Page 204 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009F-398 July 6, 2001 20:34

Main Group Elements 13

reacting silicon with Cl 2 . The SiCl 4 is a volatile liquid that the halogen with elemental silicon. The fluoride can also
can be purified by fractional distillation before reduction be synthesized by the reaction of hydrofluoric acid and
with hydrogen gas: silica:

Si (s) + 2Cl 2 (g) → SiCl 4 (l) SiO 2 + 4HF (aq) → SiF 4 (g) + 2H 2 O (l)
SiCl 4 (g) + 2H 2 (g) → Si (s) + 4HCl (g) As noted earlier, the chloride is important in the prepa-
ration of ultrapure silicon for use in transistors, but it also
Very pure silicon for electronic applications is prepared by
serves as a valuable reagent for preparing silicon esters
zone refining of the silicon obtained from SiCl 4 reduction.
and silanes:
SiCl 4 (l) + 4C 2 H 5 OH −→ Si(OC 2 H 5 ) 4 (l) + 4HCl (g)
A. Silanes (Silicon Hydrides)
(7)
Unlike carbon, silicon does not form a large number or ether
SiCl 4 + LiAlH 4 −→ SiH 4 (g) + LiCl (s) + AlCl 3 (s)
variety of hydrides; however, several colorless, volatile
Silane
compounds with the general formula Si n H 2n+2 have been (8)
synthesized and are called silanes, a name proposed by Silicon esters are important in the preparation of new high-
Alfred Stock to reflect the similarity to alkanes, C n H 2n+2 . technology ceramic materials.
A comparison of the boiling points of silanes and alka-
nes indicates the lower volatility of the former:
D. Silica

Silicon dioxide is an intriguing, highly complex com-
SiH 4 CH 4 Si 2 H 6 C 2 H 6
pound that has been the subject of more scientific scrutiny
bp ( C) −112 −162 −14 −88
◦
than any substance except water. Dozens of forms are
known: α-quartz, chalcedony, chert, agate, onyx, jasper,
Si 3 H 8 C 3 H 8 Si 4 H 10 C 4 H 10
and many others.
bp( C) +53 −42 +108 −0.5 The term silicon dioxide is a misnomer in the sense that
◦
no individual SiO 2 molecules are known. In its numerous
Despite the parallels in physical properties, silanes and crystalline modifications, SiO 2 exists in the form of gi-
alkanes are markedly different chemically. Silanes are ex- ant three-dimensional network molecules in which each
tremely reactive and ignite spontaneously in air: silicon atom is surrounded tetrahedrally by four oxygen
atoms to form a hard, rigid lattice:
SiH 4 (g) + 2O 2 (g) → SiO 2 (s) + 2H 2 O (g)
In contrast, alkanes have little affinity for most chemical O
reagents and are often called paraffins (Latin, parum, “lit-
Si
tle”; affinis, “reactivity”). The reactivity of the Si H bond O O
can be used to advantage in the synthesis of organosilicon O
compounds and in special reduction reactions.
On the Mohs hardness scale, quartz is assigned a value of
B. Silicides 7; diamond is 10.
Some of the types of SiO 2 employed for industrial pur-
Silicides are binary compounds of metals and silicon. For-
poses include high-purity α-quartz, vitreous silica, sil-
mulas are difficult to predict, and the bizarre stoichiome-
ica gels, and diatomaceous earth. The use of very pure
tries often appear to be the reaction products found on
quartz crystals in oscillators and other electromechani-
freshman examinations. Silicides of most representative
cal devices constitutes one of the most important applica-
metals except beryllium are known, and most of the tran-
tions of high-grade quartz. Such oscillators are the heart
sition elements except silver, gold, and a few others form
of quartz watches and also enable radio stations to remain
various silicides. The heavy metals mercury, thallium,
on an assigned frequency.
lead, and bismuth do not react with silicon; indeed, the
The fusion of sand and sodium carbonate yields sodium
molten metals are not even miscible with liquid silicon.
silicate (ranked number 46 among the top 50 chemicals
produced in the United States), which is converted to silica
C. Halides gel or used in the manufacture of detergents and cleaning
agents:
The tetrahalides of silicon, SiF 4 , SiCl 4 , SiBr 4 , and SiI 4 ,
are all known and can be synthesized by direct reaction of SiO 2 (s) + Na 2 CO 3 → Na 2 SiO 3 (s) + CO 2 (g)