PRE – U CHEMISTRY

SEMESTER 2SEMESTER 2

CHAPTER 5 :

GROUP 14 & COMPOUNDS

Physical properties of group 14 elements

Element Carbon Silicon Germanium Tin Lead

Classification Non–metal Metalloid Metal

Valence electron 2s²2p² 3s²3p² 4s²4p² 5s²5p² 6s²6p²

Atomic

radius(nm)0.77 1.17 1.22 1.48 1.55

Melting point(°C) 3700 1410 936 232 328

Boiling Point(°C) 4620 2680 2820 2270 1730

First ionisation

energy(kJ mol⁻¹)1085 788 760 705 714

Conductivity (Heat)Diamond (X)

Graphite (√ )

Non-

conductorConductor

Good conductor of

heat and electricityConductivity

(Electrical)Semiconductor

Stability of oxidation

states

Stability of +4 oxidation state decrease

Stability of +2 oxidation state increase

Introduction

• Group 14 is the only group which contain all 3 major classification of

elements: metal, metalloid and non metal

• The variation of Group 14 is based on the characteristic of each elements

Atomic radius

• Atomic radius increase when goes down to Group 14.

• When the number of electrons increases, the number of shell required to

fill in the electrons increase.

• This will increase the screening effect of the atom causing the effective • This will increase the screening effect of the atom causing the effective

nuclear charge decrease.

• As the attraction forces between the outermost electron and the nucleus

decrease, atomic radius become larger.

3.1.2 Melting point

• As the metallic properties of Group 14 elements are different, it has

different molecular structure.

• Carbon, silicon and germanium have gigantic molecular structure. The

melting point decrease when goes down to from carbon to germanium.

This is due to bonding length of C – C ; Si – Si ; and Ge – GeThis is due to bonding length of C – C ; Si – Si ; and Ge – Ge

• become longer, so the covalent bond become weaker as the length increase.

• As for tin (Stanum) and lead (Plumbum), they have strong metallic bond.

Lead has lower melting point than tin, as the arrangement of the metallic

atom of lead are more closely packed (faced centered cubic) than tin

(tetragonal structure) in solid lattice. The close packing of increase the

strength of the metallic bond in lead.

3.1.3 Ionisation energy

• Generally the 1st ionisation energy decrease when going down to Group

14.

• This is due to the increase of atomic size as the number of electron shells

filling in increased. When this happened, it will gradually increase the

screening effect thus decrease the effective nuclear charge of the atom.

When this occur, electron is easier to extract out from the atom.

• However the 1st ionisation energy of tin is lower than lead despite that

the atomic radius of Pb is larger than Sn. The electronic configuration of the atomic radius of Pb is larger than Sn. The electronic configuration of

Sn and Pb are as follow:

– Sn : 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p2

– Pb : 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p2 4f14 5d10 6s2 6p2

• This is due to the ineffective screening by 4f electrons in lead atom.

The nuclear charge of lead is far greater than tin. So, the effective nuclear

charge of lead will be slightly higher than tin, causing the ionisation

energy of lead is higher than tin.

3.1.4 Electrical conductivity

• Since carbon is a non metal, there’s no conductivity of carbon

(diamond) that occur on carbon. If the allotrope of carbon is graphite,

it can conduct electricity as it has one delocalised electron on carbon

(sp2 hybridisation).

• As for silicon and germanium, both of them are semiconductor as

they are metalloid. The conductivity of semiconductor can be

increase by increasing the temperature, or silicon doped with

germanium.germanium.

• As for tin and lead, since both of them are metal, they can delocalise

electron thus conducting electricity. The electrical conductivity

increase from tin to lead as the electron delocalised by lead is easier

than tin.

3.2 Chemical Properties of Group 14

3.2.1 Stability of +2 and +4 oxidation state of Group 14

• Group 14 elements can exhibit 2 oxidation states, which is +2 and +4

• The valence electron of Group 14 elements are

____ ____ ____ ____

ns2 np2

• Even though the number of valence electron of Group 14 is 4, but it will

never form M4+ ions due to its very high successive ionisation energy to

remove 4 electrons.remove 4 electrons.

• Still, the +4 oxidation state involve the involvement in the 1 s and 3 p

orbital. Whenever the energy is absorbed

____ ____ ____ ____ ____ ____ ____ ____

ns2 np2 s p3

• This can be done at lower energy level as the gaps between the s and p is

not far. When goes down to Group 14, the stability of +4 decrease due to

the far level between the s and p orbital. For example, in lead, the 6s and 6p

is separated by large d10 and f14 orbital. This effect is called as inert pair

effect. This will cause +2 oxidation states to be more stable as the 2

electron is p orbital are easier to be removed.

