. 9"$%2$: and 9I:
Rate ) k9"$%2$:9I:
If , and
-
7/26/2019 Modified Kinetics Lecture
47/68
Types of Rate ;aws
Differential Rate ;aw 3rate law5 < shows
how the rate of a reaction depends on
concentrations.
This is what we have been discussing so far
Integrated Rate ;aw < shows how the
concentrations of species in the reaction
depend on time. Graphical method to derive the rate law of
a reaction
/
-
7/26/2019 Modified Kinetics Lecture
48/68
Oero order
'f the reaction A
=roducts is azeroorder reaction, then
which yields
And a plot ofversuswill yield a straight line
withslope)
2
-
7/26/2019 Modified Kinetics Lecture
49/68
4irst order 'f the reaction A=roducts is afirst
order reaction, then
which yields
or And a plot of versuswill yield a straight line
withslope) and y-intercept)
-
7/26/2019 Modified Kinetics Lecture
50/68
(econd order
'f the reaction A=roducts is asecondorder reaction, then
which yields
A plot of versuswill yield a straight line with
slope) k and y-intercept)
5
-
7/26/2019 Modified Kinetics Lecture
51/68
INTEGRATED RATE FOR 6ARIOUSREACTION ORDERS
E l Th h d l i f th l
-
7/26/2019 Modified Kinetics Lecture
52/68
Example1 The hydrolysis of ethylnitrobenFoate by a9eous sodium hydro6idewas followed at
-
7/26/2019 Modified Kinetics Lecture
53/68
Sol(t"o!# 4or the second order reaction
Thealternati+e
methodA plot of 1C=A@t +ersus t
will gi+e astraight linewith
(lope ?k
HALF LIFE
-
7/26/2019 Modified Kinetics Lecture
54/68
HALF8LIFE
>alf life of a reaction, t1Calf-life is a con+enient way to describe
how fast a reaction occurs, especially if itis a 0rst-order process. A fast reaction hasa short half-life.
(ubstituting this for =A@t in the integratedrate e9uation for any of the order gi+esthe half life e6pression.
-
7/26/2019 Modified Kinetics Lecture
55/68
>alf-)ife4or a 0rst-order process, set =A@t5. =A@5 inintegrated rate e9uation#
NOT$% &or a first-orderprocess, the half-life
does not depend on ['(.
-
7/26/2019 Modified Kinetics Lecture
56/68
>alf-)ife-
-
7/26/2019 Modified Kinetics Lecture
57/68
COLLISION THEOR/ OF REACTION RATES
According to this theory, a chemical reactiontakes place only by collision between thereacting molecules.
;ut not all collisions are eDecti+e. %nly a smallfraction of collisions produce a reaction.
The two main conditions for a collisionbetween reacting molecules to be producti+e
areThe colliding molecules must posses suBcient
kinetic energy to cause a reaction.
The reacting molecules must collide with
proper orientation
-
7/26/2019 Modified Kinetics Lecture
58/68
COLLISION THEOR/ OF REACTION RATES
-
7/26/2019 Modified Kinetics Lecture
59/68
Acti+ation nergy
4or the rearrangement#
-
7/26/2019 Modified Kinetics Lecture
60/68
Reaction $oordinate
Jiagrams 't shows the energy
of the reactants andproducts 7and,therefore, E8.
The high point on thediagram is thetransition state. The species present at the
transition state is called the
activated co)ple*.
The ener+ +ap etween thereactants and the activated
co)ple* is the activation ener+
arrier.
5
-
7/26/2019 Modified Kinetics Lecture
61/68
the collision theoryFor the reaction#
1
-
7/26/2019 Modified Kinetics Lecture
62/68