IONIC EEACTIONS 103Equilibrium. When 0.1 mole of HC2H3O2 is dissolved in 1 liter
of water and the two opposing reactions have adjusted themselves
so that 1.4 per cent of the acid is in the ionized condition and 98.6
per cent is in the un-ionized condition, a state of equilibrium is
said to exist. Such a point of equilibrium exists for every ionic
reaction and, as in the example just cited, it is independent of the
direction from which it is approached.
Ionization is a very rapid reaction, as is, indeed, the reverse
reaction, or association; and the point of equilibrium is reached
with great rapidity. In fact, in the time taken to dissolve an
electrolyte in water, and uniformly mix the solution by stirring,
complete equilibrium is attained.
For the sake of comparison a non-ionic reaction may be cited.
If 2 volumes of hydrogen (uncombined hydrogen, not hydrogen
ions) are mixed with 1 volume of oxygen in a glass jar at ordi-
nary temperature, nothing appears to happen. Yet we know this
mixture is not in equilibrium. By the careful use of certain cata-
lyzers, the two gases will combine slowly but completely to form
water, even if the temperature is kept from rising. As we well
know, if a spark is applied to the mixture a violent reaction takes
place. Equilibrium for this reaction exists only when it has gone
practically to completion in the direction 2H 2 + O 2 —» 2H 2 O, and
yet at ordinary temperature and without catalyzers the reaction
is so slow that it will not have reached an equilibrium condition
in many years.
Equations for Ionic Reactions. Reactions involving ionized
substances cannot be adequately represented by single equations,
because such equations cannot show all the species of ions and
molecules that take part in the changes. In fact, each species of
undissociated molecule concerned requires a separate equation
to show its passage into, or out of, the ionized condition; but
these equations may be written together so as to intersect and give
a complete picture of the whole change.
In the next section eight rules to be observed in writing ionic
equations are given, and by following these rules one is able
to give, by means of the equations alone, both a fairly complete
description, and a remarkably good explanation, of the reaction.
Until the student has thoroughly mastered the ionic theory, he
should write equations in the fully ionized intersecting form for
every reaction which he studies. Later, with the practice thus