Transcript SiO 2 - Wits Structural Chemistry

THE GROUP 14 ELEMENTS
Include carbon, silicon, germanium, tin and lead.
Carbon is the building blocks of life and silicon is widely distributed in the
natural environment and tin and lead in the industry and manufacturing. C and
Si are nonmetals, Ge a metalloid and Sn and Pb are metals. Si and Ge are used
as semiconductors and optical fibres in the modern technology.
1
2
13
14
15
16
17
0
He
Li
Be
B
C
N
Mg
Al
Si
P
Ca
Ga
Ge
As
Sr
In
Sn
Sb
Ba
Tl
Pb
Bi
Ra
O
F
Ne
Properties of Elements
Atomic radius/pm
C
Si
Ge
Sn
Pb
77
117
122
162
175
73(II);
53(IV)
112(II);
69(IV)
119(II);
78(IV)
Ionic radius(M+n)/pm
Melting point/°C
3730
(graphite
sublimes)
1410
937
232
327
1st I.E.I1/kj.mol-1
1086
786
762
707
715
Electron affinity,
Ea/kj.mol-1
122
134
116
116
1.8
1.8
1.8
1.8
Eө (M4+;M2+)/V
+0.15
+1.69
Eө (M2+;M)/V
-0.14
-0.13
Pauling electronegativity 2.5
Page 318 Sh&At
IE (kJ.mol-1)
Electron
configurations
Oxidation
state
2.5
[He] 2s22p2
(II)
IV*
786
1.8
[Ne] 3s23p2
(II)
IV*
Germanium Ge
762
1.8
[Ar]
3d104s24p2
II
IV*
Tin
Sn
707
1.8
[Kr]
4d105s25p2
II
IV*
Lead
Pb
715
1.8
[Xe]
4f145d106s26p2
II*
IV
Element
Symbol
Carbon
C
1086
Silicon
Si
Element
C

Si
Ge
Sn
Pb
Abundance/ppm
180
272000
1.5
2.1
13
Relative
abundance
17
2
54
49
36
Trends from the table
Electronic configuration – ns2 np2 Generally exhibit +4 oxidation state
and changes to +2 as the group is descended.
Atomic and ionic radii increases from C to Pb.
Electronegativity: C is more electronegative than the rest. C ad Si forms strong
oxophiles and fluorophiles due to high affinities to the hard anions, O2- and F-,
respectively. All elements except lead have solid phase in a diamond structure.
The cubic form of tin – grey Sn (α-Sn) is not stable at room temperature and
converts to more stable form white tin (β-Sn).
α-Sn  β-Sn [→ - room T and ← - cooling at 13.2 °C]
Distinct chemical properties from those of other elements in a group:
C has smaller size, high electronegativity and has higher ionisation energy,
more covalent and less metallic.
- C forms long C-C chains. This property is known as catenation
- C forms flammable, gaseous hydrides, silicon and germanium hydrides are
solids
- C is the only nonmetal.
- C has a unique ability to form p-p multiple bond,(C  C, C  C, C  O,
OCCURRENCE AND RECOVERY
Two forms of pure carbon, graphite and diamond – mined. Another form is
coke – less pure form, result from pyrolysis of coal. Other forms of C –
buckminsterfullerenes – C60 (allotrope of C)
Also found in CO2 and the insoluble carbonates of calcium and
magnesium.
Silicon occurs in natural environment (26% of the earth’s crust). It’s found
in sand, quartz, amethyst, agate, clays and feldspar. Elemental Si is
obtained by high temperature reduction of silica, SiO2, by carbon;
SiO2 (s) +
2 C (s) → Si (s) + 2CO (g)
Germanium is low in abundance, tin is obtained from reduction of
cassiterite, SnO2 with coke and lead occurs in sulfide ores which are
converted into oxide and reduced by carbon.
REACTIVITY OF THE ELEMENTS
With water.
C, Si and Ge do not react.
Sn reacts with steam to give SnO2 and H2.
Pb is unaffected, probably due to a protective layer of PbO2.
With dilute acids.
C, Si and Ge are unaffected.
Sn dissolves in dilute HNO3 forming Sn(NO3)2.
