Physical Chemistry Chapter 10 1 Atkins

8/16/2019 Physical Chemistry Chapter 10 1 Atkins

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Chemical Kinetics:Chemical Kinetics:

The Rates of ReactionsThe Rates of Reactions

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Ø Chemcal kinetics is concerned with the rate of chemical reactions .

Ø Chemical kinetics deals wi th

- how r apidly reactants are consumed wnd products formed;

- how reaction r ates respond to changes in the conditions or thepresence of a catalyst;

- the identif ication of the step by which a r eaction takes place (r eaction mechani sm).

Chemical Kinetics




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Ø Two reasons for studying the rates of r eactions

- One is that the practical importance of being able to predict howquickly a r eaction mixture approaches equi l ibri um.ð The rate might depend on variable under our control (T , p,

catalyst), and we might be able to optimi ze i t by the appropr iatechoice of condi tions.

- Another is that the study of r eaction r ates leads to an under stand- ing of the mechanism of a r eaction , its analysis into a sequence ofelementar y step.

Chemical Kinetics




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Chemical Kinetics

Ø Enzyme ki netics , the study of the effect of enzymes on the rates ofreactions , is also an important wi ndow on how thesemacromolecules works.

Ø We need to cope with a wide variety of different r ates and a pr ocess

that appears to be slow may be the outcome of many faster steps.




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Empirical Chemical Kinetics

Ø The f ir st step in the investigation of the rate and mechani sm of a

reaction is the determination of the overall stoichiometry of thereaction and the identif ication of any side reactions .

Ø The next step is to determine how the concentr ations of thereactants and products change with time after the reaction has beeninitiated .

- The temperatur e of reaction mixture must be held constantthroughout the cour se of the r eaction , for otherwise the observedrate would be a meaning average of the rate for dif ferenttemperatures.

Ø The method used to moni tor the concentr ations of r eactants andproducts and their variation wi th time depends on the substances involved and the acidity.




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Empirical Chemical Kinetics

- Spectrophotometry

- The conductivi ty of the solu tion

- pH meter

- Polarimetry

- The detection of l ight emission,ti tr ation, mass spectrometer, gaschr omatography, magneticresonance.




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10.1 Spectrophotometry

Ø The key resul t for using the intensity of absorption of r adiation at a

parti cular wavelength to determine the concentration [J] of theabsorbing species is the empir ical Beer -L amber t l aw .




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10.1 Spectropjotometry

A = l og (I 0 /I ) = [J] L = - log T% T% = I /I 0 x 100%

- Α : the absorbance

- I 0 : the incident i ntensity

- I : the transmi tted intensity

- L : the length of the sample

- : the molar absorption coefficient ( "#$ )

(extinction coeff icient, %$ )

- depends on the wavelength of the incident reaction and is greatestwhere the absorption i s most i ntense .




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Ø I n a typical spectrophotometer , the absorbance is plotted as afunction of wavelength , so A may be determined dir ectly f rom thedata at a given wavelength .

10.1 Spectropjotometry

Α = [J] L




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10.2 Experimental Techniques

Ø I n a real-time analysis , the concentr ation of a system is analyzedwhile the reaction i s in progress by direct spectroscopic observationof the reaction mi xtur e .

Ø I n the f low method , the reactants are mixed as they flow together ina chamber .




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- The reaction continues as the thoroughly mixed solu tions f low

through a capil lary outl et tube at about 10 ms -1 , and dif ferentpoin ts along the tube corresponds to dif ferent times after the star tof the reaction.

- Spectrophotometr ic determination of the composition at dif ferent

positions along the tube is equivalent to the determination of thecompositi on of the r eaction mixture at dif ferent times after mixing.

- Disadvantage : a large volume of reactant

- Par ticular ly important for reactions take place very quickl y.




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Ø The stopped-f low techniques avoids this disadvantage


- The two solu tions are mixed very rapidly (< 1 ms) by injecting

them i nto a mi xi ng chamber designed to ensure that the fl ow isturbulent and that complete mixing occur s very qui ckly.




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Ø Very fast r eactions can be studied by flash photolysis .

- The sample is exposed to a bri ef f lash of l ight that ini tiates thereaction, and then the contents of the reaction chamber aremoni tored spectrophotometr ically.

- L aser : 10 -9 s (ns), 10 -12 s (pi cosecond), 10 -15 s (femtosecond),

10 -18

s (attosecond) Ø F ast r eactions are also studied by pulse radiolysis in whi ch the

f lash of electromagnetic radiation i s replaced by a shor t bur st ofhi gh velocity electrons.




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Ø I n contr ast to real-time analysis, quenching methods are based onstopping , or quenching , the reaction af ter i t has been al lowed toproceed for a cer tain time and the composition is analysis at l eisur e .

- Cooling suddenl y; adding the mixture to a large volume solvent;rapid neutr alization of an acid r eagent

- Th is method is sui table only for r eactions that are slow enough forthere to be l i ttle reaction dur ing the time i t takes to quench themixtures .




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Reaction Rates

Ø The raw data fr om exper iments to measur e reaction rates arequantiti es that are propor tional to the concentr ations or par tialpressures of reactants and pr oducts at a ser ies of times after thereaction i s ini tiated .

Ø I ntermediates can not be studied because their exi stence is f leetingor their concentration i s so low.

Ø M ore inf ormation about the reaction can be extr acted if data are obtained at a ser ies of dif ferent temperatures .




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Ø The rate of a r eaction taking place in a container of f ixed volume isdef ined in terms of the rate of change of the concentr ation of adesignated species .

