Chemical Kinetics

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Chemical Kinetics
Chapter 13
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• 13.1 ????
• 13.2 ????
• 13.3 ???? ??? ???? ??
• 13.4 ??????? ?????? ???
• 13.5 ?? ????? ????
• 13.6 ??

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Chemical Kinetics
Thermodynamics does a reaction take
place? Kinetics how fast does a reaction
proceed?
Reaction rate is the change in the concentration
of a reactant or a product with time (M/s).
DA change in concentration of A over
time period Dt
DB change in concentration of B over
time period Dt
Because A decreases with time, DA is
negative.
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DBr2 a DAbsorption
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rate a Br2
rate k Br2
rate constant
3.50 x 10-3 s-1
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PV nRT
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Reaction Rates and Stoichiometry
Two moles of A disappear for each mole of B that
is formed.
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The Rate Law
The rate law expresses the relationship of the
rate of a reaction to the rate constant and the
concentrations of the reactants raised to some
powers.
Rate k AxBy
reaction is xth order in A
reaction is yth order in B
reaction is (x y)th order overall
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rate k F2xClO2y
x 1
rate k F2ClO2
y 1
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Rate Laws

• Rate laws are always determined experimentally.

• Reaction order is always defined in terms of
  reactant (not product) concentrations.

• The order of a reactant is not related to the
  stoichiometric coefficient of the reactant in the
  balanced chemical equation.

rate k F2ClO2
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Experiment S2O82- I- Initial Rate (M/s)
1 0.08 0.034 2.2 x 10-4
2 0.08 0.017 1.1 x 10-4
3 0.16 0.017 2.2 x 10-4
rate k S2O82-xI-y
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First-Order Reactions
rate k A
k
1/s or s-1
A is the concentration of A at any time t
A0 is the concentration of A at time t0
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Decomposition of N2O5
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lnA lnA0 - kt
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First-Order Reactions
The half-life, t½, is the time required for the
concentration of a reactant to decrease to half
of its initial concentration.
t½ t when A A0/2
Q. What is the half-life of N2O5 if it decomposes
with a rate constant of 5.7 x 10-4 s-1?
How do you know decomposition is first order?
units of k (s-1)
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First-order reaction
1
2
2
4
3
8
4
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Second-Order Reactions
rate k A2
k
1/Ms
A is the concentration of A at any time t
A0 is the concentration of A at time t0
t½ t when A A0/2
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Zero-Order Reactions
rate k A0 k
k
M/s
A is the concentration of A at any time t
A A0 - kt
A0 is the concentration of A at time t0
t½ t when A A0/2
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Summary of the Kinetics of Zero-Order,
First-Order and Second-Order Reactions
A A0 - kt
0
rate k
1
rate k A
lnA lnA0 - kt
2
rate k A2
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Exothermic Reaction
Endothermic Reaction
The activation energy (Ea ) is the minimum amount
of energy required to initiate a chemical
reaction.
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Temperature Dependence of the Rate Constant
k A exp( -Ea/RT )
(Arrhenius equation)
Ea is the activation energy (J/mol)
R is the gas constant (8.314 J/Kmol)
T is the absolute temperature
A is the frequency factor
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Reaction Mechanisms
a series of simple elementary steps or elementary
reactions.
The sequence of elementary steps that leads to
product formation is the reaction mechanism.
N2O2 is detected during the reaction!
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Intermediates are species that appear in a
reaction mechanism but not in the overall
balanced equation.
An intermediate is always formed in an early
elementary step and consumed in a later
elementary step.

• The molecularity of a reaction is the number of
  molecules reacting in an elementary step.
• Unimolecular reaction elementary step with 1
  molecule
• Bimolecular reaction elementary step with 2
  molecules
• Termolecular reaction elementary step with 3
  molecules

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Rate Laws and Elementary Steps
rate k A
rate k AB
rate k A2

• Writing plausible reaction mechanisms
• The sum of the elementary steps must give the
  overall balanced equation for the reaction.
• The rate-determining step should predict the same
  rate law that is determined experimentally.

