Post on 24-Dec-2015
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