Topic 5.7 Introduction to Reaction Mechanisms: represent a reaction as a sequence of elementary steps, identify reaction intermediates and catalysts, and confirm that the steps sum to the overall equation.

What this topic is asking

The College Board (Topic 5.7) wants you to represent an overall reaction as a sequence of elementary steps (its mechanism), identify reaction intermediates and catalysts within that sequence, and verify that the steps sum to the overall equation. A mechanism is the detailed story of how a reaction actually happens at the molecular level.

What a mechanism is

Most reactions do not happen in a single collision; they proceed through a few elementary steps involving short-lived species. The mechanism tells you what those species are and the order in which bonds break and form, which the overall equation hides.

Summing the steps

This summation is the bookkeeping check on every proposed mechanism. Species that cancel are either intermediates or catalysts, depending on the order in which they appear; species that survive are the net reactants and products.

Intermediates and catalysts

The distinction is the order of appearance. An intermediate is made first, then used up (it never existed before the reaction began). A catalyst is used up first, then remade (it was there from the start and returns). Both cancel when you sum the steps, so you must read the steps in sequence to tell them apart.

Try this

Q1. In the mechanism Step 1: $\text{Cl}_2 \rightarrow 2\text{Cl}$; Step 2: $\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2$; Step 3: $\text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2$, identify a catalyst. [2 points]

• Cue. Cl is consumed in step 2 and regenerated in step 3, so Cl is the catalyst (ClO is an intermediate).

Q2. Explain how you would check that a proposed mechanism is consistent with the overall reaction. [1 point]

• Cue. Add the elementary steps and cancel common species; the result must equal the overall balanced equation.