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p-BLOCK ELEMENTS - BORON FAMILY

ELEMENTS OF GROUP 13

B -  Boron Al - Aluminium Ga - Gallium
In - Indium Tl - Thallium
Amongst the elements of this group, Al is the third most abundant element found in the earth’s crust after oxygen and silicon.

GENERAL CHARACTERISTICS

Physical properties of boron family are:-

ELECTRONIC CONFIGURATION

The elements of group 13 belong to p-block of the periodic table and these elements contain three electrons in the valence shell, therefore, their valence shell electronic configuration is ns2np1.
Element
At. No.
Electronic Configuration.
Valence Shell Configuration.  
B
5
[He] 2s2, 2p1
2s22p1
Al
13
[Ne] 3s2, 3p1
3s23p1
Ga
31
[Ar]3 3d10, 4s24p1
4s24p1
In
49
[Kr] 4d10, 5s25p1
5s25p1
Tl
81
[Xe] 4f14, 5d10, 6s2p1
6s26p1

ATOMIC RADII AND IONIC RADII

  • Atoms and their ions of group 13 elements have smaller size than those of alkaline earth metals of group-2, due to greater nuclear charge of former group than latter group.
  • Atomic radii increase on going down in the group with an abnormally at gallium and the unexpected decrease in the atomic size of Ga is due to the presence of electrons in d- orbitals which do not screen the nucleus effectively.
  • The ionic radii regularly increases from B3+ to Tl3+

DENSITY

It increases regularly on moving down the group from B to Tl

MELTING AND BOILING POINTS

  • M.P. and b. p. of group 13 elements are much higher than those of group 2 elements
Element
B
Al
Ga
In
Tl
M.p. (K)
2453
933
303
430
576
B.p. (K)
3923
2740
2676
2353
1730
  • The m.p. decreases from B to Ga and then increases, due to structural changes in the elements
  • Boron has a very high m. p. because of its three dimensional (B12- icosahedral) structure in which B atoms are held together by strong covalent bonds.
  • Low m. p. of Ga is due to the fact that it consists of only Ga2 molecules, and Ga remains liquid upto 2273K therefore it is used in high temperature thermometry.

IONISATION ENERGY

  • The first I.E. values of group 13 elements are lower than the corresponding values of the alkaline earth metals, due to the fact that removal of electron is easy in former case (p-electron) than latter (s-electron).
This results in the increase of nuclear charge. Consequently the valence electrons are more tightly held leading to high I.E. Similarly we can explain the irregularity in case of Tl on the basis of ineffective shielding of intervening electrons
  • On moving down the group IE, decreases from B to Al but the next element Ga has slightly higher IE, than Al, it again decreases in In and increases in the last element Tl as follows :
Element
B
Al
Ga
In
Tl
IE, (kJ mol–1)
800
577
578
558
590
  • The irregularity observed in case of Gallium is due to the ineffective shielding of nuclear charge because of intervening d electrons, which cause the increase in nuclear charge leading to high I.E.

INERT PAIR EFFECT

  • It is the reluctance of the s-electrons of the valence shell to take part in bonding and occurs due to ineffective shielding of the ns2 electrons by the intervening d and f electrons.
  • It increases down a group and thus the lower elements of group show lower oxidation states.

OXIDATION STATES

  • B and Al show an oxidation state of +3 only while Ga, In and Tl show oxidation states of both +1 and +3.
  • As we move down in the group 13, due to inert pair effect, the tendency to achieve +3 oxidation state goes on decreasing and the tendency to acquire +1 oxidation state goes on increasing.
  • Stability of +1 oxidation state follows the order Ga < In < Tl
  • Tl+ compounds are more stable than Tl3+ compounds.

ELECTROPOSITIVE (METALLIC) CHARACTER

  • These elements are less electropositive than the elements of the group 1 and 2 due to their smaller size and higher ionisation energies.
  • On moving down the group, the electropositive character first increases from B to Al and then decreases from Ga to Tl, due to the presence of d and f orbitals in latter elements.

