Page 218 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Main Group Elements 27

combustion introduced by Georg Stahl (1660–1734). This colorless, vile-smelling, poisonous gas whose putrid odor
theory proposed that the more phlogiston an object con- is recognized as that of rotten eggs. It can be prepared by
tained, the more easily it burned. On being burned, the treating metal sulfides with an acid or by directly com-
object lost its phlogiston and became a new substance in- bining molten sulfur and hydrogen gas at elevated tem-
capable of being burned further. Inasmuch as sulfur could peratures. A number of polysulfanes, H 2 S x (x = 2−8), are
be burned with almost no residue, it was thought to be also known, but they are unstable and readily decompose
essentially pure phlogiston. to sulfur and H 2 S.
Even in modern times it is easy to ascribe a mystical,
mythic nature to sulfur. The discovery of its numerous al-
lotropic modifications (perhaps 30 or more) would suggest C. Halides
that sulfur is the element of Proteus, the Greek sea deity
Sulfur halides include several fluorides (SF 2 ,SF 4 ,SF 6 ,
with knowledge of the past, present, and future. Proteus
S 2 F 2 , SSF 2 ,S 2 F 10 ), some chlorides (SCl 2 , SCl 4 ,S 2 Cl 2 ),
had the extraordinary ability to assume different shapes
and some poorly characterized bromides. Even a com-
and forms to avoid those who would force him to prophesy.
pound of sulfur and iodine is claimed; however, the
S I bond is notoriously unstable and cannot be formed
A. Allotropes by direct reaction of the elements.
Both SF 6 and S 2 Cl 2 are important compounds that can
Ordinary sulfur is a pale-yellow solid at room temperature
besynthesizeddirectlyfromtheelements.Unliketheother
and consists of puckered rings of S 8 molecules, but there
fluorides, most of which are readily hydrolyzed to HF and
are other cyclic allotropes with more or fewer atoms. The
an oxide of sulfur, SF 6 is a truly unreactive species. It is a
two common crystalline S 8 varieties are called orthorhom-
colorless, insoluble gas that will not burn, has no taste or
bic and monoclinic sulfur, for which the melting points are
odor, and is nontoxic. It can be bubbled through molten
listed as 113 and 119 C, respectively. These values, how-
◦
sodium, but boiling sodium will decompose it to NaF and
ever, are difficult to observe because some of the rhombic
Na 2 S. The chemical inertness of SF 6 is used to advan-
sulfur is converted to the monoclinic form, and the result-
tage in high-voltage transformers, where the compound is
ing allotropic mixture melts at only 115 C and forms a
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employed as an insulating gas. S 2 Cl 2 is used mainly to vul-
thin, straw-colored liquid still containing S 8 molecules. If
canize rubber, and for this purpose alone great quantities
liquid sulfur is heated further to temperatures near 160 C,
◦
are produced annually.
the S 8 rings rupture and the ends of the chains combine
to form S 16 ,S 24 , and S 32 species. The melt darkens to
a rich orange–brown, and a tremendous increase in vis-
D. Sulfur Oxides
cosity (10,000-fold) is observed as the long chains twist
and tangle. Above 190 C the viscosity decreases as the More than a dozen oxides of sulfur are known, but some of
◦
extended chains break into shorter fragments, and when them are rather unimportant cyclic monoxides formed by
the boiling point (444 C) is finally reached, liquid sulfur treating a puckered ring of 5 to 10 sulfur atoms with a per-
◦
is once again very thin and runny. Sulfur vapor consists oxy acid, and others are unstable acyclic species. Two of
of acyclic S 8 ,S 6 ,S 4 , and S 2 molecules, with the smaller the acyclic oxides, however, are extremely important: sul-
molecules dominating at higher temperatures. At 1000 C, fur dioxide (SO 2 ) and sulfur trioxide (SO 3 ). In the decade
◦
most of the molecules are probably S 2 , which, like O 2 of the 1980s, when acid rain became a household word,
molecules, are paramagnetic. At 2000 C, nearly half the these two compounds and their roles in acid rain forma-
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diatomic S 2 molecules are estimated to be dissociated into tion became as familiar to the educated laity as carbon
sulfur atoms. monoxide and carbon dioxide.
Although sulfur is found in the native state, it is really Sulfur dioxide is made on a vast industrial scale by burn-
quite reactive and combines directly with most other ele- ing sulfur, by burning H 2 S, or by roasting sulfide ores as
ments at elevated temperatures. It does not, however, com- part of the pyrometallurgy of zinc, molybdenum, and other
bine directly with certain nonmetals (iodine, tellurium, ni- metals. The burning of high-sulfur coals and fuel oil serves
trogen,thenoblegases)orwithunreactivemetals(iridium, as the major source of SO 2 produced as an environmental
platinum, gold). pollutant.
Most of the commercially prepared SO 2 is converted to
SO 3 , which is subsequently used to manufacture sulfuric
B. Sulfanes
acid, H 2 SO 4 . Other uses of SO 2 , albeit on a much-smaller
The hydrides of sulfur are called sulfanes. Hydrogen sul- scale, include its application as a refrigerant, food preser-
fide, H 2 S, is the most important and most stable. It is a vative, and bleaching agent.