Reaction Mechanism

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Reaction Mechanism
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Reaction Mechanism

• Process by which a reaction occurs
• Reaction occurring in a single event or step its
  called an elementary reaction
• Total reaction may by a sequence of elementary
  reactions
• The elementary reactions in a multi-step
  mechanism must always add to give the chemical
  equation of the overall reaction

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• Substances that are formed in one elementary step
  and consumed in the next are called intermediates
• Number of molecules that participate as reactants
  in an elementary reaction define the moleculartiy
• Unimolecular one reactant
• Bimolecular two
• Termolecular - three

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Example

• It has been proposed that ozone is converted into
  oxygen gas through the mechanism below a) write
  the equation for the overall reaction b)
  identify any intermediates c) describe the
  molecularity of each step
• O3 ? O2 O
• O3 O ? 2O2

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Rate

• It is not the overall reaction that determines
  the rate law rather the elementary reactions
• Rate law of an elementary reaction is based on
  its molecularity
• In general the order for each reactant in an
  elementary reaction is equal to its coefficient
  in the elementary reaction

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Example

• If the following reaction occurs in a single
  elementary step, predict the rate law
• O3 NO ? NO2 O2

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Rate-Determining Step

• If reaction occurs in a series of elementary
  steps, one step is often much slower than the
  others
• The slow step limits the speed of the reaction
  called the rate determining step

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Example

• A reaction occurs according to the mechanism
  below, write the rate law
• NO2 NO2 ? NO3 NO (slow)
• NO3 CO ? NO2 CO2 (fast)

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• This reaction takes place in three steps

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Ea

• First step is fast
• Low activation energy

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Ea
Second step is slow High activation energy
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Ea
Third step is fast Low activation energy
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Second step is rate determining
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Intermediates are present
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Activated Complexes or Transition States
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Mechanisms and rates

• There is an activation energy for each elementary
  step.
• Slowest step (rate determining) must have the
  highest activation energy.