REDUCING CHARACTER

It decreases down the group from Al to Tl because of the increase in electrode potential values for M3+/M. Therefore it follows the order.
Al > Ga > In > Tl
Element B
Al
Ga
In
Tl
Eº (V) at 298 K for M3+/M
–1.66
–0.56
–0.34
+1.26

COMPLEX FORMATION

Due to their smaller size and greater charge, these elements have greater tendency to form complexes than  the s-block elements.

NATURE OF COMPOUNDS

  • The tendency of formation of ionic compounds increases from B to Tl.
  • Boron forms only covalent compounds where as Al can form both covalent (e.g., anhydrous AlCl3) and ionic compounds (e.g., hydrated AlCl3) because when anhydrous AlCl3 is hydrated, the hydration energy released is sufficient to overcome the ionisation energy of Al.
  • Gallium forms mainly ionic compounds although anhydrous GaCl3 is covalent.

CHEMICAL PROPERTIES

Crystalline B is unreactive whereas amorphous B is reactive. It reacts with air at 700ºC as follows :
  • 4B + 3O2 2B2O3;
2B + N2 2BN
  • Al is stable in air due to the formation of protective oxide film.

Some properties are given below

ACTION OF AIR

  • 4Al + 3O2 2 Al2O3
  • Thallium is more reactive than Ga and In, due to the formation of unipositive ion Tl+.
    4Tl + O2 2 Tl2O

REACTION WITH NITROGEN

The elements of group 13 form their corresponding nitrides with the reaction of nitrogen. These nitrides undergo hydrolysis with steam and evolve NH3

ACTION OF WATER

  • Both B and Al do not react with water  but amalgamated aluminium does react with H2O evolving H2.
2Al(Hg)  + 6H2O   2Al(OH)3 + 3H2 +Hg
Aluminium amalgam
  • Ga and In do not react with pure cold or hot water but Tl forms an oxide on the surface.

REACTION WITH ALKALIES

  • Boron dissolves in alkalies on fusion and gives borates.
2B + 6NaOH 2Na3BO3 + 3H2
  • Al and Ga dissolve in concentrated alkalies on heating and form meta-aluminate and gallate respectively.
Al + 2NaOH + 2H2O   2NaAlO2 + 3H2
Sod. meta aluminate
Ga  + 2NaOH + 2H2O 2NaGaO2 +3H2
Sod. gallate

REACTION WITH CARBON

  • B and Al form carbides with carbon on heating. ;      
  • Aluminium carbide is ionic and forms methane with water, while boron carbide is covalent having molecular formula B12C3 and it is very hard, hence used as an abrasive.

HYRIDES

  • Elements of group 13 do not combine directly with H2 to form hydrides, therefore, their hydrides have been prepared by indirect methods for example.
BF3 + 3LiBH4    2B2H6 + 3LiF
diborane
  • Boron forms a number of hydrides which are called boranes with general formula BnHn+4(e.g., B2H6) and BnHn+6 (e.g., B4H10)
  • Boranes catch fire in the presence of oxygen with the evolution of heat energy
B2H6 + 3O2 B2O3  + 3H2O ; H = – 2008 kg/mol
  • Boranes are hydrolysed by water as follows :
B2H6 + 6H2O 2H3BO3 + 6H2
  • Boranes are quite stable but the stability of hydrides of Al, Ga, In, and Tl decreases on moving down the group because the strength  of the M–H bond decreases with increasing size of the element.
  • Structure of diborane : The simplest boron hydride, i.e. BH3 does not exist as such but exists as a dimer, B2H6, having following structure
 
  • In above structure B atoms are in sp3 hybrid state. There are six B-H bonds out of which four B-Ha bonds are normal covalent bonds (two centre electron - pair bonds i.e, 2c–2e) present in the same plane while rest two B-Hb bonds behave as bridges containing three centre two electron pairs bonds i.e., 3c–2e (known   as banana bonds) and present above and below the plane of the molecules which do not have sufficient number of electrons to form normal covalent bonds are called electron - deficient molecules. Ex. B2H6.
  • Aluminium forms a polymeric hydride of general formula (AlH3)x which decomposes into its elements on heating.
  • B, Al and Ga form complex anionic hydrides, like lithium borohydride, Li[BH4], lithium aluminium hydride , Li[AlH4] etc. due to the presence of a vacant p-orbital in their outermost shells which accepts electron pair from H ion. XH3 + H [XH4] where X= B, Al and Ga.

