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Molecularity is defined as the number of molecules required to collide simultaneously to form a product in an elementary reaction.

In a unimolecular elementary reaction, do two molecules of the same species collide or does a single molecule dissociate?

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    $\begingroup$ ?!? If two molecules collide, you have a bimolecular reaction. $\endgroup$
    – Karl
    Jul 7 at 7:56
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    $\begingroup$ Just for future reference, As homework is considered literal homework, exams, self-study questions, puzzles, worked examples etc. $\endgroup$
    – Poutnik
    Jul 7 at 8:06
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    $\begingroup$ @Karl Is reaction $\ce{A + A ->[collision \Delta E ] A^{*} + A -> B + C + A}$ unimolecular ? $\endgroup$
    – Poutnik
    Jul 7 at 8:50
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    $\begingroup$ @Karl Lakedaimonians would say IF.... A* is not identifiable intermediate, it is just A with higher then ground energy. Compounds have dozens of different energy states. Many reactions use A as implicit A*, A -> B + C is always A* -> B + C as A does not have enough energy. // My point since the beginning was that unimolecular reaction can be triggered by molecular collision as one of possible form to reach activation energy. I have thought it was clear. $\endgroup$
    – Poutnik
    Jul 7 at 10:36
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    $\begingroup$ What is called a unimolecular reaction in textbooks is normally (a) an isomerisation such as cis $\ce{<=>}$ trans isomerisation or dissociation $A\to$ products or in the gas phase (b) the reaction $\ce{A + M<=> A^* + M, A^*\to }$products usually described by the Lindemann scheme or more advanced by RRKM theory. Here $A^*$ is a collisionally activated molecule. $\endgroup$
    – porphyrin
    Jul 7 at 16:41
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Truly collisionless unimolecular reaction are rare, as even highly unstable molecules must be triggered somehow to gain the reaction activation energy.

This is frequently obtained as an energy gain by thermal collision with other molecule, or by a photon absorption. Collisions are usually much more frequent, but photons are usually much more energetic than thermal collisions.

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