Encyclopedia of Chemistry

spontaneous reaction A reaction that will continue
to proceed without any outside energy. Spontaneous
reactions are those that will take place by themselves,
e.g., precipitation of calcium carbonate in the stalac-
tites in a cave over millions of years.

square planar Molecules and polyatomic ions with
one atom in the center and four atoms at the corners of
a square.

square planar complex When metal is in the center
of a square plane, and the LIGANDdonor atoms are at
each of the four corners.

square plane SeeCOORDINATION.

stability constant An equilibrium constant that
expresses the propensity of a species to form from its
component parts. The larger the stability constant, the
more STABLEis the species. The stability constant (for-
mation constant) is the reciprocal of the instability con-
stant (dissociation constant).

stable A term describing a system in a state of equi-
librium corresponding to a local minimum of the
appropriate thermodynamic potential for the specified
constraints on the system. Stability cannot be defined
in an absolute sense, but if several states are in princi-
ple accessible to the system under given conditions,
that with the lowest potential is called the stable state,
while the other states are described as metastable.
Unstable states are not at a local minimum. Transitions
between metastable and stable states occur at rates that
depend on the magnitude of the appropriate activation
energy barriers that separate them.

standard electrode potential The potential recorded
by an electrode in which all reacting species are present
at unit activity (approximately 1 Mand 1 atm).

standard molar enthalpy of formation The
enthalpy change when a mole of substance is formed

from its elements in their most stable form under stan-

dard conditions.

standard molar volume The volume occupied by 1

mole of an ideal gas under standard conditions; 22.4

liters.

starch A polysaccharide containing glucose (long-

chain polymer of amylose and amylopectin) that is the

energy storage reserve in plants.

state function A function whose value depends only

on the state of a substance and not on the path by

which the state was reached, e.g., energy, enthalpy,

temperature, volume, and pressure.

stationary state SeeSTEADY STATE.

steady state(stationary state) (1) In a kinetic analy-

sis of a complex reaction involving UNSTABLEinterme-

diates in low concentration, the rate of change of each

such INTERMEDIATEis set equal to zero, so that the rate

equation can be expressed as a function of the concen-

trations of CHEMICAL SPECIESpresent 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] 0

which, since [A] 0 is constant, implies:

–d[X]/dt = d[A]/dt + d[D]/dt.

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, (d[X]/dt = 0)

allows the elimination of [X] from the kinetic equa-

tions, whereupon the rate of reaction is expressed:

AX

X+C D

k

k

k

1

1

2

• ←⎯⎯⎯→⎯

⎯→⎯

steady state 255