Molecularity refers to the number of molecules that come together to react in a single elementary step of a chemical reaction. It is an important concept in the study of chemical kinetics and helps in understanding the mechanisms by which reactions occur.
Molecularity can be classified into different categories based on the number of reacting species:
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Unimolecular: Involves a single molecule undergoing a reaction. For example, the decomposition of a compound into products.
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Bimolecular: Involves two reacting molecules. This can be two identical molecules (like A + A) or two different molecules (like A + B).
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Termolecular: Involves three reacting molecules. While this is theoretically possible, termolecular reactions are rare because the likelihood of three molecules colliding at the same time is comparatively low.
Reactions that involve more than three molecules in a single elementary step are generally considered improbable and are not typically categorized by molecularity.
It's important to note that molecularity is distinct from reaction order, which is determined by the sum of the powers of concentration terms in the rate law and can be fractional or zero.
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