• The graph below shows the relative stability of +2 and +4 oxidation state in

Group 14

C Si Ge Sn Pb

• From the graph, we can conclude that +4 more stable than +2 for carbon,

silicon, germanium and tin while +2 more stable than +4 for lead

• From the angle of electrochemistry, the standard reduction potential, E0red,

of +4 to +2 oxidation state of Group 14 elements is shown below

– Ge4+ (aq) + 2 e- � Ge2+ (aq) E0red = – 1.60 V

– Sn4+ (aq) + 2 e- � Sn2+ (aq) E0red = + 0.15 V

– Pb4+ (aq) + 2 e- � Pb2+ (aq) E0red = + 1.69 V

• As the E0red value become more positive, it mean from Ge2+ to Pb2+, the

stability increase, Sn2+ and Pb2+ are preferably to stay as +2 oxidation state. stability increase, Sn and Pb are preferably to stay as +2 oxidation state.

Therefore, Pb4+ is a strong oxidising agent while Ge4+ is a strong reducing

agent.

• Pb4+ (aq) + 2 e- � Pb2+ (aq) (preferably to reduce) (oxidising agent)

• Ge2+ (aq) � Ge4+ (aq) + 2 e- (preferably to oxidise) (reducing agent)

• All Group 14 elements (except carbon) can make use of their empty

d-orbitals to form complex ions.

• Example : SiF62- ; GeCl6

2- ; SnCl62- ; PbCl6

2- .

• Central atom of Group 14 can use of the empty d orbital to expand

their valency, from 4 to 6.

• Carbon cannot form complex ions because ………………………….

……………………………………………………………….

carbon which contain only

2 shells can allocate a maximum of 8 electrons

3.2.2 Catenation

• Catenation is the ability of an element to form bonds between atoms of the

same element

• Carbon is unique in its ability to catenate to form stable long chain or even

ring compound.

• For catenation to occur, the metal M–M bond must be strong. The bond

energy of the M–M must must be similar in strength to those between M

and other particular element, e.g. M–O bond.

Element M – M M = M M ≡ M M – OElement M – M M = M M ≡ M M – O

Carbon 350 610 840 360

Silicon 222 - - 464

Germanium 188 - - 360

• Carbon, with the smallest size formed the strongest M–M bonds, plus it is

as strong as in M–O bond.

• The M–M for other elements are too weak. Furthermore Carbon can form

double and even triple bond. double and even triple bond.

• So, only carbons are able to catenate to form series of long chain known

as organic compounds.

• However, for silicon, since the Si–O bonds are stronger than Si–Si, hence

Si–O catenates to form the following chain as found in SiO2.

• The ability to catenate decrease as the size of atom increase

3.3 Oxide of Group 14

• Group 14 elements form oxides with the formulae XO and XO2 in relation

with the +2 and +4 oxidation states.

• The tendency to form dioxides, XO2 , decreases down the group showing

the increases stability of the +2 oxidation states.

Element

+2 oxidation state +4 oxidation state

FormulaStructure and

bonding

State of

matterFormula

Structure and

bonding

State of

matter

Simple Simple

Carbon CO covalent

molecule

Gas CO2 covalent

molecule

Gas

Silicon SiO

Simple

covalent

molecule

Unstable

gasSiO2

Giant covalent

molecule

Solid with high

melting and

boiling points

Germaniu

mGeO - - GeO2

Molecular/

ionicSolid

Tin SnOIonic

compoundSolid SnO2

Ionic

compound

Solid with high

melting and

boiling points

Lead PbOIonic

compoundSolid PbO2

Ionic

compound

Solid, Easily

decompose

3.3.1 Carbon Oxides

• Carbon monoxide, CO can be produced by passing carbon dioxide

gas,CO2, over heated carbon.

CO2 (g) + C (s) 2CO (g)

• An alternative method is by reducing the methanoic acid using

concentrated sulphuric acid.

• HCOOH (l) CO (g) + H2O (l)

• In industry, it is prepared in a big scale by passing steam over heated coke.

The mixture of hydrogen and carbon monoxide gases produced formed

water gas’‘water gas’ and used as fuel

C (s) + H2O (l) � CO (g) + H2 (g) [ Water gas ]

• Carbon monoxide is a toxic gas. Chemically, it is neutral and dissolves

partially in water. Carbon monoxide can exists in three possible structures.

The main covalent bond in the molecule is in sp hybridisation.

• It is toxic since it will combines with hemoglobin to

produce carboxyhemoglobin, which usurps the space in hemoglobin

that normally carries oxygen, but is ineffective for delivering oxygen

to bodily tissues. Furthermore, carbon monoxide is easily absorbed by

lung.

