Preferred Label : steady state;
Detailed label : steady state (stationary state);
IUPAC definition : in a kinetic analysis of a complex reaction involving unstable intermediates in low
concentration, the rate of change of each such intermediate is set equal to zero,
so that the rate equation can be expressed as a function of the concentrations of
chemical species present in macroscopic amounts. for example, assume that x is an
unstable intermediate in the reaction sequence: conservation of mass requires that:
[a] [x] [d] [a]0which, since [a]0 is constant, implies: [x] t [a]
t [d] t . since [x] is negligibly small, the rate of formation of d is essentially
equal to the rate of disappearance of a, and the rate of change of[x] can be set equal
to zero. applying the steady state approximation ( [x] t 0) allows the elimination
of [x] from the kinetic equations, whereupon the rate of reaction is expressed:
[d] t [a] t k1 k2 [a] [c] k-1 k2 [c]; in a stirred flow reactor a steady state implies a regime so that all concentrations
are independent of time.;
Scope note : the steady-state approximation does not imply that [x] is even approximately constant,
only that its absolute rate of change is very much smaller than that of [a] and [d].
since according to the reaction scheme [d] t k2 [x] [c] , the assumption
that [x] is constant would lead, for the case in which c is in large excess, to the
absurd conclusion that formation of the product d will continue at a constant rate
even after the reactant a has been consumed.;
Origin ID : S05962;
Automatic exact mappings (from CISMeF team)
- See also
in a kinetic analysis of a complex reaction involving unstable intermediates in low
concentration, the rate of change of each such intermediate is set equal to zero,
so that the rate equation can be expressed as a function of the concentrations of
chemical species present in macroscopic amounts. for example, assume that x is an
unstable intermediate in the reaction sequence: conservation of mass requires that:
[a] [x] [d] [a]0which, since [a]0 is constant, implies: [x] t [a]
t [d] t . since [x] is negligibly small, the rate of formation of d is essentially
equal to the rate of disappearance of a, and the rate of change of[x] can be set equal
to zero. applying the steady state approximation ( [x] t 0) allows the elimination
of [x] from the kinetic equations, whereupon the rate of reaction is expressed:
[d] t [a] t k1 k2 [a] [c] k-1 k2 [c]
in a stirred flow reactor a steady state implies a regime so that all concentrations
are independent of time.
