white metallic *transition element;
a.n. 75; r.a.m. 186.2; r.d. 20.53; m.p.
3180°C; b.p. 5627 (estimated)°C. The
element is obtained as a by-product
in reÜning molybdenum, and is used
in certain alloys (e.g. rhenium–
molybdenum alloys are superconducting). The element forms a number of complexes with oxidation
states in the range 1–7. It was discovered by Walter Noddack (1893–1960)
and Ida Tacke (1896–1978) in 1925.
A
• Information from the WebElements site
rheology The study of the deformation and Ûow of matter.
rheopexy The process by which
certain thixotropic substances set
more rapidly when they are stirred,
shaken, or tapped. Gypsum in water
is such a rheopectic substance.
rhodinol See aminophenol.
rhodium Symbol Rh. A silverywhite metallic *transition element;
a.n. 45; r.a.m. 102.9; r.d. 12.4; m.p.
1966°C; b.p. 3727°C. It occurs with
platinum and is used in certain platinum alloys (e.g. for thermocouples)
and in plating jewellery and optical
reÛectors. Chemically, it is not attacked by acids (dissolves only slowly
in aqua regia) and reacts with nonmetals (e.g. oxygen and chlorine) at
red heat. Its main oxidation state is
+3 although it also forms complexes
in the +4 state. The element was discovered in 1803 by William Wollaston (1766–1828).
A
• Information from the WebElements site
RIA See radioimmunoassay.
riboÛavin See vitamin b complex.
ribonucleic acid See rna.
ribose A *monosaccharide,
C 5 H 10 O 5 , rarely occurring free in nature but important as a component
of *RNA (ribonucleic acid). Its derivative deoxyribose, C 5 H 10 O 4 , is equally
important as a constituent of *DNA
(deoxyribonucleic acid), which carries the genetic code in chromosomes.
ribulose A ketopentose sugar (see
monosaccharide), C 5 H 11 O 5 , that is
involved in carbon dioxide Üxation in
photosynthesis as a component of
*ribulose bisphosphate.
ribulose bisphosphate (RuBP) A
Üve-carbon sugar that is combined
with carbon dioxide to form two
three-carbon intermediates in the
Ürst stage of the light-dependent reactions of *photosynthesis (see
calvin cycle). The enzyme that mediates the carboxylation of ribulose
bisphosphate is ribulose bisphosphate carboxylase.
Rice–Herzfeld mechanism A
mechanism enabling complex chain
reactions to give simple rate laws in
chemical kinetics. An example of a
Rice–Herzfeld mechanism occurs
with the pyrolysis of acetaldehyde,
the mechanism consisting of initiation, propagation (in two steps), and
termination. This leads to the experimental result that the overall reaction is three-halves order in CH 3 CHO.
To ascertain that such a mechanism
is correct, either the steady-state approximation is used or the differential equations for the reaction rates
are solved numerically. It is possible, as in this example, for the
Rice–Herzfeld mechanism to give a
correct description of the main part
of the reaction but not to take account of reactions that produce
by-products. The Rice–Herzfeld
mechanism is named after F. O. Rice
and K. F. Herzfeld, who put forward
the scheme in 1934.
Rice–Ramsperger–Kassel theory
(RRK theory) A statistical theory used
461
Rice–Ramsperger–Kassel theory
r
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a.n. 75; r.a.m. 186.2; r.d. 20.53; m.p.
3180°C; b.p. 5627 (estimated)°C. The
element is obtained as a by-product
in reÜning molybdenum, and is used
in certain alloys (e.g. rhenium–
molybdenum alloys are superconducting). The element forms a number of complexes with oxidation
states in the range 1–7. It was discovered by Walter Noddack (1893–1960)
and Ida Tacke (1896–1978) in 1925.
A
• Information from the WebElements site
rheology The study of the deformation and Ûow of matter.
rheopexy The process by which
certain thixotropic substances set
more rapidly when they are stirred,
shaken, or tapped. Gypsum in water
is such a rheopectic substance.
rhodinol See aminophenol.
rhodium Symbol Rh. A silverywhite metallic *transition element;
a.n. 45; r.a.m. 102.9; r.d. 12.4; m.p.
1966°C; b.p. 3727°C. It occurs with
platinum and is used in certain platinum alloys (e.g. for thermocouples)
and in plating jewellery and optical
reÛectors. Chemically, it is not attacked by acids (dissolves only slowly
in aqua regia) and reacts with nonmetals (e.g. oxygen and chlorine) at
red heat. Its main oxidation state is
+3 although it also forms complexes
in the +4 state. The element was discovered in 1803 by William Wollaston (1766–1828).
A
• Information from the WebElements site
RIA See radioimmunoassay.
riboÛavin See vitamin b complex.
ribonucleic acid See rna.
ribose A *monosaccharide,
C 5 H 10 O 5 , rarely occurring free in nature but important as a component
of *RNA (ribonucleic acid). Its derivative deoxyribose, C 5 H 10 O 4 , is equally
important as a constituent of *DNA
(deoxyribonucleic acid), which carries the genetic code in chromosomes.
ribulose A ketopentose sugar (see
monosaccharide), C 5 H 11 O 5 , that is
involved in carbon dioxide Üxation in
photosynthesis as a component of
*ribulose bisphosphate.
ribulose bisphosphate (RuBP) A
Üve-carbon sugar that is combined
with carbon dioxide to form two
three-carbon intermediates in the
Ürst stage of the light-dependent reactions of *photosynthesis (see
calvin cycle). The enzyme that mediates the carboxylation of ribulose
bisphosphate is ribulose bisphosphate carboxylase.
Rice–Herzfeld mechanism A
mechanism enabling complex chain
reactions to give simple rate laws in
chemical kinetics. An example of a
Rice–Herzfeld mechanism occurs
with the pyrolysis of acetaldehyde,
the mechanism consisting of initiation, propagation (in two steps), and
termination. This leads to the experimental result that the overall reaction is three-halves order in CH 3 CHO.
To ascertain that such a mechanism
is correct, either the steady-state approximation is used or the differential equations for the reaction rates
are solved numerically. It is possible, as in this example, for the
Rice–Herzfeld mechanism to give a
correct description of the main part
of the reaction but not to take account of reactions that produce
by-products. The Rice–Herzfeld
mechanism is named after F. O. Rice
and K. F. Herzfeld, who put forward
the scheme in 1934.
Rice–Ramsperger–Kassel theory
(RRK theory) A statistical theory used
461
Rice–Ramsperger–Kassel theory
r
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