OXIDES

  • Except Tl all the elements of group 13 form oxides (general formula, M2O3) on heating with oxygen
4M + 3O2 2M2O3 (M = B to In)
  • Tl forms thallium (I) oxide, Tl2O which is more stable than thallium  (III) oxide, Tl2O3 because of inert pair effect.
  • B2O3 and Al2O3 can also be prepared by following method :
2Al(OH)3 Al2O3 + 3H2O
4Al(NO3)3 2Al2O3 + 12NO2 + 3O2
  • The hardest alumina, a–Al2O3, which is stable at high temperature, resistant to hydration and attacked by acids, is called corundum and is used as an abrasive where as –Al2O3 is known as activated alumina and used in column chromatography.

HYDROXIDES

Amalgamated aluminium reacts with H2O and forms its hydroxide.
2Al(Hg)         + 6H2O Al(OH)3 + 3H2 + Hg
Aluminium amalgam
However, B(OH)3 can also be prepared by the reaction of B2O3 and H2O
B2O3 + 3H2O 2B(OH)3 + 2H3BO3

NATURE OF OXIDES AND HYDROXIDES

  • B(OH)3 or H3BO3 is soluble in water, whereas all other hydroxides are practically insoluble in water.
  • On moving down the group, there is a change from acidic to amphoteric and then to basic character of oxides and hydroxides of group 13 elements.

PREPARATION OF HALIDES

  • All the elements of boron family (except thallium which forms thallous monohalides ) form trihalides of type MX3 where X= F, Cl, Br and I.
B2O3 + 3C + 3Cl2 2BCl3 + 3CO
Al2O3 + 3C + 3Cl2 2AlCl3 + 3CO
  • All the boron trihalides, BX3 and aluminium trihalides AlX3 (except AlF3 which is ionic) are covalent compounds whereas former exist as only monomers and latter as dimers, because boron atom is too small to coordinate with four large halide ions and in case of much smaller F ion , the energy released during the formation of the bridge structure is not sufficient for the cleavage of the typical  p-p bond in BF3.
       BX3
Al2X6
  • BF3 is a colourless gas, BCl3 and BBr3 are colourless fuming liquids whereas BI3  is a white fusible solid at room temperature.
  • The covalent character of trihalides decreases on moving from Ga to Tl.
  • Hybridisation of Boron in BCl3 is sp2

NATURE OF TRIHALIES

  • Trihalides of group 13 elements behave as lewis acids due to having a strong tendency to accept a pair of electrons.
  • The relative strength of lewis acids of boron trihalides increases in the order :
BF3 < BCl3 < BBr3 < BI3.
  • The halides of group 13 elements behave as lewis acids and the acidic character decreases as follows:
BX3 > AlX3 > GaX3 > InX3 (where X=Cl, Br or I)
  • BF3 and anhydrous AlCl3 are used as a catalyst in Friedel Crafts reactions.
  • TlCl3 decomposes to TlCl and Cl2 above 40ºC and hence acts as an oxidising agent, whereas TlBr3 converts into Tl[TlBr4] at room temperature.
TlCl3 TlCl + Cl2 ;    2TlBr3Tl[TlBr4] +Br2
While TlI3 is an ionic compound containing Tl(I) and   ions.

ANOMALOUS BEHAVIOUR OF BORON

Boron shows anomalous behaviour as compared to other member of the group, due to the following reasons :
  • It has smallest size in the group.
  • It has high ionisation energy.
  • It has highest electronegativity in the group.
  • It does not have any vacant d-orbital in valence shell

