Kinetics Books Ta Ver

8/16/2019 Kinetics Books Ta Ver

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Chemical

Kinetics

Chemical Kinetics

John D. Bookstaver

St. Charles Community College

St. Peters, MO

200, Prentice !all, "nc.

Modified by S.A. Green, 2006

Chemistry, The Central Science, Cha#ter $%,

$0th e&ition

'heo&ore (. Bro)n* !. +ugene (eMay, Jr.*

an& Bruce +. Bursten


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Chemical

Kinetics

Kinetics

• Stu&ies the rate at )hich a chemical

#rocess occurs.

• Besi&es inormation a-out the s#ee& at)hich reactions occur, kinetics also

she&s light on the reaction mechanism

e/actly how the reaction occurs.


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Chemical

Kinetics

Outline1 Kinetics

Reaction Rates !o) )e measure rates.

Rate Laws!o) the rate &e#en&s on amountso reactants.

Integrated Rate Laws

!o) to calc amount let or time to

reach a given amount.

Half-life!o) long it takes to react 03 oreactants.

Arreni!s "#!ation !o) rate constant changes )ith '.

Mecanis$s(ink -et)een rate an& molecularscale #rocesses.


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Chemical

Kinetics

4actors 'hat 5ect 6eaction 6ates• %oncentration of Reactants

5s the concentration o reactants increases, so &oes thelikelihoo& that reactant molecules )ill colli&e.

• &e$'erat!re 5t higher tem#eratures, reactant molecules have more kinetic

energy, move aster, an& colli&e more oten an& )ith greater

energy.

• %atalysts S#ee& r/n -y changing

mechanism.


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Chemical

Kinetics

6eaction 6ates

6ates o reactions can -e &etermine& -y

monitoring the change in concentration oeither reactants or #ro&ucts as a unction o

time. (A) *s t

6/n Movie


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Chemical

Kinetics

6eaction 6ates

"n this reaction, the

concentration o

-utyl chlori&e,%+H%l, )as

measure& at various

times, t.

%+H%laq 7 !2Ol C%!8O!aq 7 !Claq (%+H%l) M


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Chemical

Kinetics

6eaction 6ates

'he a*erage rate o

the reaction over

each interval is thechange in

concentration &ivi&e&

-y the change in time1

C%!8Claq 7 !2Ol C%!8O!aq 7 !Claq A*erage Rate, Ms


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Chemical

Kinetics

6eaction 6ates

• 9ote that the average

rate &ecreases as the

reaction #rocee&s.

• 'his is -ecause as the

reaction goes or)ar&,

there are e)er

collisions -et)een

reactant molecules.

C%!8Claq 7 !2Ol C%!8O!aq 7 !Claq


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Chemical

Kinetics

6eaction 6ates

• 5 #lot o concentration

vs. time or this reaction

yiel&s a curve like this.

• 'he slo#e o a line

tangent to the curve at

any #oint is the

instantaneous rate at

that time.



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Chemical

Kinetics

6eaction 6ates

• 'he reaction slo)s

&o)n )ith time -ecause

the concentration o the

reactants &ecreases.



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Chemical

Kinetics

6eaction 6ates an& Stoichiometry

• "n this reaction, the ratio

o C%!8Cl to C%!8O! is

$1$.

• 'hus, the rate o

disappearance o

C%!8Cl is the same as

the rate o appearance

o C%!8O!.


6ate :;∆


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Chemical

Kinetics


• >hat i the ratio is not $1$?

!2g 7 "2g 2 !"g • Only $@2 !" is ma&e or each !2 use&.


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Chemical

Kinetics


• 'o generaliAe, or the reaction

a 5 7 bB c C 7 d D

6eactants &ecrease Pro&ucts increase


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Chemical

Kinetics

Concentration an& 6ate

+ach reaction has its o)n euation that

gives its rate as a unction o reactant

concentrations.

this is calle& its Rate Law

'o &etermine the rate la) )e measure the rateat &ierent starting concentrations.


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Chemical

Kinetics


Com#are +/#eriments $ an& 21

)hen


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Chemical

Kinetics


(ike)ise, com#are +/#eriments an& 1

)hen


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Chemical

Kinetics


'his euation is calle& the rate la),an& k is the rate constant.


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Chemical

Kinetics

6ate (a)s

• 5 rate la) sho)s the relationshi# -et)een the reactionrate an& the concentrations o reactants. 4or gas;#hase reactants use P 5 instea& o


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Chemical

Kinetics

6ate (a)s

• +/#onents tell the or&er o the reaction )ithres#ect to each reactant.

