INORGANIC CHEMISTRY

BORON FAMILY
Boron
Occurence: Boron is not found free in nature. In the combined state, it is found as the salts ofboric acid.
The important minerals of boron are:
(i) Borax(Tincal) : Na2
B4
O7
·10H2
O – It is found in Tibet, Ceylon, California and Kashmir.
(ii) Colemanite: Ca2
B6
O11
·5H2
O –– It is found inAsia Minor and America.
Panderinite: Ca2
B6
O11
·3H2
O
(iii) Boracite: 2Mg3
B8
O15
·MgCl2
– It is found in Stass-furt, Germany.
(iv) Boronatro calcite: CaB4
O7
·NaBO2
·8H2
O–It is found in Chile.
(v) Kernite (Rasorite): Na2
B4
O7
·4H2
O–It is found in Mojave dessert inAmerica.
(vi) Boric acid: H3
BO3
–It occurs in the jets of steam called soffioni escaping from ground in the
volcanic regions of Tuscany.
Boron is also present to some extent in plants and sometimes in coal ash.
Extraction: Boron is extracted from borax and colemanite minerals. The extraction involves two steps:
(i) Preparation of boric anhydride, B2
O3
, from one ofthe minerals.
(ii) Reduction of B2
O3
.
1st Step
(a) Preparation of B2
O3
from borax:
The finelyground borax is heated with concentrated hydrochloric acid or concentrated sulphuric
acid when sparingly soluble orthoboric acid separates out.
Na2
B4
O7
+ 2HCl  2NaCl + H2
B4
O7
Borax Tetraboric acid
Na2
B4
O7
+ H2
SO4
 Na2
SO4
+ H2
B4
O7
H2
B4
O7
+ 5H2
O  4H3
BO3
Orthoboric acid
Orthoboric acid is strongly heated to get B2
O3
2H3
BO3
 B2
O3
+ 3H2
O
(b) Preparation of B2
O3
from colemanite:
(i) The powdered mineral is fused with sodium carbonate.
Ca2
B6
O11
+ 2Na2
CO3
 2CaCO3
+ Na2
B4
O7
+ 2NaBO2
Colemanite
The fused mass is extracted with hot water. CaCO3
remains as insoluble. The fiterate containing
Sborax and sodium metaborate is put to crystallisation when borax is obtained. The remaining
solution is treated wth carbon dioxide which converts sodium metaborate into borax.
4NaBO2
+ CO2
 Na2
B4
O7
+ Na2
CO3
The borax is then converted into B2
O3
in the manner described above.
(ii) The other method ofconversion of colemanite into B2
O3
involves the suspension of mineral
in water and passing of sulphur dioxide gas into the suspension.
Ca2
B6
O11
+ 4SO2
+ 4 H2
O  2Ca(HSO3
)2
+ H4
B6
O11
H4
B6
O11
+ 7 H2
O  6H3
BO3
–––––––––––––––––––––––––––––––––––––––––––––––––––––
Ca2
B6
O11
+ 11 H2
O + 4SO2
 2Ca(HSO3
)2
+ 6H3
BO3

On concentration and cooling, crystals of boric acid separate out. These crystals from B2
O3
on
strong heating.
2H3
BO3
 B2
O3
+ 3H2
O
2nd step
Reduction of B2
O3
: The reduction of boric anhydride (B2
O3
), can be done with sodium, potassium or
magnesium. The boric anhydride is mixed with sodium, potassium or magnesium
powder and heated in a covered crucible.
B2
O3
+ 6Na  2B + 3 Na2
O
B2
O3
+ 6K  2B + 3K2
O
B2
O3
+ 3Mg  2B + 3MgO
The fused mass is stirred with iron rod as to oxidise the unreacted sodiumor potassium. T h e
mass is then boiled with dil. HClwhen insoluble boron powder is obtained. It is washed with water
and made dry. This is the amorphous variety of boron.
Modern method (Electrolyte method):
Boron is obtained these days by the electrolysis of a fused mixture containing boric anhydride,
magnesium oxide and magnesium fluoride at 11000
C. The electrolysis is done in carbon crucible
which acts as anode and iron rod is used as cathode. The magnesium discharged at cathode
reduces B2
O3
to boron.
2MgO  2Mg + O2
B2
O3
+ 3Mg  2B + 3MgO
Boron thus obtained is heated electrically in vacuum at 11000
C when the impurities are volatilised
off and pure boron is obtained.
Crystalline variety ofboron is obtained by the reduction of B2
O3
with aluminium powder.
B2
O3
+ 2Al  2B + Al2
O3
Aluminium is removed by heating the fused mass with NaOH solution.
Physical Properties:
Boron exists in two allotropic forms, (a) crystalline and (b) amorphous. Crystalline boron isblack
and chemically inert in nature. It is very hard in nature. It is very hard in nature.
Amorphous boron is brown and chemically active. It melts at 23000
C and boils at 25500
C.
It is difficult to fuse it. It is bad conductor of heat and electricity. Its density is 2.4 g mL–1
.
Chemical Properties:
(i) Action of air and oxygen:
Amorphous form when heated in air or oxygen at 7000
C, burnsAmorphous form when heated in
air or oxygen at 7000
C, burns with a reddish flame forming a mixture of oxide and nitride.
4B + 3O2
 2B2
O3
2B + N2
 2BN
(ii) Action of water:
Boron is not affected by water under ordinary conditions, however, when steam is passed over red
hot boron, hydrogen is liberated.
2B + 3H2
O  B2
O3
+ 3H2

