pseudo first order
(gcide) | First-order \First`-or"der\, a.
decaying at an exponential rate; -- a mathematical concept
applied to various types of decay, such as radioactivity and
chemical reactions.
Note: In first order decay, the amount of material decaying
in a given period of time is directly proportional to
the amount of material remaining. This may be expressed
by the differential equation: dA/dt = -kt where dA/dt
is the rate per unit time at which the quantity (or
concentration) of material (expressed as A) is
increasing, t is the time, and k is a constant. The
minus sign in front of the "kt" assures that the amount
of material remaining will be decreasing as time
progresses. A solution of the differential equation to
give the quantity A shows that: A = e^-kt where e is
the base for natural logarithms. Thus this type of
decay is called exponential decay. In certain chemical
reactions that are in fact second-order, involving two
reactants, the conditions may be chosen in some cases
so that one reactant is vastly in excess of the other,
and its concentration changes very little in the course
of the reaction, so that the reaction rate will be
approximately first order in the more dilute reactant;
such reactions are called pseudo first order.
[PJC] |