• All the structures of carbon monoxide show that oxygen atom possessing

lone pair of electrons. That is why carbon monoxide is a good ligand. In

a complex consisting of transition metals as the centre ion.

• The carbon monoxide molecules form ligands by donating a lone

pair of electrons through dative bonds. These complexes are known

as carbonyl compounds.

• Example : tetracarbonylnickel (II) ion, [Ni(CO)4]2+

2+CO

Ni

CO

CO

CO

OC

Carbon dioxide, CO2.

• Carbon monoxide is an unstable molecule. In contact with air, it is easily

oxidised to carbon dioxide gas and released into the atmosphere.

2 CO (g) + O2 (g) � 2 CO2 (g)

• Carbon dioxide, CO2, is a colourless and odourless gas. Unlike carbon

monoxide, carbon dioxide is non-toxic. It is slightly acidic and it forms

non-polar linear molecule with structure O=C=O. The main covalent bond

is also in sp hybridisation. The intermolecular forces are weak van der

waals’ forces. Carbon dioxide has a low boiling point and sublimes at low waals’ forces. Carbon dioxide has a low boiling point and sublimes at low

temperatures. At room temperature pressure, carbon dioxide exists as a gas

• Common uses of carbon dioxide includes the following :

– Carbon dioxide used in fire extinguishers are stored under high

pressure. So, when the gas is suddenly released into the atmosphere,

sudden expansion and cooling of the gas causes a dense layer of

inflammable gas to form on top of the flame, blocking off oxygen

supply and thus halts burning.

– Carbon dioxide is also used in making aerated drinks and beverages

and in the manufacture of baking soda, NaHCO3.

– Solid carbon dioxide or dry ice is used as a refrigerant as well as for – Solid carbon dioxide or dry ice is used as a refrigerant as well as for

producing ‘stage effects’ as it sublimes immediately to produce a cloudy

and misty environment

3.2.2 Silicon Oxides

• Two well known silicon oxides are silicon monoxides, SiO,and silicon

dioxide,SiO2.

• Silicon monoxide, SiO, is an unstable compound at room temperature.

When silicon dioxide, SiO2, and silicon, Si, are heated together under a

vacuum to a high temperature (1250ºC), silicon monoxide will be

produced. However, the reaction is reversible when the temperature cools

down. The reverse reaction is a disproportionation reaction in which the

same element, Si, is reduced and oxidised at the same time.

2 SiO (g) Si (s) + SiO2 (s)

Oxidation state : +2 0 +4

• Silicon dioxide, SiO2, is a very stable compound with high boiling and

melting points. It exists in three different forms at different temperatures.

The three forms, namely quartz, tridimite and cristobalite exist

simultaneously at different temperatures as shown below.

• In the silicon dioxide structures, each

silicon atom is connected to four oxygen

atoms and each oxygen atom shares two

tetrahedrons. The basic unit of the structure

is a SiO4 tetrahedron but since each oxygen

atom is shared between two basic units, the

empirical formula is again SiO2 with

coordination ratio of 4 : 2

• The cristobalite structure shows similarities

to the diamond structure. It is simply to the diamond structure. It is simply

inserting an oxygen atom in the middle of

each Si-Si bond.

• Silicon dioxide is an important component

in ceramics because of its unique

properties. It is inert to attacks from acids

and alkalis, do not corrode, hard but brittle

& stable as a lot of heat energy is required

to break the strong bonds. All these

properties resemble diamond in many

aspects

3.2.3 Germanium oxide

• Germanium (II) oxide, GeO, is obtained from the reduction of germanium

(IV) ion. Reduction must be carried out under inert conditions because any

presence of air will easily oxidise germanium (II) ions back to the more

stable germanium (IV) ions. As in the case of all divalent compounds of

germanium, disproportionation occurs during oxidation.

2 Ge (II) Ge (IV) + Ge

• Germanium oxide, like all the other lower metal oxides of the group, is

more basic and ionic than their dioxide counterparts. Germanium oxide

shows certain acidic properties, however tin (II) oxide and lead (II) oxide shows certain acidic properties, however tin (II) oxide and lead (II) oxide

are amphoteric. The basic properties become prominent with increase in

proton number.

• Germanium dioxide, GeO2, is more stable than germanium oxide.

Germanium dioxide shows both acidic and basic properties (amphoteric).

As going down Group 14, the acidic properties of the dioxide decreases

giving a decreasing order of acidity, GeO2 > SnO2 > PbO2.

