1. The Circuitous Route to an Overall Reaction
Reaction Mechanisms
The Circuitous Route to an Overall Reaction

2. Elementary Steps
A series of simple reactions that represent the progress of the overall reaction at the molecular level.
The reaction order can be determined by inspection.
Can not be proven, only supported. However, some mechanisms can be disproved.

3. Example 2NO(g) + Cl2(g)  2NOCl(g)
* I have know idea if NOCl2 is observed as the reaction progresses. I am merely stating this so as to illustrate how a mechanism might be proposed. Any similarity between the above statement and reality is purely coincidental.
It is observed that NOCl2(g) is present during the course of the reaction.*

4. Possible mechanism for 2NO(g) + Cl2(g)  2NOCl(g)
Step 1: NO + Cl2  NOCl2(g) slow
Step 2: NOCl2 + NO  2NOCl fast

5. The elementary steps must add up to the overall reaction.
Two requirements must be satisfied for the proposed mechanism to be possible:
The elementary steps must add up to the overall reaction.
The rate law of the overall reaction must agree with the rate law of the “rate determining step” (the slowest step) of the proposed mechanism.

6. Do the elementary steps add up?
Step 1: NO + Cl2  NOCl2(g)
Step 2: NOCl2 + NO  2NOCl
2NO(g) + Cl2(g)  2NOCl(g)
YES!!

7. Does the rate law of the overall reaction agree with the slow elementary step?
Experiment
[NO]
[Cl2]
Rate M/s 1
1.0
3.0
4.6 x 102 2
1.5
2.3 x 102 3
2.0
9.2 x 102
For overall reaction, rate = k[NO][Cl2]

8. Determine the rate law of the slow elementary step
Step 1: NO + Cl2  NOCl2(g) slow
The reaction step is bimolecular since there are two molecules reacting.
Remember that the reaction order for an elementary step can be determined by inspection.
For this elementary step, rate = k[NO][Cl2]