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1 Aoucemets, Nov. 9 th Lecture PRS Quiz topic: results Chemical through Kietics July 9 are posted o the course website. Chec agaist Kietics LabChec agaist Kietics Lab O Fial Exam, NOT 3 Review Exam 3 ad exam rd Exam aalysis July 9, Chapter 4 WebCT set posted (Homewor ) Friday July 3 PRS Course Review Ch. 4.6(b), 4(b) Brig Due Wed. PRS uit 4 th ad, 700 calculator Wrap-up course Moday, Nov Moday July 6 Fial Exam 3 rd Exam, usual format; topics: :50-5:40 Chs. 9-3 (omit (this room) 9.5-6,.4-5, 3.6-7) -4 Practice Crib sheets Exam, from posted Exam,, 3 ad Chapter 4 Wedesday, Nov 4 Part A (-6), B (7-9), C (0-3), D (4) RSVP your attedace This aoucemet to be posted /30/004 OFB Chapter 4

2 Chapter 4: Chemical Kietics 4- Rates of Chemical Reactios 4- Rates & Cocetratios 4-3 The Depedece of Cocetratios o Time 4-4 Reactio Mechaisms 4-5 Reactio Mechaism ad Rate Laws 4-6 Effect of Temperature o Reactio Rates 4-7 Kietics of Catalysis /30/004 OFB Chapter 4

3 Chapter 4: Chemical Kietics Thus far, we have bee usig ad talig about chemical reactios Reactats, products, ad how much is ivolved Chemical equilibrium is dyamic Chemical Kietics* cocers how fast a reactio proceeds Kietics Rates of Chemical Reactios Ad how to deduce reactio mechaisms from observed s of reactios Activatio Eergy (Role of Catalysts) How to aalyze, quatify, ad predict observed reactio s? *Kietics, Kiematics, Ciematics, Kio /30/004 OFB Chapter 4 3

4 Reactio Mechaisms Most reactios, as writte, actually proceed through a series of steps Each Step is called a elemetary reactio Classes of elemetary reactios:. Uimolecular (a sigle reactat) A B + C (a decompositio). Bimolecular (very commo) A + B products 3. Termolecular (less liely evet) A + B + C products /30/004 OFB Chapter 4 4

5 [ X average reactio t mol / L mol uits are mol L s L s [ X f [ X i [ X average t t t f i s To measure s, moitor disappearace of reactats or appearace of products e.g., NO + CO NO + CO rx /30/004 [NO [CO t t [NO [CO + + t t OFB Chapter 4 5

6 Role of stoichiometry To measure s we could moitor the disappearace of reactats or appearace of products e.g., NO + F NO F + [ NO t [ NO t F [ F t /30/004 OFB Chapter 4 6

7 OFB Chapter 4 7 /30/004 For a Geeralized Reactio aa + bb cc + dd t D d t C c t B b t A a [ [ [ [ + t X t t X X i f i f [ [ [ average NO + CO NO + CO

8 Order of a Reactio N O + decompositio 5 NO O [ N O5 Called a Rate Expressio Or Rate Law Called a Rate costat f(temperature) e.g., aa products [A /30/004 A th order reactio OFB Chapter 4 8

9 aa aa /30/004 Summary aa + bb cc + dd [A [ A [ B a t b t [ C [ D + c t d t products + bb products [A m [B Overall order m + Called a Rate Expressio Or Rate Law th order reactio m th order i [A ad th order i [B OFB Chapter 4 9

10 Example 4- At elevated temperatures, HI reacts accordig to the chemical equatio HI H + I at 443 C, the of reactio icreases with cocetratio of HI, as show i this table. Data [HI Rate Poit (mol L - ) (mol L - s - ) x x x 0 - a) Determie the order of the reactio with respect to HI ad write the expressio b) Calculate the costat ad give its uits c) Calculate the istataeous of reactio for a [HI 0.000M /30/004 OFB Chapter 4 0

11 /30/004 HI H + I Data [HI Rate Poit (mol L - ) (mol L - s - ) ( ) 3 4 ( ) ( ) ([ HI ) ([ HI ) x x x 0 - a) Determie the order of the reactio with respect to HI ad write tae the theexpressio ratio of ay two data poits 3.0x0 7.5x0 4 secod [HI [HI [HI order OFB Chapter i aswer HI to ad part A

12 Example 4- b) Calculate the costat ad give its uits [HI aswer to part A [HI 30 L mol Example 4-7.5x0 - s - 4 mol ( mol aswer to B L L c) Calculate the istataeous of reactio for a [HI 0.000M ( - -)( - 30 L mol s mol L ). x 0 [HI -4 mol L - s aswer to C s ) - /30/004 OFB Chapter 4