GeO2 (aq) + 4 HCl (aq) � GeCl4 (aq) + 2 H2O (aq)

GeO2 (aq) + 2 NaOH (aq) � Na2GeO3 (aq) + H2O (l)

3.2.4 Tin Oxide

• Tin (II) oxide, SnO, is a deep green solid compound which is relatively

unstable at room temperature. When heated up in air, it is easily oxidised to

tin (IV) oxide, SnO2. Tin (IV) oxide exists naturally in the Earth’s crust as

a mineral ore from which tin can be mined or extracted from it. Carbon is

used to extract tin.

SnO2 (s) + C (s) Sn (s) + CO2 (g)

• Tin (IV) oxide, SnO2, is only slightly more stable than tin (II) oxide. Both

are amphoteric. Only Sn4+ can exist as free ions.

• Tin can exist in 2 oxides, which is tin (II) oxide (SnO) and tin (IV) oxide

(SnO2). Both of the oxides of tin are amphoteric oxide.

• When tin (II) oxide, SnO react with acid and base

SnO (s) + 2H+ (aq) Sn2+ (aq) + H2O (l)

SnO (s) + OH- (aq) + H2O (l) [Sn(OH)3]- (aq)SnO (s) + OH (aq) + H2O (l) [Sn(OH)3] (aq)

Trihydroxystannate (II) ion

• When tin (IV) oxide, SnO2 react with acid and base

SnO2 (s) + 4 H+ (aq) Sn4+ (aq) + 2H2O (l)

SnO2 (s) + 2 OH- (aq) + 2 H2O (l) Sn(OH)6

2- (aq)

Lead Oxide

• Lead exists in different states of oxides, lead (II) oxide, PbO, lead (IV)

oxide, PbO2, and trilead tetroxide, Pb3O4.

• Lead (II) oxide, PbO is the most stable form of all the lead oxides. All

the other oxides are unstable and easily decompose on heating to form the

more stable lead (II) oxides.

2PbO2 (s) 2 PbO (s) + O2 (g)

Brown Yellow

2Pb3O4 (s) 6 PbO (s) + O2 (g)3 4 2

Red Yellow

• When lead (II) oxide is heated in air to a temperature of 400oC, it will form

Pb3O4 first. On further heating to 470oC, Pb3O4 will revert back to lead (II)

oxide.

3 PbO2 (s) + O2 (g) Pb3O4 (s) 6PbO (s) + O2 (g)

• Lead (IV) oxide is a powerful oxidising agent as it readily loses its oxygen

atom to form a more stable lead (II) oxide.

2 PbO2 (s) 2 PbO (s) + O2 (g)

• One of the example of this oxidising reaction is the oxidation of the iodide ion,I- to

iodine, I2.

• Trilead tetroxide is also known as ‘red lead’ because of its brilliant scarlet colour.

It consists of a mixture of two moles of lead (II) oxide and one mole of lead( (IV)

oxide. Thus, it behaves chemically like a mixture of the two. It is an oxidising

agent which easily reduced to its metal. It is used as a red pigment in

– oil paints,especially as a first coating for structural steelworks,

– putty with linseed oil for joints in pipes and plates,

– making glass and pottery-glazes, and matches

• Like tin oxide, Lead (II) oxide is also an amphoteric oxide. When react • Like tin oxide, Lead (II) oxide is also an amphoteric oxide. When react

with acid or base,

In acidic solution : PbO (s) + 2H+ (aq) Pb2+ (aq) + H2O (l)

In alkaline solution : PbO (s) + 2OH- (aq) + H2O (l) [Pb(OH)4]2- (aq)

Tetrahydroxyplumbate(II) ion

• Lead (IV) oxide is also an amphoteric oxide

In acidic solution : PbO2 (s) + 4 H+ (aq) Pb4+ (aq) + 2 H2O (l)

Cold, concentrated

In alkaline solution : PbO2 (s) + 2 OH- (aq) PbO3

2- (aq) + H2O (l)

Hot, concentrated Plumbate (IV) ion

3.4 Group 14 Chlorides

• All the Group 14 elements form

tetrachloride with formula MCl4

and have a tetrahedral structure.

• The MCl₄ are simple molecules which held together by weak van der

waals forces.Hence,all the compounds have low boiling points and are

liquids under room conditions.

• The stability of the MCl₄ compounds decreases down the group as the M-

Cl covalent bond becomes weaker when the atom becomes larger. This is

₄

Cl covalent bond becomes weaker when the atom becomes larger. This is

due to the bond energy decreases when the bond length becomes longer.

• The inert pair effect of bigger atoms like lead gives rise to stable +2

oxidation states. The electrons in the s orbitals show extra stability and are

harder to be removed. Ions like Pb²⁺ are stable compared to its Pb4+ ions.

