empirical rate laws [compatibility mode]

8/11/2019 Empirical Rate Laws [Compatibility Mode]

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Empirical rate laws.

Master Courses I SemesterAdvanced Physical Chemsitry Chemical Kinetics

Assoc. Prof. Eng.Alexandra Csavdri, PhD.


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Basic concepts of


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Some basic conceptsof chemical kinetics

Advance variable (number of moles,

concentration, conversion) Limitative component

Reaction rate r

Rate law (differential, integral)

Significance of reaction order (n) and ratecoefficient (kobs)

Rate dependence on T:

in

ii

Cobskdt

idC

i

r ==

1

=

TR

aEAk exp


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Examples of rate laws- Units ofkobs dependent on n -

n Rate law [k] - example

0 r = k mole.dm-3.s-1

1 r = k [H2O2] s-1

1.5 r = k [C2H5OH]1.5 mole-0.5.dm1.5.s-1

2, I r = k [N2O]2 mole-1.dm3.s-1

2, II r = k [CH3COOC2H5][NaOH] mole-1.dm3.s-1

3 r = k [NO]2[O2] mole-2.dm6.s-1

2 3 r = k [H2O2][I] + k [H2O2][I

][H+] ---


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Experimental detn. of n and kobs

Isolation of reactant - Degeneration of n

Experimental methods: Differential methods: (C,r) / (P,r) data

Integral methods: (t,C) / (t,P) data(P = Property of reaction mixture; it reflects thereaction advance)

Rate dependence on T Ea Arrhenius type linearization


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Experimental information

Reaction mechanism

Partial reaction orders Experimental rate law

Activation energy

Influence of solvent (ionic strength, dielectric

constant) Product distribution

Effect of catalyst / inhibitor


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Dependence r = f(C) for various n

r = kobsC

n

rn>1

ln(r) = ln(kobs) + n ln(C)ln(r)

Experimental detn. of n and kobsExperimental data

C

n=0

n


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Zeroth

Order (n=0), r = kobs Species does not affect reaction rate

Species is involved in the reaction mechanismAFTERthe rate determining step

Example:

CH3COCH3 + I2 CH3COCH2I + H+ + I

(acid autocalaysis)

r = k [CH3COCH3] [H+] f ([I2])

CH3COCH3 + H+ [CH3C(OH)CH3]

+ (fast)

[CH3C(OH)CH3]+ CH3C(OH)CH2 + H

+ (rate determining)

CH3C(OH)CH2 + I2 CH3COCH2I + H+ + I

r

f ([I2]); nIodine = 0


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First Order (n=1), r = kobsC Species is involved in the reaction mechanism as

a WHOLE molecule, ONCE, either in the rate

determining step itself or in another step thatprecedes it

1 molecule in 1 step

Example:

CH3COOC2H5 + NaOH CH3COONa + C2H5OH

r = k [CH3COOC2H5] [NaOH]


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Second Order (n=2), r = kobsC2

Example:

2 NO + O2 2 NO2 ; r = k [NO]2 [O2]

2 NO N2O2 (fast)

N2O2 + O2 2 NO2 (rate determining)I

NO + O2 NO3 (fast)

NO3 + NO 2 NO2 (rate determining)II


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Subunitary Order (n


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Subunitary Order (n=0.5), r = kobsC1/2

Species dissociates in two identical parts(radicals) prior the rate determining step

Species is involved in the reaction mechanism asthe RADICAL, either in the rate determining step

Often indicates reaction mechanisms involvingfree radicals

Stoechiometry of involvement: 1 molecule breaks into 2 identical radicals in 1 step further: 1 radical involved in 1 step


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Subunitary Order (n=0.5), r = kobsC1/2

Example:

MB + H2A MBH + A + H+

]2[][2/1]2)[(2

2/11][

++==

AHHMBKkKMBdr

(MB+)2 2 MB+ (K1) (fast)

MB+

+ H+

MBH+

(K2) (fast)MBH+ + H2A Products + H

+ (k) (rate determining)

][21 +

+ HKdt

MB = metylene blue, H2A = ascorbic acid


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Overunitary Order (n>1), r = kobs

Cn

I. Parallel Reactions:

- Mechanism contains at least 2 different reaction

paths: of n=1 and of n=2 with respect to species- rate law: r = k1C + k2C

2

II. Combination between first order

elementary step and the one plus typemechanism

- Species involved once, as a WHOLE, in 1 step prior

further involvement in one plus type mechanism

- rate law: CCk

Ckr

,1+

=


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References1. I. Bldea, Cinetica chimica si mecanisme de

reactie. Baze teoretice si aplicatii, PresaUniversitara Clujeana, Cluj-Napoca, 2002, pg.11-20, 35-47

2. I. Bldea, Deducerea mecanismului de reactie,Presa Universitara Clujeana, Cluj-Napoca,

2008, pg. 69-99, 205-211

empirical rate laws [compatibility mode]

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