A few points of difference are :
  • It is a non-metal while other members of the group are metallic in character.
  • It shows allotropy as crystalline and amorphous while other members do not.
  • It has the highest m. p. and b.p. amongst the elements of group 13.
  • It forms only covalent compounds while other members form both ionic and covalent compounds.
  • It forms a number of hydrides which are quite stable while those of other members are less stable.
  • The halides of boron exist as monomers while AlCl3 exists as a dimer.
  • The oxides and hydroxides of boron are weak acidic while those of aluminium are amphoteric and those of other elements are basic.
  • It does not react with steam but other elements decompose steam.
  • It can be oxidised by concentrated HNO3 while aluminium is passive due to the formation of oxide layer on the surface. 2B +6HNO3      2H3BO3 + 6NO2
Boric acid

DIAGONAL RELATIONSHIP BETWEEN BORON AND SILICON

Boron shows resemblance with its diagonal element silicon of group 14. Some of the important points are given below :
  • Both B and Si are non- metals.
  • Both are semiconductor.
  • Both do not form ions, i.e., B3+ and Si4+.
  • Both B and Si form covalent hydrides : boranes i.e., B2H6, B4H6 etc. and silanes, i.e., SiH4, Si2H6 etc. respectively, which catch fire when exposed to air.
B2H6 + 3O2 B2O3 + 3H2O
SiH4 + 2O2 SiO2 + 2H2O
  • Both form covalent, and volatile halides which fume in moist air due to release of HCl fumes.
BCl3 + 3H2O H3BO3 + 3HCl ­
SiCl4 + 4H2O H4SiO4 + 4HCl ­
  • Both form solid oxides which are acidic and dissolve in alkalies forming borates and silicates respectively.
B2O3 + 2NaOH 2NaBO2 + H2O
SiO2 + 2NaOHNa2SiO3 + H2O      
  • Both react with electropositive metals and form binary compounds, which yield mixture of boranes and silanes on hydrolysis. Both B and Si form hydroxides boric acid B(OH)3 and silicic acid Si(OH)4 respectively which are weak acids.
3Mg + 2B Mg3B2
Mg3B2 Mixture of boranes
2Mg + Si Mg2Si
Mg2Si   mixture of silanes

METALLURGY OF BORON

OCCURRENCE AND IMPORTANT MINERALS OF BORON

It does not occur in the free state in nature. It forms electron deficient compounds.
Its important minerals are :
  • Borax (or Tincal), Na2B4O7.10H2O   or Na2[B4O5(OH)4]8H2O
  • Kernite, Na2B4O7.2H2O or Na2[B4O5(OH)4]  
  • Orthoboric-acid H3BO3
  • Colemanite, Ca2B6O11.5H2O or Ca2[B3O4(OH)3]2 2H2O
  • Boracite, 2Mg3B8O15. MgCl2

ISOLATION

Elemental boron in the form of dark brown powder is obtained by following methods :
  • By reduction of boric oxide with highly electropositive metals like K, Mg, Al, Na etc. in the absence of air
B2O3 + 6K 2B + 3K2O
  • By the reaction of boron halides with hydrogen at high temperature.
2BCl3 + 3H2 2B + 6HCl
  • By thermal decomposition of boron triiodide over red hot tungsten filament.
  • By thermal decomposition of boron hydrides

USES OF BORON

  • As a semiconductor
  • Boron steel or boron carbide rods are used to control the nuclear reactions.

COMPOUNDS OF BORON

BORON HYDRIDES OR BORANES

Boron forms hydrides of the type BnHn+4 and BnHn+6 which are called Boranes.

PREPARATION
  • 8BF3 + 6LiH B2H6 + 6LiBF4
  • 4BCl3 + LiAlH42B2H6+ 3AlCl3 + 3LiCl
PROPERTIES
  • All boranes are called as electron – deficient compounds because boron in boranes never completes its octet.
  • On reaction with water boric acid is formed.