• 'his reaction is

First-order in


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Chemical

Kinetics

"ntegrate& 6ate (a)s

Consi&er a sim#le $st or&er r/n1 5 B

!o) much 5 is let ater time t ? "ntegrate1

Dierential orm1


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Chemical

Kinetics


'he integrate& orm o irst or&er rate la)1

Can -e rearrange& to give1


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Chemical

Kinetics


Mani#ulating this euation #ro&uces

)hich is in the orm y : mx 7 b


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Chemical

Kinetics

4irst;Or&er Processes

" a reaction is irst;or&er, a #lot o ln


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Chemical

Kinetics


Consi&er the #rocess in)hich methyl isonitrile is

converte& to acetonitrile.

C!E9C C!EC9

!o) &o )e kno) this

is a irst or&er r/n?


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Chemical

Kinetics


'his &ata )as

collecte& or thisreaction at $8F.8GC.

C!E9C C!EC9

Does

rate:k


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Chemical

Kinetics


• >hen ln is #lotte& as a unction o time, a

straight line results.'he #rocess is irst;or&er.

k is the negative slo#e1 .$ × $0

;

s;$

.


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Chemical

Kinetics

Secon&;Or&er Processes

Similarly, integrating the rate la) or a

#rocess that is secon&;or&er in reactant 51

also in the orm y : m/ 7 b

6earrange, integrate1


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Chemical

Kinetics


So i a #rocess is secon&;or&er in 5, a

#lot o $@


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Chemical

Kinetics

Determining r/n or&er

'he &ecom#osition o 9O2 at E00GC is &escri-e& -ythe euation

9O2 g 9O g 7 $@2 O2 g

an& yiel&s these &ata1

'ime s


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Chemical

Kinetics

Ira#hing ln


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Chemical

Kinetics


5 gra#h o $@


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Chemical

Kinetics

!al;(ie

• !al;lie is &eine&as the time reuire&

or one;hal o a

reactant to react.

• Because


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Chemical

Kinetics

!al;(ie

4or a irst;or&er #rocess, set


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Chemical

Kinetics

!al;(ie; 2n& or&er 4or a secon&;or&er #rocess, set


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Chemical

Kinetics

Outline1 Kinetics

4irst or&er Secon& or&er Secon& or&er

RateLaws

Integrate

d RateLaws

com#licate&

Half-life com#licate&


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Chemical

Kinetics

'em#erature an& 6ate

• Ienerally, as tem#erature

increases, so &oes the

reaction rate.• 'his is -ecause k is

tem#erature &e#en&ent.


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Chemical

Kinetics

'he Collision Mo&el

• "n a chemical reaction, -on&s are

-roken an& ne) -on&s are orme&.

• Molecules can only react i they colli&e)ith each other.


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Chemical

Kinetics

'he Collision Mo&el

4urthermore, molecules must colli&e )ith the

correct orientation an& )ith enough energy to

cause -on& -reakage an& ormation.


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Chemical

Kinetics

5ctivation +nergy

• "n other )or&s, there is a minimum amount o energyreuire& or reaction1 the activation energy, # a.

• Just as a -all cannot get over a hill i it &oes not roll

u# the hill )ith enough energy, a reaction cannot

occur unless the molecules #ossess suicient energyto get over the activation energy -arrier.


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Chemical

Kinetics

6eaction Coor&inate Diagrams

"t is hel#ul tovisualiAe energychangesthroughout a#rocess on areaction coor&inate&iagram like this

one or therearrangement omethyl isonitrile.


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Chemical

Kinetics

6eaction Coor&inate Diagrams

• "t sho)s the energy othe reactants an&#ro&ucts an&,thereore, ∆# .

• 'he high #oint on the&iagram is the transitionstate.

• 'he s#ecies #resent at the transition state is

calle& the activate& com#le/.• 'he energy ga# -et)een the reactants an& the

activate& com#le/ is the activation energy

-arrier.


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Chemical

Kinetics

Ma/)ellBoltAmann Distri-utions

• 'em#erature is

&eine& as a

measure o theaverage kinetic

energy o the

molecules in a

sam#le.• 5t any tem#erature there is a )i&e

&istri-ution o kinetic energies.


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Chemical

Kinetics


• 5s the tem#erature

increases, the curve

lattens an&

-roa&ens.• 'hus at higher

tem#eratures, a

larger #o#ulation o

molecules has

higher energy.


