1. Lecture 1405 Reaction Mechanism and Catalysis
Chemistry, The Central Science, 11th edition
Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten
Lecture 1405 Reaction Mechanism and Catalysis
John D. Bookstaver
St. Charles Community College
Cottleville, MO
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2. How Do Reactants Become Products?
Reaction Mechanisms
How Do Reactants Become Products?
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3. For a reaction to occur, particles must collide with sufficient energy and the proper orientation.
What is chance that “many” particles will collide simultaneously with this energy and orientation?
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4. Most reactions seem to occur as a result of a single collision.
Thus, a chemical reaction must occur in a series of single steps called elementary reactions (processes).
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5. 1 particle – unimolecular 2 particles – bimolecular Etc.
The number of particles that serve as reactants in an elementary reaction defines its molecularity.
1 particle – unimolecular
2 particles – bimolecular
Etc.
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6. Reaction Mechanisms
The molecularity of a process tells how many molecules are involved in the process.
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7. Mechanism
a set of steps that try* to explain how a reaction takes place
*Chemist’s Intuition

8. Reaction Mechanism
The set of bond-making and bond breaking steps that lead from reactants to products.
Rate studies give information about reaction mechanisms.

9. The mechanism MUST be consistent with experimentally validated RATE LAW

10. Types of mechanism steps
fast steps - not rate determining
slow steps - rate determining
equilibrium - can be rate determining

11. Equilibium
A  B
ie.
A  B and B  A

12. Mechanism must pass two tests to be accepted
Sum of mechanism steps produces the net equation.
Rate laws of all rate determining steps must be combined to produce experimental rate law.

13. Rate laws do NOT use the reactant coefficients of the balanced net equation
Rate laws DO use the reactant coefficients of the balanced rate determining step(s)

14. The rate equation for any individual step is defined by the stoichiometry of that step.

15. Multistep Mechanisms
In a multistep process, one of the steps will be slower than all others.
The overall reaction cannot occur faster than this slowest, rate-determining step.
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16. Rate laws reflect the chemistry of the slowest step in a multistep reaction
& also the equilibrium chemistry, if present, that occurs prior to the slow step.

17. Rate Data and Mechanisms
The rate of the reaction is limited by the slow reaction step in the mechanism
The experimental rate law shows species upon which reaction rate depends.
If the species in the rate law are reactants in the slow step in a proposed mechanism, this is evidence that the proposed mechanism could be the one operating.

18. Reaction: CO + NO2  CO2 + NO
(all species are gases)
Rate Law: R = k [NO2]2

19. Possible Mechanisms for NO2 + CO  NO + CO2

20. Possible Mechanisms NO2 + CO  NO + CO2
(2a) 2NO2  N2O4
(2b) N2O4 + 2CO  2CO2 + 2NO
(slow)
The N2O4 is called an intermediate.

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22. Possible Mechanisms NO2 + CO  NO + CO2
c. 2NO2  NO3 + NO (slow)
NO3 + CO  NO2 + CO2 (fast) 
Proposed Mechanism Step 1
Proposed Mechanism Step 2

23. Experimental Evidence 1 Experimental Evidence 2
How can we determine if a possible mechanism is “correct”?
Experimental Evidence 1
Experimental Evidence 2

24. Possible Mechanisms NO2 + CO  NO + CO2
d. 2NO2  2NO + O2 (slow)
2 CO + O2  2CO2 (fast) 

25. What is a Catalyst and How Do They Speed Reaction Rates?
Catalysis
What is a Catalyst and How Do They Speed Reaction Rates?
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26. changes the rate by lowering the value of Ea
Catalyst
changes the rate by lowering the value of Ea
H2O2 Decomposition
Catalytic Converter

27. Catalysts
Catalysts increase the rate of a reaction by decreasing the activation energy of the reaction.
Catalysts change the mechanism by which the process occurs.
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28. Enzymes Enzymes are catalysts in biological systems.
The substrate fits into the active site of the enzyme much like a key fits into a lock.
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29. Misconceptions about catalysts
1. Inhibiting catalysis exist
2. Catalysts do not take part in reactions