The rate-determining step is the slowest step in
the sequence of steps leading to product
formation.
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Sequence of Steps in Studying a Reaction Mechanism
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Q. The experimental rate law for the reaction
between NO2 and CO to produce NO and CO2 is rate
kNO22. The reaction is believed to occur via
two steps
What is the equation for the overall reaction?
What is the intermediate?
NO3
What can you say about the relative rates of
steps 1 and 2?
rate kNO22 is the rate law for step 1 so
step 1 must be slower than step 2
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Chemistry In Action Femtochemistry


CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
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catalyst
k A exp( -Ea/RT )
Catalyzed
Uncatalyzed
ratecatalyzed gt rateuncatalyzed
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In heterogeneous catalysis, the reactants and the
catalysts are in different phases.

• Haber synthesis of ammonia
• Ostwald process for the production of nitric acid
• Catalytic converters

In homogeneous catalysis, the reactants and the
catalysts are dispersed in a single phase,
usually liquid.

• Acid catalysis
• Base catalysis

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Haber Process
Gerhard Ertl
N2(g) ? N2(adsorbed) N2(adsorbed) ? 2N(adsorbed)
H2(g) ? H2(adsorbed) H2(adsorbed) ?
2H(adsorbed) N(adsorbed) 3H(adsorbed)?
NH3(adsorbed) NH3(adsorbed) ? NH3(g)
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Fritz Haber (1868 1934)
Haber ??? ?? 1??? ?? ?? ??. ?? ????? 35? Haber
?? ?? ??? ???. ?? ??? ???? 12. ? ?? ??? ?? ??
1/3? ??? ???? ??. Haber?? ???? ????? ??? ????
?????. 1918? ?? ??? ??
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Ostwald Process
Pt catalyst
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Catalytic Converters
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Enzyme Catalysis
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rate k ES
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Michaelis-Menten Mechanism
k1 k-1
k2

• E S ES ? E P

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Michaelis-Menten Mechanism
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??????? ??
??? ??
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??????? ??
?? ??? ???? ??? gt ????? ????
A A0 - kt
0
rate k
1
rate k A
lnA lnA0 - kt
2
rate k A2
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??????? ??
?? ??? ?? - Arrhenius equation
k A exp( -Ea/RT )
Ea is the activation energy (J/mol), ??? ??
R is the gas constant (8.314 J/Kmol)
T is the absolute temperature
A is the frequency factor, ???
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??????? ??
??????? ?? ??
rate k A
rate k AB
rate k A2
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??????? ??
?? ??? ??, ?? ??? ???? ??
Haber process
?? ?? ??
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??1-1
350??? ????? ????? 4.6010-4 s-1 ??. ??????? 104
kJ/mol ? ?, ????? 8.8010-4 s-1 ? ?? ??? ?????.
R(gas constant) 8.314 J/Kmol
(1) 644 K (2) 356 K (3) 371 K (4)
?________K ??? ?? ??? ?? ??? ??? ??.
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??1-2
??? 295 K?? 305 K? ? ? ????? ? ?? ?? ??? ??. ?
??? ??????? ?????.
R(gas constant) 8.314 J/Kmol
(1) 52 kJ/mol (2) 105 kJ/mol (3) 190
kJ/mol (4) ?________ kJ/mol ?? ??? ?? ??? ???
??.
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??1-3
????? ??? ???? ??? ??? 1/8? ???? ? ??? ??????
???(t1/2)? ?? ????, ?? ???? k? ?? ?????.
(1) 8 t1/2 (2) 4 t1/2 (3) 3 t1/2 (4)
?____ t1/2 (A) ln(8)/k (B) ln(4)/k (C)
ln(2)/k (D) ln(2) k ?? ??? ?? ??? ??? ??.