Page 48 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNOAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION
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Salts. The structure of numerous salts in the solid state has been investigated
by means of X-rays and by other methods, and it has been shown that they are
composed of charged atoms or groups of atoms held together in a crystal lattice;
they are said to be ionic compounds. When these salts are dissolved in a solvent
of high dielectric constant such as water, or are heated to the melting point,
the crystal forces are weakened and the substances dissociate into the pre-existing
charged particles or ions, so that the resultant liquids are good conductors of
electricity; they are referred to as strong electrolytes. Some salts, however,
exemplified by cyanides, thiocyanates, the halides of mercury and cadmium,
and by lead acetate, give solutions which show a significant electrical conductance,
but which is not as great as that shown by solutions of strong electrolytes of
comparable concentration. Solutes showing this behaviour are referred to as
weak electrolytes: they are generally covalent compounds which undergo only
limited ionisation when dissolved in water:
BA=B++A-
Acids and bases. An acid may be defined as a substance which, when dissolved
in water, undergoes dissociation with the formation of hydrogen ions as the
only positive ions:
HCle H+ +Cl-
HN03 = H+ + NO;
Actually the hydrogen ion H+ (or proton) does not exist in the free state in
aqueous solution; each hydrogen ion combines with one molecule of water to
form the hydroxonium ion, H30+. The hydroxonium ion is a hydrated proton.
The above equations are therefore more accurately written:
HC1+H20= H30+ +Cl-
HN03 + H20 = H30+ +NO;
The ionisation may be attributed to the great tendency of the free hydrogen
ions H+ to combine with water molecules to form hydroxonium ions.
Hydrochloric and nitric acids are almost completely dissociated in aqueous
solution in accordance with the above equations; this is readily demonstrated
by freezing-point measurements and by other methods.
Polyprotic acids ionise in stages. In sulphuric acid, one hydrogen atom is
almost completely ionised:
H2S04+H20=H30+ +HS04
The second hydrogen atom is only partially ionised, except in very dilute
solution:
HS04 +H20eH30+ +Soi-
Phosphoric(V) acid also ionises in stages:
H3P04+H20= H30+ +H2P04
H2P04 + H20 = H30+ + HPO2-
HPOZ- +H20 e H30+ + PO:-
The successive stages of ionisation are known as the primary, secondary, and