1. Reaction Mechanisms
Chapter 6.6

2. Reaction Mechanisms
The sequence of events that describes the actual process by which reactants become products is called the reaction mechanism
The balanced chemical equation does not tell us how reactants becomes products; it is a summary of the overall process
The sum of the elementary steps must give the overall balanced equation for the reaction
Most chemical reactions occur by a sequence of simpler reactions

3. Reaction Mechanisms
Consider the reaction between nitrogen dioxide and carbon monoxide:
A reaction mechanism is a series of elementary steps by which a chemical reaction occurs
A reaction intermediate is an entity that is neither a reactant nor a product but is formed and consumed during the reaction sequence
The rate determining step is the step in a reaction mechanism that determines the rate of the overall reaction; the slowest step
NO3 is the reaction intermediate
 This is the Rate Determining Step (RDS)

4. Rate Law and Elementary Reactions
The molecularity of a process tells how many individual entities are involved in the process (Termolecular steps are very rare because the chances of three molecules coming into contact at the same time are miniscule)

5. Rate Law and Elementary Reactions
Recall that rate law equations cannot be written using information from an overall balanced equation for a reaction (i.e. it must be determined experimentally)
This is not the case for elementary steps;
Rate Law equations can be written directly from the balanced equations representing elementary steps
Elementary step with 1 reactant  always 1st order
Elementary step with 2 reactants  always 2nd order
Elementary Step
Rate Law Equation
A  products
Rate = k[A]
A + A  products
Rate = k[A]2
A + B  products
Rate = k[A][B]

6. Rate Determining Step
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
The rate law equation for the overall reaction is the same as the rate law equation for the rate determining step

7. Reaction Mechanisms
In order for a reaction mechanism to be plausible, it must satisfy two requirements:
Summing the elementary steps in the reaction mechanism must give the overall balanced equation for the reaction
The reaction mechanism must agree with the experimentally determined rate law

8. Potential Energy Diagram for a Reaction Mechanism
2NO(g) + 2H2(g)  N2(g) + 2H2O(g) rate = k[H2 ] The mechanism for the above reaction occurs in three steps:
 The rate determining step (step #2 in this case) always has the highest activation energy

9. Example 1: Identifying the Rate Determining Step
The rate law of a chemical reaction was found to be: rate = k[NO(g)][Cl2(g)]
The following elementary steps have been proposed:
NO(g) + Cl2(g)  NOCl2(g)
NOCl2(g) + NO(g)  2NOCl(g)
Determine the overall reaction
Identify any reaction intermediates
Determine the slowest step in the mechanism. Explain your choice
Determine the rate law equation for the last step.

10. Homework
Page 387 #1-9 Unit Test Review: p. 406 #1-8, #22-33, 35-37, 39, 40, 41-56, 59, 61-67, 72, The Unit Test is on Wednesday May 18!