Kolmemolekulaarsed

Kolmemolekulaarsed refers to a class of chemical reactions where three molecules participate in the rate-determining step. These reactions are distinct from unimolecular (one molecule involved) and bimolecular (two molecules involved) reactions. The rate law for a typical elementary kolmemolekulaarsed reaction will show the concentration of three reactants raised to the power of one, leading to a third-order rate dependence. For example, a reaction A + B + C -> Products might have a rate law of Rate = k[A][B][C].

The probability of three molecules colliding simultaneously with the correct orientation and sufficient energy to react is significantly lower than for bimolecular collisions. This inherent low probability often means that kolmemolekulaarsed reactions proceed at slower rates compared to reactions with lower molecularity, assuming similar activation energies and concentrations. Consequently, such reactions are less common as elementary steps in reaction mechanisms.

However, reactions that appear to be kolmemolekulaarsed can often be explained by a series of simpler, faster bimolecular steps, with the overall reaction being a composite mechanism. In some cases, a termolecular collision might involve two identical molecules and one different molecule (e.g., 2A + B -> Products, with a rate law of Rate = k[A]^2[B]), or three identical molecules (e.g., 3A -> Products, with a rate law of Rate = k[A]^3). Understanding the molecularity of a reaction is crucial for predicting its rate and for elucidating its underlying reaction mechanism.