THE GROUP 14 ELEMENTS 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.

He

Li Be B C N O F Ne

Mg Al Si P

Ca Ga Ge As

Sr In Sn Sb

Ba Tl Pb Bi

Ra

1 2 13 14 15 16 17 0

Properties of ElementsProperties of ElementsC Si Ge Sn Pb

Atomic radius/pm 77 117 122 162 175

Ionic radius(M+n)/pm 73(II); 53(IV)

112(II); 69(IV)

119(II); 78(IV)

Melting point/°C 3730 (graphite sublimes)

1410 937 232 327

1st I.E.I1/kj.mol-1 1086 786 762 707 715Electron affinity, Ea/kj.mol-1

122 134 116 116

Pauling electronegativity 2.5 1.8 1.8 1.8 1.8

Eө (M4+;M2+)/V +0.15 +1.69

Eө (M2+;M)/V -0.14 -0.13

Page 318 Sh&At

Element SymbolIE (kJ.mol-1) Electron

configurationsOxidation

state

Carbon C 1086 2.5 [He] 2s22p2 (II) IV*

Silicon Si 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 C Si Ge Sn Pb

Abundance/ppm 180 272000 1.5 2.1 13

Relative abundance

17 2 54 49 36

Trends from the tableTrends from the table

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, C S, C N ), with self and other elements

Electronic configuration – ns2 np2 Generally exhibit +4 oxidation state and changes to +2 as the group is descended.

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 ELEMENTSREACTIVITY 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 USESIMPORTANT 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. Germaniumused 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 CARBONFORMS OF CARBONDIAMOND AND GRAPHITEDIAMOND AND GRAPHITE

Carbon clustersCarbon clustersFullerene metal complexesFullerene metal complexesPartially crystalline carbonPartially crystalline carbon

SELF STUDY

CARBON COMPOUNDSCARBON COMPOUNDSHYDRIDES

They form tetravalent hydrides – hydrocarbons and silanes. Simplest hydrocarbon – alkanes (general formula – CnH2n + 2). Methane is thesimplest 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 highresistant 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

Melting point/ °C

-187 -23 90 171 decompose

Biling point/ °C -128 77 190 Sublime

ΔrGø/kJ.mol-1 -879 -65 +48 > 0

Characteristics reactions of C-X Characteristics reactions of C-X bonds (X = halogen)bonds (X = halogen)

CH3-X

CH3MgBr

Mg

CH3ZnCl

Zn[IrL2Cl(CO)

[Co(CN)5(CH3)]3-

+ [Co(CN)5I]3-

Co(CN)5]3-

X = I-

Fe(η5-C5H5)(CO)2(CH3)]- + I-

[Fe(η5-C5H5)(CO)2]2-

X = I-

Ir

H3CCO

L

L ClCl

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

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, PbOPbO2 i.e. lead(II) lead(IV) oxide. It is used in paint to prevent the rusting of iron and steel.

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.

OXIDES OF Ge, Sn AND Pb OXIDES OF Ge, Sn AND Pb

+II CO SiO GeO SnO PbO

neutral ? acidic amphoteric amphoteric

gas solid solid solid solid

The +2 oxidation state becomes more stable from Ge to Pb.

+IV 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

NH3, RMgX C═OH2N

H2NCO2

NaHCO3

NaOH

CaCO3

CaO

[Co(tren)(OCO2)(OH2)]

Co(OH2)2(tren)]3+

RMgXXMg C R

O

O

[(R3P)2Ni(N2)]

Ni

R3P

OC

O

PR3

[(Li)2W(CO)5]

W

CO

OC

O

OC

COOC CO

Li

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 C2

2- 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 TCa(l) + 2 C(s) (>2000 °C) → CaC2(s)Reaction with metal oxide and carbon at high TCaO(l) + 3 C(s) (2000 °C) → CaC2(s) + CO(g)Reaction of ethyne (acetylene) with a metal-ammonia solution2 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 COMPOUNDSEXTENDED Si-O COMPOUNDS

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. Otherexamples; 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 channelsgiving rise to useful properties such as ion exchange and molecular absorption. Are crystalline aluminosilicate having open structures with apetures of moleculardimensions. Zeolites are a subclass of molecular sieves having an alumino-silicate framework with cations (Group 1 and 2) trapped inside tunnels or cages.

ALUMINOSILICATESAluminium may replace Si atom in a silicate framework to form an alumino-silicates. Additional cations includes H+, Na+, Ca2+

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

Further reading: Organosilicon and organometallic componds on pages 342 – 344.

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 petro-Chemical intermediates, shape selective alkylation and isomerization of aromatics for petroleum and polymer intermediates.

Exercises:

SiCl2 RLi H2O

RMgBr Δ

H2O

A

F

E

B

D

C

A = SiCl4

B = SiCl3RC = RSi(OH)3

D = RSiOSiRE = SiR4

F = SiO2

Complete the reaction scheme by giving the products A - F

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

CC B

H

BH

H

H

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.

B-H UNITS: 4B-H UNITS: 2B-H-B UNITS: 4B-B UNITS: 1

B

B

B

B

H

H

HH

H

H

H H

H H