52 SIMPLE COMPOUNDS.Thus sulphuric acid anhydride is now prepared directly by the contact process
(No. 28), nitric oxide from the elements in the atmosphere (combustion of air),
ammonia from nitrogen and hydrogen, and aluminium chloride from the
metal (cf. No. 43.)
REACTIVITY AND DEGREE OP DISSOCIATION. It is a general principle that
the reactivity of a substance is determined by a previous breaking down
(dissociation), to a greater or less extent, in the same sense as that in which
the reaction in question takes place. Phosphorus pentachloride, for example,
has a chlorinating effect, and sulphur trioxide an oxidizing effect, only when
under the prevailing conditions the one is partly dissociated into free chlorine,
the other into free oxygen, even in the absence of any substance to be chlo-
rinated or oxidized. Conversely, a tendency shown by substances to enter
into reaction may be considered as an indication of the preexistence of a
corresponding dissociation. The dissociation of binary substances, which in
fact often takes place in stages, is essentially the reverse of their synthesis.
PC1 6 = PC1 3 + Cl 2
PC1 3 = P + 3 Cl
2 SO 3 = 2 SO 2 + O 2 (No. 28 )
DISSOCIATION AND STATE OF EQUILIBRIUM. If it is true that reactivity is
dependent upon a certain ability to dissociate, it becomes important to study
the conditions favoring the formation and those favoring the decomposition
of substances. To take a concrete example, — When does the reaction
2 SO 2 + O 3 = 2 SO 3
take place, and when
2 SO 3 = 2 SO 2 + O 2?
In this connection, another principle which is likewise of very general
importance has been established, — namely, that a reaction never takes place
completely in one direction; at most the chemical change may proceed chiefly
in a definite direction until when the reaction comes to a standstill (i.e., when
equilibrium is reached) the products of dissociation and the undissociated
compound exist together side by side, forming the so-called equilibrium-
mixture. The percentage composition of an equilibrium-mixture is charac-
terized by the fact that the same values are obtained irrespective of whether
at the start a mixture of the pure components or the pure compound itself is
present. Thus, for example, the same mixture of SO 3 , SO 2 , and O 2 is obtained
whether equivalent amounts (e.g.! formula weights) of SO 2 and O 2 are allowed
to react, or an equivalent quantity of SO 3 is allowed to decompose under the
same conditions of temperature and volume.
2 SO 3 <=> 2 SO 2 + 0 2
should be read: sulphur trioxide "in equilibrium with" sulphur dioxide and
oxygen.
Inasmuch as all reactions are, strictly speaking, reversible, it is theoretically
impossible to prepare perfectly pure compounds; for a compound can only
exist as a stable substance when it is in equilibrium with its products of disso-
ciation. For practical purposes, however, it is true that (1) in the equilibrium
mixture the percentage content of dissociation products, or in the other case
the fraction of undissociated substance, is frequently so extremely small that it