Enduring Understanding 4.B: Reaction Mechanism

  • Reactions proceed at a molecular level, through elementary steps.
  • These elementary steps can be unimolecular, or can involve collisions between two or more molecules.
  • In a bimolecular (or higher order) elementary step, the collision must have both sufficient energy and be in a favorable orientation for the reaction to happen.
  • The order of an elementary step depends on the number of molecules involved - a unimolecular step is first order, a bimolecular collision is second order, etc...
  • Elementary steps involving collisions between three or more particles are rare.
  • Example: The following particulate diagram illustrates the elementary reaction

  • NO2Cl + Cl → NO2 + Cl2



  • The upper diagram shows a collision between a NO2Cl molecule and a Cl atom. However, the orientation of the collision is not correct and no reaction occurs.
  • The lower diagram shows a collision, but in a more suitable orientation. The reaction then occurs.
  • Because it is a bimolecular collision, the rate law of this elementary reaction would be k[Cl][NO2]

  • Here is an energy diagram of the NO2Cl + Cl → NO2 + Cl2 reaction.


  • The two colliding species must have sufficient energy to overcome the activation energy barrier, Ea.
  • The high point in the energy curve represents the transition state, or activated complex, where bonds between the two Cl atoms and the NO2 have partially formed, partially broken bonds.
  • The energy released by the reaction is Erxn. It does not depend on Ea.



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Reaction Rates



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