BORAX OR SODIUM TETRABORATE DECAHYDRATE, NA2B4O7.10H2O OR NA2[B4O5(OH)4]8H2O

PREPARATION
  • It occurs naturally as tincal in dried up lakes of Sri Lanka , USA and India.
  • By the boiling of mineral colemanite with a solution of Na2CO3.
  Ca2B6O11 + 2Na2CO3 2CaCO3 + 2NaBO2 + Na2B4O7
Colemanite                                                                 Borax
Above NaBO2 can be reconverted by passing CO2 through it.
4NaBO2 + CO2 Na2CO3+ Na2B4O7

PROPERTIES
  • Its aqueous solution is basic in nature due to hydrolysis.
Na2B4O7+ 7H2O 2NaOH + 4H3BO3
  • On heating with ethyl alcohol and conc. H2SO4, it gives volatile vapours of triethylborate which burn with a green edged flame.
Na2B4O7+ H2SO4 + 5H2O Na2SO4+ 4H3BO3
         H3BO3 + 3C2H5OH B(OC3H5)3 + 3H2O
    Triethylborate
  • Action of heat : Na2B4O7.10H2O     
   Na2B4O7 2NaBO2        + B2O3
  Anhydrous         Sodium Metaborate    Boric anhydride
Glassy mass (Borax bead)

Borax bead is used for the detection of coloured basic radicals under the name borax bead test in which on heating borax bead combines readily with a number of coloured transition metal oxides such as Co, Ni, Cr, Cu, Mn etc. to form the corresponding metaborates which possess characteristic colours.
CoSO4  CoO +SO3 ;
CoO +B2O3  Co(BO2)2
  cobalt metaborate (blue)
Basic radical of a salt Cu Fe Co Cr Ni
Colours of metaborates Blue Green Blue Green Brown

USES
It is used in making optical and hard glasses and in the borax bead test.

BORIC ACID OR ORTHOBORIC ACID, H3BO3 OR B(OH)3

PREPARATION
  • By treating borax with dil HCl or dil H2SO4
Na2B4O7 + 2HCl + 5H2O 2NaCl +4H3BO3
  • By passing SO2 through a mixture of powdered mineral colemanite in boiling water.
Ca2B6O11 + 4SO2 + 11H2O 2Ca(HSO3)2 +6H3BO3

PROPERTIES
  • It is a very weak monobasic acid. It does not act as a proton donor but accepts a hydroxyl ion i.e., it behaves as a lewis acid.
H3BO3 + H2O [B(OH)4]+ H +
  • With C2H5OH and conc. H2SO4, it gives triethylborate.
H3BO3 + 3C2H5OHB(OC2H5)3 +3H2O
  • With NaOH, it gives sodium metaborate
H3BO3 + NaOH NaBO2 + 2H2O
  • Heating effect
H3BO3 HBO2   2B4O7   2O3
Orthoboric acid Metaboric acid      Tetraboric acid    Boron trioxide
                      (Boric anhydride)
USES
As an antiseptic and eye lotion under the name Boric lotion, and as a food preservative.

STRUCTURE
It has a layer structure in which planar units are linked by H- bonding, as shown in fig.

BORON HALIDES , BX3, (WHERE, X= F, Cl, Br OR I)

PREPARATION
By the reaction of boron and halogens at high temperature.
2B + 3X2 2BX3

PROPERTIES
  • BF3 and BCl3 are gases, BBr3 is a volatile liquid and BI3 is a solid at room temperature.
  • These are covalent in nature and act as lewis acids. The decreasing order of acid strength is. BI3 > BBr3 > BCl3 > BF3

BORAZINE OR BORAZOLE OR TRIBORINE TRIAMINE, B3N3H6

It is a colourless liquid  having a six membered ring of alternating B and N atoms. It is also called inorganic benzene. It is prepared by B2H6 as follows:
3B2H6 + 6NH3  2B3N3H6 + 12H2
The p electrons in borazine are only partially delocalised. It is much more reactive than benzene, because there is a retention of partial negative charge by nitrogen atoms in latter case. It is isosteric (presence of same number of atoms and electrons) with benzene.
 
    Borazine                      
Benzene

METALLURGY OF ALUMINIUM

OCCURRENCE AND IMPORTANT MINERALS

It is the most abundant metal found in the earth’s crust. It does not occur in the free state in nature. Its important minerals are :
  • Bauxite, Al2O3. 2H2O
  • Diaspore,  Al2O3. H2O
  • Corundum, Al2O3
  • Cryolite, Na3 AlF6
  • Alunite or alum stone, K2SO4. Al2(SO4)3. 3Al(OH)3
  • Feldspar, K2O. Al2O3 6SiO2
  • Mica, KAl3Si3O10(OH)2
  • Kaolinite, Al2O3. 2SiO2 .2H2O

EXTRACTION

Aluminium metal is extracted from bauxite. It involves following steps.
  1. Purification of bauxite : Bauxite usually contains silica as impurity. These impurities must be removed before electrolysis, since aluminium, once prepared, cannot be freed of other metals by refining it. The bauxite is first purified by any of the following processes depending upon the nature of impurities present in it.

