Download Chemistry FIFTH EDITION by Steven S. Zumdahl University of Illinois

Chemistry
FIFTH EDITION
by Steven S. Zumdahl
University of Illinois
Copyright©2000 by Houghton
Mifflin Company. All rights reserved.
1
Chemistry
FIFTH EDITION
Chapter 12
Chemical Kinetics
Schedule:
http://www2.fultonschools.org/teacher/warrene/
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2
Chemical Kinetics
• The area of chemistry that
concerns reaction rates.
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3
Main Goal of Chemical Kinetics
To understand the steps by which a reaction
takes place.
That is,
REACTION MECHANISM
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Section 12.6
REACTION MECHANISMS
MOST CHEMICAL RXNS. OCCUR BY A SERIES
OF STEPS called the REACTION MECHANISM
EACH STEP IN THE SERIES IS A SINGLE
MOLECULAR EVENT called an
ELEMENTARY REACTION.
THE SET OF ELEMENTARY REACTIONS
IS THE
REACTION MECHANISM.
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Overall Balanced Equation:
NO2 (g) + CO (g)  NO (g) + CO2 (g)
Reaction Mechanism:
Step 1: NO2 + NO2 (g)  NO3 (g) + NO (g)
Step 2: NO3 + CO (g)  NO2 (g) + CO2 (g)
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Figure 12.9
A Molecular Representation of the Elementary Steps
in the Reaction of NO2 and CO
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The Sum of the Set of Elementary Reactions
GIVES
the Overall Effect represented by
a NET Chemical Equation.
What is happening on the Molecular Level
is best represented by the
Reaction Mechanism and is
often more Complicated than that represented by
the Simple Chemical Equation.
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REACTION INTERMEDIATE
• SPECIES PRODUCED DURING A RXN.
THAT DOES NOT APPEAR IN THE NET
EQUATION BECAUSE IT REACTS IN A
SUBSEQUENT STEP IN THE
MECHANISM.
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Figure 12.9
A Molecular Representation of the Elementary Steps
in the Reaction of NO2 and CO
NO3
NO3 is a reaction intermediate.
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10
ELEMENTARY STEPS
RATE LAW DEPENDS ON MOLECULARITY
(i.e., the # of species that must collide to produce the
reaction indicated by that step.)
UNIMOLECULAR STEP:
Rxn. involving one molecule; always 1st order.
BIMOLECULAR:
Rxn by the collision of 2 species;
Always 2nd order.
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• TERMOLECULAR:
Rxn. by the collision of 3 species;
Very rare
Probability of 3 molecules colliding
simultaneously is very small.
See Table 12.7 on page 550.
Examples of Elementary Steps.
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12
REACTION MECHANISM
SERIES OF ELEMENTARY STEPS THAT
MUST SATISFY TWO REQUIREMENTS:
1. SUM OF ELEMENTARY STEPS MUST GIVE
THE OVERALL BALANCED EQUATION FOR
THE REACTION.
2. MECHANISM MUST AGREE WITH THE
EXPERIMENTALLY DETERMINED RATE LAW.
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First Requirement is met.
Overall Balanced Equation:
NO2 (g) + CO (g)  NO (g) + CO2 (g)
Reaction Mechanism:
Step 1: NO2 + NO2 (g)  NO3 (g) + NO (g)
Step 2: NO3 + CO (g)  NO2 (g) + CO2 (g)
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RATE OF THE REACTION
• In Multi-step reactions, reaction is only as fast
as its slowest step.
• RATE IS ONLY DEPENDENT ON THE
SLOWEST STEP,
KNOWN AS THE RATE DETERMINING
STEP.
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Reaction Mechanism:
Step 1: NO2 + NO2 (g)  NO3 (g) + NO (g)
Step 2: NO3 + CO (g)  NO2 (g) + CO2 (g)
• In Proposed Mechanism,
assume Step 1 is the slow step,
the rate determining step.
assume Step 2 is a fast step.
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For Step 1 (an elementary step),
Rate of formation of NO3 =
 [NO3] = k [NO2]2
t
This agrees with experimentally determined
Rate law.
Rate Mechanism may be correct (not proven.)
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Let’s do #49, 51 & 52 on page 5571
Read Handout
The Rate Law and the Mechanism
Do Exercise 14.12
Put in folder on Friday January 16.
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18
Section 12.7
A Model for Chemical Kinetics
• RATE OF RXN. DEPENDS ON
TEMPERATURE.
ROUGH RULE OF THUMB:
IN MANY CASES,
RATE DOUBLES (approx.)
for every 10 °C Increase.
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Figure 12.10
A Plot Showing the
Exponential Dependence
of the Rate Constant on
Absolute Temperature
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A MODEL FOR
CHEMICAL KINETICS
COLLISION THEORY:
IN ORDER FOR A RXN. TO OCCUR,
REACTANT MOLECULES MUST COLLIDE WITH
(1)AN ENERGY GREATER THAN SOME MINIMUM
VALUE
(2) AND WITH PROPER ORIENTATION.
ACTIVATION ENERGY ( Ea):
MINIMUM ENERGY OF COLLISION
REQUIRED FOR 2 MOLECULES TO REACT.
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Collision Theory
• Key Idea: Molecules must collide to react.
• However, only a small fraction of collisions
produces a reaction. Why?
• Arrhenius: An activation energy must be
overcome.
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• CONSIDER REACTION
2BrNO (g)  2 NO (g) + Br2 (g)
Energy comes from the KE possessed by the
reacting molecules before they collide.
During the collision, KE changed to PE &
used to distort molecules, break bonds &
rearrange atoms.
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Figure 12.11
Change in Potential Energy
Exothermic
Top of PE Hill: Activated complex or Transition State which
is the arrangement of atoms found at the top of the
PE Hill
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Figure 12.12
Plot Showing the Number of Collisions with a Particular Energy at T1 and T2, where T2>T1
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• ORIENTATION OF MOLECULES
ALSO IMPORTANT
DURING COLLISIONS.
Observed Reaction Rates are still smaller than
the rate of collisions with enough activation
energy.
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Figure 12.13
Several Possible Orientations for a Collision
Between Two BrNO Molecules
Some collision orientations lead to rxn & other do not!
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REQUIREMENTS FOR REACTANTS TO
COLLIDE & SUCCESSFULLY REARRANGE
TO FORM PRODUCTS
• 1) COLLISION ENERGY MUST EQUAL
OR EXCEED THE ACTIVATION ENERGY.
• 2) RELATIVE ORIENTATION OF
REACTANTS MUST ALLOW
FORMATION OF ANY NEW BONDS
NECESSARY TO PRODUCE THE
PRODUCTS.
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ARRHENIUS EQUATION
k = A
-Ea/RT
e
A = z p = frequency factor
where z = collision frequency (changes slowly
with temp).
p = steric factor, reflects the fraction of
collisions with effective orientations.
k = rate constant
Ea = activation energy
T = temperature
R = gas constant
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-Ea/RT
e
• Fraction of collisions with sufficient energy to
produce a reaction.
• Changes rapidly with Temperature.
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ARRHENIUS EQUATION
ln (k) = [-(Ea/R) (1/T) ] + ln A
Plot of ln k versus 1/T
gives a straight line.
Slope = -Ea/R
Y- intercept = ln A
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Figure 12.14
Plot of ln(k) Versus
1/T for the Reaction
2N2O5(g) 
4NO2(g) + O2(g)
Slope = - Ea /R
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ARRHENIUS EQUATION
(Another Form)
• ln (k2/k1) = Ea/R [ 1/T1 – 1/T2 ]
Ea can be calculate from values of k at two
different temperatures.
Homework: #53-57 all, 59-63 all
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Section 12.8
Catalysis
• Catalyst: A substance that speeds up a
reaction without being consumed
• Read
• Write a detailed summary –
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Section 12.8
Catalysis
• Catalyst: A substance that speeds up a
reaction without being consumed
• Enzyme: A large molecule (usually a
protein) that catalyzes biological reactions.
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How do
They Work?
Figure 12.15
Energy Plots for a
Catalyzed and an
Uncatalyzed
Pathway for a
Given Reaction
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Figure 12.16
Effect of a Catalyst
on the Number of
Reaction-Producing
Collisions
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Catalysts
• Lower Activation Energy,
BUT
does not affect the
E, energy difference between the products
and the reactants.
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Figure 12.15
Energy Plots for a
Catalyzed and an
Uncatalyzed
Pathway for a
Given Reaction
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Catalysts
• Homogeneous catalyst: Present in the same
phase as the reacting molecules.
• Heterogeneous catalyst: Present in a
different phase than the reacting molecules.
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Heterogeneous Catalysts
-- most often involves gaseous reactants being
adsorbed on the surface of a solid catalyst.
EXAMPLE:
Hydrogenation of ethylene
H2C==CH2 + H2

