From rate law to reaction mechanism

1
From rate law to reaction mechanism Products of a
reaction can never be produced faster than the
rate of the slowest elementary reaction - rate
determining step Experimental data for the
reaction between NO2 and F2 indicate a
second-order rate Overall reaction 2 NO2(g)
F2(g) ? 2FNO2(g) Rate k NO2 F2 From the
rate law reaction does not take place in one
step How can a mechanism be deduced from the rate
law?
2
Rate determining step must involve NO2 and F2 in
11 ratio Possible mechanism Step 1 NO2(g)
F2(g) ? FNO2(g) F(g) slow Step 2 NO2(g)
F(g) -gt FNO2(g) fast Overall 2 NO2(g) F2(g)
? 2FNO2(g) Rate for step 1 k1 NO2 F2 rate
determining step Rate for step 2 k2 NO2 F
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For the reaction 2 H2(g) 2NO(g) ? N2(g)
H2O(g) The observed rate expression is rate
kNO2 H2 The following mechanisms have been
proposed. Based on the rate law can any
mechanism be ruled out?
k1
Mechanism I 2 H2(g) 2NO(g) ? N2(g) H2O(g)
k2
Mechanism II H2(g) NO(g) ? N(g) H2O(g) slow
k3
NO(g) N(g) ? N2(g) O(g) fast
k4
O(g) H2 (g) ? H2O(g) fast
k5
Mechanism III H2(g) 2NO(g) ? N2O(g)
H2O(g) slow
k6
H2(g) N2O(g) ? N2(g) H2O(g) fast
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Mechanism I rate k1H22 NO2 not possible
Mechanism II rate k2H2 NO not possible
Mechanism III rate k5H2 NO2 possible If
mechanism III is a possible mechanism, try to
detect N2O experimentally to confirm mechanism.
5
Mechanism involving an initial fast reaction that
produces an intermediate, followed by a slower
second step. Rate law of an elementary step must
be written with respect to the reactants only an
intermediate cannot appear in the rate law 2
NO(g) O2(g) ? 2 NO2(g) rate k NO2 O2
Experiments indicate that an intermediate is
involved in this reaction Step 1
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Rate law for second step k2 NO OONO Cannot
be compared with experimental rate law since OONO
is an intermediate Rate of production of OONO
k1 NO O2 Rate of consumption of OONO to NO
and O2 k-1 OONO Rate of forward and reverse
of 1 gt rate of 2 equilibrium is established in 1
before significant amount of OONO consumed to
form NO2 At equilibrium k1 NO O2 k-1 OONO
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Rate k2 NO OONO k2 NO (K NO O2)
k2 K NO2 O2 k NO2 O2
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Stratospheric Ozone Chapman mechanism
(1930s) (1) O2 light ? 2O. (2) O. O2 M ?
O3 M (3) O3 light ? O. O2 (4) O. O3 ? 2
O2 For O3 concentration to be in balance Rate of
production of O3 by reactions (1) and (2) must
equal rate at which is O3 consumed in (3) and
(4) In the 1960s data published showed that
reaction (4) was much too slow to balance levels
of O3
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Paul Crutzen realized that rate constants could
not explain measured distribution of ozone in the
stratosphere. Using experimental data, Crutzen
presented a model NO O3 ? NO2 O2 NO2 O. ?
NO O2 Overall O. O3 ? 2O2 Contributed to
reaction (4) in Chapman mechanism
1995 Nobel Prize in Chemistry
10
Kinetics and Equilibrium

• For a reaction which occurs in a single
  elementary step

NO(g) O3 (g) NO2 (g) O2 (g)
Rate of forward reaction k1 NO O3 Rate of
reverse reaction k-1 NO2 O2 At
equilibrium rate of forward reaction rate
of reverse reaction k1 NOeq O3eq k-1
NO2eq O2eq where the eq denotes equilibrium
concentrations
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where K is the equilibrium constant
12
For the reaction
K (N2 H2O2) /( NO2 H22)
The reaction occurs through three elementary
reactions
NO(g) NO(g) N2O2(g)
N2O2(g) H2(g) N2O(g) H2O(g)
N2O(g) H2(g) N2(g) H2O(g)
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At equilibrium k1 NO2eq k-1 N2O2eq k2
N2O2eq H2eq k-2 N2Oeq H2Oeq k3
N2Oeq H2eq k-3 N2eq H2Oeq
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Chain reactions
Reaction which proceeds through a series of
elementary steps, some of which are repeated many
times.
A highly reactive intermediate reacts to produce
another highly reactive intermediate
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Chain reactions include reactions in Atmosphere
- ozone depletion Explosions Polymerization Nuclea
r Fission Anti-oxidants Many chain reactions
involve free-radicals - atoms or molecules with
one or more unpaired electron formed by
homolytic cleavage of a covalent bond
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Overall H2(g) Cl2(g) ? 2 HCl(g) Cl2(g)
light ? 2 ?Cl(g) initiation ?Cl(g) H2 (g)
? HCl(g) ?H(g) propagation ?H(g) Cl2(g) ?
HCl(g) ?Cl(g) propagation ?Cl(g) ?Cl(g)
? Cl2(g) termination ?H(g) ?H(g) ?
H2(g) termination ?H(g) ?Cl(g) ?
HCl(g) termination
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Stratospheric Ozone depletion by
chlorofluorocarbons (CFCs)
CCl2F2 hn ? CF2Cl Cl? CCl2F2 O? ? CF2Cl
ClO?
reactions which generate free radicals
?Cl O3 ? ?ClO O2
?ClO ?O ? ?Cl O2
Overall Reaction
?O O3 -gt 2O2
Ozone Depletion
1995 Nobel Prize in Chemistry
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Polymerization
initiation
propagation
termination