• Tetrachloromethane or carbon tetrachloride,CCl₄,is very stable to heat

whereas lead (IV) chloride, PbCl₄, (yellow liquid) is unstable and will

slowly decompose to lead (II) chloride (white solid) and chlorine gas, even

at low temperatures.

PbCl4� PbCl2 + Cl2

3.4.1 Physical properties and thermal stability of Group 14 chloride

• All Group 14 chlorides are liquid at room temperature. They are mostly

volatile liquid with high vapour pressure

• The melting points of Group 14 chlorides generally increase as the

molecular mass increase. Though, carbon tetrachloride shows greater

melting point than most of the Group 14 chlorides due to the short bond

Tetrachloride CCl4 SiCl4 GeCl4 SnCl4 PbCl4

Melting points (oC) -23 -70 -50 -33 -15

melting point than most of the Group 14 chlorides due to the short bond

length between C–Cl, which required a higher temperature is required to

overcome the forces of attraction between the CCl4 molecules.

• MBr4 or MI4 does not exist as ………………………………………………

………………………………….....................................................................

• When goes down to Group 14, the bond length of M – Cl increase as the

atomic size of group 14 increase. This will causes the covalent bond to be

weakened and decrease the stability.

the atomic size of Br and I are too big for

Carbon. So the product formed will not be stable. (steric hindrance)

• CCl4, SiCl4 and GeCl4 are stable at high temperature and does not

decomposed easily. Tin (IV) chloride, SnCl4 and lead (IV) chloride, PbCl4,

on the other hand, decomposed when heated

SnCl4 (l)

PbCl4 (l)

3.4.2 Hydrolysis of Group 14 chloride

• All Group 14 chloride can undergoes hydrolysis except carbon

tetrachloride, CCl4.

• Hydrolysis of Group 14 chloride involves the attachment of water

molecules in empty d-orbital first before the Cl is removed from the Si–Cl

bond. Then, the hydrogen atom from the water attract the chlorine atom

from SiCl4 away and left oxygen atom attached to silicon to form silicon

(IV) oxide

3.4.4 Freons

• Freon is a trade name given to a group of chlorofluorocarbon compounds

normally known as CFC.

• Freon is formed when one or more chlorine atoms from the

tetrachloromethane, CCl₄, is/are substituted by one or more fluorine

atom/atoms.

• The common freons produced are freon-11, freon-12, freon-13, and freon-

14. When only one chlorine atom is substituted by the fluorine atom, the

compound formed is known as trichlorofluoromethane, CFCl₃ or freon-11.

Freon-12 refers of two substitution of the chlorine atoms in CCl₄ to form

₂ ₂ ₂ ₃ ₃

₂ ₄ ₂

₃

Freon-12 refers of two substitution of the chlorine atoms in CCl₄ to form

CF₂Cl₂. Same as freon-11 and freon-12, freon-13, C₂F₃Cl₃ and freon-14,

C₂F₄Cl₂, are all volatile liquids , readily liquefied, relatively inert, non-

toxic and non-combustible.

• They are used as coolants in refrigerators and air conditioners. Large

quantities of CFC are also used as aerosol propellants in spray cans like

hair spray and insecticide.

• The harmful and destructive effects of these chlorofluoro compounds on

the ozone layer in the stratosphere have attracted much study on these CFC

compounds since the mid 1970s

• The harmful and destructive effects of these chlorofluoro compounds on

the ozone layer in the stratosphere have attracted much study on these CFC

compounds since the mid 1970s

• In the stratosphere, the UV radiation has a wavelength of 175-220nm that

is strong enough to dissociate the CFC compounds

CFCl3 (g) � CFCl2• + Cl•

• The chlorine radical, Cl●, is very reactive and will continue to attack the

ozone molecules in the stratosphere

Cl• + O3� ClO • + O2 ……………………….. (1)3 2

ClO• + O � Cl• + O2 ……………………….. (2)

• Although the ClO• species acts only as an intermediate, its presence is

greatly responsible for the destruction and hence the depletion of the

ozone layer

• The ozone layer absorbs the harmful ultraviolet (UV) radiation from the

sun so that only a minimal amount penetrates through to reach the

atmosphere. Too much UV radiation is harmful as it causes skin cancer and

damages vegetation

3.5 Carbon composite in industry

• Composite compounds are formed when two or more materials are mixed

or combined together to give improved qualities over the original

substance. For example, substances like metal or alloy , ceramic and

polymer when combined together will form composites that display all the

good properties of the components

• Some common composite substances used in everyday like are concrete,

high fibre plastics, superconductors and photochromatic glass.

