which is soluble in water and in
ethanol but insoluble in diethyl
ether; cubic; r.d. 1.80; m.p. 840°C. It
is made industrially by reducing
potassium sulphate with carbon at
high temperatures in the absence of
air. In the laboratory it may be prepared by the reaction of hydrogen
sulphide with potassium hydroxide.
The pentahydrate is obtained on crystallization. Solutions are strongly alkaline due to hydrolysis. It is used as
an analytical reagent and as a depilatory. Potassium sulphide is generally
regarded as a hazardous chemical
with a Üre risk; dusts of K 2 S have
been known to explode.
potassium sulphite A white crystalline solid, K 2 SO 3 , soluble in water
and very sparingly soluble in
ethanol; r.d. 1.51; decomposes on
heating. It is a reducing agent and is
used as such in photography and in
the food and brewing industries,
where it prevents oxidation.
potassium superoxide (potassium
dioxide) A yellow paramagnetic
solid, KO 2 , produced by burning
potassium in an excess of oxygen; it
is very soluble (by reaction) in water,
soluble in ethanol, and slightly soluble in diethyl ether; m.p. 380°C.
When treated with cold water or dilute mineral acids, hydrogen peroxide is obtained. The compound is a
powerful oxidizing agent and on
strong heating releases oxygen with
the formation of the monoxide, K 2 O.
potential barrier A region containing a maximum of potential that
prevents a particle on one side of it
from passing to the other side. According to classical theory a particle
must possess energy in excess of the
height of the potential barrier to pass
it. However, in quantum theory there
is a Ünite probability that a particle
with less energy will pass through
the barrier (see tunnel effect). A potential barrier surrounds the atomic
nucleus and is important in nuclear
physics; a similar but much lower
barrier exists at the interface between semiconductors and metals
and between differently doped semiconductors. These barriers are important in the design of electronic
devices.
potential energy See energy.
potential-energy curve A graph
of the potential energy of electrons
in a diatomic molecule, in which the
potential energy of an electronic
state is plotted vertically and the interatomic distance is plotted horizontally, with the minimum of the curve
being the average internuclear distance. The *Morse potential gives a
good analytic description of a potential-energy curve. The dissociation
energy and certain quantities of interest in vibrational spectroscopy can
be found from the potential-energy
curve.
potential-energy surface A multidimensional surface used in the
theory of electronic states of polyatomic molecules and chemical reactions. A *potential-energy curve is
the simplest type of potential-energy
surface. Each internuclear distance in
the molecule or reacting system is
represented by one dimension, as is
the potential energy. This means that
if a nonlinear molecule has N atoms
its potential-energy surface is a
(3N – 6)-dimensional surface in a
(3N – 5)-dimensional space. In the
case of a linear molecule the potential-energy surface is a (3N – 5)-dimensional space. If the surface has a
minimum then that electronic state
is stable. It is possible for there to be
more than one minimum. In potential-energy surfaces for chemical reactions the reactants and products
are frequently ‘valleys’ connected by
a region of higher energy, which is
potassium sulphite
436
p
www.AzShimi.ir www.AzShimi.com
ethanol but insoluble in diethyl
ether; cubic; r.d. 1.80; m.p. 840°C. It
is made industrially by reducing
potassium sulphate with carbon at
high temperatures in the absence of
air. In the laboratory it may be prepared by the reaction of hydrogen
sulphide with potassium hydroxide.
The pentahydrate is obtained on crystallization. Solutions are strongly alkaline due to hydrolysis. It is used as
an analytical reagent and as a depilatory. Potassium sulphide is generally
regarded as a hazardous chemical
with a Üre risk; dusts of K 2 S have
been known to explode.
potassium sulphite A white crystalline solid, K 2 SO 3 , soluble in water
and very sparingly soluble in
ethanol; r.d. 1.51; decomposes on
heating. It is a reducing agent and is
used as such in photography and in
the food and brewing industries,
where it prevents oxidation.
potassium superoxide (potassium
dioxide) A yellow paramagnetic
solid, KO 2 , produced by burning
potassium in an excess of oxygen; it
is very soluble (by reaction) in water,
soluble in ethanol, and slightly soluble in diethyl ether; m.p. 380°C.
When treated with cold water or dilute mineral acids, hydrogen peroxide is obtained. The compound is a
powerful oxidizing agent and on
strong heating releases oxygen with
the formation of the monoxide, K 2 O.
potential barrier A region containing a maximum of potential that
prevents a particle on one side of it
from passing to the other side. According to classical theory a particle
must possess energy in excess of the
height of the potential barrier to pass
it. However, in quantum theory there
is a Ünite probability that a particle
with less energy will pass through
the barrier (see tunnel effect). A potential barrier surrounds the atomic
nucleus and is important in nuclear
physics; a similar but much lower
barrier exists at the interface between semiconductors and metals
and between differently doped semiconductors. These barriers are important in the design of electronic
devices.
potential energy See energy.
potential-energy curve A graph
of the potential energy of electrons
in a diatomic molecule, in which the
potential energy of an electronic
state is plotted vertically and the interatomic distance is plotted horizontally, with the minimum of the curve
being the average internuclear distance. The *Morse potential gives a
good analytic description of a potential-energy curve. The dissociation
energy and certain quantities of interest in vibrational spectroscopy can
be found from the potential-energy
curve.
potential-energy surface A multidimensional surface used in the
theory of electronic states of polyatomic molecules and chemical reactions. A *potential-energy curve is
the simplest type of potential-energy
surface. Each internuclear distance in
the molecule or reacting system is
represented by one dimension, as is
the potential energy. This means that
if a nonlinear molecule has N atoms
its potential-energy surface is a
(3N – 6)-dimensional surface in a
(3N – 5)-dimensional space. In the
case of a linear molecule the potential-energy surface is a (3N – 5)-dimensional space. If the surface has a
minimum then that electronic state
is stable. It is possible for there to be
more than one minimum. In potential-energy surfaces for chemical reactions the reactants and products
are frequently ‘valleys’ connected by
a region of higher energy, which is
potassium sulphite
436
p
www.AzShimi.ir www.AzShimi.com