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Homework Help: Is the unimolecular elementary step generally more quicker than

  1. Nov 21, 2013 #1
    Is the unimolecular elementary step generally more quicker than a two body collision. Also technically how are they possible, if a reaction only occurs when a collision takes place, and since it is one body what can it possibly collide with.
  2. jcsd
  3. Nov 22, 2013 #2


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    Staff: Mentor

    I don't think so, it can be linked to some unlikely intramolecular regrouping to a different tautomer, in which case unimolecular step can be quite slow. It may also happen that the reaction requires some forbidden transition, like between a triplet and singlet state.

    It collides with everything around all the time, and these collisions can excite the molecule. So technically it is collision that is a source of activation energy, but the reaction is still intramolecular.
  4. Nov 23, 2013 #3
    Comparing rates of steps described by different order rate laws is tough because rates depend on concentrations of the relevant species.

    Rate constants are a better parameter to use to describe how quickly an elementary step should go because they contain information about all of the other factors affecting rates aside from the concentration dependence. So all things being equal, a higher rate constant means a 'more efficient' reaction. This doesn't mean that a higher rate constant means a faster rate under any conditions due to the concentration dependence of the rate.
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