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Chemical

Kinetics


• " the &otte& line re#resents the activation

energy, as the tem#erature increases, so &oes

the raction o molecules that can overcome

the activation energy -arrier.

• 5s a result, the

reaction rate

increases.


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Chemical

Kinetics


'his raction o molecules can -e oun& through the e/#ression1

)here $ is the gas constant an& T is the tem#erature in Kelvin .


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Chemical

Kinetics

5rrhenius +uation

Svante 5rrhenius &evelo#e& a mathematicalrelationshi# -et)een k an& # a1

)here % is the reuency actor , a num-er that

re#resents the likelihoo& that collisions )oul&

occur )ith the #ro#er orientation or reaction.


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Chemical

Kinetics

5rrhenius +uation

'aking the natural

logarithm o -oth

si&es, the euation

-ecomes$

$T

y : mx 7 b>hen k is &etermine& e/#erimentally at

several tem#eratures, # a can -e calculate&

rom the slo#e o a #lot o ln k vs. $@T .


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Chemical

Kinetics

Outline1 Kinetics

4irst or&er Secon& or&er Secon& or&er

RateLaws

Integrate

d RateLaws com#licate&

Half-life com#licate&

k(T)


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Chemical

Kinetics

6eaction Mechanisms

'he seuence o events that &escri-es

the actual #rocess -y )hich reactants

-ecome #ro&ucts is calle& the reactionmechanism.


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Chemical

Kinetics

6eaction Mechanisms

• 6eactions may occur all at once or

through several &iscrete ste#s.

• +ach o these #rocesses is kno)n asan elementary reaction or elementary

#rocess.


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Chemical

Kinetics

6eaction Mechanisms

• 'he molecularity o a #rocess tells ho) manymolecules are involve& in the #rocess.

• 'he rate la) or an elementary ste# is )ritten&irectly rom that ste#.


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Chemical

Kinetics

Multiste# Mechanisms

• "n a multiste# #rocess, one o the ste#s )ill

-e slo)er than all others.

• 'he overall reaction cannot occur aster than

this slo)est, rate;&etermining ste#.


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Chemical

Kinetics

Slo) "nitial Ste#

• 'he rate la) or this reaction is oun&

e/#erimentally to -e

6ate : k


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Chemical

Kinetics

Slo) "nitial Ste#

• 5 #ro#ose& mechanism or this reaction isSte# $1 9O2 7 9O2 9OE 7 9O slo)

Ste# 21 9OE 7 CO 9O2 7 CO2 ast

• 'he 9OE

interme&iate is consume& in the secon& ste#.

• 5s CO is not involve& in the slo), rate;&etermining ste#, it &oes

not a##ear in the rate la).


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Chemical

Kinetics

4ast "nitial Ste#

• 'he rate la) or this reaction is oun&

e/#erimentally to -e

• Because termolecular : trimolecular#rocesses are rare, this rate la)suggests a t)o;ste# mechanism.


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Chemical

Kinetics

4ast "nitial Ste#

• 5 #ro#ose& mechanism is

Ste# $ is an eq&ilibri&m;

it inclu&es the or)ar& and reverse reactions.


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Chemical

Kinetics

4ast "nitial Ste#

• 'he rate o the overall reaction &e#en&s

u#on the rate o the slo) ste#.• 'he rate la) or that ste# )oul& -e

• But ho) can )e in&


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Chemical

Kinetics

4ast "nitial Ste#

• 9OBr 2 can react t)o )ays1

>ith 9O to orm 9OBr

By &ecom#osition to reorm 9O an& Br 2

• 'he reactants an& #ro&ucts o the irst

ste# are in euili-rium )ith each other.

• 'hereore,

6ate' : 6ater


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Chemical

Kinetics

4ast "nitial Ste#

• Because 6ate' : 6ater ,

k $


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Chemical

Kinetics

4ast "nitial Ste#

Su-stituting this e/#ression or


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Chemical

Kinetics

Catalysts

• Catalysts increase the rate o a reaction -y&ecreasing the activation energy o the

reaction.

• Catalysts change the mechanism -y )hich

the #rocess occurs.


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Chemical

Kinetics

Catalysts

One )ay a

catalyst can

s#ee& u# a

reaction is -y

hol&ing the

reactants together

an& hel#ing -on&sto -reak.


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Chemical

Kinetics

+nAymes

• +nAymes arecatalysts in

-iological systems.

• 'he su-strate its

into the active site othe enAyme much

like a key its into a

lock.

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Kinetics Books Ta Ver