Chemistry

Session Objectives

1. Physical properties of group 13 elements

2. Halides of group 13 elements

3. Hydrides of group 13 elements

4. Oxides of group 13 elements

5. Extraction of aluminum

6. Compounds and uses of aluminum

7. Physical properties of group 14 elements

Elements of group 13

B-Boron = [He]2s22p1

Al-Aluminium = [Ne]3s23p1

Ga-Gallium = [Ar]3d104s24p1

In-Indium = [Kr]4d105s25p1

Tl-Thallium = [Xe]4f145d106s26p1

Abundance and Occurrence of group 13 elementsAluminium is the most abundant metal known and third most abundant element by mass in the earth’s crust.

Boron is a rare element and occurs as concentrated deposits of Borax, Kernite .

Gallium is twice as abundant but indium and thallium are less common.

Gallium M.P. = 29.78oC

Semiconductor ApplicationsGallium Arsenide

Atomic and ionic radii

Element B Al Ga In Tl

Atomic radii (A°) 0.80 1.24 1.24 1.50 1.55

Ionic radii (A°) 0.20 0.52 0.60 0.81 0.95

In case of Ga, In and Tl, there are 10-d electrons in the penultimate shell causing weaker screening of nuclear charge and thus the increase in size is not very large.

Ionisation Energy

B (6887)

Al (5137)

Ga (5520)

In (5082)

Tl (5437)

The ionisation energy values do not decrease smoothly down the group. The decrease from B to Al is the usual trend on descending the group associated with increased size.The poor shielding by d-electrons affect the value for latter elements.

Oxidation States

Maximum oxidation number shown by group 13 elements is +3.Boron and aluminum generally have +3 oxidation state in its compounds.The stability of lower oxidation state (+1 ) increases as we move down the group due to inert pair effect.

The decreasing stability of higher oxidation state is due to the fact that the bond energy decreases from Al to Tl,and the energy required to unpair the ns2 electrons is not compensated by the energy released in forming two additional bonds

Illustrative Example

What will be the order of stability of trivalent cations among Ga, In, Tl?

The correct order will be Ga3+ > In3+ > Tl3+

Solution :

Illustrative Problem

Why there is similarity in atomic radii of Al and Ga ?

Due to poor shielding effect of 3d electrons in Ga,the atomic radii of Al and Ga is almost same.

Solution :

Hydrides of group 13 elements

Forms hydrides of MH3 type.

Thermal stability decreases as we move down the group.

BH3 > AlH3 > GaH3 > InH3 > TlH3

Weak Lewis acids.

Readily form adducts with strong Lewis bases.

Also form tetrahydrido anions, eg. [MH4]-.

Lithium tetrahydrido- aluminate(III), LiAlH4 is most important compound which is obtained by the reaction.

2Et O3 44LiH AlCl LiAlH 3LiCl

LiAlH4 is a white crystalline solid and soluble in diethyl ether and isused as reducing agent in organic chemistry.

The electronic structure of diborane

Three centre two electron bonds.

Halides of group 13 element

With halogens,group 13 elements give binary halides.

Fluorides are ionic and have high melting points.

Chlorides, bromides and iodides are covalent with low melting point.

33MX(s) M(s) M (aq) 3X (aq)

Monohalides, GaX, InX and TlX are known for X= Cl, Br and I because stability of +1 oxidation state increases down the group.

GaX and InX disproportionate in water

F — B

F

F

N — H

H

H

F — B N — H

F

F

H

H

+

sp2 sp3 sp3 sp3

Lewis acid character: BBr3 > BCl3 > BF3.

Halides of group 13 element

BF3 is the weakest lewis acid because it is less electron deficient due to back donation of electrons from F atoms.

Trihalides are strong Lewis bases.

Structure of AlCl3.

Vapour density measurements show that AlCl3 is monomeric over 800°. Structure of AlCl3 is planar triangular and the bonding is covalent. Below 400°C it exists as a dimer. In solid state also it exists as a dimer and is non conducting. Its structure is

Cl

ClAlAlCl

Cl Cl

Cl

AlCl3.6H2O is used as deodorant and antiperspirant.

A

Cl

Cl

Cl

Cl

Al

Cl

Cl

Boron chloride exists as a monomer while in the same group anhydrous aluminium chloride exists as dimer, why?

Illustrative Example

Solution

BCl3 cannot form a dimer because B atom is too small that it is unable to attract four large sized halide ions while Al can do so because of the larger size.

Illustrative Example

What property of anhydrous AlCl3 makes it a very good preparative reagent in organic chemistry?

Solution

Anhydrous AlCl3 acts as a Lewis-acid and accepts lone pair of electrons, which makes it a useful reagent in organic chemistry.

