1. Unit 1: Reaction Kinetics
Lesson 8: Reaction Mechanisms

2. 1,2,3... Reaction!
Assuming a collision between two particles is certain to occur, the probability of a three-particle collision is 1000 times less likely, and a four-particle collision is almost impossible.
Complex reactions, therefore, must involve a series of steps. The actual sequence of steps that make up an overall reaction is called the reaction mechanism.

3. Reaction Mechanism Example
The reaction 4 HBr + O2 → 2 H2O + Br2 occurs in three steps (called elementary processes):
1) HBr + O2 → HOOBr (found to be slow)
Record steps on chalkboard for later reference.

4. 2) HOOBr + HBr → 2 HOBr (found to be fast) 3) HOBr + HBr → H2O + Br2 (found to be fast)
Have students draw mechanisms, then go over answers - 20 min

5. Brain Break!

6. Volume (mL) poured through in 15 s
Rate (mL/s)
Small Funnel
Medium Funnel
Large Funnel
Medium, Small, then Large
Demo 12.I.16 (have students TPS predictions) – 10 min

7. Notes about Reaction Mechanisms (Using the Same Example)
The slowest step is the rate-determining step (Step 1 in our example). Only changes to the rate of this step will affect the overall rate.
HOOBr and HOBr are called intermediates. They are stable molecules (unlike activated complexes), but react quickly in this particular reaction. At any given time, there is very little HOOBr or HOBr present, since they react very quickly to form the products of the reaction.

8. Adding Elementary Processes
The elementary processes in a reaction mechanism add up to the overall reaction when the species that appear on both sides are cancelled out:
HBr + O2 → HOOBr
HOOBr + HBr → 2 HOBr
2 HOBr + 2 HBr → 2 H2O + 2 Br2
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5 min

9. Homework:
Pg. 28 #46-52
5 min