Elementarschritte

Elementarschritte, also known as elementary steps or elementary reactions, are fundamental chemical transformations that occur in a single molecular event. They represent the simplest possible reaction pathways, involving the breaking and forming of a specific set of chemical bonds in a single, concerted step. In contrast to an overall reaction, which may involve multiple stages, an elementarschritt is a fundamental building block.

The concept of elementarschritte is crucial in understanding reaction mechanisms. Many complex chemical reactions proceed through a sequence of several elementarschritte. The sum of these individual steps leads to the observed overall reaction. By identifying and characterizing these elementary steps, chemists can gain insights into the detailed molecular dynamics of a reaction, including the intermediates formed, the transition states involved, and the factors that influence reaction rates.

A key characteristic of an elementarschritt is that its molecularity, which is the number of molecules or atoms that collide and react in that single step, is typically unimolecular, bimolecular, or at most, termolecular. Unimolecular steps involve a single molecule undergoing transformation, such as isomerization or dissociation. Bimolecular steps involve the collision and reaction of two molecules. Termolecular steps, though less common, involve the simultaneous collision of three species. Reactions involving four or more species colliding at once are exceedingly rare and statistically improbable.

The rate law for an elementarschritt can be directly derived from its stoichiometry. For example, a unimolecular elementarschritt $A \rightarrow Products$ would have a rate law of rate = $k[A]$, and a bimolecular elementarschritt $A + B \rightarrow Products$ would have a rate law of rate = $k[A][B]$. This direct relationship between rate law and stoichiometry is a defining feature that distinguishes elementary steps from overall reactions, whose rate laws often do not directly reflect their overall stoichiometry.