• Composite compound is one type of solid that does not have a crystal

lattice. Other such solid is the amorphous solid which include carbon black, lattice. Other such solid is the amorphous solid which include carbon black,

glass and polymer.

• Carbon composites are formed when amorphous carbon is heated to a high

temperature to form graphite fibres. This fibre then interwined with plastics

to form a structure which is strong, stretchable and yet chemically stable

known as carbon composites

• Kevlar is one of the synthetic graphite fibres that is considerable

importance in the plastic industry. It has high tensile strength and is more

modulus than fibreglass. Kevlar is used for structures that require stiffness,

high abrasion resistance and lightweight. Example of uses are Kevlar ropes

used as lightweight boat hulls such as in canoes and aircrafts, and also

canvas for tents.

• Kevlar maybe be used with epoxy or vinyl ester resin (polymer) as in

fabrics for high performance reinforcement in automotive and marine

applications. The firemen suits and the bulletproof jackets are all made of

this type of material.this type of material.

Other composite compounds

• Concrete is another composite of Group 14 elements which is a mixture of

cement, sand, brick and water. Concrete is hard but brittle and is able to

withstand heavyweights. The structure of concrete can be further

strengthened by laying steel wires to hold the components cement, sand,

and brick closer together.

• Plastics, if strengthened by adding fiberglass to its structures, form a

composite material that has broad uses in the building industry and in

making sports equipment because composite material is light yet strong.

• Photochromatic glasses are formed when photochromatic chemicals like

silver chloride are added to transparent glass (silicon dioxide and silica ).

This type of glass turns darker under sunlight and will revert back to its

colourless state when out of the sun. This special property makes

photochromatics glasses suitable for making sunglasses as they protect

eyes from ultraviolet rays of the sun. Window screens of cars and glass

window of residential houses also make use of this property of

photochromatic glass to absorb unwanted rays so that the interior of cars

and houses remain cool.

• Ceramics and a few metal compounds are able to conduct electricity with

negligible resistance when cooled to a certain temperature. These are

superconductors and are normally used in high technology facilities like

the bullet train and some modern medical appliances.

3.6 Silicon and Silicates

• Silicon exists naturally in the Earth’s crust as - silicates,aluminosilicates

and silica.

• Basic structural unit of silicates, SiO44- (Silicon atom is surrounded by 4 O

atoms to form a tetrahedron).

• Silicon uses sp3 hybridisation in its bonding with oxygen.

• Different ways of arrangement will result in rings, single and double

chains, sheets and 3-D network.

• Basic understanding to define silicates structures:• Basic understanding to define silicates structures:

(a) All structures are constructed from SiO4 tetrahedral.

(b) The tetrahedral are joined to each other by corners only.

(c) Resulting charges on the SiO44- anion depends on the number of

corners shared

• Most basic - consist only one simple SiO 4- unit.• Most basic - consist only one simple SiO44- unit.

• The 4 electrons needed to complete the octet are obtained from metal

atoms (like Mg2+ and Ca2+) and give the anion its quadruple negative

charge. Examples:

(a) zircon,ZiSiO4

(b) olivine, Mg2SiO4

(c) garnet,Ca2Al2(Si3O9)

Different Type of Silicates

Pyrosilicates Ring silicates / Cyclosilicates

It’s formed when 2 SiO44- unit joined

together using an oxygen atom as

the bridging atom.

Also known as soro-silicates or

disilicates.

Examples:

(a) thortveitite,Sc2Si2O7.

(b) barycilite,Pb Si O

It’s formed when 3 basic SiO44 -

units joined together by sharing 2

of its oxygen atoms in a compact

manner.

Unit cell - (SiO3)n2n- unit.

Examples:

(a) bentonite,BaTiSi3O9.

(b) beryl, Al Be Si O(b) barycilite,Pb3Si2O7 (b) beryl, Al2Be3Si6O18

3.6.1 Single Chain Silicates/Pyroxenes

• It’s formed when each SiO44- tetrahedral unit sharing 2 of its oxygen

atoms with the next unit to form a long chain of (SiO3)n2n-. 2 pyroxene

chains joined together by the corners will form amphiboles, with anionic

structure [(Si4O11)6-]n. structure : 2-dimensional. Si-O bond is strong

within the chain and weak between the chains. Hard but brittle (bonds

between the chains can easily be broken).

• Asbestos is an important amphibole.

– Resistant towards heat and fire, inert towards attacks from acids and

alkalis, fibrous (amphiboles sheets are separable),and not malleable.alkalis, fibrous (amphiboles sheets are separable),and not malleable.

• Tremolite is also an amphibole with formula Ca2Mg5(Si4O11)2(OH)2.

– can be cleft into strands,which is separated into thin fibres and later on

woven into cloth.