Oxides of group 13 elements

Group 13 elements form oxides of the type M2O3.

Basic character increases as we move from Aluminium to thallium.Only B2O3 shows the acidic nature.

B2O3 + 2NaOH 2NaBO2 + H2O

Al2O3 + 6HCl 2AlCl3 + 3H2O

Al2O3 + 2NaOH 2NaAlO2 + H2O

An element X which occurs in the second period has an outer electronic structure s2p1. What are the formula and acid-base character of its oxides?

Illustrative Example

The element with outer electronic configuration s2p1 belongs to group III in the periodic table.

Eg: B, Al, Ga, In and Tl.

B2O3 acidic

Al2O3

Ga3O2

amphoteric

In2O3 basic

Tl2O3 basic

On moving down the group there is a gradual change from acidic to amphoteric.

Therefore, answer is X2O3, acidic.

Solution

Hydroxides of group 13 elements

3 2 4 2 4 3 2

3 4

2Al(OH) (s) 3H SO (aq) Al (SO ) (aq) 6H O(l)

2Al(OH) (s) NaOH(aq) Na[Al(OH) ](aq)

Aluminium and gallium hydroxides show amphoteric behaviour.

In contrast Tl(OH)3 is insoluble in water and Tl(OH) is strong base.

Many of Tl(I) compounds are similar to corresponding alkali metal compounds.

Group 13 elements form hydroxides of the type M(OH)3

Chemical properties of group 13 metals

Note that thallium is the most reactive element in this family, and is the only one that reacts directly with liquid water.Note also that aluminum and gallium are amphoteric.

Reactant Reaction with group 13 elements (M)

OxygenNitrogenHalogen(X2) Water Acid  Base

4M(s) + 3O2(g) 2M2O3(s)2M(s) + 3N2(g) 2MN(s), M=B, Al2B(s) + 3X2(g,l,s) 2BX3(g)2M(s) + 3X2(g,l,s) M2X6(g), M=Al, Ga, In2Tl(s) + 3X2(g,l,s) 2TlX(s)2Tl(s) + 2H2O(l) 2Tl(OH)(aq)+H2(g)2M(s)+6H3O

+(aq)2E3+(aq)+6H2O(g)+3H2(g) M=Al, Ga, Tl2M(s)+6H2O(l)+2OH- 2E3+(aq)+6H2O(g)+3H2(g) M=Al, Ga,

Aluminum: Al

KAl(SO4)2•12H2O

Identified in 1807 it was not isolated until 1828.

Wöhler

AlCl3 + 3K Al + 3KCl

Important minerals areBauxite, Al2O3.xH2O where x=1 to 3;Cryolite, Na3AlF6

Orthoclase, KAlSi3O8

Mica (muscovite), KAl2(Si3AlO10)(OH)2;Beryl, Be3Al2Si6O18; andCorundum, Al2O3

Third most abundant element; 8.3% in earth crust after oxygen.

Only commercial source is bauxite, a hydrated impure oxide.

Alumina

Ruby

Cr3+ in place of some Al ions.

Sapphire

Fe3+ and Ti4+ impurities.

Topaz

With Fe3+ impurities.

Alumina is white but it can be coloured by the addition of impurities

Alumina exists in two crystalline forms –Al2O3 or corundum and Al2O3

Extraction of aluminium

1.Purification of bauxite

2.Electrolysis of alumina

Purification can be done in three ways.

1.Baeyer’s process

2.Hall’s process

3.Serpeck’s process

Baeyer’s process

Al2O3 2H2O+2NaOH

Bauxite ore

2NaAlO2 + 3H2O

Sodium aluminate

Roasted ore is digested with conc. NaOH underpressure at 150°C which forms sodium meta aluminate (NaAlO2).

Ferric oxide and silica are removed by filtration.

Baeyer’s process

ppt Fe(OH)3Precipitated Al(OH)3.

Al(OH)3 Al2O3 + 3H2O Heat

The NaAlO2 solution is agitated with freshly precipitated Al(OH)3

NaAlO2 undergoes hydrolysis with the formation of Al(OH)3 precipitate.

2 2 3NaAlO 2H O Al(OH) NaOH

Hall’s process

2 3 2 3 2 2Al O Na CO 2NaAlO CO

Bauxite, is fused with Na2CO3 to form NaAlO2.

2 2 2 2 332NaAlO CO 3H O 2Al OH Na CO

NaAlO2 is warmed to 50°-60°C and CO2 is circulated through it. Al(OH)3 is separated as precipitate.

The fused mass is extracted with water where Fe2O3 and SiO2 remain as insoluble residue.