Pb dissolves in dilute HCl forming sparingly soluble PbCl2 and quite readily
in dilute HNO3 forming Pb(NO3)2 and oxides of nitrogen. Reaction with hot
HCl is faster because the PbCl2 is soluble in hot water. Pb also reacts with
organic acids like ethanoic acid and ethanedioic acid. Pb does not dissolve
in dilute H2SO4 because of a surface coating of insoluble PbSO4.
With concentrated acids.
Diamond does not react with concentrated acids but graphite reacts with conc.
HNO3 and concentrated HF/HNO3 mixture.
Si is oxidized and fluorinated by concentrated HF/HNO3.
Ge dissolves slowly in hot concentrated H2SO4 and in HNO3.
Sn reacts with several concentrated acids.
Pb does not dissolve in concentrated HCl because of a surface coating of
PbCl2
With alkalis.
C does not react.
Si reacts slowly with cold aqueous solutions of NaOH and readily in hot
solutions, giving silicates, [SiO4]4-.
Sn and Pb react slowly with cold and rapidly with hot to give stannates,
Na2[Sn(OH)6], and plumbates, Na2[Pb(OH)6], i.e. Sn and Pb are amphoteric
(to some extent Si is also amphoteric).
With halogens.
Diamond does not react but graphite reacts with F2 at 500.
Si and Ge react readily forming volatile SiX4 and GeX4.
Sn and Pb are less reactive: Sn reacts with Cl2 and Br2 in the cold and with F2
and I2 on warming. Pb reacts with F2 in the cold, forming PbF2, and with Cl2
on heating, giving PbCl2.
IMPORTANT USES
Elemental C in the form of coal and coke is used as fuel and reducing agent
in the recovery of metals from their ores (see occurrence and recovery).
Graphite used as lubricant and in pencils and diamond used in cutting tools.
Silicon used in integrated circuits, computer chips, and solar cells. Germanium
used in the construction of transistors due to its purification other than Si and
also for its low band gap (0.71 eV Ge, 1.11 eV Si). Tin is resistant to corrosion
and used to plate steel in cans. Bronze (Sn and Cu) and solder (Sn and Pb)
are alloys containing tin. Lead is used in plumbing (illegal due o lead poisoning).
Its high density leads to use in ammunition and as shielding ionizing radiation.
FORMS OF CARBON
DIAMOND AND GRAPHITE
Carbon clusters
Fullerene metal complexes
Partially crystalline carbon
SELF STUDY
CARBON COMPOUNDS
HYDRIDES
They form tetravalent hydrides – hydrocarbons and silanes.
Simplest hydrocarbon – alkanes (general formula – CnH2n + 2). Methane is the
simplest alkane; CH4. It is a colourless, odourless gas and found in large
amounts natural underground deposits. Extracted as natural gas and used as
domestic and industrial fuel.
CH4 (g) + O2 (g) → CO2 (g) + H2O (l)
ΔcombHø = -882 kJ.mol-1
Silanes forms long chains similar to C although the longest chain length is
four, Si4H10 They have greater than number of electrons, strong intermolecular
attractive forces and are less volatile than the hydrocarbons. Silanes are more
reactive and are prepared commercially by reducing silica, SiO2 with Al under
high pressure of H2 in a molten salt mixture of NaCl and AlCl3.
6 H2(g) + 3 SiO2(s) + 4 Al(s) → 3 SiH4(g) + 2 Al2O3(s)
Thermal stability decreases from germane (GeH4) to stannane (SnH4) and plumbane
(PbH4). Germane and stannane can be prepared by the reaction of the tetrachloride
with LiAlH4 in tetrahydrofuran. Protolysis of a magnesium/lead alloy gives traces of PbH4.
HALIDES OF CARBON
Generally form tetrahalides and Pb forms stable dihalide. Tetrahalomethanes
vary from highly stable and volatile CF4 to the thermally unstable CI4. All
tetrahalomethane are thermodynamically unstable to hydrolysis.
