Chemistry of the Elements3GROUP FOUR ELEMENTS

C

Si

GeSn`

Pb

FIGURE 3.1 Elements In Group Four (IV)

Metals are good conductors of electricity with atomic conductance (atomic electrical conductivity) greater than 10-3 ohm-1 cm-1. Their conductivity slowly falls as the temperature rises.

Metalloids are poor conductors of electricity with some atomic conductance usually less than 10-3 but greater than 10-5 ohm-1 cm-1. The conductivity of the metalloids increase as the temperature rises. It is also considerably affected by the presence of impurities.

Non metals are usually or virtually non conductors. Their atomic conductance is usually less than 10-10 ohm-1 cm-1.

The element carbon exists in the form of allotropes. Graphite is a poor conductor of electricity and would be classified as a metalloid. Diamond, however, is an insulator and is therefore classified as a non metal.

3.1PHYSICAL PROPERTIES OF GROUP IV

Carbon(C)Silicon(Si)Germanium(Ge)Tin(Sn)Lead(Pb)

Electronic Structure2s22p23s23p24s24p25s25p26s26p2

Atomic Radius0.0770.1170.1220.1410.154

Electronegativity2.51.81.81.81.8

M.P / oC3730 d1410937232327

B.P / oC4830 d2680283022701730

Density / g cm-32.26 gr, 3.51 d2.335.327.311.44

Conductivity Fairly Good gr Non Cond dSemiconductorSemiconductorGoodGood

Electrical Conductivity / Ohm-1 m-11 X 10-71 X 1062 X 1068 X 1065 X 106

Thermal Conductivity / J cm-1 s-1 K-10.24 gr0.840.590.630.35

Enthalpy Change Hoat / KJ mol-1 +716 gr+456+376+302+195

1st I.E / KJ mol-11086787760707715

Principal Ox. No.+4+4+2, +4+2, +4+2, +4

Type Of StructureGiant MoleculesGiant Molecular, Similar To DiamondGiant Molecular, Similar To DiamondGiant MetallicGiant Metallic

TABLE 3.1 Physical Properities Of Group IV

Carbon and silicon are non metals and give acidic oxides. Those of germanium, tin and lead are amphoteric. Although some lead oxides are definitely basic, covalency dominates with carbon and silicon and then ionic with tin and lead.

The most striking feature of the compounds of the group IV elements is the existence of two oxidation states, +2 and +4. The relative stabilities of the +2 and +4 oxidation states vary. In carbon and silicon compounds, the +4 is very stable relative to +2. Germanium forms oxides in both +4 and +2 states. However, GeO2 is rather more stable than GeO. GeO2 does not act as an oxidizing agent and GeO is readily converted to GeO2.

In tin compounds, the +4 is only slight more stable than the +2 state. Thus aqueous tin (II) ions are mild reducing agents. They will convert Mercury (II) ions to mercury and iodine to iodide.

Sn2+ (aq) + Hg2+ (aq)Sn4+ (aq) + Hg (l)

Sn2+ (aq) + I2 (aq)Sn4+ (aq) + 2I- (aq)

In lead compounds, however, +2 is unquestionably more stable, PbO2 is a strong oxidizing agent, whilst PbO is relatively stable. Thus PbO2 can oxidise hydrochloric acid to chlorine and hydrogensulphide to sulphur.

PbO2 (s) + 4HCl (aq) PbCl2 (aq) + Cl2 (g) + 2H2O (l)

There is a steady increase in the stability of the lower oxidation state relative to the higher oxidation state on moving down the graph from Carbon to Lead.

Relative Stability Falls

+4 State

Relative Stability Rises+2 State

CSiGeSnPb

FIGURE 3.1 Relative Stability Of The +4 And +2 Oxidation States Of Group IV

The greater stability of the +4 and +2 oxidation state with respect to +4 state as the atomic number rise is well illustrated by the standard electrode potentials of the M4+(aq) /M2+(aq) system for germanium, tin and lead.

Ge4+ + 2e-Ge2+ ; Eo = -1.6V

Sn4+ + 2e- Sn2+ ; Eo = +0.15 V

Pb4+ + 2e-Pb2+ ; Eo = +1..8 V

As the electrode potential gets more positive from Ge4+ to Pb4+, the oxidized form is more readily reduced to the +2 state.

All group IV elements have four electrons in their outermost shell and therefore show an oxidation state of +4 but more forms cation of +4, M4+ ion in the solid state. This is due to the high ionization energy involved in removing the four electrons. Consequently the bonding in the tetravalent compounds is predominantly covalent. Compounds of tin and lead in which the group IV element has an oxidation number of +2 (e.g. PbF2, PbCl2, PbO) are normally regarded as ionic.

In these compounds, the Sn2+ and Pb2+ ions are formed by the loss of the two p electrons from the 5p2 and 6p2 subshell. The two s electrons remain relatively stable and unreactive on the field subshell. This is referred to as the inner pair effect.

3.2REACTIONS OF TETRACHLORIDES WITH WATER

All the chlorides (except CCl4) are readily hydrolyzed. Tetrachloromethane will not react with water but SiCl4 is immediately converted to silica.

