Download CH225 Inorganic Chemistry I

CH226.18
•Compounds of Group IV elements: Oxides and oxoanions
Silicon dioxide
Silica, SiO2, is a hard solid and has a high melting point (1710 °C). SiO2 possesses
four single Si–O bonds resulting in an extended three-dimensional covalent lattice.
Note: Four Si–O bonds (4 × 111 kcal/mol) are better than two Si=O bonds (2 × 153
kcal/mol) by 138 kcal/mol. 1 calorie = 4.184 joules
Si
Si
O
O
Si
Si
Si O Si
O
O
O
O
Si
Si
Compare this with CO2 (gas):
O═C═O
Two C=O bonds (2 × 193 kcal/mol) are better than
four C–O bonds (4 × 86 kcal/mol) by 42 kcal/mol
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Sulfides
Carbon disulfide, CS2 is a highly flammable and volatile liquid (b.p. = 46 °C); it has a
low autoignition temperature (90 °C) and catches fire easily. The worldwide
production of CS2 is ca. 1 million tonnes; most of it is used in the production of
viscose fiber and cellophane film.
S
C
S
Silicon disulfide is prepared by heating Si with sulfur. SiS2 is very different from CS2.
Silicon disulfide (like SiO2) is a polymeric material, but it adopts a 1-dimensional
structure different from that of SiO2:
S
Si
Si
S
S
S
Si
Si
S
S
S
Si
S
Silicon disulfide is instantly hydrolyzed in water.
SiS2 + 2H2O → SiO2 + 2H2S
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Silicates
The three most abundant elements in the Earth's crust:
Oxygen (46.1%), Silicon (28.2%), and Aluminum (8.2%).
Minerals which combine Si and O are called silicates, and they are the most
abundant minerals on Earth.
About 95% of the continental crust rocks are composed of feldspars or quartz.
Mineral
Idealized Formula
Olivine
(Mg,Fe)2SiO4
Pyroxene group
(Mg,Fe)SiO3
Amphibole group
Ca2(Fe,Mg)5Si8O22(OH)2
Biotite
K(Mg,Fe)3AlSi3O10(OH)2
Muscovite
KAl2(AlSi3O10)(OH)2
Orthoclase
KAlSi3O8
Plagioclase
(Ca,Na)AlSi3O8
Micas
Feldspars
Quartz
SiO2
Examples of orthoclase gems:
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Structure of silicates
Silicates have structures based on SiO4 tetrahedra, which can assemble in a variety
of different ways by sharing the oxygen atoms. Each terminal O atom contributes –1
to the overall charge:
O
4O
Si
O
O
O
[SiO4]
4-
O
O
O
Si
Si
O
6O
O
[O3Si-O-SiO3]
6-
Examples: open or cyclic chains, [SiO3]n(2n+2)– or [SiO3]n2n– , respectively.
[SiO3]612– in beryl, Be3Al2[SiO3]6
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Asbestos: Tremolite
Mineral tremolite, of the empirical formula Ca2Mg5(Si8O22)(OH)2, contains chains
[Si4O11]n6n–. Tremolite is a fibrous mineral, one of those which were given the
commercial name asbestos.
(Each red sphere represents an O atom)
A sample of tremolite from the
Smithsonian Museum of Natural History
Note: Small tremolite crystals are needle-like. When dust-size particles are inhaled,
the crystals penetrate into the lung tissue by piercing the walls of the alveoli. Once
the particles penetrate the lung tissue, they stay in the lungs permanently. There is
no way to remove them. Eventually these crystals will cause a scarring of the lungs,
called asbestosis, or cause a cancer of the lining (pleura) of the lung, called
mesothelioma. Both of these diseases are incurable and both are terminal.
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Silicates: Talc
Many important silicate minerals are known. Talc (derived from the Persian via Arabic
talq) is a mineral composed of hydrated magnesium silicate with the chemical
formula Mg6[Si8O20](OH)4. Due to its softness and other special characteristics, talc
has an important function in several products: body powder, pharmaceutics, makeup, paper, paint and varnish, ceramics, china, waste water treatment.
[Si8O20]8–
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Aluminosilicates
Aluminosilicates are formed when Al atoms replace some of the Si atoms in the
silicates.
O
O Si
O
O
O
isoelectronic
fragments
O Al
O
O
Si / Al− replacement changes the charge by –1, and an additional cation (H+, Na+,
½Ca2+) is required for every Al atom.
A great variety of different aluminosilicates exist. Some important ones:
Molecular sieves: crystalline aluminosilicates having open structures with cages of
molecular dimensions. They absorb only molecules that are smaller than the cage
dimensions so can be used to separate molecules of different size.
Zeolites: a subclass of molecular sieves, with metal cations trapped inside tunnels
and cages. Zeolites are very important in heterogeneous catalysis and in solid state
chemistry.
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Zeolites
The structure of H-ZSM-5 zeolite (Al0.08Si23.92O48) is typical of a zeolite in possessing
cavities which can accommodate guest molecules. Below, there are two orthogonal
views of the zeolite lattice (Si, Al are purple and O is red).
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Silicones
Hydrolysis of MeSiCl3 and Me2SiCl2 yields silicones which are oligomeric products
containing tetrahedral groups in which each O atom is in a Si-O-Si bridge:
Me
Me
O
Si
O
Me
Si
O
O
O
Silicone polymers have a range of structures, e.g.
Me
Me Me
Si
O
Me Me
Si
O
Me Me
Si
O
Me Me
Si
O
Me
Si
O
O
Silicones find industrial uses as greases, sealants, varnishes, waterproofing
materials. Silicone resins are tough, flexible and inert. Silicone products are crucial
ingredients in personal care products: silicones are the components of shampoos
and conditioners that improve the softness and silkiness of hair, and are also used in
shaving foams, toothpastes, cosmetics, hair styling gels and bath oils.
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