CX4 + 2 H2O(l) → CO2(g) + 4 HX(aq)
However C-F bond react slowly and making the fluorocarbon polymers are high
resistant to water. Tetrahalomethanes can be reduced by alkali metals (strong
reducing agents); CCl4(l) + 4Na(s) → 4NaCl(s) + C(s) ΔrGø = -249 kJ.mol-1
CF4
CCl4
CBr4
CI4
-187
-23
90
171
decompose
Biling point/ °C -128
77
190
Sublime
ΔrGø/kJ.mol-1
-65
+48
>0
Melting point/
°C
-879
Characteristics reactions of C-X
bonds (X = halogen)
CO
H3C
L
Ir
CH3ZnCl
L
CH3MgBr
Cl
Cl
Zn
[IrL2Cl(CO)
Mg
CH3-X
Co(CN)5]3-
X = I)]3-
[Co(CN)5(CH3
+
[Co(CN)5I]3-
[Fe(η5-C5H5)(CO)2]2X = IFe(η5-C5H5)(CO)2(CH3)]- + I-
OXYGEN AND SULFUR COMPOUNDS
Important oxides – CO and CO2, and less familiar oxide such as
carbon suboxide – O=C=C=C=O.
Sulfur compounds have similar structure to oxides, CS and CS2.
Si contains Si-O-Si as in silica, silicates and silicone polymers.
CO is a colourless, odourless, poisonous gas. It is a reducing agent e.g.
PbCl2 + CO + H2O  Pb + CO2 + 2 HCl
Preparation is by dehydrating formic acid with concentrated H2SO4:
H . COOH + H2SO4  CO + H2O
CO is toxic because it forms a stable complex with haemoglobin.
CO is an important fuel because it evolves a large amount of heat when
it burns in air.
2 CO + O2  2 CO2
CO2 - colourless, odourless. It is a major industrial gas. Sold in liquid form and
solid forms. CO2 is obtained from fermentation in breweries, from calcining
limestone in lime kilns, from coal burning in electric power station.
GeO2, SnO2, PbO2 are amphoteric and therefore react with alkali to give
germanates, stannates and plumbates respectively. Germates have
complicated structures but stannates and plumbates contain [Sn(OH)6]2-.
The three are insoluble in acids except in the presence of complexing agent
such as F- or Cl- when complex ions like [GeF6]2-, and [SnCl6]2- are formed.
Other oxide of lead is red lead, Pb2O3 which is actually a mixed oxide,
PbOPbO2 i.e. lead(II) lead(IV) oxide. It is used in paint to prevent the rusting
of iron and steel.
OXIDES OF Ge, Sn AND Pb
The +2 oxidation state becomes more stable from Ge to Pb.
+II
+IV
CO
SiO
GeO
SnO
PbO
neutral
?
acidic
amphoteric
amphoteric
gas
solid
solid
solid
solid
CO2
SiO2
GeO2
SnO2
PbO2
weakly acid
acidic
acidic
(less than SiO2)
amphoteric amphoteric
gas
solid
solid
solid
solid
covalent
covalent
covalent
Ionic
ionic
Characteristics reactions of CO2
R3P
PR3
Ni
O
CaCO3
NaHCO3
C
O
CaO
O
XMg
[(R3P)2Ni(N2)]
NaOH
C
R
RMgX
CO2
H2N
H2N
O
Co(OH2)2(tren)]3+
[(Li)2W(CO)5]
CO
[Co(tren)(OCO2)(OH2)]
C═O
NH3, RMgX
OC
W
OC
CO
CO
O
C
Li
O
COMPOUNDS WITH NITRIDES AND CARBIDES
The cyanide ion, CN- forms complexes with many d-metal ions, its coordination
to the active sites of enzymes accounts for its high toxicity. An example is
hydrogen cyanide, which is highly volatile and highly poisonous. Silicon nitride,
Si3N4 is inert and very hard and is used in high temperature ceramic material.
Carbides can be classified as saline, metallic and metalloid carbides.
Saline carbides – ionic solids and they are formed by elements in
Groups 1
and 2 and by Al. Are classified as follows; graphite intercalation compounds
such as KC8, dicarbides which contains C22- anion and methides which
contains C4- anion. Be2C and Al4C3 are the borderline between saline and
metalloid.
