Download Reaction mechanisms and catalysts

REACTION MECHANISMS
AND CATALYSTS
6.4
Reaction Mechanisms
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The transition state theory is used to explain the
process in a reaction that results from an effective
collision.
The collision of two particles may result in the
formation of a new temporary unstable substance that
behaves as a temporary complex in the formation of
a more stable product
This high energy unstable state is called an activated
complex.
Within the activated complex the bonds are not
completely formed or broken which represents a
maximum potential energy state called the transition
state.
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Transition state
Activated complex
Threshold
energy
Potential
Energy (kJ)
Ea
Products
DH
Reactants
Reaction Progress (s)
3
Reaction Mechanisms

We tend to focus on the start and end of chemical
reactions. The pathway from start to finish is often
comprised of intermediate or elementary steps.
Elementary steps only involve one, two or possibly three
particles in a collision.
 Unstable compounds may be produced in these elementary
steps and are called reaction intermediates.
 The slowest of these elementary steps is the rate determining
step.
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The sum of these elementary steps represents the
overall sequence or reaction mechanism.
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The overall reaction may be comprised of individual
elementary reaction steps. Each involving their own
energy changes and rates.
4 HBr + O2
2 H2O + 2 Br2
Overall impression of
the reaction’s progress
Potential
Energy
(kJ)
Actual progression of
the reaction through 3
intermediate reactions
4 HBr + O2
2 H2O + 2 Br2
Reaction Progress (s)
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The overall reaction may be comprised of individual
elementary reaction steps. Each involving their own
energy changes and rates.
4 HBr + O2
2 H2O + 2 Br2
Activated complexes
Reaction intermediates
Potential
Energy
(kJ)
4 HBr + O2
2 H2O + 2 Br2
Reaction Progress (s)
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1.
2.
3.
HBr(g) + O2(g)  HOOBr
HOOBr
(g)
HOBr
HOOBr(g) + HBr(g)  2 HOBr
(g)
2x(HOBr(g) + HBr(g)  H2O(g) + Br2(g))
4 HBr(g) + O2(g)  2 H2O(g) + 2 Br2(g)
Potential
Energy
(kJ)
4 HBr + O2
2 H2O + 2 Br2
Reaction Progress (s)
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(slow)
(fast)
(fast)
Rate Determining Step
Elementary Reaction Steps
1.
2.
3.
HBr(g) + O2(g)  HOOBr(g)
HOOBr(g) + HBr(g)  2 HOBr(g)
2x(HOBr(g) + HBr(g)  H2O(g) + Br2(g))
4 HBr(g) + O2(g)  2 H2O(g) + 2 Br2(g)
Reaction
Intermediates
Ea
Potential
Energy
(kJ)
HOOBr
4 HBr + O2
HOBr
DH
2 H2O + 2 Br2
Reaction Progress (s)
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(slow)
(fast)
(fast)
Rate Law Equations for Elementary
Reactions

For elementary reactions, the exponents in the rate
law equation are the same as the stoichiometric
coefficients for each reactant in the chemical
equation.
Proposing and Evaluating Mechanisms

When chemists propose a mechanism, they must
satisfy the following criteria:
 The
equation for the elementary steps must combine to
give the equation for the overall reaction
 The proposed elementary steps must be reasonable
 The mechanism must support the experimentally
determined rate law.
Rate-Determining Step and the Rate
law
Example:
The rate law will depend on the rate-determining
reaction.
A catalyst reduces the overall activation energy for
the reaction by producing alternative reaction
intermediates with lower energy states.
Overall reaction
Uncatalyzed reaction
path
Potential
Energy
(kJ)
Catalyzed reaction
path
Uncatalyzed
activation energy
Catalyzed
activation energy
Reaction Progress (s)
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
Example:
Reaction of sodium potassium
tartrate with H2O2, catalyzed
by cobaltous chloride.
 CoCl2 is pink in solution.
 Contents of beaker turn dark
greed, suggesting the formation
of a reaction intermediate.
 CoCl2 is regenerated when the
reaction is over.

Explaining & Applying Chemical Kinetics

Increasing the temperature of the system increases
the number of particles with sufficient energy to
react.
ET
Number
of
particles
New particles with sufficient
energy to effectively collide in
the chemical reaction.
Particles with sufficient
energy to effectively collide in
the chemical reaction.
Kinetic energy (kJ)
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Explaining & Applying Chemical Kinetics

An increase in concentration results in more particles
with the required threshold energy being added to
the system.
Increased
concentration
ET
Number
of
particles
New particles with sufficient
energy to effectively collide in
the chemical reaction.
Particles with sufficient
energy to effectively collide in
the chemical reaction.
Kinetic energy (kJ)
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Explaining & Applying Chemical Kinetics

A catalyst reduces the threshold energy and
consequentially, activation energy .
ET Catalyzed
ET
Number
of
particles
New particles with sufficient
energy to effectively collide in
the chemical reaction.
Particles with sufficient
energy to effectively collide in
the chemical reaction.
Kinetic energy (kJ)
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Explaining & Applying Chemical Kinetics

A catalyst reduces the activation energy
and threshold energy.
ET
Ep
ET Catalyzed
(kJ)
Reaction Progress
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Homogeneous Catalysts
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Exists in the same phase as the reactant.
Usually catalyze gaseous and aqueous reactions.
Heterogeneous Catalysts
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Exist in a phase different from the phase of the
reaction.
Example: addition of hydrogen to an organic compound
that contains C=C bonds: ___________.
Without a catalyst, the reaction is very slow.
Ethylene and hydrogen molecules form bonds with the
metal surface. This weakens the bonds of the hydrogen
and ethylene
The H-H bonds of the hydrogen molecules break and are
somewhat stabilized because of their attraction to the metal.
 Hydrogen atoms react with ethylene, forming ethane.
