SIMPLE COMPOUNDS. 53
becomes negligible, and that (2) the rate at which certain substances decom-
pose after they have once been prepared "pure" is often so extremely slow
that measurable quantities of the dissociation products are formed only after a
very long time.^1
MASS-ACTION LAW. The state of equilibrium which combining or decom-
posing substances reach is dependent (1) upon the nature of the reacting
substances, (2) upon their masses, and (3) upon the temperature. The influ-
ence exerted by the masses upon the state of equilibrium can be expressed
mathematically by the so-called Law of Mass Action (Guldberg and Waage,
1867). According to this law, the product of the concentrations of the sub-
stances which are upon the right-hand side of the sign of equilibrium, divided
by the product of the concentrations of the substances on the left-hand side,
is a constant at a given temperature. The concentration is usually expressed
as the number of gram-molecules of substance which are contained in a unit
of volume. If A is the formula of a substance, it is customary to express the
concentration of A by inclosing it in brackets [A]; then if (A) represents the
actual amount of substance present expressed in gram-molecules, and v
the volume, we have c. \
[A] -£>If A and B are two substances which by reacting together form two new
substances C and D, with which they finally come to equilibrium,
A + B <=> C + D,
then the mass-action law is expressed as follows:
[C] [D] _ K
[A] [B] *•
If, however, two or more molecules (a, 6, etc., being the numbers) of any of
the substances enter into the reaction, then the concentration of these sub-
stances must be taken a, b, etc., times in the mass-action-law equation. Thus
if a molecules of A react with b molecules of B to form c molecules of C and d
molecules of D, the equation becomes:
[C]c [T>f R
[A]° [Bf
APPLICATION OP THE MASS-ACTION LAW. The value of the mass-action
law for the manufacturing chemist becomes apparent when with its aid the
yields are predicted that can be obtained in the preparation of a substance at
a given temperature but with varying proportions of the reacting materials.
This is particularly well illustrated by measurements of Bodenstein and Pohl
with regard to the contact-process for the manufacture of sulphuric acid.
Sulphur dioxide and oxygen react within a reasonable interval of time to form
sulphur trioxide only when in the presence of catalyzers; the presence of the
catalyzer, however, has no effect upon the equilibrium which is finally reached.
2 SO 2 + 0 2 <=> 2 SO 3.
1
In old collections of organic preparations, the amount of impurities which
have arisen from a self-decomposition of the material is often very considerable.