– *layered structure contributes to its hardness and brittleness

• Single chain in pyroxenes

• Double chain in amphiboles

3.6.2 Silicate sheets

• Silicate It’s formed when 3 corners of the SiO44- unit are shared by 3 other

units to form a 2-dimensional infinite sheet.

• Unit formula:(Si2O5)n2n- - found in clay substances like talc, mica and

kaolinite.

• Pyrophyllite is forrmed when 2 layers of SiO4 tetrahedral and 1 layer of

AlO6 octahedral are linked together in a ratio of 2:1.

– ideal composition:Al2Si4O10(OH)2.

• Talc is formed if the 2 aluminium ions in pyrophyllite are replaced by 3 • Talc is formed if the 2 aluminium ions in pyrophyllite are replaced by 3

magnesium ions.

– formula: Mg3Si4O10(OH)2.

– electrically neutral.

– it’s a giant molecule with strong covalent properties.

– The molecules in talc are arranged in parallel sheets giving the rise to

flaky textures

Mica/Aluminosilicates

• consists of anion sheets held together by cations (K+ or Mg2+) present in

between the sheets.

• Negative charge among the layers appears because 1/4 of the silicon ions

in pyrophyllite is being replaced by an aluminium ion-[Al3Si3O10(OH)2]-

• Example:muscovite (KAl2(OH)2[AlSi3O10]5-) *also known as white mica

Kaolin

• higher quality kaolinite (clay soils).• higher quality kaolinite (clay soils).

• It’s formed when some SiO2 units are removed from micas,

[Al3Si3O10(OH)2]-,so that one layer of SiO4

• tetrahedral share corners with one layer of AlO6 octahedral to form a ratio

of 1:1 ,Al2Si2O3(OH)4

• Clay is formed when it’s mixed with water.-making porcelain vases, cups,

china plates and white ceramic tiles.It can also be used to treat

indigestion

Montmorillonite

• Molecular formula: (Mg0.33Al1.67)(OH)2(Si2O5)20.33-.

• Consists of identical sheets.

• If one of the aluminium ion in it is replaced by one magnesium ion, the

resulting unit cell will have a positive charge which can be balanced out

by absorbing different ions into its structure.

• Forms semi-solid gel in the presence of water.

• Also acts as a cation exchange.(e.g. to exchange one mole of Na+ with

one mole of H+ so that the surface becomes acidic, one mole of one mole of H+ so that the surface becomes acidic, one mole of

montmorillonite is required.

• At high temperature(150oC), montmorillonite loses its water molecules

and acidity. The layered structure of its silicate will collapse. Because of

this, zeolite is preferred to montmorillonite in industries especially when

the reactions required high pressure and temperature

3.6.3 Three – Dimensional Silicates / Framework Structure

• It’s formed when all the 4 angles of the silicate anion unit are shared by 4

other units to form 3-D lattice.

• It’s found in quartz and cristobalite.

• Basic formula: (SiO2)n with neutral lattice.

• Quartz is regarded as a giant covalent molecule.

• Besides quartz,all the other members are aluminosilicates because of the

presence of aluminium atoms in their structures.

• When silicon atoms are subsituted by aluminium atoms, the resulting • When silicon atoms are subsituted by aluminium atoms, the resulting

structure is a giant anion because of the difference in charges (Al3+and

Si4+).

• If 1/4 of the silicon atoms in 4 SiO2 is replaced by an aluminium atom, a

big anion with formula [AlSi3O8]- is formed.

• The negative charge will be neutralised by cations as in orthoclase,

K(AlSi3O8).

3.6.4 Silicones

• A group of polyorganosiloxanes containing the Si-C bond in long chains

of -Si-O-Si- structures with

• alkyl or aryl groups attached to the silicon atoms.

• It’s a long chain polymer and the structure varies when the conditions of

reaction (hydrolysis and polymerisation)change.

– includes forming cross-links between the polymer chains by different

alkyl groups.

– The type of alkyl groups attached can be determined by the types of – The type of alkyl groups attached can be determined by the types of

Grignard reagents used.

– Polymers with cross-link bridges are harder and withstand heat

better than single chain silicones.

• Silicones are prepared from silicon tetrachloride and methylmagnesium

iodide (Grignard reagent) in ether

Step 1 : Reaction of silicon tetrachloride with Grignard reagent

SiCl4 (l) + 2 CH3–MgI (l) �Si(CH3)2Cl2 + 2 MgClI

Step 2 : Dimethyldichlorosilane then undergoes hydrolysis to form hydroxyl Step 2 : Dimethyldichlorosilane then undergoes hydrolysis to form hydroxyl

compound Si(CH3)2Cl2 (l) + 2 H2O � Si(CH3)2(OH)2 + 2 HCl

Step 3 : Product of hydrolysis undergoes condensation polymerisation to

displace water molecules

Application of silicones

• It’s non-toxic, colourless, odourless and resists to physical and chemical

changes caused by heat, cold and pressure.