Rate = | [J ]| / t = |d [J ]| / dt

- [J ] is the change in the molar concentration of the specied J thatoccurs dur ing the time interval t.

- All r ates are positive.

10.3 The Definition of Rates




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Ø The instantaneous rate of the reaction ! i ts rate at a specif ic instant.

- The instantaneous rate of consumption of a reactant is the slope ofi ts molar concentr ation plotted against the time , with the slopeevaluated as the tangent to the graph at the instant of interest andreported as a positive quanti ty .




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- The instantaneous rate of formation of a product is also the slope

of the tangent to the graph of i ts molar concentr ation plotted , andalso reported as a positive quanti ty .

- The steeper the slope in ei ther case, the greater the rate of thereaction.

Ø [J] : moles / dm 3 ; t : second ; rate : moles dm -3 s -1 (M s -1 )

- The instantaneous rate : υ

Ø The var ious reactants in a given r eaction ar e consumed at dif ferentrates , and the var ious products are also formed at dif ferent rates .

- These r ates are related by the stoichiometr y of the reaction.




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Ø We have to be careful to specify exactly what species we mean whenwe repor t a reaction r ate .

Ø The most sophisticated defini tion of a unique r ate of a r eaction is interms of the stoichiometric numbers , ν J , that appear in thechemical equation .

- Stoichiometr ic numbers are the stoichiometr ic coefficients but

wr itten as positive for products and as negative for reactants .

- The rate is always positive because whenever [J] / t i s negative,so is the stoichiometr ic number .

= (1/ ν J ) d[J]/dt




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Ø A compli cation : i f the reactants form a slowly decayingintermediate, the products do not f orm at the same rate as thereactants tur n into the intermediates.

- Complication ð advantage : the observation that the consumptionand formation r ates are not r elated by the r eaction stoichiometry is

a good sign that a long-lived intermediate is involved in the r eaction.




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10.4 Rate Laws and Rate Constants

Ø The rate of r eaction is often found to be propor tional to the molarconcentr ation of the reactants raised to a simple power .

- I t may be found that the rate is dir ectly propor tional to theconcentrations of the r eactants A and B.

υ = k r [A] [B ]

- The coefficient k r is call ed the rate constant ( r ate coeff icient ).

- The rate constant is independent of the concentr ations of thespecies taking par t in the reaction but depends on the temperature .

Ø An empir ically determined equation is called the " rate law " of thereaction .

- A rate law is an equation that expr esses the rate of reaction interms of the molar concentr ations of r eactants and/or products .




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Ø The units of k r are always to conver t the product of concentrationsinto a rate expressed as a change in concentr ation divided by time .

- Ex. υ = k r [A] [B]

[A] , [B] : mol dm -3 (M )

k r : dm 3 mol -1 s -1 (M -1 s -1 )

- I n gas-phase studies concentr ations are commonly expr essed inmolecul es cm -3 , so the rate constant f or reaction above woul d beexpressed in cm 3 molecule -1 s -1 .




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(Self-test 10.1)




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Ø Once we know the rate law and the rate constant of the reaction,

- we can predict the rate of the reaction for an given composition ofthe reaction mix tur e ;

- we can use a rate law to predict the concentr ation of the reactantsand products at any time after the start of the reaction .

- An observed rate law is also an important guide to the mechanism of the reaction, for any proposed mechanism must be consistentwith it.




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- A rate law provides a basis for the classif ication of reactionsaccording to their kinetics.

- Reactions belonging to the same class have simi lar ki netic behavior- their rates and the concentrations of the reactants and productsvary with compositi on in a simil ar way.

Ø This classif ication i s based on their order , the power to whi ch the

concentr ation of a species is raised in the rate law .- F ir st order ( '()* ) in A : υ = k r [A]

- F ir st order in A and fir st order in B : υ = k r [A] [B]

- Second or der ( +()* ) in A : υ = k r [A] 2

Ø The overall order of a r eaction with a rate law of the formυ = k r [A]

a [B] b [C] c is the sum , a+b+c , of the orders of al l thecomponents.

10.5 Reaction Order




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10.5 Reaction Order




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Ø A reaction need not have an i ntegral order , and many gas-phasereactions do not .

- Ex. υ = k r [A] 1/2 [B] ð half -order (1/2) in A ; f ir st-order i n B ;

thr ee-halfs (3/2) order overall

Ø I f a rate law is not of the form υ = k r [A] a [B] b [C] c ## , thereaction does not have an overall order .

- Ex. H 2 (g) + Br 2 (g) 2 H Br (g)

10.5 Reaction Order




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10.5 Reaction Order

- A typical r ate law for the action of an enzyme E on a substr ate .

K M ; a constant

Ø Under cer tain cir cumstances a compli cated rate law without anoverall order may simplif y into a law with a def ini te order .

- [S]


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10.5 Reaction Order

Ø A r ate law is established exper imentally , and cannot in general beinf er red fr om the chemical equation f or the reaction .

- Ex. H 2 (g) + Br 2 (g) 2 H Br (g)

- The rate law does happen to reflect the reaction stoichi ometr y.

H 2 (g) + I 2 (g) 2 H I (g)

υ = k r [H 2 ] [I 2 ]




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2 nd Exam

Date : 12/9/2014 ( ,+ )

Ti me : 19:00 ~ 22:00 PM Place : C01-202

Physical Chemistry Chapter 10 1 Atkins

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Transcript of Physical Chemistry Chapter 10 1 Atkins