# What happens in a unimolecular reaction? [closed]

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?

• ?!? If two molecules collide, you have a bimolecular reaction.
– Karl
Jul 7 at 7:56
• Just for future reference, As homework is considered literal homework, exams, self-study questions, puzzles, worked examples etc. Jul 7 at 8:06
• @Karl Is reaction $\ce{A + A ->[collision \Delta E ] A^{*} + A -> B + C + A}$ unimolecular ? Jul 7 at 8:50
• @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. Jul 7 at 10:36
• 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. Jul 7 at 16:41