Bayer’s process :
Finely powdered bauxiteRoasted ore
                      
The reactions involved are given below.
Al2O3.2H2O +2NaOH   2NaAlO2 + 3H2O
Bauxite            Sod. meta aluminate
NaAlO2 + 2H2O Al(OH)3 ¯ + NaOH
2Al(OH)3  Al2O3 + 3H2O
        Alumina

Hall’s process :
Bauxite (Fine powder) solution
ppt. Al(OH)3  Pure Al2O3
The reactions involved are given below.
Al2O3. 2H2O + Na2CO3
  2NaAlO2 + CO2 + 2H2O
2NaAlO2 + 3H2O+ CO2  
     2Al(OH)3 ¯ + Na2CO3

Serpek’s process :
This process is employed when silica content of ore is high.
Finely powdered bauxite silica reduced to Si which volatalises  
  + Alumina form aluminium nitride AlN
                                
   Pure Al2O3   Al(OH)3
                    (ppt)
                
The involving reactions are given below.
Al2O3. 2H2O + N2 + 3C    2AlN + 3CO ­ + 2H2O
SiO2 + 2C Si ­ + 2CO ­
  AlN + 3H2O  Al(OH)3 ¯ + NH3
  1. Electrolysis of fused alumina (Hall and Heroult’s process) : Since pure alumina is a bad conductor of electricity, therefore it is dissolved in molten cryolite. Na3AlF6 and fluorspar, CaF2 to decrease its fusion temperature. The molten electrolyte is covered with a layer of powdered coke to prevent oxidation and loss of heat due to radiation.
The reactions are :
Na3AlF6 3NaF + AlF3
AlF3 Al3+ + 3F
At cathode :  Al3+ + 3e   Al
At anode : F e+ F
2Al2O3 + 12F  4AlF3 + 3O2
2C + O2 2CO ;  
    2CO + O2 2CO2
  1. Refining of aluminium : It is refined by Hoope’s electrolytic process, which is carried out in a graphite lined bath which acts as the anode and carbon cathodes are used. The refining cell consists of three fused layers of different densities.
  • The bottom layer is of molten impure aluminium (anode)
  • The middle layer is of fused cryolite and barium fluoride
  • The upper layer is of pure aluminium (cathode)
There is a new method of extraction of Aluminium in which the purification of the oxide is not of much importance. In this method, AlCl3 vapour is passed through the fused oxide at 1000ºC
2Al2O3 + 2AlCl3               6AlCl + 3O2
                                          Aluminium monochloride
The above aluminium monochloride vapour is unstable when cooled and disproportionates below at 800ºC.
3AlCl AlCl3 + 2Al

USES

  • A mixture of aluminium powder and aluminium nitrate is known as Ammonal and is used in bombs.
  • A mixture of Al powder in linseed oil is used as silver paint.
  • The reduction of metal oxides by aluminium is called aluminothermy or thermite process or Goldschmidt aluminothermite process.

COMPOUNDS OF ALUMINIUM

ANHYDROUS ALUMINIUM CHLORIDE, AlCl3 (or Al2Cl6)

PREPARATION
It can not be prepared by heating AlCl3. 6H2O because of its hydrolysing tendency by its own water as below.
2AlCl3. 6H2O  2Al(OH)3 + 6HCl
2Al(OH)3  Al2O3 + 3H2O
However, it can be prepared by following methods:
  • By passing dry chlorine or HCl gas over
    heated Al.
2Al + 3Cl2  2AlCl3
2Al + 6HCl 2AlCl3 + 3H2
  • By heating a mixture of alumina and carbon in a current of dry chlorine.
Al2O3 + 3C + 3Cl2  2AlCl3 + 3CO

PROPERTIES
  • It fumes in moist air due to hydrolysis
AlCl3 + 3H2O   Al(OH)3 + 3HCl
The resulting solution is acidic due to the formation of HCl.
  • It behaves as lewis acid.
  • It is a covalent solid and dissolves in organic solvents like C6H6 etc.    

