von Fraunhofer, that result from the
absorption by elements in the solar
chromosphere of some of the wavelengths of the visible radiation emitted by the hot interior of the sun.
free electron See electron.
free-electron approximation
The approximation resulting from
the assumption that electrons in
*metals can be analysed using the
*kinetic theory of gases, without taking the periodic potential of the
metal into account. This approximation gives a good qualitative account
of some properties of metals, such as
their electrical conductivity. At very
low temperatures it is necessary to
use quantum statistical mechanics
rather than classical statistical mechanics. The free-electron approximation does not, however, give an
adequate quantitative description of
the properties of metals. It can be improved by the nearly free electron approximation, in which the periodic
potential is treated as a perturbation
on the free electrons.
free energy A measure of a system’s ability to do work. The Gibbs
free energy (or Gibbs function), G, is
deÜned by G = H – TS, where G is the
energy liberated or absorbed in a reversible process at constant pressure
and constant temperature (T), H is
the *enthalpy and S the *entropy of
the system. Changes in Gibbs free energy, ∆G, are useful in indicating the
conditions under which a chemical
reaction will occur. If ∆G is positive
the reaction will only occur if energy
is supplied to force it away from the
equilibrium position (i.e. when ∆G =
0). If ∆G is negative the reaction will
proceed spontaneously to equilibrium.
The Helmholtz free energy (or
Helmholtz function), F, is deÜned by F
= U – TS, where U is the *internal energy. For a reversible isothermal
process, ∆F represents the useful
work available.
free radical An atom or group of
atoms with an unpaired valence electron. Free radicals can be produced
by photolysis or pyrolysis in which a
bond is broken without forming ions
(see homolytic fission). Because of
their unpaired valence electron, most
free radicals are extremely reactive.
See also chain reaction.
freeze drying A process used in
dehydrating food, blood plasma, and
other heat-sensitive substances. The
product is deep-frozen and the ice
trapped in it is removed by reducing
the pressure and causing it to sublime. The water vapour is then removed, leaving an undamaged dry
product.
freezing mixture A mixture of
components that produces a low
temperature. For example, a mixture
of ice and sodium chloride gives a
temperature of –20°C.
freezing-point depression See
depression of freezing point.
Frenkel defect See crystal defect.
Frenkel–Kontorowa model A
one-dimensional model of atoms,
such as xenon, adsorbed on a periodic substrate, such as graphite. This
model, which can be used to investigate the nature of the lattice formed
by the adsorbed gas, was invented in
1938 by Y. I. Frenkel and T. Kontorowa and independently in 1949 by
F. C. Frank and J. H. Van der Merwe.
The Frenkel–Kontorowa model can
be used to investigate the phase transition between a *commensurate lattice and an *incommensurate lattice.
freon See chlorofluorocarbon.
Freundlich isotherm An isotherm
for adsorption with the form θ =
c 1 P
1/c2 , where the fractional coverage
235
Freundlich isotherm
f
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absorption by elements in the solar
chromosphere of some of the wavelengths of the visible radiation emitted by the hot interior of the sun.
free electron See electron.
free-electron approximation
The approximation resulting from
the assumption that electrons in
*metals can be analysed using the
*kinetic theory of gases, without taking the periodic potential of the
metal into account. This approximation gives a good qualitative account
of some properties of metals, such as
their electrical conductivity. At very
low temperatures it is necessary to
use quantum statistical mechanics
rather than classical statistical mechanics. The free-electron approximation does not, however, give an
adequate quantitative description of
the properties of metals. It can be improved by the nearly free electron approximation, in which the periodic
potential is treated as a perturbation
on the free electrons.
free energy A measure of a system’s ability to do work. The Gibbs
free energy (or Gibbs function), G, is
deÜned by G = H – TS, where G is the
energy liberated or absorbed in a reversible process at constant pressure
and constant temperature (T), H is
the *enthalpy and S the *entropy of
the system. Changes in Gibbs free energy, ∆G, are useful in indicating the
conditions under which a chemical
reaction will occur. If ∆G is positive
the reaction will only occur if energy
is supplied to force it away from the
equilibrium position (i.e. when ∆G =
0). If ∆G is negative the reaction will
proceed spontaneously to equilibrium.
The Helmholtz free energy (or
Helmholtz function), F, is deÜned by F
= U – TS, where U is the *internal energy. For a reversible isothermal
process, ∆F represents the useful
work available.
free radical An atom or group of
atoms with an unpaired valence electron. Free radicals can be produced
by photolysis or pyrolysis in which a
bond is broken without forming ions
(see homolytic fission). Because of
their unpaired valence electron, most
free radicals are extremely reactive.
See also chain reaction.
freeze drying A process used in
dehydrating food, blood plasma, and
other heat-sensitive substances. The
product is deep-frozen and the ice
trapped in it is removed by reducing
the pressure and causing it to sublime. The water vapour is then removed, leaving an undamaged dry
product.
freezing mixture A mixture of
components that produces a low
temperature. For example, a mixture
of ice and sodium chloride gives a
temperature of –20°C.
freezing-point depression See
depression of freezing point.
Frenkel defect See crystal defect.
Frenkel–Kontorowa model A
one-dimensional model of atoms,
such as xenon, adsorbed on a periodic substrate, such as graphite. This
model, which can be used to investigate the nature of the lattice formed
by the adsorbed gas, was invented in
1938 by Y. I. Frenkel and T. Kontorowa and independently in 1949 by
F. C. Frank and J. H. Van der Merwe.
The Frenkel–Kontorowa model can
be used to investigate the phase transition between a *commensurate lattice and an *incommensurate lattice.
freon See chlorofluorocarbon.
Freundlich isotherm An isotherm
for adsorption with the form θ =
c 1 P
1/c2 , where the fractional coverage
235
Freundlich isotherm
f
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