Molecularity

Not to be confused with Molarity.

In chemistry, molecularity is the number of molecules that come together to react in an elementary (single-step) reaction[1] and is equal to the sum of stoichiometric coefficients of reactants in the elementary reaction with effective collision (sufficient energy) and correct orientation.[2] Depending on how many molecules come together, a reaction can be unimolecular, bimolecular or even trimolecular.

The kinetic order of any elementary reaction or reaction step is equal to its molecularity, and the rate equation of an elementary reaction can therefore be determined by inspection, from the molecularity.[1]

The kinetic order of a complex (multistep) reaction, however, is not necessarily equal to the number of molecules involved. The concept of molecularity is only useful to describe elementary reactions or steps.

  1. ^ a b Atkins, P.; de Paula, J. Physical Chemistry. Oxford University Press, 2014
  2. ^ Temkin, O. N. State-of-the-Art in the Theory of Kinetics of Complex Reactions. In Homogeneous Catalysis with Metal Complexes: Kinetic Aspects and Mechanisms, John Wiley and Sons, ltd, 2012