fractional distillation (fractionation) The separation of a mixture of
liquids by distillation. Effective separation can be achieved by using a
long vertical column (fractionating
column) attached to the distillation
vessel and Ülled with glass beads.
Vapour from the liquid rises up the
column until it condenses and runs
back into the vessel. The rising
vapour in the column Ûows over the
descending liquid, and eventually a
steady state is reached in which
there is a decreasing temperature
gradient up the column. The vapour
in the column has more volatile components towards the top and less
volatile components at the bottom.
Various fractions of the mixture can
be drawn off at points on the column. Industrially, fractional distillation is performed in large towers
containing many perforated trays. It
is used extensively in petroleum
reÜning.
fractionating column See fractional distillation.
fractionation See fractional distillation.
francium Symbol Fr. A radioactive
element belonging to *group 1 (formerly IA) of the periodic table; a.n.
87; r.d. 2.4; m.p. 27±1°C; b.p.
677±1°C. The element is found in
uranium and thorium ores. All 22
known isotopes are radioactive, the
most stable being francium–223. The
existence of francium was conÜrmed
by Marguerite Perey in 1939.
A
• Information from the WebElements site
Franck–Condon principle A principle governing the intensity of transitions in the vibrational structure
during an electronic transition in a
molecule. The principle states that
since nuclei are much heavier and
move much more slowly than electrons (see born–oppenheimer approximation), an electronic transition occurs much more rapidly than the
time required for the nuclei to respond to it. Therefore, in a diagram
showing the electronic states of the
molecule as a function of internuclear distance, the most intense electronic transition is represented by a
vertical line. For this reason a transition obeying the Franck–Condon
principle is called a vertical
transition; when it occurs the relative
positions of the nuclei remain unchanged. The Franck–Condon principle is named after James Franck
(1882–1964), who stated it in 1925,
and Edward Condon, who formulated it mathematically in terms of
quantum mechanics in 1928.
Frankland, Sir Edward (1825–99)
British organic chemist who was the
Ürst to produce organometallic compounds (zinc dialkyls). He is remembered as the originator of the theory
of valency and introduced a method
of writing structural formulas.
Frasch process A method of obtaining sulphur from underground
deposits using a tube consisting of
three concentric pipes. Superheated
steam is passed down the outer pipe
to melt the sulphur, which is forced
up through the middle pipe by compressed air fed through the inner
tube. The steam in the outer casing
keeps the sulphur molten in the
pipe. It was named after the Germanborn US chemist Hermann Frasch
(1851–1914).
Fraunhofer, Josef von
(1787–1826) German physicist, who
trained as an optician. In 1814 he observed dark lines in the spectrum of
the sun (see fraunhofer lines). He
also studied diffraction.
Fraunhofer lines Dark lines in the
solar spectrum, discovered by Josef
fractional distillation
234
f
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liquids by distillation. Effective separation can be achieved by using a
long vertical column (fractionating
column) attached to the distillation
vessel and Ülled with glass beads.
Vapour from the liquid rises up the
column until it condenses and runs
back into the vessel. The rising
vapour in the column Ûows over the
descending liquid, and eventually a
steady state is reached in which
there is a decreasing temperature
gradient up the column. The vapour
in the column has more volatile components towards the top and less
volatile components at the bottom.
Various fractions of the mixture can
be drawn off at points on the column. Industrially, fractional distillation is performed in large towers
containing many perforated trays. It
is used extensively in petroleum
reÜning.
fractionating column See fractional distillation.
fractionation See fractional distillation.
francium Symbol Fr. A radioactive
element belonging to *group 1 (formerly IA) of the periodic table; a.n.
87; r.d. 2.4; m.p. 27±1°C; b.p.
677±1°C. The element is found in
uranium and thorium ores. All 22
known isotopes are radioactive, the
most stable being francium–223. The
existence of francium was conÜrmed
by Marguerite Perey in 1939.
A
• Information from the WebElements site
Franck–Condon principle A principle governing the intensity of transitions in the vibrational structure
during an electronic transition in a
molecule. The principle states that
since nuclei are much heavier and
move much more slowly than electrons (see born–oppenheimer approximation), an electronic transition occurs much more rapidly than the
time required for the nuclei to respond to it. Therefore, in a diagram
showing the electronic states of the
molecule as a function of internuclear distance, the most intense electronic transition is represented by a
vertical line. For this reason a transition obeying the Franck–Condon
principle is called a vertical
transition; when it occurs the relative
positions of the nuclei remain unchanged. The Franck–Condon principle is named after James Franck
(1882–1964), who stated it in 1925,
and Edward Condon, who formulated it mathematically in terms of
quantum mechanics in 1928.
Frankland, Sir Edward (1825–99)
British organic chemist who was the
Ürst to produce organometallic compounds (zinc dialkyls). He is remembered as the originator of the theory
of valency and introduced a method
of writing structural formulas.
Frasch process A method of obtaining sulphur from underground
deposits using a tube consisting of
three concentric pipes. Superheated
steam is passed down the outer pipe
to melt the sulphur, which is forced
up through the middle pipe by compressed air fed through the inner
tube. The steam in the outer casing
keeps the sulphur molten in the
pipe. It was named after the Germanborn US chemist Hermann Frasch
(1851–1914).
Fraunhofer, Josef von
(1787–1826) German physicist, who
trained as an optician. In 1814 he observed dark lines in the spectrum of
the sun (see fraunhofer lines). He
also studied diffraction.
Fraunhofer lines Dark lines in the
solar spectrum, discovered by Josef
fractional distillation
234
f
www.AzShimi.ir www.AzShimi.com