the “rate of conversion” and is appropriate when the
use of concentrations is inconvenient, e.g., under con-
ditions of varying volume. In a system of constant vol-
ume, the rate of reaction is equal to the rate of
conversion per unit volume throughout the reaction.
For a STEPWISE REACTIONthis definition of “rate of
reaction” (and “extent of reaction,” π) will apply only
if there is no accumulation of intermediate or forma-
tion of side products. It is therefore recommended that
the term rate of reactionbe used only in cases where it
is experimentally established that these conditions
apply. More generally, it is recommended that, instead,
the terms rate of disappearanceor rate of consumption
of A (i.e., –d[A]/dt,the rate of decrease of concentra-
tion of A) or “rate of appearance” of P (i.e., d[P]/dt,
the rate of increase of concentration of product P) be
used, depending on the concentration change of the
particular CHEMICAL SPECIESthat is actually observed.
In some cases, reference to the CHEMICAL FLUX
observed may be more appropriate.
The symbol v(without lettered subscript) should
be used only for rate of reaction; vwith a lettered sub-
script (e.g., vA) refers to a rate of appearance or rate of
disappearance (e.g., of the chemical species A).
See alsoCHEMICAL RELAXATION; LIFETIME; ORDER
OF REACTION.
reactants The materials consumed in a chemical
reaction.
reacting bond rules (1) For an internal motion of a
MOLECULAR ENTITYcorresponding to progress over a
TRANSITION STATE(energy maximum), any change that
makes the motion more difficult will lead to a new
molecular geometry at the energy maximum, in which
the motion has proceeded further. Changes that make
the motion less difficult will have the opposite effect.
(This rule corresponds to the HAMMOND PRINCIPLE).
(2) For an internal motion of a molecular entity
that corresponds to a vibration, any change that tends
to modify the equilibrium point of the vibration in a
particular direction will actually shift the equilibrium
in that direction.
(3) Effects on reacting bonds (bonds made or bro-
ken in the reaction) are the most significant. The bonds
nearest the site of structural change are those most
strongly affected.
reaction SeeCHEMICAL REACTION.reaction coordinate A geometric parameter that
changes during the conversion of one (or more) reac-
tant MOLECULAR ENTITIESinto one (or more) product
molecular entities and whose value can be taken for a
measure of the progress of an ELEMENTARY REACTION
(for example, a bond length or bond angle or a com-
bination of bond lengths and bond angles; it is some-
times approximated by a nongeometric parameter,
such as the BOND ORDERof some specified bond). In
the formalism of TRANSITION-STATE theory, the reac-
tion coordinate is that coordinate—in a set of curvi-
linear coordinates obtained from the conventional
ones for the reactant—which, for each reaction step,
leads smoothly from the configuration of the reactants
through that of the transition state to the configura-
tion of the products. The reaction coordinate is typi-
cally chosen to follow the path along the gradient
(path of shallowest ascent/deepest descent) of poten-
tial energy from reactants to products.
The term has also been used interchangeably with
the term TRANSITION COORDINATE, applicable to the
coordinate in the immediate vicinity of the potential
energy maximum. Being more specific, the name transi-
tion coordinateis to be preferred in that context.
See also POTENTIAL-ENERGY PROFILE; POTENTIAL-
ENERGY(REACTION) SURFACE.reaction mechanism SeeMECHANISM.reaction path (1) A synonym for MECHANISM.
(2) A trajectory on the POTENTIAL-ENERGY
SURFACE.
(3) A sequence of synthetic steps.reaction ratio The relative amounts of reactants and
products involved in a reaction.reaction stage A set of one or more (possibly experi-
mentally inseparable) REACTION STEPs leading to or
from a detectable or presumed reaction INTERMEDIATE.232 reactants