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Reaction Intermediate

Ozone depletetion

Ozone (O 3 ) 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:

& \text{O}_3 + \text{ultraviolet light} \rightarrow \text{O}_2 + {\text{O} \cdot} (\text{free radical}) \ \text{slow reaction} \\& {\text{O} \cdot} + \text{O}_3 \rightarrow 2 \text{O}_2 \ \text{fast reaction}

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


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 \text{NO}(g)+ \text{O}_2(g) \rightarrow 2 \text{NO}_2(g)

It may seem as though this reaction would occur as the result of a collision between two NO molecules with one O 2 molecule. However, careful analysis of the reaction has detected the presence of N 2 O 2 during the reaction. A proposed mechanism for the reaction consists of two elementary steps:

Step 1: 2 \text{NO}(g) \rightarrow \text{N}_2 \text{O}_2(g)

Step 2: \text{N}_2 \text{O}_2(g)+ \text{O}_2(g) \rightarrow 2 \text{NO}_2(g)

In the first step, two molecules of NO collide to form a molecule of N 2 O 2 . In the second step, that molecule of N 2 O 2 collides with a molecule of O 2 to produce two molecules of NO 2 . The overall chemical reaction is the sum of the two elementary steps:

& \qquad \qquad \ 2 \text{NO} (g) \rightarrow \cancel{\text{N}_2 \text{O}_2(g)} \\& \underline{\;\; \cancel{\text{N}_2\text{O}_2(g)} + \text{O}_2(g) \rightarrow 2 \text{NO}_2(g) \;\;} \\& \ \ 2 \text{NO}(g)+ \text{O}_2(g) \rightarrow 2 \text{NO}_2(g)

The N 2 O 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.

Nitrogen dioxide (left) and dinitrogen tetroxide (right).


  • The role of intermediates in reaction mechanisms is described.


Read the material at the link below and answer the questions at the end:



  1. What is the intermediate in the reaction described above?
  2. Do we see this intermediate in the actual reaction mix?
  3. Where do we first see an intermediate in a reaction mechanism?
  4. What happens to the intermediate?

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