H3C—CH3
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Figure 12.17
Heterogeneous
Catalysis of the
Hydrogenation
of Ethylene
Main function of catalyst
-- weaken the H—H bonds
by formation of
metal – H interactions.
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Heterogeneous Catalysis
Steps:
• 1. Adsorption and activation of the
reactants.
• 2. Migration of the adsorbed reactants
on the surface.
• 3. Reaction of the adsorbed
substances.
• 4. Escape, or desorption, of the
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Other Examples of Heterogeneous Catalysis
(1) Oxidation of SO2 (g) and SO3 (g)
(2) Catalytic Converter for Automobile Exhaust
Solid catalyst is a mixture of catalysts
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Figure 12.18
Catalytic Converter
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Homogeneous Catalysis
• Catalyst is in the same phase as the reacting
molecule.
• Examples
(1) NO
(2) Freon
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Enzymes: Nature’s Catalysts
Enzymes are large molecules specifically tailored to
facilitate a given type of reaction.
Usually enzymes are proteins, biomolecules
constructed from -amino acids
See page 596
Proteins: “Polymers of amino acids”
Body makes specific proteins from amino acids that
come from the proteins that we eat.
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Figure 12.19
The Removal of the End Amino Acid from a
Protein by Reaction with a Molecule of Water
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Figure 12.20
The Structure of the Enzyme Carboxypeptidase-A
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49
• Enzyme,
Carboxypeptidase-A catalyzes this
reaction.
Homework: Do it!!

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50