SiCl4 (l) + 2H2O (l) SiO2 (s) + 4H+ (aq) + 4Cl- (aq)

The difference is explained by assuming that one of the lone pairs on a water molecule can overlap with one of the empty 3d orbitals or the silicon atoms. The 3d orbitals are much higher in carbon, so bonding cannot occur between them and water molecules. Once electron density is fed into the silicon atom, the chlorine atoms can detach themselves by inverting into chloride ions and the SiCl4 is destroyed.

Very Large Gap

Small Gap

Very Small Gap

5p4p3p2p

FIGURE 3.2 Differences Between The 2p and 3d Energy Of Carbon

The diagram illustrates the large difference between the 2p and 3d energy of carbon. The carbon 3d orbitals are so high in energy that they cannot be used in bonding. The 3p and 3d for silicon is relatively small, so silicon can use the 3d orbital in bonding.

ElementTypical HalidesComplex Halides

CarbonCCl4None, d orbitals needed.

SiliconSiCl4SiF62-

GermaniumGeCl4, GeCl2GeFe22-, GeCl62-

TinSnCl4, SnCl2SnF62-, SnCl42-, SnCl62-

LeadPbCl4, PbCl2PbCl42-, PbCl62-

TABLE 3.2 Typical & Complex Halides Of Group IV

Similar compounds are given with fluorine, chlorine, bromine and iodine. PbBr4, PbI4, do not exist.

GeCl4 + 4H2O Ge(OH)4 + 4HCl

SnCl4 + 4H2OSn(OH)4 + 4HCl

PbCl4 + 4H2OPb(OH)4 + 4HCl

*PbCl4 must be kept below 5 oC or it dissociates.

PbCl4PbCl2 + Cl2

ClCl

-+SiClSiClCl

ClClClOH2

+H . .

- O :

H

ClCl

-+SiClSiClCl

ClClOHOH2

+H . .

- O :

H

ClCl

-+SiOHSiClCl

ClClOHOH2

+H . .

- O :

H

ClOH

-+SiOHSiClCl

ClClOHOH2

+H . .

- O :

H

ClOH

-+SiOHSiOHCl

ClCl OH

+H . .

- O :

H

FIGURE 3.3 Reaction Mechanism For SiCl4 And H2O

THE DIOXIDESGROUP IV ELEMENTS WITH +4 OXIDATION STATE

Oxides

CO2SiO2GeO2SnO2PbO2

Boling Point (oC)-78259012001900Decomposes On Heating

StructureSimple MolecularGiant MolecularIntermediate Between Giant And Ionic

NatureACIDIC

Reaction with Alkalis giving XO32- salts.

CO2 + 2OH- CO32- + H2O

SiO2 + 2OH- SiO3 + H2O

AMPHOTERIC

React with fused alkalis giving XO32- salts.

SnO2 + 2OH SnO32- + H2O

PbO2 + 2OH SnO32- + H2O

React with concentrated acid forming +4 salts.

SnO2 + 4H+ Sn4+ (aq) + H2O

PbO2 + 4HCl PbCl4 + 2H2O

TABLE 3.3 Reaction Mechanism For SiCl4 And H2O

THE MONOXIDESGROUP IV ELEMENTS WITH +2 OXIDATION STATE

Oxides

COSiOGeOSnOPbO

Boling Point (oC)-1911470

StructureSimple MolecularNeutral Oxides

Predominantly IonicAmphoteric Oxides

NatureReacts with neither acids nor alkalis.

Reacts with acids to form salts:

PbO + 2H+ Pb2+ + H2OSnO + 2H+ Sn2+ + 2H2O

Reacts with alkalis to forms salts:

PbO + OH- Pb(OH)3-+ H2OTrihyrdroxyplumbate (II)

SnO + OH- + H2O Sn(OH)3-Trihydroxystannate (II)

TABLE 3.3 Reaction Mechanism For SiCl4 And H2O

3.5CERAMICS

The word ceramics literally means heat resistant and traditional ceramic materials are ones such as porcelain which are fired during manufacture. Hardened by heat are the best known ceramics and are based on clay, such as pottery.

Clays are found naturally and contain a number of minerals such as kaolinite, Al2Si2O5(OH)4, an alumino silicate. Clay contains crystals. When water is added it acts as a lubricant, allowing the crystals to slide over one another. This makes the clay easy to shape. If the water is gently dried, the shaped articles become hard, but if more water is added the clay becomes moldable again. If the dry clay is then heated to a temperature of around 1000 oC (fired), chemical changes occur and a glass is formed, which glues the clay crystals together. These chemical changes are not reversible and the fired ceramic article can never be remolded. It will neither melt nor will it react with oxygen in air, as it is already an oxide.

Ceramics are good insulators of both heat and electricity and are brittle. This last property is because they have giant structures which are either covalently or ionically bonded.

Brittleness is the result of tiny cracks present on the surface.

Under load these cracks increase in size and lead to failure of the material.

No Crack. Each chain of atoms shares the load.

A small crack if found. Each load is placed on the next chain which breaks.

More load. Eventually the structure breaks.

FIGURE 3.4 The Effect Of A Load On A Crack Within A Ceramic Structure

A small surface crack (Griffith crackio) can dramatically reduce the strength of a material under tension.

FIGURE 3.5 Structure Of Kaolinite

Copyright Pooran Appadu