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12.12: Reaction Intermediate

Last updated

Jul 18, 2022
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- 12.11: Reaction Mechanisms and the Elementary Step
- 12.13: Molecularity

Theodore Chan
Fullerton College

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Ozone $\left( \ce{O_3} \right)$ depletion in the atmosphere is of significant concern. This gas serves as a protection against the ultraviolet rays of the sun. Ozone is naturally depleted in addition to the depletion caused by human-made chemicals. The depletion reaction is a two-step process:

$\ce{O_3} + \text{ultraviolet light} \rightarrow \ce{O_2} + \ce{O} \cdot \: \text{(free radical) slow reaction}\nonumber$

$\ce{O} \cdot + \ce{O_3} \rightarrow 2 \ce{O_2} \: \text{fast reaction}\nonumber$

The free radical is not a part of the overall equation, but can be detected in the lab.

Intermediate

Reaction mechanisms describe how the material in a chemical reaction gets from the initial reactants to the final products. One reaction that illustrates a reaction mechanism is the reaction between nitrogen monoxide and oxygen to form nitrogen dioxide:

$2 \ce{NO} \left( g \right) + \ce{O_2} \left( g \right) \rightarrow 2 \ce{NO_2} \left( g \right)\nonumber$

It may seem as though this reaction would occur as the result of a collision between two $\ce{NO}$ molecules with one $\ce{O_2}$ molecule. However, careful analysis of the reaction has detected the presence of $\ce{N_2O_2}$ during the reaction. A proposed mechanism for the reaction consists of two elementary steps:

Step 1: $2 \ce{NO} \left( g \right) \rightarrow \ce{N_2O_2} \left( g \right)$

Step 2: $\ce{N_2O_2} \left( g \right) + \ce{O_2} \left( g \right) \rightarrow 2 \ce{NO_2} \left( g \right)$

In the first step, two molecules of $\ce{NO}$ collide to form a molecule of $\ce{N_2O_2}$ . In the second step, that molecule of $\ce{N_2O_2}$ collides with a molecule of $\ce{O_2}$ to produce two molecules of $\ce{NO_2}$ . The overall chemical reaction is the sum of the two elementary steps:

$\begin{align*} 2 \ce{NO} \left( g \right) &\rightarrow \cancel{\ce{N_2O_2} \left( g \right)} \\ \cancel{\ce{N_2O_2} \left( g \right)} + \ce{O_2} \left( g \right) &\rightarrow 2 \ce{NO_2} \left( g \right) \\ \hline 2 \ce{NO} \left( g \right) + \ce{O_2} \left( g \right) &\rightarrow 2 \ce{NO_2} \left( g \right) \end{align*}\nonumber$

The $\ce{N_2O_2}$ molecule is not part of the overall reaction. It was produced in the first elementary step, then reacts in the second elementary step. An intermediate is a species which appears in the mechanism of a reaction, but not in the overall balanced equation. An intermediate is always formed in an early step in the mechanism and consumed in a later step.

Two clear bottles with green caps, one filled with a dark amber liquid and the other with a light yellow liquid, placed side by side on a light surface. — Figure $\PageIndex{1}$ : Nitrogen dioxide (left) and dinitrogen tetroxide (right). (CC BY-NC; CK-12)

Summary

Reaction mechanisms describe how the material in a chemical reaction gets from the initial reactants to the final products.
An intermediate is a species which appears in the mechanism of a reaction, but not in the overall balanced equation.

Intermediate

Summary

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