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.
dissociation The breakdown of a
molecule, ion, etc., into smaller molecules, ions, etc. An example of dissociation is the reversible reaction of
hydrogen iodide at high temperatures
2HI(g) ˆ H 2 (g) + I 2 (g)
The *equilibrium constant of a reversible dissociation is called the
dissociation constant. The term ‘dissociation’ is also applied to ionization
reactions of *acids and *bases in
water; for example
HCN + H 2 O ˆ H 3 O
+ + CN
–
which is often regarded as a straightforward dissociation into ions
HCN ˆ H
+ + CN
–
The equilibrium constant of such a
dissociation is called the acid dissociation constant or acidity constant,
given by
K a = [H
+ ][A
– ]/[HA]
for an acid HA (the concentration of
water [H 2 O] can be taken as constant). K a is a measure of the strength
of the acid. Similarly, for a nitrogenous base B, the equilibrium
B + H 2 O ˆ BH
+ + OH
–
is also a dissociation; with the base
dissociation constant, or basicity constant, given by
K b = [BH
+ ][OH
–
]/[B]
For a hydroxide MOH,
K b = [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 maintained at a constant high temperature in a closed container, the
dissociation pressure at that temperature is the pressure of carbon dioxide
from the equilibrium
CaCO 3 (s) ˆ CaO(s) + CO 2 (g)
distillation The process of boiling
a liquid and condensing and collecting the vapour. The liquid collected
is the distillate. It is used to purify
liquids and to separate liquid mixtures (see fractional distillation;
steam distillation). See also destructive distillation; extractive
distillation.
distilled water Water puriÜed by
distillation so as to free it from dissolved 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
–6 siemens cm
–1 at
18°C (sometimes called conductivity
water). The limiting conductivity is
due to self ionization: H 2 O ˆ H
+ +
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 crysdissociation
182
d
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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.
dissociation The breakdown of a
molecule, ion, etc., into smaller molecules, ions, etc. An example of dissociation is the reversible reaction of
hydrogen iodide at high temperatures
2HI(g) ˆ H 2 (g) + I 2 (g)
The *equilibrium constant of a reversible dissociation is called the
dissociation constant. The term ‘dissociation’ is also applied to ionization
reactions of *acids and *bases in
water; for example
HCN + H 2 O ˆ H 3 O
+ + CN
–
which is often regarded as a straightforward dissociation into ions
HCN ˆ H
+ + CN
–
The equilibrium constant of such a
dissociation is called the acid dissociation constant or acidity constant,
given by
K a = [H
+ ][A
– ]/[HA]
for an acid HA (the concentration of
water [H 2 O] can be taken as constant). K a is a measure of the strength
of the acid. Similarly, for a nitrogenous base B, the equilibrium
B + H 2 O ˆ BH
+ + OH
–
is also a dissociation; with the base
dissociation constant, or basicity constant, given by
K b = [BH
+ ][OH
–
]/[B]
For a hydroxide MOH,
K b = [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 maintained at a constant high temperature in a closed container, the
dissociation pressure at that temperature is the pressure of carbon dioxide
from the equilibrium
CaCO 3 (s) ˆ CaO(s) + CO 2 (g)
distillation The process of boiling
a liquid and condensing and collecting the vapour. The liquid collected
is the distillate. It is used to purify
liquids and to separate liquid mixtures (see fractional distillation;
steam distillation). See also destructive distillation; extractive
distillation.
distilled water Water puriÜed by
distillation so as to free it from dissolved 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
–6 siemens cm
–1 at
18°C (sometimes called conductivity
water). The limiting conductivity is
due to self ionization: H 2 O ˆ H
+ +
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 crysdissociation
182
d
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