STRUCTURE
It exists as dimer Al2Cl6 in which each Al atom is tetrahedrally surrounded by four Cl atoms as below.  

USES
  • As a catalyst in Friedel - Craft reactions
  • As a mordant in dyeing

ALUMINIUM OXIDE OR ALUMINA, Al2O3

It is the most stable compound of aluminium and occurs in nature as colourless corundum and several coloured oxides, (when present in combination with different metal oxides) like ruby (red), topaz (yellow), sapphire (blue), amethyst (violet) and emerald (green), which are used as precious stones (gems).

THERMITE

A mixture of aluminium powder and ferric oxide in the ratio 1: 3.

ALUMINIUM SULPHATE, Al2(SO4)3

It is used for obtaining H2S in pure form and for making fire proof clothes.

ALLOYS OF ALUMINIUM

Alloy
Composition
Properties
Uses
Duralumin
Light, tough, ductile
Aeroplanes and automobile parts
Aluminium Bronze
Light, strong, golden lustre
Coins, jewellery
Alcald
Duralumin coated with pure aluminium
corrosion resistant, strong
aircraft industry
Magnalium
Light, tough and strong
Balance beams and machinery
Alnico
Highly Magnetic
Permanent magnets

ALUMS

The term alum is given to double sulphates of the type X2SO4, Y2(SO4)3. 24H2O where X represents a monovalent cation such as Na+,K+ and NH Rb+, Cs+, Ag+ while Y is a trivalent cation such a Al3+, Cr3+ and Fe3+, Co3+, Ga3+, V3+, Ti3+. (Li+ is too small to be accommodated in the lattice)

General formula :
or

Some important alums are :
  • Potash alum K2SO4. Al2(SO4)3. 24H2O
  • Sodium alum Na2SO4. Al2(SO4)3. 24H2O
  • Ammonium alum (NH4)2SO4. Al2(SO4)3. 24H2O
  • Ferric alum (NH4)2SO4. Fe2(SO4)3. 24H2O
  • Chrome alum K2SO4. Cr2(SO4)3. 24H2O

Out of these, potash alum is the most important which is prepared in the laboratory by mixing hot solutions of equimolar quantities of K2SO4 and Al2(SO4)3. The resulting solution on  concentration and crystallization gives potash alum (emperical formula is KAl(SO4)2.12H2O).

Pseudo alums : When monovalent element of ordinary alums is replaced by a bivalent element eg Mn2+, Fe2+, Mg2+, Cu2+ or Zn2+, the alums are called pseudo alums.
Examples : FeSO4, Al2(SO4)3.24H2O
Ferrous aluminium pseudo alum
MnSO4.Al2(SO4)3.24H2O
Manganese aluminium pseudo alum

PROPERTIES
  • Potash alum is a white crystalline compound.
  • The aqueous solution of all alums is acidic due to hydrolysis of Al2(SO4)3, Cr2(SO4)3 or Fe2(SO4)3 as given below
Al2(SO4)3 + 6H2O 2Al(OH)3 + 3H2SO4
  • On heating all alums lose water of crystallization
    and swell up. The anhydrous alum is known as burnt alum.
  • Ionisation of aqueous solution of a double salt is as
K2SO4. Al2(SO4)2. 24H2O K+ + 2Al3+ + 3SO + 24H2O

USES
  • In purification of water
  • For sizing of paper
  • As a styptic to stop bleeding
  • As a mordant in dyeing and tanning of leather

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Best IIT JEE Coaching Institute in Delhi | NEET Coaching in East Delhi | NEET Coaching in West Delhi: p-Block Elements - Boron Family | Chemistry Notes for IITJEE/NEET
p-Block Elements - Boron Family | Chemistry Notes for IITJEE/NEET
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