Like the rate of consumption,
the rate of formation of a specified product may be defined in two ways:

- As the time derivative of the amount of a product. Thus for a product Y, present at any time in amount . the rate of its formation may be given by: This definition is particularly appropriate for open systems.
- For kinetics in closed systems it is more usual to define a rate of formation per unit volume, denoted : When the volume is constant this reduces to: When the volume is not constant the relationship may be differentiated to give: and the rate of formation becomes: A rate of formation may be specified even for a reaction of time dependent stoichiometry or of unknown stoichiometry.

Source:

PAC, 1996, *68*, 149*
(A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC
Recommendations 1996))
* on page 181