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 © 2009, Prentice-Hall, Inc.

How Do Reactants Become Products? Reaction Mechanisms How Do Reactants Become Products? © 2009, Prentice-Hall, Inc.

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? © 2009, Prentice-Hall, Inc.

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). © 2009, Prentice-Hall, Inc.

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. © 2009, Prentice-Hall, Inc.

Reaction Mechanisms The molecularity of a process tells how many molecules are involved in the process. © 2009, Prentice-Hall, Inc.

Mechanism a set of steps that try* to explain how a reaction takes place *Chemist’s Intuition

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

The mechanism MUST be consistent with experimentally validated RATE LAW

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

Equilibium A  B ie. A  B and B  A

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.

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)

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

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. © 2009, Prentice-Hall, Inc.

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.

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.

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

Possible Mechanisms for NO2 + CO  NO + CO2

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

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

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

What is a Catalyst and How Do They Speed Reaction Rates? Catalysis What is a Catalyst and How Do They Speed Reaction Rates? © 2009, Prentice-Hall, Inc.

changes the rate by lowering the value of Ea Catalyst changes the rate by lowering the value of Ea H2O2 Decomposition Catalytic Converter

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. © 2009, Prentice-Hall, Inc.

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. © 2009, Prentice-Hall, Inc.

Misconceptions about catalysts 1. Inhibiting catalysis exist 2. Catalysts do not take part in reactions