Reaction Mechanisms
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+ Reaction Mechanisms 1 Learning Goals: I will become familiar with reaction mechanisms and how they relate to the rate law
description
Learning Goals: I will become familiar with reaction mechanisms and how they relate to the rate law . Reaction Mechanisms. Reaction Mechanisms. A series of steps by which a chemical reaction occurs Elementary step: only involves one, two or possibly three particles in a collision . - PowerPoint PPT Presentation
Transcript of Reaction Mechanisms
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Reaction Mechanisms
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Learning Goals:I will become familiar with reaction mechanisms and how they relate to the rate law
+ Reaction Mechanisms A series of steps by which a chemical reaction occurs
Elementary step: only involves one, two or possibly three particles in a collision.
Rate Determining Step: slowest step; determines the rate of the overall reaction
Reaction Intermediate : not a reactant or a product but is an unstable compounds formed and used up in a reaction.
Activation Complex: high energy unstable state
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Potential Energy
(kJ)
Reaction Progress (s)
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
Actual progression of the reaction through 3 intermediate reactions
4 HBr + O2
2 H2O + 2 Br2
Overall impression of the reaction’s progress
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Potential Energy
(kJ)
Reaction Progress (s)
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
4 HBr + O2
2 H2O + 2 Br2
Activated complexes
Reaction intermediates
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+1. HBr(g) + O2(g) HOOBr(g) (slow)2. HOOBr(g) + HBr(g) 2 HOBr(g) (fast)3. 2x(HOBr(g) + HBr(g) H2O(g) + Br2(g)) (fast)
4 HBr(g) + O2(g) 2 H2O(g) + 2 Br2(g)
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Potential Energy
(kJ)
Reaction Progress (s)
4 HBr + O2
2 H2O + 2 Br2
HOOBrHOBr
+ Elementary Reaction Steps Rate Determining Step
1. HBr(g) + O2(g) HOOBr(g) (slow)2. HOOBr(g) + HBr(g) 2 HOBr(g) (fast)3. 2x(HOBr(g) + HBr(g) H2O(g) + Br2(g)) (fast)
4 HBr(g) + O2(g) 2 H2O(g) + 2 Br2(g)
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Potential Energy
(kJ)
Reaction Progress (s)
4 HBr + O2
2 H2O + 2 Br2
Ea
DH
HOOBrHOBr
Reaction Intermediates
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Potential Energy
(kJ)
Reaction Progress (s)
A catalyst reduces the overall activation energy for the reaction by producing alternative reaction intermediates with lower energy states.
Uncatalyzed reaction path
Overall reaction
Catalyzed reaction path
Uncatalyzed activation energyCatalyzed
activation energy
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+ Let’s Try It:
Write out the elementary steps for the reaction above and find the overall reaction equation.
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+ ExampleA proposed mechanisms for the
reaction is show below: I2(g) 2I(g) slow
H2(g) + 2I(g) 2HI(g) fast (a) what is the overall reaction? (b) are there any intermediates? (c) what is the rate law equation?
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+Explaining & Applying Chemical Kinetics Increasing the temperature of the system increases the
number of particles with sufficient energy to react.
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Number of
particles
Kinetic energy (kJ)
ET
Particles with sufficient energy to effectively collide in
the chemical reaction.
New particles with sufficient energy to effectively collide in
the chemical reaction.
+Explaining & Applying Chemical Kinetics An increase in concentration results in more particles with
the required threshold energy being added to the system.
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Number of
particles
Kinetic energy (kJ)
ET
Particles with sufficient energy to effectively collide in
the chemical reaction.
New particles with sufficient energy to effectively collide in
the chemical reaction.Increased
concentration
+Explaining & Applying Chemical Kinetics
A catalyst reduces the threshold energy and consequentially, activation energy .
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Numberof
particles
Kinetic energy (kJ)
ET
Particles with sufficient energy to effectively collide in
the chemical reaction.
New particles with sufficient energy to effectively collide in
the chemical reaction.
ET Catalyzed
+Explaining & Applying Chemical KineticsA catalyst reduces the activation energy and threshold energy.
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Ep
(kJ)
Reaction Progress
ET
ET Catalyzed
