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rate law (empirical differential rate equation) reaction coordinate is typically chosen to follow
Anexpressionfortherateofreactionofaparticu- the path along the gradient (path of shallowest
lar reaction in terms of concentrations of chem- ascent/deepest descent) of potential energy from
ical species and constant parameters (normally reactants to products.
rate coefficients and partial orders of reaction) The term has also been used interchangeably
only. For examples of rate laws see equations with the term transition coordinate, applicable
(1)-(3) under kinetic equivalence. to the coordinate in the immediate vicinity of the
potential energy maximum. Being more specific,
rate of change (of a function f(t), with respect the name transition coordinate is to be preferred
to t) (1) Average rate of change over [t , t ] in that context.
2
1
is
f(t ) − f(t ) n
2 1 reaction-diffusion equation Let U ⊂ R
. n
t − t 1 be open and u : U × R → R . The reaction-
2
diffusion equation for u is
(2) Instantaneous rate of change at t = t is
1
u − u = f (u).
df t
(t ).
1
dt
reaction path (1) A synonym for mech-
anism.
reactant A substance that is consumed in
(2) A trajectory on the potential-energy
the course of a chemical reaction. It is some-
surface.
times known, especially in the older literature, as
(3) A sequence of synthetic steps.
a reagent, but this term is better used in a more
specialized sense as a test substance that is added
reaction’s compounds The set of molecular
to a system in order to bring about a reaction or to
species, including catalysts, which participate in
see whether a reaction occurs (e.g., an analytical
that reaction. See compound’s reactions.
reagent).
reactive (reactivity) As applied to a chem-
reaction Any chemical or biochemical ical species, the term expresses a kinetic prop-
transformation of matter and energy. erty. A species is said to be more reactive or
Comment: This definition includes both spon- to have a higher reactivity in some given con-
taneous and catalyzed reactions. The word text than some other (reference) species if it has
“transformation” is used in the general English a larger rate constant for a specified elemen-
sense, not the mathematical or linguistic sense. tary reaction. The term has meaning only by
reference to some explicitly stated or implicitly
reaction coordinate A geometric param- assumed set of conditions. It is not to be used
eter that changes during the conversion of one for reactions or reaction patterns of compounds
(or more) reactant molecular entities into one in general.
(or more) product molecular entities and whose The term is also more loosely used as a phe-
value can be taken for a measure of the progress nomenological description not restricted to ele-
of an elementary reaction (for example, a bond mentary reactions. When applied in this sense
length or bond angle or a combination of bond the property under consideration may reflect not
lengths and/or bond angles; it is sometimes only rate, but also equilibrium, constants.
approximated by a nongeometric parameter,
such as the bond order of some specified bond). reduced viscosity (of a polymer) The ratio
In the formalism of “transition-state theory,” the of the relative viscosity increment to the mass
reaction coordinate is that coordinate in a set of concentration of the polymer, c, i.e., η /c, where
i
curvilinear coordinates obtained from the con- η is the relative viscosity increment.
i
3 −1
ventional ones for the reactants which, for each Notes: (1)The unit must be specified; cm g
reaction step, leads smoothly from the configur- is recommended.
ationofthereactantsthroughthatofthetransition (2) This quantity is neither a viscosity nor a
state to the configuration of the products. The pure number. The term is to be looked on as

© 2003 by CRC Press LLC
© 2003 by CRC Press LLC