13 Similarly for two or more cocetratios aa + bb products [ A m [ B order is p m + uits of are mol ( p ) ( L p ) s Example A + B C tae the ratio of ay two ([X ) ([X ) data poits iitial Rate [A [B mol L - s -.0x0-4.0x0-4.8x0-6.0x x x0-6.0x x x0-5 /30/004 OFB Chapter 4 3

14 For Example A + B C iitial Rate [A [B mol L - s -.0x0-4.0x0-4.8x0-6.0x x x0-6.0x x x0-5 A m B ([X ) ([X ) [ [ tae the ratio st Whe [A is costat (.0x 0-4 ), Whe [B icreases times 3 ad icreases times 3 of ay two data poits /30/004 [ B [ B [ A [ B OFB Chapter 4 4

15 For Example A + B C iitial Rate [A [B mol L - s -.0x0-4.0x0-4.8x0-6.0E x x0-6.0x x x0-5 /30/004 m [ A [ B d Whe [B is costat (3.0x 0-4 ), [A icreases times & icreases times 4 [ A m [ m 4 4 [ A [ B tae the ratio ay m ([X ) ([X ) m OFB Chapter 4 5 of two data What is the rx order with respect to A? B? Overall? poits

16 Ca ow solve for [A [A [x0 [B 4.8x0 [B 6.8x0 L 6 [x0 mol 4 - s - uits of are ( L p ) mol ( p ) s Actually Example 3-4 NO + O NO [ NO [ O /30/004 OFB Chapter 4 6

17 4-3 The Depedece of Cocetratios o Time First Order Reactios i geeral, for first order aa products [A a t [A if [A [A 0 is the iitial coc of [A 0 e t a term that dimiishes A For ow, just accept this importat formula [A 0 /30/004 OFB Chapter 4 7

18 Itegd Rate Law for a st Order Reactio [A [A 0 e t If we tae the l of both sides l[ A l[ A 0 t Recall y mx + b or y b + mx A plot of l [A vs t will be a straight lie with the Itercept l[a 0 Slope - /30/004 OFB Chapter 4 8

19 l[ A l[ A 0 t First Order Reactio l [A} (i mol / L 0.0E+00.0E E E Slope - Time (i secods).7 x 0-5 s - /30/004 OFB Chapter 4 9

20 for a Secod Order Reactios /30/004 I Geeral, A From calculus [A t - [A o Itegd Rate Law d Order Reactio + This is also the equatio for a straight lie y mx +b for secod order [A t products [A slope [A OFB Chapter 4 0 t itercept + [ A [ A o 0

21 Slope Or /30/004 ½ slope OFB Chapter 4

22 [A t + [ A o Secod Order Reactio /[A (L /mol) Slope Time (i secods) /30/004 OFB Chapter 4

23 Summary Itegd Rate Laws First Order Reactios l[ A l[ A 0 t Secod Order Reactios t + [A [ A o /30/004 OFB Chapter 4 3

24 I practice, if we do t ow the order of the reactio [A plot both If a plot of l [A vs t is a straight lie, the the reactio is st order l [A} (i mol / L First Order Reactio 0.0E+00.0E E E Time (i secods) /[A (L /mol) Secod Order Reactio Time (i secods) If a plot of / [A vs t is a straight lie, the the reactio is d order /30/004 OFB Chapter 4 4

25 I a study of the reactio of pyridie (C 5 H 5 N) with methyl iodide (CH 3 I) i a bezee solutio, the followig iitial reactio s were measured at 5ºC for the differet cocetratios of the two reactats. [C 5 H 5 N [CH 3 I Rate (mol L - ) (mol L - ) (mol L - s - ).0 x x x x x x x x x 0-6 a) Write the law for this reactio b) Calculate the costat ad give its uits c) Predict the iitial reactio that would be see i a solutio i which [C 5 H 5 N 5.0 x 0-5 M ad [CH 3 I.0 x 0-5 M /30/004 OFB Chapter 4 5

26 [C 5 H 5 N [CH 3 I Rate (mol L - ) (mol L - ) (mol L - s - ).0 x x x x x x x x x 0-6 a) Write the law for this reactio 3 ( ) [CH 3I 3 ([CH I ) x 0 x ( -4 4 x 0 ) ( -4 x 0 ) /30/004 [ CH I [ C H N OFB Chapter 4 6

27 [C 5 H 5 N [CH 3 I Rate (mol L - ) (mol L - ) (mol L - s - ).0 x x x x x x x x x 0-6 a) Write the law for this reactio /30/004 3 x x 0 4 () -6-7 () [CH [CH I 3 I 3 [C H 5 5 N [C H N 5 5 ( -4 )( - 4 x 0 x 0 ) ( -4 x 0 )( -4 x 0 ) OFB Chapter 4 7 [ C H [ CHI N

