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 supercon-
ducting). The element forms a num-
ber of complexes with oxidation
states in the range 1–7. It was discov-
ered by Walter Noddack (1893–1960)
and Ida Tacke (1896–1978) in 1925.
A
- Information from the WebElements site
rheologyThe study of the defor-
mation andÛow of matter.
rheopexyThe process by which
certain thixotropic substances set
more rapidly when they are stirred,
shaken, or tapped. Gypsum in water
is such a rheopectic substance.
rhodinolSee aminophenol.
rhodiumSymbol Rh. A silvery-
white 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 plat-
inum alloys (e.g. for thermocouples)
and in plating jewellery and optical
reÛectors. Chemically, it is not at-
tacked by acids (dissolves only slowly
in aqua regia) and reacts with non-
metals (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 dis-
covered in 1803 by William Wolla-
ston (1766–1828).
A
- Information from the WebElements site
RIASee radioimmunoassay.
riboÛavinSee vitamin b complex.
ribonucleic acid See rna.
ribose A *monosaccharide,
C 5 H 10 O 5 , rarely occurring free in na-
ture but important as a component
of *RNA (ribonucleic acid). Its deriva-
tive deoxyribose, C 5 H 10 O 4 , is equally
important as a constituent of *DNA
(deoxyribonucleic acid), which car-
ries the genetic code in chromo-
somes.
ribuloseA ketopentose sugar (see
monosaccharide), C 5 H 11 O 5 , that is
involved in carbon dioxideÜxation 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 re-
actions of *photosynthesis (see
calvin cycle). The enzyme that me-
diates the carboxylation of ribulose
bisphosphate is ribulose bisphos-
phate carboxylase.Rice–Herzfeld mechanismA
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 initia-
tion, propagation (in two steps), and
termination. This leads to the experi-
mental result that the overall reac-
tion is three-halves order in CH 3 CHO.
To ascertain that such a mechanism
is correct, either the steady-state ap-
proximation is used or the differen-
tial equations for the reaction rates
are solved numerically. It is pos-
sible, 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 ac-
count 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 used461 Rice–Ramsperger–Kassel theory
r