Second order reactions

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Second order reactions

• A2 B2 ? 2AB
• Can show energetic and orientation requirements
• Two objects are colliding, if a reaction is going
  to take place, it will be during collision

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First order reactions

• Broken down into two parts
• There will be collisions between the molecule and
  something else (container wall, other molecules)
  which result in energy transfer - used to
  overcome activation energy barrier. A high
  energy species is formed, then reaction takes
  place. Collision only forms high energy species,
  doesnt make reaction happen.
• Some collisions do not provide enough energy to
  overcome EA, but others do (depicted with excited
  yellow molecule)

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First order reactions (cont.)

• The excited molecule can do one of two things
• React (reaction here would be AB ? A B)
• Collide with other molecules with energy lost
  (de-excitation)

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Zero order reactions

• Illustrated by process with a heterogeneous
  catalyst. Reaction is A2 B2 ? 2AB
• Reaction is zero order in the white molecules
  because catalyst surface is saturated with white
  molecules.
• Reaction does depend on the red molecule
  concentration more red molecules adsorbing to
  surface faster reaction

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