102 THE THEORY OF IONIZATIONamong substances that are ionized are clearly electrical in nature.
On the other hand, a very large number of organic substances (that
is, compounds of carbon) are not perceptibly ionized even in
water solution. At ordinary temperature such substances do not
enter into rapid chemical reaction as electrolytes do.
Non-Ionic Reactions. This term is applied to all slow reactions
in which none of the substances involved show any measurable
degree of ionization; there seems to be no great advantage in
trying to deal with them from the electrical viewpoint.
Ionic Reactions. All reactions among ionized substances clearly
involve electrical forces, and it is the purpose of this outline to
deal with various types of ionic reactions.
Ionization as a Reversible Reaction. The ionization of a sub-
stance is itself a chemical reaction. Pure anhydrous acetic acid
does not possess the properties of an acid: it does not react with zinc
or calcium carbonate, nor does it turn blue litmus red. Yet if a
little acetic acid is dissolved in water it acquires the properties
of an acid: it dissolves zinc with evolution of hydrogen, it dis-
solves calcium carbonate with evolution of carbon dioxide, and it
turns blue litmus red. These new properties betoken the presence
of new substances, the ions, and that the reaction HC2H3O2—>
H+ + C2H3O2 ~~ has taken place. From the preceding table it
-will be seen that if 0.1 mole, or 6 grams, of acetic acid is dis-
solved in 1 liter of water 1.4 per cent of the acetic acid molecules
are ionized.
To show that this reaction may take place in the opposite
direction let us take 1 liter of a 0.2 molal sodium acetate solution.
This solution has no odor of acetic acid. Sodium acetate is a
strong electrolyte, and we may regard it as 100 per cent ionized in
this fairly dilute solution. Let us now mix with this solution 1
liter of 0.2 molal hydrochloric acid, another strong electrolyte
which we may regard as 100 per cent ionized. We will now have
brought together into 2 liters the ions of 0.2 mole of sodium
chloride, which remains ionized, and 0.2 mole of acetic acid. The
latter, however, does not remain fully ionized but immediately
comes to the equilibrium point at which 1.4 per cent only of the
total acetic acid is ionized (the solution is 0.1 molal in acetic acid).
That acetic acid has actually been formed, H+ + C2H3O2" —*
HC2H3O2, can be perceived by the odor of acetic acid which is
now apparent.