Basic premise of kinetics

It can easily be thought that, in order to react with each other, molecules have to be very close to each other at a given time. This means they must positioned so that they exert mutual influences that can provoke ehanges. 

This reaction involves three molecules as reactants. It is clear that if this reaction occurred in a single step, it would imply that the three molecules (reagents) meet (or are very close to each other) at the same time to give rise to a reaction that is followed by the simultaneous transfer of two electrons. This situation is thought to be unlikely. Indeed the probabihty of collision of three gas molecules can be calculated through the kinetic theory of gases and is very low, certainly well below the number of molecules that react per second, which can be obtained from the reaction speed that readily becomes explosive. We therefore prefer a path that is a 

Note 2.1.- The above reaction could have been written by dividing the stoichiometric coefficients by two, which would have led to the necessity for only two molecules to collide - a phenomenon that is much more common and sufficient with regard to the order of magnitude of reaction speeds. Such a collision will have produced half a nitrogen molecule, however, which is unacceptable given that there is no such thing as half a water molecule. 

Such observations can be made for a large number of reactions, which leads us to admit the following premise that can be considered to be the fundamental principle of kinetics. 

Principle.- Aity real reaction is a more or less complex combination of elementary steps. 

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