• Has water repellant properties and is an electric insulator.

• Properties can be modified by attaching different organic groups to the

main structure framework

• Silicone fluids- is used as lubricant in areas with low temperature as well as

in hot areas.It’s used over waterproofing fabrics and in polish for cars and

furniture.furniture.

• Silicon rubbers- can be converted to elastomers by vulcanisation and cross-

linking of the chains.

• Resins- insulation in electric motors and transformers.

• It’s added to paints to increase the resistance against weather tear and heat.

• Form lacquers for coating frying pans so that food do not stick on the

surface.

• It’s also used as grease for lubricating glass tubes and burettes heads

3.7 Glass

• Solids are most stable in crystalline form. However, if the solid is formed

rapidly ( like sudden cooling of crystals), the crystals formed will not have

a regular lattice. This type of solid is known as amorphous or non-

crystalline.

• Glass is an amorphous solid as it does not have a regular three-dimensional

arrangement of atoms. Glass is commonly referred to an optically

transparent fusion product of inorganic materials cooled rapidly to a rigid

state without undergoing crystallisation.

• Fusion – process of mixing molten silicon dioxide, with its supplementary • Fusion – process of mixing molten silicon dioxide, with its supplementary

compounds such as sodium oxide, boron oxide or certain transition metals

for colours and other properties.

• The main component of glasses is silicon dioxide, or simply sand. Other

components are boron oxide, aluminium oxide and others are only

supplementary additives, added to change the properties of the glasses

produced. Some important glasses with special purpose are listed below

• Quartz glass = silicon oxide SiO2. It has a low coefficient of thermal

expansion but its important property lies in its transparency to a wide range

of wavelengths. It is used in mirrors, glass rods and quartz lenses

• Pyrex = borosilicate glass = boron + silicon oxides which give the glass a

rather high coefficient of thermal expansion (more resistance towards

thermal expansion when heated or cooled quickly). As such, the glass will

not crack when heated to a high temperature or break when contracted in a

freezer. This type of glass is used as test tubes and boiling flasks in the

laboratory as well as cooking wares in the kitchen

• Soda glass = sodium silicate + calcium silicate. Soda glass is passive to

attack from chemical substances and is very brittle. Bottles made from

soda glass are used to store non-reactive drinks and sauces like cordials

and soya sauces. Since soda glass has a low melting point, it can be

moulded into desired shapes and sizes. Bulbs, window-panes, drinking

glasses, some cups and plates are made of soda glass. Products made from

soda glass are cheaper and breakable.

• Some laboratory apparatus like microscopic glass plate, glass dish and

glass tubing are all very fragile. The properties of soda glass can be

upgraded by adding some metal oxides like lead oxide or boron oxide to upgraded by adding some metal oxides like lead oxide or boron oxide to

enhance properties like colour and hardness

• Lead glass = flint glass = potassium silicate and lead silicate which

gives the glass a high refractive index. This type of glass is also known as

crystal glass because of its glittering appearance. It is used in decorative

glassware and windows. Lead glass is used in the laboratory as high

refractive lens and prisms for optical studies

• Bio glass = crown glass = phosphorus pentoxide + silicon oxide. It gives

a smaller refractive index than flint glass but can be combined with flint

glass to produce the desired refractive index. It is therefore important as

implants in medical surgery at it has almost the same composition as bones

• Water glass = fusion of sand, silicon oxide and sodium carbonate to

produce sodium silicates which are soluble in water. The molten water

glass is used to preserve eggs and as adhesives

3.9 Uses of Tin

1. This is a white metal with low melting point of 232 C. It is soft, ductile

and malleable. It is widely used to make tin foil and wires.

2. Tin forms several alloys with other metals. For example,

(a) bronze (90% copper, 10% tin) which is used in engines, electrical

fuse and door knobs

(b) pewter (75% tin, 25% lead) used for souvenirs

(c) solder ( 50% tin, 50% lead) used for soldering

3. Pure tin is used in making cans for storing food ( so called canned food ) 3. Pure tin is used in making cans for storing food ( so called canned food )

and tubes for storing toothpaste, creams and medicines.

4. Almost half of the tin produced is used to plate iron or mild steel to

prevent the iron or steel from rusting. Tin plating is used in canned food as

well.

5. Thin sheets of tin are used as wrapping in cigarette boxes