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CHEM 636 Fal l 2 6 Exam

1 hort nswers (5 Pts each)(a) What is th e Principle of Microscopic Reversibility?

(b) Which complex would be expected to undergo more rapid CO substitution? Why?Fe CO)5

(c) It took 150 sec for 50 of particular reagent to decompose. If the initialconcentration is 0.050 and the decomposition reaction follows second-orderkinetics, what is the value of the ra t e constant?

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HEM636 Fall 2006 xam I Pg

(d) For a re ac tio n which obeys first-order kin etics and has a l value of OO x 10-~sec- t2s0C, w h a t is the value of the half-life a t 25 C?

(e) A typ ical diffusion controlled bim ole cu lar rate constant (include un its ) at ambienttem per atur e in an organic solvent is: I lo o1 ~ i ~

f) S on e proce eds from a D(dissoc iative) mechanism to an A(associative) mechanismwhat a re the trends in AH', AS', and AV . At this point do not w rry abourprocesses

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CHEM 636 - Full 2 6 Exam Pg 3

2. Derive the rate law for the equilibrium process below which involves the intramolecularisomerization of an octahedral metal carbonyl derivative.

Given the kinetic data for the forward reaction (cis + rans) at various temperatures, i-e.,ko sd kl k-1 , and if the equilibrium constant I ,measured at 64.8 C of 5 3 s not verytemperature dependent between 60 and 80C, calculate AH and AS for the cis+ ransprocess. Draw an energy vs reaction coordinate profile and clearly indicate which isomer isthermodynamically more stable. Would you expect the value of o be greater or less than5 3 if n-Bu3P is replaced by the smaller PMe3 ligand? Explain.

Temp. ( C) sd104sec-'

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y = -12091x + 21.254

R2= 0.9983

-14.8

-14.6

-14.4

-14.2

-14

-13.8

-13.6

-13.4

-13.2

-13

-12.8

0.00282 0.00284 0.00286 0.00288 0.0029 0.00292 0.0029

1 / T

l

n(

k/T)

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CHEM 636 Fal l 2 6 - xum

3. For the ligand substitution processes desc ribed below, bsds a composite of two reactionpath waysMo(CO)G+ PPh3 -+ M o ( C O ) ~ P P ~ ~CO (1)

kobsd = {kl + k2 [PPh3]}

Pt(dien)CI + Br- -+ Pt(dien)Br + CI-(dien = a tridentate amine ligand)

kobsd = {kl + k2[Bf]}a) Com pare and contrast the significance of and the relative importanc e of the two termsin the rate-laws for each process.

II L e Mo-c~ i s s O ~ - ~ ~ L t ,O(CO)-> Mo(0)5 + C Q ) W4,~f ~hd s . -4-L k'k on5ck si . TA. h s ~ d i 4 ~ l p m -

~ Wo w ) ~s \ ~ c l hF I ~ T3vi ,.Hu. b- sw& S h L 4 - L , . , ~ ~ ~ h ~-koLv 3ac - i h L k i ~ U ~ ~ \ / c l b ~ ~ < + , , ~ ~ .

4 u5;2 m d ay-r + a . 'D~SLb) Assuming both reactions are spontaneous as writteni) Fo r reaction 1) draw the reaction profile for the process pr oce edi ng via kiAcco unt for the fact that the intermediate in this process do e s not discriminatebetween incoming ligands.

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HEM636 all 2 6 Exum P g 6

ii) For reaction 2) draw the reaction profile for the process proceeding via k2where bond-breaking is the rate-determining step.

- hoc) Is AV for the k l pathway in reaction 1) negative or positive? Exp lain. W ould an

increase in pressure retard or enh ance the process?

d) Is AV for the k2 pathway in reaction 2) negative or positive? Explain. Wou ld anincrease in pressure retard or enh ance the process?

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CHEM 636 Fa l l 2006 xam 1 Pg

4 a) The ligand substitution reaction below was shown to occur vi a d issociative D)process.

Th e kinetic data in the table contains the observed rate constants as a function of[NHC sHlo] and [CO] for reactions run at 28C. Deriv e the rate law for this process.From the data in the table calculate the com petition ratio of the five coordinateintermediates for piperidine NHC 5Hlo) vs CO.

a[CO ] is in large excess

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y = 3668x + 1567.6R

2= 0.9987

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0 2 4 6 8

[NHC5H10] / [CO]

1

/k

obsd

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HEM636 Fall 2 6 xam Pg

b) What is known about the reactivity of five-coordinate intermediates of this typewith incoming ligands in hydrocarbon and in perfluorohydrocarbon solvents?