were made by the Norwegian-born
US chemist Lars Onsager (1903–76).
Debye–Hückel theory A theory to
explain the nonideal behaviour of
electrolytes, published in 1923 by
Peter *Debye and Erich Hückel
(1896–1980). It assumes that electrolytes in solution are fully dissociated and that nonideal behaviour
arises because of electrostatic interactions between the ions. The theory
shows how to calculate the extra free
energy per ion resulting from such
interactions, and consequently the
activity coefÜcient. It gives a good description of nonideal electrolyte behaviour for very dilute solutions, but
cannot be used for more concentrated electrolytes.
Debye–Scherrer method A technique used in *X-ray diffraction in
which a crystal in powder form is
Üxed to a thin Übre or thin silica
tube, which is then rotated in the
path of a beam of monochromatic Xrays. A circular diffraction ring,
called the Debye–Scherrer ring, concentric with the undeÛected beam
is formed. The diffraction pattern is
recorded on a cylindrical Ülm, which
has its axis parallel to the axis of rotation of the material. The Debye–
Scherrer method is used to obtain information about the material. The
grains of the powdered crystal must
be much larger than the atomic dimensions in order for them to diffract X-rays.
Debye temperature See debye
theory of specific heat.
Debye theory of speciÜc heat A
theory of the speciÜc heat capacity of
solids put forward by Peter *Debye in
1912, in which it was assumed that
the speciÜc heat is a consequence of
the vibrations of the atoms of the lattice of the solid. In contrast to the
*Einstein theory of speciÜc heat,
which assumes that each atom has
the same vibrational frequency,
Debye postulated that there is a continuous range of frequencies that
cuts off at a maximum frequency ν D ,
which is characteristic of a particular
solid. The theory leads to the conclusion that the speciÜc heat capacity of
solids is proportional to T
3
, where T
is the thermodynamic temperature.
This result is in very good agreement
with experiment at low temperatures.
A key quantity in this theory is the
Debye temperature, θ D , deÜned by θ D =
hν D k, where h is the Planck constant
and k is the Boltzmann constant. The
Debye temperature is characteristic
of a particular solid. For example, the
Debye temperature of sodium is 150
K and the Debye temperature of copper is 315 K.
Debye–Waller factor A quantity
that characterizes the effect of lattice
vibrations on the scattering intensity
in X-ray diffraction by crystals. The
existence of the Debye–Waller factor
was pointed out and calculated by
Peter *Debye in 1913–1914 and Ivar
Waller in 1923–1925. Because the
amplitude of lattice vibrations increases with temperature, it was
thought in the very early days of Xray diffraction studies that the diffraction pattern would disappear at
high temperatures. The work of
Debye and Waller showed that the
lattice vibrations at higher temperatures reduce the intensities of the diffracted radiation but do not destroy
the diffraction pattern altogether.
deca- Symbol da. A preÜx used in
the metric system to denote ten
times. For example, 10 coulombs = 1
decacoulomb (daC).
decahydrate A crystalline hydrate
containing ten moles of water per
mole of compound.
Debye–Hückel theory
164
d
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US chemist Lars Onsager (1903–76).
Debye–Hückel theory A theory to
explain the nonideal behaviour of
electrolytes, published in 1923 by
Peter *Debye and Erich Hückel
(1896–1980). It assumes that electrolytes in solution are fully dissociated and that nonideal behaviour
arises because of electrostatic interactions between the ions. The theory
shows how to calculate the extra free
energy per ion resulting from such
interactions, and consequently the
activity coefÜcient. It gives a good description of nonideal electrolyte behaviour for very dilute solutions, but
cannot be used for more concentrated electrolytes.
Debye–Scherrer method A technique used in *X-ray diffraction in
which a crystal in powder form is
Üxed to a thin Übre or thin silica
tube, which is then rotated in the
path of a beam of monochromatic Xrays. A circular diffraction ring,
called the Debye–Scherrer ring, concentric with the undeÛected beam
is formed. The diffraction pattern is
recorded on a cylindrical Ülm, which
has its axis parallel to the axis of rotation of the material. The Debye–
Scherrer method is used to obtain information about the material. The
grains of the powdered crystal must
be much larger than the atomic dimensions in order for them to diffract X-rays.
Debye temperature See debye
theory of specific heat.
Debye theory of speciÜc heat A
theory of the speciÜc heat capacity of
solids put forward by Peter *Debye in
1912, in which it was assumed that
the speciÜc heat is a consequence of
the vibrations of the atoms of the lattice of the solid. In contrast to the
*Einstein theory of speciÜc heat,
which assumes that each atom has
the same vibrational frequency,
Debye postulated that there is a continuous range of frequencies that
cuts off at a maximum frequency ν D ,
which is characteristic of a particular
solid. The theory leads to the conclusion that the speciÜc heat capacity of
solids is proportional to T
3
, where T
is the thermodynamic temperature.
This result is in very good agreement
with experiment at low temperatures.
A key quantity in this theory is the
Debye temperature, θ D , deÜned by θ D =
hν D k, where h is the Planck constant
and k is the Boltzmann constant. The
Debye temperature is characteristic
of a particular solid. For example, the
Debye temperature of sodium is 150
K and the Debye temperature of copper is 315 K.
Debye–Waller factor A quantity
that characterizes the effect of lattice
vibrations on the scattering intensity
in X-ray diffraction by crystals. The
existence of the Debye–Waller factor
was pointed out and calculated by
Peter *Debye in 1913–1914 and Ivar
Waller in 1923–1925. Because the
amplitude of lattice vibrations increases with temperature, it was
thought in the very early days of Xray diffraction studies that the diffraction pattern would disappear at
high temperatures. The work of
Debye and Waller showed that the
lattice vibrations at higher temperatures reduce the intensities of the diffracted radiation but do not destroy
the diffraction pattern altogether.
deca- Symbol da. A preÜx used in
the metric system to denote ten
times. For example, 10 coulombs = 1
decacoulomb (daC).
decahydrate A crystalline hydrate
containing ten moles of water per
mole of compound.
Debye–Hückel theory
164
d
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