28 I a study of the reactio of pyridie (C 5 H 5 N) with methyl iodide (CH 3 I) i a bezee solutio, the followig iitial reactio s were measured at 5ºC for the differet cocetratios of the two reactats. [C 5 H 5 N [CH 3 I Rate (mol L - ) (mol L - ) (mol L - s - ).0 x x x x x x x x x 0-6 a) Write the law for this reactio [ CH I[ C5H5 3 N b) Calculate the costat ad give its uits c) Predict the iitial reactio that would be see i a solutio i which [C 5 H 5 N 5.0 x 0-5 M ad [CH 3 I.0 x 0-5 M /30/004 OFB Chapter 4 8

29 Chemical Equilibrium A direct coectio exists betwee the equilibrium costat of a reactio that taes place i a sequece of steps ad the costats i each step. a.) at equil: forward reactio reverse reactio b.) K eq f / r /30/004 f aa + bb cc + r Forward reactio f f r a c eq dd Reverse reactio.).) [A [B [C [A OFB Chapter 4 9 a b [D [B d b r [C K c r f [D [A [C d c a [B [D d b

30 OFB Chapter 4 30 /30/004 a.) at equil: forward reverse b.) K eq f / r (sometimes / - ) A geeral reactio to illust this priciple. A (g) + B (g) C (g) + D (g) Suppose the reactio proceeds by the followig two step mechaism.) A (g) + A (g) A (g).) A (g) + B (g) C (g) + D (g) - - Rate - - [A Rate - - [C[D Rate [A Rate [A [B [ [ A A K [ [ [ [ B A D C K [ [ [ [ [ [ [ [ [ [ B A D C B A D C A A K K K K

31 PRS Quiz, Questio I a study of the reactio of compoud A ad compoud B i solutio, the followig iitial reactio s were measured at 5ºC for the differet cocetratios of the two reactats. [A [B Rate (mol L - ) (mol L - ) (mol L - s - ).0 x x x x x x x x x 0-5 Q What is the order of the reactio with respect to compoud A? /30/004 OFB Chapter 4 3

32 /30/004 OFB Chapter 4 3

33 PRS Quiz, Solutio I a study of the reactio of compoud A ad compoud B i solutio, the followig iitial reactio s were measured at 5ºC for the differet cocetratios of the two reactats. [A [B Rate (mol L - ) (mol L - ) (mol L - s - ).0 x x x x x x x x x 0-5 Q What is the order of the reactio with respect to compoud A? /30/004 36x0 4 9x0 ([A 3 ) ([A ) ( -4 x0 ) ( -4 x0 ) OFB Chapter

34 PRS Quiz, Questio I a study of the reactio of compoud A ad compoud B i solutio, the followig iitial reactio s were measured at 5ºC for the differet cocetratios of the two reactats. [A [B Rate (mol L - ) (mol L - ) (mol L - s - ).0 x x x x x x x x x 0-5 Q What is the order of the reactio with respect to compoud B? /30/004 OFB Chapter 4 34

35 /30/004 OFB Chapter 4 35

36 PRS Quiz, Solutio I a study of the reactio of compoud A ad compoud B i solutio, the followig iitial reactio s were measured at 5ºC for the differet cocetratios of the two reactats. [A [B Rate (mol L - ) (mol L - ) (mol L - s - ).0 x x x x x x x x x 0-5 Q What is the order of the reactio with respect to compoud B? x /30/004 3x ([B ) ([B ) ( -4 3x0 ) ( -4 x0 ) OFB Chapter 4 36

37 PRS Quiz, Questio 3 I a study of the reactio of compoud A ad compoud B i solutio, the followig iitial reactio s were measured at 5ºC for the differet cocetratios of the two reactats. [A [B Rate (mol L - ) (mol L - ) (mol L - s - ).0 x x x x x x x x x 0-5 Q3 What is the expressio for the above reactio?. Rate [A. Rate [A[B 3. Rate [A [B 4. Rate [A [B /30/004 OFB Chapter 4 37

38 /30/004 OFB Chapter 4 38

39 PRS Quiz, Solutio 3 I a study of the reactio of compoud A ad compoud B i solutio, the followig iitial reactio s were measured at 5ºC for the differet cocetratios of the two reactats. [A [B Rate (mol L - ) (mol L - ) (mol L - s - ).0 x x x x x x x x x 0-5 Q3 What is the expressio for the above reactio?. Rate [A. Rate [A[B 3. Rate [A [B 4. Rate [A [B /30/004 OFB Chapter 4 39

40 f aa + bb cc + r dd a [ A t b f [ B t [A + c m [B [ C t d [ D t Called a Rate Expressio Or Rate Law f called costat a.) at equil: forward reverse b.) K eq f / r /30/004 Forward reactio Reverse reactio.).) f f r [A a [B b c [C [D [A [B d r [C K c r f [D [A [C d OFB a Chapter b 4eq 40 c a [B [D d b

