Reaction Mechanisms

1
Reaction Mechanisms

• A reaction mechanism is step or series of steps
  that make up a reaction.
• Molecularity refers to the number of reactant
  molecules (ions, atoms or molecules) involved in
  an elementary step or making up an activated
  complex.
• Simple reaction will have one step where complex
  reactions will consist of a number of steps.
• Consider the production of NO2 from NO and O2.
• 2 NO O2 ? 2 NO2
• This reaction consists of elementary steps.
• 1) 2 NO ? N2O2 (fast, molecularity 2)
• 2) N2O2 O2 ? 2 NO2 (slow, molecularity 2)
• 2 NO O2 ? 2 NO2
• N2O2 is a reaction intermediate which is short
  lived and difficult to isolate.

2

• Often it is difficult to determine the actual
  mechanism of an reaction. In this case it must
  be determined through experimentally determined
  relationships.
• The rate determining step is the slowest step in
  the reaction mechanism.
• The rate determining step will therefore have the
  highest activation energy.

3
Chain Reactions

• These reaction have mechanisms which involve a
  step or steps where a reaction intermediate is
  continually being regenerated.
• Many consist of chain initiation, chain
  propagation and chain termination.
• Chain termination occurs when the reactants are
  used up and the intermediates combine with
  themselves

4
Catalyzed Reactions

• Catalysts can provide a lower activation energy
  by producing an intermediate.
• The intermediate that is formed in many cases is
  a species with an unpaired electron and is known
  as a radical.
• Radicals are highly reactive due to the unpaired
  electron.
• This is what occurs when CFCs are released into
  the atmosphere and aid in the depletion of the
  ozone.
• Enzymes are biological catalysts which are
  usually large protein molecules with a distinct
  3-D shape which provides an active site where
  substrates (i.e. the reactants) can interact with
  each other.