Reaction Intermediate

Learning Objectives

  • Define intermediate.
  • Describe the role of intermediates in reaction mechanisms.

Ozone depletetion

The ozone radical is an intermediate that in the decomposition of ozone by ultraviolet radiation

Ozone waves. Courtesy of NASA/Wikimedia.

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.

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 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}_2text{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.

Caramel/burnt orange colored liquid in a glass bottle on the left and yellow/honey colored liquid in the right.

Figure 1. Nitrogen dioxide (left) and dinitrogen tetroxide (right). User:Greenhorn1/Wikipedia.

Summary

  • The role of intermediates in reaction mechanisms is described.

Practice

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

http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Rate_Laws/Reaction_Mechanisms/Reaction_Mechanisms

Review

  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?

 Glossary

  • intermediate: A species which appears in the mechanism of a reaction, but not in the overall balanced equation.