1. Chapter 6.6 Reaction Mechanisms

2. What is a Reaction Mechanism?
Balanced chemical reactions provide an overall summary of a chemical reactions, but most chemists would agree that chemical reactions occur by way of sequence of simpler reactions.
An elementary step is a step involving a 1-, 2-, or 3- entity collision that cannot be explained in terms of simpler reactions.
A reaction mechanism is the series of elementary steps by which a chemical reaction occurs

3. Rate-Determining Step
The slow step is called the rate-determining step.
This is because it determines the rat of the overall reaction, much like the slowest runner on a relay team determines the overall speed.

4. Reaction Intermediate
Not that NO3 gas was formed in the first step, but is consumed in the second step. We call this a reaction intermediate, which is an entity that is neither a reactant or product, but is formed and consumed during the reaction sequence.

5. Reaction Steps Most reaction steps have either 1 or 2 entities.
1 entity – a single entity collides with the sides of the container and breaks apart
2 entities – collide with each other and the sides of the container

6. What about 3 Entities?
Very rare because all 3 entities would have to collide at the same time
Ex. Velcro ball scenario

7. Rate Law Equations and Elementary Steps

8. To be Plausible, A Reaction Mechanism must Satisfy 2 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.

9. The first elementary step is the slow step, which means it is the rate-determining step.
This is most likely because it has a fairly high activation energy.
What would be the rate law equation?

10. Example 1
Given the balanced equation and the experimentally determined rate law equation: 2NO2(g) + F2(g)  2 NO2F(g) rate = k[NO2(g)][F2(g)] A suggested mechanism for this reaction is Does this mechanism satisfy the two requirements for a plausible reaction meachism?

11. Identify 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 fast step.

12. HOMEWORK
P. 386 #1 – 4
P. 387 #3, 4, 7, 8