68 ACIDS, BASES, AND SALTS.
represent, therefore, reciprocal processes. It is important in the preparation
of salts to know which of these reactions preponderates. The mass-action
law applied to this reversible reaction gives:
[HR] [MOH]
[MR] [HOH]Salts, acids, bases, and water are dissociated electrolytically according to the
following equations:
- MR = M+ + R~
- HR = H+ + R~
- MOH = M+ + OH-
- HOH = H+ + OH-
The corresponding dissociation constants, K\, Ki, K 3 , Kt, may be given as
follows:[M+] [R-] , [H+] [R-]. _ [M+] [OH-], [H+] [0H~](^1) [MR] ' (^2) [HR] ' (^3) [MOH] ' l [HOH] '
If, in the above equations of hydrolysis, the values for the concentration of
undissociated MR, HR, MOH, and HOH obtained respectively from the last
four expressions are inserted, the equation
K — ^' ' Kt
K K
is obtained. This equation expresses all the essential phenomena in neu-
tralization and hydrolysis. It shows that hydrolysis will be greater if the
value of Ki becomes larger; or, in other words, if the temperature is raised, since
the dissociation of water, although extremely small, increases quite rapidly
with rise of temperature. The hydrolysis is also greater as K* and Kz are
smaller, or in other words as the acid and base in question are weaker. Inas-
much as the weakness of the base usually increases with the valence of the
metal, it follows that salts of the tervalent metals (Fe, Al, Sb, Bi) are hydrolyzed
to a greater extent than the salts of metals having a lower valence (Mn, Ba,
etc.) (cf. Nos. 40, 41). The halides of the quadrivalent metals (Sn, Si, Ti)
are hydrolyzed to an especially "marked extent (cf. Nos. 50 to 52), and it is,
therefore, possible to prepare such salts pure only when water is completely
excluded. If sulphur chloride is regarded as the sulphur salt of hydrochloric
acid (cf. No. 45), this compound then forms the extreme of the hydrolyzable
chlorides, inasmuch as the metallic character of the element combined with
chlorine has entirely disappeared.
Salts of weak acids and weak bases, such as, for example, the sulphides of
aluminium (Nos. 4 and 5) and titanium, can only be prepared when out of
contact with water. After they have been prepared synthetically in a crys-
tallized form, they can be kept for some time without change on account of the