If the equilibrium mixture of a
chemical reaction is disturbed by a sudden change, especially of some external parameter (such as temperature,
pressure or electrical field strength), the system will readjust itself to a new position
of the
chemical equilibrium or return to the original position, if the perturbation is temporary. The readjustment
is known as chemical
relaxation. In many cases, and in particular when the displacement from equilibrium is slight,
the progress of the system towards equilibrium can be expressed as a first-order law:
where
and
are the equilibrium concentrations of one of the chemical species involved in the
reaction before and after the change in the external parameter, and
is its concentration at time
. The time parameter
, named
relaxation time, is related to the rate constants of the chemical reaction involved. Measurements
of the
relaxation times by
relaxation methods [involving a
temperature jump (T-jump),
pressure jump, electric field jump or a periodic disturbance of an external parameter, as in ultrasonic
techniques] are commonly used to follow the kinetics of very fast reactions.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1096
in NMR
![C t − C eq 2 = [ C eq 1 − C eq 2 ] e − t τ](../../mathml/C01035-0.png){kind=small}
