Reaction Kinetics (7)

1
Reaction Kinetics (7)
Physical Chemistry

• Xuan Cheng
• Xiamen University

2
Physical Chemistry
Reaction Kinetics
Thermodynamic Aspects
Activated Complex Theory structure of the
activated complex (?)
The concepts an equilibrium between the
reactants and the activated complex
The activation process is expressed in terms of
thermodynamic function
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Physical Chemistry
Reaction Kinetics
Thermodynamic Aspects
In terms of concentration
For the process
By analogy to (23.27), define
Gibbs energy of activation
Rate constant
Thermodynamic version of the TST expression for
the rate constant.
4
Physical Chemistry
Reaction Kinetics
Thermodynamic Aspects
Enthalpy of activation
Entropy of activation
Gibbs energy of activation
5
Physical Chemistry
Reaction Kinetics
Thermodynamic Aspects
6
Physical Chemistry
Reaction Kinetics
Thermodynamic Aspects
7
Physical Chemistry
Reaction Kinetics
Catalysis
Catalyst a substance that increases the rate of
a reaction and can be recovered chemically
unchanged at the end of the reaction.
A catalyst
Lower the activation energy
Avoiding the slow, rate-determining step of the
uncatalysed reaction
Results in a higher reaction rate at the same
temperature
Activation energies of catalysed reactions
Reaction
Catalyst
Ea / kJ?mol-1
None
184
Au
105
Pt
59
None
350
W
162
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Physical Chemistry
Reaction Kinetics
Catalysis
E1ltEa E2ltEa
(a) Participate the reaction
(b) Lower the activation energy
(c) High selectivity
Cant change the reaction direction and reaction
equilibrium
(2) The catalysts cant change standard
equilibrium constants
(3) The catalysts change both forward and
backward reaction rates
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Physical Chemistry
Reaction Kinetics
Catalysis
Ethanol Conversion
25 types of products
10
Physical Chemistry
Reaction Kinetics
Homogeneous Catalysis
The kinetics of the bromide-catalysed
decomposition of hydrogen peroxide
The reaction is believed to proceed through the
following pre-equilibrium
(fast)
(in the equilibrium constant, the activity of H2O
has been set equal to 1.)
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Physical Chemistry
Reaction Kinetics
Types of Homogeneous Catalysis
Heterogeneous catalysis normally depends on at
least one reactant being adsorbed (usually
chemisorbed) and modified to a form in which it
readily undergoes reaction.
Acid catalysis
Brønsted acid
Homogeneous catalysis
reacts
(many organic reactions)
Base catalysis
Brønsted base
reacts
Autocatalysis
The acceleration of a reaction by the products.
The reaction rate increases as products are
formed.
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Physical Chemistry
Reaction Kinetics
Autocatalysis
Homework No. 1
The rate law
At what time is the reaction rate a maximum?
Integration by partial fractions, using
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Physical Chemistry
Reaction Kinetics
Heterogeneous Catalysis
Heterogeneous catalysis normally depends on at
least one reactant being adsorbed (usually
chemisorbed) and modified to a form in which it
readily undergoes reaction.
The Eley-Rideal Mechanism
A surface-catalysed reaction, a gas-phase
molecule collides with another molecule adsorbed
on the surface.
Pressure of non-adsorbed gas B
Surface coverage of adsorbed gas A
14
Physical Chemistry
Reaction Kinetics
The Michaelis-Menten Mechanism
Enzyme Catalysis
The proposed mechanism is
The rate of an enzyme-catalysed reaction in which
a substrate S is converted into products P is
found to depend on the concentration of the
enzyme E even through the enzyme undergoes no net
change.
The total enzyme concentration is
Only a little enzyme is added
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Physical Chemistry
Reaction Kinetics
The Michaelis-Menten Mechanism
Enzyme Catalysis
The proposed mechanism is
The Michaelis constant
The Michaelis equation
16
Physical Chemistry
Reaction Kinetics
The Michaelis-Menten Mechanism
Enzyme Catalysis
Plot r vs S
1.When SgtgtKM, r k2E0, independent of S,
zero-order in S.
k2E0 is maximum velocity
k2 is maximum turnover number
2.When S?8?, r rmk2E0.
3.When SltltKM, r k2E0S/KM, is proportional
to S and E0.
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Physical Chemistry
Reaction Kinetics
The Michaelis-Menten Mechanism
Enzyme Catalysis
Plot 1/r vs 1/S, KM and rm can be determined.
18
Physical Chemistry
Reaction Kinetics
Unimolecular Reactions
The Lindemann-Hinshelwood Mechanism
The isomerization of cyclopropane
follows first-order kinetics
A molecule acquires enough energy to react as a
result of its collisions with other molecules.
However, collisions are simple bimolecular
events.
The problem how can they result in a first-order
rate law?
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Physical Chemistry
Reaction Kinetics
Unimolecular Reactions
The Lindemann-Hinshelwood Mechanism
The L-H mechanism proposed that a reactant
molecule A becomes energetically excited by
collision with another A molecule.
The energized molecule might lose its excess
energy by collision with another A molecule.
Alternatively, the excited molecule might shake
itself apart and form products P (it might
undergo the unimolecular decay).
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Physical Chemistry
Reaction Kinetics
Unimolecular Reactions
The Lindemann-Hinshelwood Mechanism
The steady-state approximation to the net rate of
formation of A.
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Physical Chemistry
Reaction Kinetics
Unimolecular Reactions
The Lindemann-Hinshelwood Mechanism
This is not first-order
If the rate of deactivation by (A, A) collision
is much greater than the rate of unimolecular
decay
first-order
second-order
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Physical Chemistry
Reaction Kinetics
Improvements of the Theory
RRKM Mechanism
The L-H mechanism agrees in general with the
switch in order of unimolecular reactions, but
does not agree in detail.
A plot of 1/k against 1/A expects to be a
straight line.
The graph has a pronounced curvature when a
larger value of k (small 1/k) at higher pressure
(low 1/A)
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Physical Chemistry
Reaction Kinetics
Improvements of the Theory
RRKM Mechanism
The L-H mechanism fails to recognize that
specific excitation of the molecule may be
required before reaction occurs.
In the isomerization of cyclopropane, the crucial
step is the breaking of one C-C bond, and this
occurs when that bond is highly vibrationally
excited.
discrepancy
In the collision leading to excitation, the
excess energy is shared among all the bonds and
the rotation of the molecule, so the
isomerization occurs only after the energy
accumulates in the critical bond.
The existence of this delay energized molecule
A vs activated state A?
24
Physical Chemistry
Reaction Kinetics
Improvements of the Theory
RRKM Mechanism
The unimolecular part of the L-H mechanism need
to be modified to
The values of the various rate constants are
related to the numbers and frequencies of the
vibrational modes.
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Physical Chemistry
Reaction Kinetics
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Physical Chemistry
Reaction Kinetics
Reactions in solution
Reading Assignments
Chemically-controlled Reactions
Diffusion-controlled Reactions
Mixed-control Reactions
23.2 23.8 23.12 23.17 23.21
Homework
27
Good Luck!

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