Al(OH)3 Al2O3 + 3H2O Heat

The precipitate is removed by filteration,washed and ignited to ger alumina

Serpeck’s process

AlN is hydrolysed with water into Al(OH)3 which when ignited ,gives alumina.

This process is used when silica is present in considerable amount in bauxite ore. The ore is mixed with coke and heated at 1800° C in presence of nitrogen, where AlN (aluminium nitride) is formed. Silica is reduced to silicon which volatilises off at this temperature

2 3 2Al O 3C N 2AlN 3CO

2SiO 2C Si 2CO

2 3 3AlN 3H O Al(OH) NH

Hall-Heroult Process

2Al2O3 + 3C 4Al(l) + 3CO2(g)

He was only 23!!

3

2

22

Cathode : Al (melt) 3e Al(l)

Anode : C(s) O (melt) CO(g) 2e

C(s) 2O (melt) CO (g) 4e

Role of cryolite in electrolysis

Cryolite Na3AlF6

Cryolite improves the electrical conductivity of the cell as Al2O3 is a poor conductor.In addition , the cryolite serves as an added impurity and lowers the melting point of the mixture to about 1140K

Properties of aluminum

Metallic, strong, excellent electrical conductor, strongly reducing, low density.

Aluminum reacts with both acids and bases.

Al + 6HCl 2AlCl3 + 3H2

Al + 2NaOH +H2ONaAlO2 + H2

Chemical properties

1.Action of air

Reactive metal but becomes unreactive due to formation of oxide film.

heat2 2 34Al 3O 2Al O

2. Reaction with water:

Not attacked by pure water but easily corroded by saline water.

3. Reaction with metallic oxides: Thermite reaction

Has strong affinity for oxygen, so reduces the oxide of metals. The reaction is exothermic and can be used for welding.

heat2 3 2 3

heat2 3 2 3

Fe O 2Al 2Fe Al O

Cr O 2Al 2Cr Al O

Uses of aluminum

aluminium is used for because

aircraft light, strong, resists corrosion

other transport such as ships' superstructures, container vehicle bodies, tube trains (metro trains)

light, strong, resists corrosion

overhead power cables (with a steel core to strengthen them)

light, resists corrosion, good conductor of electricity

saucepanslight, resists corrosion, good appearance, good conductor of heat

Alums

2 6 2 6 4 2General formula is [M(H O) ][Al(H O) ](SO )

where M is singly charged cation.

Potash alum is used as a mordant in dyeing and printing of textiles.

Alums are used for softening of hard water.

Anomalous behaviour of the first element of a group.

i. Small sizeii. high electronegativityiii. non-availability of d-orbitals.

Anomalous behaviour

C – Carbon = [He] 2s2 2p2

Si – Silicon = [Ne] 3s2 3p2

Ge – Germanium = [Ar] 3d10 4s2 4p2

Sn – Tin = [Kr] 4d10 5s2 5p2

Pb – Lead = [Xe] 4f14 5d10 6s2 6p2

Elements of group 14

Atomic Radii and Atomic Volume

C Si Ge Sn Pb

Atomic radius (A°) 0.77 1.11 1.22 1.41 1.44

Atomic volume (ml) 3.4 11.4 13.6 16.3 18.27

Radii and volume increase gradually on moving down the group.

Ionisation Energy

The ionisation potential decreases gradually from carbon to lead but not systematically. The value of lead is slightly higher than expected due to lanthanide contraction.

C SI Ge Sn Pb

I.P. (eV) 11.2 8.1 7.8 7.3 7.4

Oxidation state

Oxidation state: +4

Due to inert pair effect Sn and Pb exhibit +2 state

4 4 4Ge Sn Pb

2 2 2Ge Sn Pb

Order of stability for +2 oxidation state is

Self linking property of an element to form chains of covalent bonds with atoms of the same elements.

C >> Si >> Ge Sn > Pb

Catenation

Allotropy: Except Pb, all others show allotropy.

Carbon: diamond, graphite, charcoal.

Allotropy

Si and Ge: Crystalline and amorphous forms.

Sn: -Sn(Grey) , -Sn(white)

Known since 3000B.C.

-Sn -Sn

“white” tin Metallic in appearance. Tetragonal structure. Highly crystalline.

“grey” tin crumblyCubic structure

Allotropy

Illustrative Problem

What is the percentage of lead in lead pencil ?

Zero

Solution :

Illustrative Example

SiF6 2– is known but SiCl6 2- is not. Why is it so?

SiCl6 2–is not known due to following reasons(i) due to smaller size of F,steric repulsions will be less in

SiF6 2-.

(ii) Interaction of lone pair of electrons of F with Si is stronger than that of lone pair electrons of Cl.

Solution :

Thank you