Reactions of Saline Carbides
Direct reaction at high T
Ca(l) + 2 C(s) (>2000 °C) → CaC2(s)
Reaction with metal oxide and carbon at high T
CaO(l) + 3 C(s) (2000 °C) → CaC2(s) + CO(g)
Reaction of ethyne (acetylene) with a metal-ammonia solution
2 Na(am) + C2H2(g) → Na2C2(s) + H2(g) (ours in mild conditions)
The carbides also reacts with the weak acid water to produce ethyne;
CaC2(s) + 2 H2O(l) → Ca(OH)2(s) + HC≡CH(g)
Metallic carbides (interstitial carbides) – have metallic conductivity
and lustre and they are formed by the d-block elements. d-Metallic carbides
are often hard materials with the carbon atom octahedrally surrounded by
metal atoms.
Example is tungsten carbide (WC) which is used for cutting tools and also
high-pressure apparatus. Cementite, Fe3C is a major constituent of steel
and cast iron.
Silicides: silicon-metal compounds contain isolated Si, tetrahedral or Si4, or
hexagonal nets of Si atom. Examples are Fe3Si and K4Si4.
EXTENDED Si-O COMPOUNDS
ALUMINOSILICATES
Aluminium may replace Si atom in a
silicate framework to form an aluminosilicates. Additional cations includes H+,
Na+, Ca2+
Layered and three dimensional aluminosilicates – are primary constituents of
clay and some minerals. Mineral kaolinite, Al2(OH)4Si2O5, used in china clay.
The electrically neutral layers are held together by a weak hydrogen bonds,
and so the mineral cleaves and incorporates water in between the layers. Other
examples; pyrophyllite Al2(OH)2Si4O10 and mineral talc – Mg3(OH)2Si4O10.
More examples and figures on page 339.
Molecular sieves – Zeolite aluminosilicate have large open cavities or channels
giving rise to useful properties such as ion exchange and molecular absorption.
Are crystalline aluminosilicate having open structures with apetures of molecular
dimensions. Zeolites are a subclass of molecular sieves having an aluminosilicate framework with cations (Group 1 and 2) trapped inside tunnels or cages.
Composition and properties of some molecular
sieves
Molecular
sieve
Composition
Diameter
(bottleneck)/
pm
Chemical properties
A
Na12[(AlO2)12(SiO2)12].xH2O
400
Absorbs small molecules; ion
exchanger, hydrophilic
X
Na86[(AlO2)86(SiO2)106].xH2O
800
Absorbs medium-sized
molecules; ion exchanger,
hydrophilic
Chabazite
Ca2[(AlO2)4(SiO2)8].xH2O
400 – 500
Absorbs small molecules; ion
exchanger, hydrophobic
ZSM-5
Na3[(AlO2)3(SiO2)93].xH2O
550
Moderately hydrophobic
ALPO-5
AlPO4.xH2O
800
Moderately hydrophobic
Silicalite
SiO2
600
Hydrophobic
Some uses of Zeolites;
- Ion exchange – water softeners in detergents.
- Absorption of molecules – selective gas separation and gas chromatography.
- Solid acids – cracking high molecular weight hydrocarbons for fuel and petroChemical intermediates, shape selective alkylation and isomerization of
aromatics for petroleum and polymer intermediates.
Further reading:
Organosilicon and organometallic componds
on pages 342 – 344.
Exercises:
Complete the reaction scheme by giving the products A - F
F
H2O
Si
Cl2
A
RLi
RMgBr
E
B
H2O
C
Δ
D
A = SiCl4
B = SiCl3R
C = RSi(OH)3
D = RSiOSiR
E = SiR4
F = SiO2
Compare bonding between diamond and boron:
They both as C and B atoms have four orbitals one 2s and three 2p’s available
for bonding. However C has four electrons to form four bonds (2c, 2e bonds)
with other C atoms in a diamond. B atom has one less electron and hence to
use all its electrons or orbitals by forming 3c, 2e bonds with another neighboring
B atom.
C6 : 1s2 2s2 2p2 (2px 2py 2pz)
B5 : 1s2 2s2 2p1 (2px 2py 2pz)
C
C
C
H
B
C
H
C
H
H
B
H
H
Boranes: Count the number of skeletal electrons in B4H10. ;
Four B-H units accounts for 8 electrons, six additional H contributes 6 electrons
giving a total of 14.
H
H
H
H
B
H
H
B
B
H
B
H
H
H
B-H UNITS: 4
B-H UNITS: 2
B-H-B UNITS: 4
B-B UNITS: 1