Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 1 of 61 14-8 Theoretical Models for...

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


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.


Activation Energy


Kinetic Energy


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.


Collision Theory


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.


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


Arrhenius Plot

N2O5(CCl4) → N2O4(CCl4) + ½ O2(g)

= -1.2104 KR

-Ea

-Ea = 1.0102 kJ mol-1


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


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.


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.


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]


Slow Step Followed by a Fast Step


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]


Catalytic Converters

Dual catalyst system for oxidation of CO and reduction of NO.

CO2 + N2CO + NOcat


14-5 Catalysis


Worked Examples Follow:


CRS Questions Follow:


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.


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.


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)


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


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?


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


Textbook End of Chapter ?’s: P.611- #1, 3, 11, 13, 17, 19, 21, 33, 47, 51, 55, 100, 101, 102, 103, 104, 105

Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 1 of 61 14-8 Theoretical Models for...

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