sipative system the system is moving
towards a state of equilibrium, which
can be regarded as moving toward a
point attractor in phase space; this is
equivalent to moving towards the
minimum of the free energy, F.
dissociationThe breakdown of a
molecule, ion, etc., into smaller mol-
ecules, ions, etc. An example of disso-
ciation is the reversible reaction of
hydrogen iodide at high tempera-
tures
2HI(g) ˆH 2 (g) + I 2 (g)
The *equilibrium constant of a re-
versible dissociation is called the
dissociation constant. The term ‘dis-
sociation’ is also applied to ionization
reactions of *acids and *bases in
water; for example
HCN + H 2 O ˆH 3 O++ CN–
which is often regarded as a straight-
forward dissociation into ions
HCN ˆH++ CN–
The equilibrium constant of such a
dissociation is called the acid dissoci-
ation constant or acidity constant,
given by
Ka= [H+][A–]/[HA]
for an acid HA (the concentration of
water [H 2 O] can be taken as con-
stant). Kais a measure of the strength
of the acid. Similarly, for a nitroge-
nous base B, the equilibrium
B + H 2 O ˆBH++ OH–
is also a dissociation; with the base
dissociation constant, or basicity con-
stant, given by
Kb= [BH+][OH–]/[B]
For a hydroxide MOH,
Kb= [M+][OH–]/[MOH]dissociation pressure When a
solid compound dissociates to give
one or more gaseous products, the
dissociation pressure is the pressure
of gas in equilibrium with the solid
at a given temperature. For example,when calcium carbonate is main-
tained at a constant high tempera-
ture in a closed container, the
dissociation pressure at that tempera-
ture is the pressure of carbon dioxide
from the equilibrium
CaCO 3 (s) ˆCaO(s) + CO 2 (g)distillationThe process of boiling
a liquid and condensing and collect-
ing the vapour. The liquid collected
is the distillate. It is used to purify
liquids and to separate liquid mix-
tures (see fractional distillation;
steam distillation). See also de-
structive distillation; extractive
distillation.distilled water Water puriÜed by
distillation so as to free it from dis-
solved salts and other compounds.
Distilled water in equilibrium with
the carbon dioxide in the air has a
conductivity of about 0.8 × 10 –6
siemens cm–1. Repeated distillation in
a vacuum can bring the conductivity
down to 0.043 × 10 –6siemens cm–1at
18 °C (sometimes called conductivity
water). The limiting conductivity is
due to self ionization: H 2 O ˆH++
OH–. See also deionized water.disulphur dichloride (sulphur
monochloride)An orange–red liquid,
S 2 Cl 2 , which is readily hydrolysed by
water and is soluble in benzene and
ether; r.d. 1.678; m.p. –80°C; b.p.
136 °C. It may be prepared by passing
chlorine over molten sulphur; in the
presence of iodine or metal chlorides
sulphur dichloride, SCl 2 , is also
formed. In the vapour phase S 2 Cl 2
molecules have Cl–S–S–Cl chains.
The compound is used as a solvent
for sulphur and can form higher
chlorosulphanes of the type
Cl–(S)n–Cl (n <100), which are of
great value in *vulcanization
processes.
disulphuric(VI) acid (pyrosulphuric
acid)A colourless hygroscopic crys-dissociation 182d