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