© Ultrafast Systems LLC /Vernier Software & Technology
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elapsed. Thus the rate constant provides the experimenter with a useful measure of the velocity of a
reaction, and it is this quantity that we usually seek in a study of chemical kinetics.
5.4
Order of Reaction
In Equation 7, the concentrations of A and B both appear to the first power, and we say that this
reaction is first order in [A] and first order in [B], and that it is second order overall. However the rate
expression for the elementary process 3 will be
2
[ ]
[ ][ ]
[ ]
d A
k A A
k A
dt
Ms
-1
Equation 8
For this reaction it is seen that the concentration of A appears as the 2
nd
power, and we say that
the reaction is second order in [A] and second order overall.
Also, the symbolic elementary process
A
products
would be expected to have a rate expression such as
[ ]
[ ]
d A
k A
dt
Ms
-1
Equation 9
and the reaction would be said to be first order in [A] (and first order overall). Orders higher than 2 and
also fractional and zero orders are found in special cases. These are not considered here because of lack
of relevance to the Vernier Flash Photolysis Spectrometer concept.
SPECIAL NOTE:
It is important to realize that assignment of order by inspection is only valid for
elementary reactions; we cannot do this for stoichiometric equations.
