ACIDS, BASES, AND SALTS. 67equilibrium with the ions, and at the dilution v the concentration of the
to the mass-action law the following relation holds:i 2
v ' = k, or yz—-—T— = k. (Ostwald's Law of Dilution.)
1 — a (1 — a) V
V
The maximum possible value for a, the degree of dissociation, is 1, and this
represents complete dissociation; it is evident that this is approached in pro-
portion as v is made large. Experiment 37 illustrates a case of dissociation
with progressive dilution.
Ostwald's dilution law fairly accurately describes the behavior of little
ionized acids and bases, but it fails with the highly ionized acids and bases and
with salts, which in general are highly ionized. Indeed, De Bye and others
hold that salts in solution are J.00 per cent ionized and that the apparent lack
of complete ionization in concentrated solutions, computed from conductivity
and osmotic measurements, is due to certain factors restricting the free motion
of the ions.
IONIZATION TENDENCY, OE ELECTEOAFFINITY. A measure of the force
with which the electron is held to the material atom of an ion is given by the
potential which is necessary to effect the discharge of the ion in an electrolysis
(decomposition potential). This force is variously known as ionization
tendency or electroaffinity. In the following table, the tension in volts is given
which is necessary for the discharge of a few of the metal ions from their
normal solutions, on the basis of the discharge-potential of hydrogen being
taken arbitrarily as zero.^1
Mg + 1.482 Cd + 0.420 Pb + 0.151
Al + 1.276 Fe + 0.344 Cu - 0.329
Mn + 1.075 Co + 0.232 Ag - 0.753
Zn + 0.770 Ni + 0.228
In this potential series the distinction which has always been recognized
between noble and base metals is again expressed. Elements with a high
ionizing tendency possess to a marked degree the ability to form simple ions.
Simple ions of the noble metals are less stable, so that the number and impor-
tance of the simple salts of these metals is much less than that of their com-
plex salts. (Cf. Chapters IV and V.)
HYDEOLYTIC DISSOCIATION, OE HYDEOLYSIS. The formation of a salt by
the neutralization of an acid and a base is a reversible process. If M denotes
a metal and R an acid radical, then the reaction of equilibrium is:
Neutralization
MOH + HR *± MR + H 2 O
Hydrolysis
If the equation is read from right to left it represents an hydrolysis, i.e., the
breaking up of a salt into an acid and a base. Neutralization and hydrolysis
(^1) The choice of an arbitrary zero-point is necessary, since it is only differ-
ences in potential which can actually be measured.