18.3 Reaction Mechanisms
18.3 Reaction Mechanisms
Lesson Objectives
- Describe how the elementary steps of a chemical reaction combine to produce the overall reaction mechanism.
- Be able to identify intermediates and catalysts when they appear in a reaction mechanism.
- Write a rate law for an elementary step, and identify the molecularity of the reaction.
- Identify the rate-determining step of a reaction.
- Analyze the potential energy diagram for a multi-step reaction.
Lesson Vocabulary
- elementary step
- intermediate
- molecularity
- rate-determining step
- reaction mechanism
Check Your Understanding
Recalling Prior Knowledge
- What is a rate law?
- What is the effect of using a catalyst on the rate of a reaction?
Chemical reactions rarely occur in one simple step. More commonly, many small steps combine in a specific
sequence, resulting in the overall reaction. In this lesson, you will be introduced to reaction mechanisms and how
they are related to rate laws.
Elementary Steps
The overall balanced equation for a chemical reaction does not always tell us how a reaction actually proceeds. In
many cases, the overall reaction takes place in a series of small steps. Anelementary step(or elementary reaction)
is one step in a series of simple reactions that show the progress of a reaction at the molecular level. Areaction
mechanismis the sequence of elementary steps that together comprise an entire chemical reaction. As an analogy,
consider the route that you might take while driving to the grocery store. That route may consist of several turns,
similar to the elementary steps. The overall reaction specifies only the beginning point (your house) and the final
destination (the store), with no information about the specifics in between.