Man's physical universe

xanabras

UNIT VIII

SECTION 5

ELECTROVALENT REACTIONS CONSIST OF REACTIONS

BETWEEN IONS

Introduction.

In the last Section it was pointed out that many reactions are electrovalent

in nature and that electrovalent compounds are held together

by the attraction of oppositely charged ions. What evidence is there

for the existence of ions? Why do some substances ionize while others

do not? Such questions will be considered in this Section, as we consider

one of the greatest and, incidentally, one of the most incomplete

and least satisfactory theories of physical science — namely, the

theory of electrolytic dissociation.

The Theory of £lectrol3rtic Dissociation Explains Many Phenomena.

i. The Irregularities of Solutions of Electrovalent Compounds.

In the study of the properties of solutions it is observed that electrovalent

compounds show abnormal behavior; for example, it is readily

shown that for solutions of equal numbers of molecules of solute in a

given amount of solvent, the freezing-point lowering is greater, the

boiling-point elevation is greater, and the lowering in vapor pressure

is greater in the case of polar compounds than in the case of covalent

compounds. It may also be observed that the osmotic pressures of

solutions of electrovalent compounds are always higher than the

osmotic pressures of solutions of covalent compounds of equivalent

concentration.

If Avogadro's law can be extended to solutions, these abnormalities

of solutions of electrovalent compounds must be ascribed to there

being more particles present than in solutions of covalent compounds

of the equivalent concentration, because all of these properties are

a function of the number of particles present rather than of their nature.

If more particles are present, they must be produced by the dissociation

(splitting) of molecules.

It may be observed that reactions between solutions of electrovalent

compounds take place almost instantaneously, whereas reactions be-

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