In a mixture of reagents prepared to react two situations may occur for non equilibrium reactions:

A) The reagents are in the same molar proportions as in . In this case the reagents are in stoichiometric proportions, and at the end, only the products will be present.

B) One of the reagents is in default (in less than the stoichiometric molar proportion). It is named the limiting reagent. The reaction ends when this reagent is consumed and the system contains the products and the excess reagent.