41 4-5 Reactio Mechaism & Rate Laws Typically with a reactio oe of several elemetary step reactio is the slowest step. This is called the Rate Determiig Step (RDS) Case #: Whe the RDS occurs first, the first step is slow ad determies the of the overall reactio..).) NO NO + F + F NO Stepis the RDS NO [NO F + F F [F (slow) (fast) /30/004 NO + F NO F OFB Chapter 4 4

42 .).) NO NO + F + F Stepis NO NO the RDS [NO F + F F [F (slow) (fast) E e r g y slow fast F + NO NO + F NO F Reactio Progress /30/004 NO + F NO F OFB Chapter 4 4

43 Case #: Whe the RDS occurs after oe or more Fast steps, mechaisms are ofte sigaled by a reactio order greater tha two. The slow step determies the overall of the reactio. Overall NO reactio + O NO 3 molecule reactio. Is it A Termolecular or Bimolecular reactios? Three way collisios are rare. Try a two step mechaism..).) NO N O + NO N + O - O NO (fast) (slow) [N O [O But N O is a reactive itermediate /30/004 OFB Chapter 4 43

44 .) NO +.) N O NO + O - N O NO (fast) (slow) [N O [O E slow e fast r g y N O + O NO /30/004 NO Reactio Progress NO + O NO OFB Chapter 4 44

45 .) NO +.) N O NO N + O - O NO (fast) (slow) /30/004 [N O [O Need to express [itermediates i terms of other reactats forward [NO reverse at equilibrium recall K [NO - [N - [N O [NO - that forward O [N OFB Chapter 4 45 O reverse O K [NO [N

46 .) NO +.) N O NO + O - N O NO (fast) (slow) [N O [O O K [NO [N Substitutig for [N O i the expressio above K [NO [O /30/004 OFB Chapter 4 46

47 .) NO +.) N O NO N + O - O NO (fast) (slow) [N O [O K [NO [O E e r g fast slow K y N O + O NO /30/004 NO Reactio Progress NO + O NO OFB Chapter 4 47

48 Reactio Mechaism Itermediates Trasitio states Trasitio States E e r g y slow fast itermediates fast Reactio Progress /30/004 OFB Chapter 4 48

49 Trasitio States /30/004 OFB Chapter 4 49

50 4-6 The Effect of Temperature o Reactio Rates Arrehius Equatio A e E a RT where E is the Activatio eergy a uits are eergy per mole ad A is the "pre - expoetial is a costat ad has factor" the uits of /30/004 OFB Chapter 4 50

51 Arrehius Equatio Ae Ea RT l la y mx + b E a RT l A Arrehius Plot Slope - E a / R /T (K - ) /30/004 OFB Chapter 4 5

52 Trasitio State E e r g y E f a E r a E mius exothermic Reactio Progress E E af -E a r The Activatio Eergy (E a ) is the miimum collisio eergy that reactats must have i order to form products /30/004 OFB Chapter 4 5

53 Trasitio State E e r g y E a f E a r E positive edothermic Reactio Progress E E af -E a r The Activatio Eergy (E a ) is the miimum collisio eergy that reactats must have i order to form products /30/004 OFB Chapter 4 53

54 Chapter 4 Chemical Kietics Catalyst provides a lower eergy path, but it does ot alter the eergy of the startig material ad product rather it chages the eergy of the trasitio (s), i the reactio A catalyst has o effect o the thermodyamics of the overall reactio Ihibitor is a egative catalyst. It slows the of a reactio frequetly by barrig access to path of low E a ad thereby forcig the reactio to process by a path of higher E a. /30/004 OFB Chapter 4 54

55 /30/004 OFB Chapter 4 55

56 Kietics of Catalysis A catalyst has o effect o the thermodyamics of the overall reactio It oly provides a lower eergy path Examples Pt ad Pd are typical catalysts for hydrogeatio reactios (e.g., ethylee to ethae coversio) Ezymes act as catalysts Phases Homogeous catalysis the reactats ad catalyst are i the same catalyst (gas or liquid phase) Heterogeeous catalysis reactio occurs at the boudary of two differet phases (a gas or liquid at the surface of a solid) /30/004 OFB Chapter 4 56

57 Chapter 4 Chemical Kietics Example / exercise 4-, 4-, 4-3, 4-4, 4-5, 4-6, 4-7, 4-8, 4-9 Problems 7, 9,, 5, 9,, 3, 5, 37, 4, 43, 45, 5, 53 /30/004 OFB Chapter 4 57

58 First Order Reactios A useful cocept for st order reactios (oly) is the halflife i.e., the time for ½ [A 0 (see boo for the derivatio) t / t / Applies oly to First order reactios TRY Examples 4-5 ad Exercise /30/004 OFB Chapter 4 58