CAS CH 110 Lecture Notes - Lecture 30: Rate-Determining Step, Elementary Reaction, Reaction Mechanism
25 Apr 2017
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Document Summary
We cannot write a rate law from a chem ical equation: the reason is all but the simplest reactions are the outcome of several, and sometimes many, steps called elementary reactions. Elementary reactions describes a distinct event, often a collision of particles. Understanding how reactions takes place, requires us to propose a reaction mechanism, a sequence of elementary reactions. One-step mechanism: two o3 molecules collide and rearrange into three. O2 molecules: o3 + o3 o2 + o2 + o2. Second step, the o attacks another o3 molecule making two more o2: The free o atom is a reaction intermediate, (it is produced in one step but is used up in a later step) a species that plays a role in a reaction but does not appear in the chemical equation. The two equa-tions for the elementary reactions add together to give the equation for the overall reaction. Step 1 is a unimolecular reaction, because only one reactant molecule participates.
