rate of formation, v n , y or v c , y

Like the rate of consumption, the rate of formation of a specified product may be defined in two ways:
  1. As the time derivative of the amount of a product. Thus for a product Y, present at any time in amount n Y. the rate of its formation may be given by:
    v n Y = d n Y d t
    This definition is particularly appropriate for open systems.
  2. For kinetics in closed systems it is more usual to define a rate of formation per unit volume, denoted v c Y:
    v c Y = 1 V d n Y d t
    When the volume is constant this reduces to:
    v c Y = 1 V d n Y d t = d Y d t
    When the volume is not constant the relationship n Y = Y V may be differentiated to give:
    d n Y = V d Y + Y d V
    and the rate of formation becomes:
    v c Y = d Y d t + Y V d V d t
    A rate of formation may be specified even for a reaction of time dependent stoichiometry or of unknown stoichiometry.
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 181