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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 1 of 61
14-8 Theoretical Models for Chemical Kinetics
Kinetic-Molecular theory can be used to calculate the collision frequency. In gases 1030 collisions per second. If each collision produced a reaction, the rate would be about 106
M s-1. Actual rates are on the order of 104 M s-1.
◦ Still a very rapid rate.
Only a fraction of collisions yield a reaction.
Collision Theory
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 2 of 61
Activation Energy
For a reaction to occur there must be a redistribution of energy sufficient to break certain bonds in the reacting molecule(s).
Activation Energy: The minimum energy above the average kinetic energy
that molecules must bring to their collisions for a chemical reaction to occur.
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Activation Energy
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Kinetic Energy
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Collision Theory
If activation barrier is high, only a few molecules have sufficient kinetic energy and the reaction is slower.
As temperature increases, reaction rate increases.
Orientation of molecules may be important.
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Collision Theory
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Transition State Theory
The activated complex is a hypothetical species lying between reactants and products at a point on the reaction profile called the transition state.
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 8 of 61
14-9 Effect of Temperature on Reaction Rates
Svante Arrhenius demonstrated that many rate constants vary with temperature according to the equation:
k = Ae-Ea/RT
ln k = + lnAR
-Ea
T
1
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 9 of 61
Arrhenius Plot
N2O5(CCl4) → N2O4(CCl4) + ½ O2(g)
= -1.2104 KR
-Ea
-Ea = 1.0102 kJ mol-1
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 10 of 61
Arrhenius Equation
k = Ae-Ea/RT ln k = + ln AR
-Ea
T
1
ln k2– ln k1 = + ln A - - ln AR
-Ea
T2
1
R
-Ea
T1
1
ln = - R
-Ea
T2
1
k2
k1
T1
1
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 11 of 61
14-10 Reaction Mechanisms
A step-by-step description of a chemical reaction. Each step is called an elementary process.
Any molecular event that significantly alters a molecules energy of geometry or produces a new molecule.
Reaction mechanism must be consistent with: Stoichiometry for the overall reaction. The experimentally determined rate law.
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 12 of 61
Elementary Processes
Unimolecular or bimolecular. Exponents for concentration terms are the same
as the stoichiometric factors for the elementary process.
Elementary processes are reversible. Intermediates are produced in one elementary
process and consumed in another. One elementary step is usually slower than all the
others and is known as the rate determining step.
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 13 of 61
Slow Step Followed by a Fast Step
H2(g) + 2 ICl(g) → I2(g) + 2 HCl(g)dt
= k[H2][ICl]d[P]
Postulate a mechanism:
H2(g) + 2 ICl(g) → I2(g) + 2 HCl(g)
slowH2(g) + ICl(g) HI(g) + HCl(g)
fastHI(g) + ICl(g) I2(g) + HCl(g)
dt= k[H2][ICl]
d[HI]
dt= k[HI][ICl]
d[I2]
dt= k[H2][ICl]
d[P]
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Slow Step Followed by a Fast Step
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 15 of 61
Fast Reversible Step Followed by a Slow Step
2NO(g) + O2(g) → 2 NO2(g)dt
= -kobs[NO2]2[O2]d[P]
Postulate a mechanism:
dt = k2[N2O2][O2]
d[NO2]
fast 2NO(g) N2O2(g)k1
k-1
slow N2O2(g) + O2(g) 2NO2(g)k2
dt= k2 [NO]2[O2]
d[I2]
k-1
k12NO(g) + O2(g) → 2 NO2(g)
K =k-1
k1=
[NO]
[N2O2]
= K[NO]2
k-1
k1= [NO]2[N2O2]
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Catalytic Converters
Dual catalyst system for oxidation of CO and reduction of NO.
CO2 + N2CO + NOcat
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14-5 Catalysis
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Worked Examples Follow:
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CRS Questions Follow:
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Ene
rgy
reaction coordinate
A+B
X+Y
Ene
rgy
reaction coordinate
A+B
X+Y
Ene
rgy
reaction coordinate
A+BX+Y
Ene
rgy
reaction coordinate
A+B
X+Y
The reaction between A and B is determined to be a fairly fast reaction and slightly exothermic. Which of the following potential energy surfaces fit this description?
1.
4.3.
2.
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 24 of 61
Ene
rgy
reaction coordinate
A+B
X+Y
Ene
rgy
reaction coordinate
A+BX+Y
Ene
rgy
reaction coordinate
A+B
X+Y
The reaction between A and B is determined to be a fairly fast reaction and only slightly exothermic. Which of the following potential energy surfaces fit this description?
4.3.
2.
Ene
rgy
reaction coordinate
A+B
X+Y
1.
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 25 of 61
Ene
rgy
reaction coordinate
1. H = 60 kJ mol-1
R
P2. H = -60 kJ mol-1
5. H = 140 kJ mol-1
4. H = -80 kJ mol-1
3. H = 80 kJ mol-1
A particular reaction was found to have forward and reverse activation energies of 60 and 140 kJ mol-1, respectively. The enthalpy change for the reaction is, (do not use a calculator)
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 26 of 61
Ene
rgy
reaction coordinate
1. H = 60 kJ mol-1
R
P
A particular reaction was found to have forward and reverse activation energies of 60 and 140 kJ mol-1, respectively. The enthalpy change for the reaction is, (do not use a calculator)
2. H = -60 kJ mol-1
5. H = 140 kJ mol-1
4. H = -80 kJ mol-1
3. H = 80 kJ mol-1
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 27 of 61
En
erg
y
reaction coordinate
4.
2.
3.
A+B
X+Y
En
erg
y
reaction coordinate
1.
A+B
X+Y
En
erg
yreaction coordinate
A+B
X+Y En
erg
y
reaction coordinate
A+BX+Y
In which diagram to the right does the dashed line best represent the catalyzed version of the reaction’s potential energy profile?
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Prentice-Hall © 2007General Chemistry: Chapter 14Slide 28 of 61
In which diagram to the right does the dashed line best represent the catalyzed version of the reaction’s potential energy profile?
En
erg
y
reaction coordinate
4.
2.
3.
A+B
X+Y
En
erg
y
reaction coordinate
1.
A+B
X+Y
En
erg
yreaction coordinate
A+B
X+Y En
erg
y
reaction coordinate
A+BX+Y
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Textbook End of Chapter ?’s: P.611- #1, 3, 11, 13, 17, 19, 21, 33, 47, 51, 55, 100, 101, 102, 103, 104, 105