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:
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:
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:
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:
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:
- What is the intermediate in the reaction described above?
- Do we see this intermediate in the actual reaction mix?
- Where do we first see an intermediate in a reaction mechanism?
- What happens to the intermediate?