(iii) Action of acids:
Boron is not affected by non-oxidising acids. It is attacked by oxidising acids like conc. H2
SO4
and
HNO3
.
B + 3HNO3
 H3
BO3
+ 3NO2

2B + 3H2
SO4
 2H3
BO3
+ 2SO2

(iv) Action of alkalies:
Boron dissolves in fused alkalies, liberating hydrogen.
2 + 6NaOH  2Na3
BO3
+ 3H2


(v) Action of metals:
Boron combines with strongly electropositive metals at high temperatures to form borides.
3Mg + B2
 Mg3
B2
(vi) Action of non-metals:
Boron forms B2
S3
when heated with sulphur. It forms extremelyhard substance boron carbide(B4
C),
when heated with carbon in an electric furnace. Boron combines directlywith chlorine and bromine
at higher temperatures.
(vii) Reducing nature:
It is a powerful reducing agent.
3CO2
+ 4B  2B2
O3
+ 3C
3SiO2
+ 4B  2B2
O3
+ 3Si
Uses: Boron is used :
(i) as a deoxidiser in the casting of copper.
(ii) for making boron steels which are very hard and are used as control rods in atomic reactors.
(iii) as a catalytic agent.
Boron steel or boron carbide rods are used to control the nuclear reactions. Boron has a very high
cross-section to capture the neutrons. According to another concept boron absorbs neutron to
make the boron having even number of neutrons.
5
B10
+ 0
n1
 5
B11
Compounds of Boron
1. Boron Trioxide, B2
O3
It is also called sesquioxide. It is an anhydride of orthoboric acid.
It is prepared by burning boron in oxygen,
4B + 3O2
 2B2
O3
or heating orthoboric acid to redness,
2H3
BO3
 B2
O3
+ 3H2
O
It is a white hygroscopic solid. It absorbs moisture (becoming opaque from transparent galssy
mass) and finally converted into boric acid.
B2
O3
+ H2
O  2HBO2
(Metaboric acid)
HBO2
+ H2
O  H3
BO3
(Orthoboric acid)
(B2
O3
is, thus, the anhydride of orthoboric acid)
It is an acidic oxide. It combines with metal oxides when fused and forms metaborates.
Some of the metaborates have characterstic colours (This is the basis of borax bead test.)
CuO + B2
O3
 Cu(BO2
)2
[The volatile part of the salt is displaced by B2
O3
CuO·SO3
+ B2
O3
 CuO·B2
O3
+ SO3
(CuSO4
) blue bead
[Cu(BO2
)2
]
Cr2
O3
·3SO3
+ 3B2
O3
 Cr2
O3
·3B2
O3
+ 3SO3
]
[Cr2
(SO4
)3
] Green bead
[2Cr(BO2
)3
]
It is reduced by magnesium.
B2
O3
+ 3Mg 
 
Heated 2B + 3MgO
When reacted with strongly acidic oxides, it behaves as a base.
2B2
O3
+ P4
O10
 4BPO4
2. Borax (Sodium Tetraborate)Na2
B4
O7
.10H2
O
(i) It is also called Tincalor Suhaga. Tincal conatins about 45% of borax.Advantage is taken of its

higher solubilityin hot water and purification. The naturaltincalis dissolved in hot water and insoluble
impurities are filtered off. The solution is concentrated and cooled when crystals of borax are
obtained.
(ii) Borax can be obtained from colemanite mineral
(iii) From boric acid:
Small quantities of borax are obatined from boric acid by neutralising it with soda ash.
4H3
BO3
+ Na2
CO3
 Na2
B4
O7
+ 6H2
O + CO2
Properties : Borax is known in three forms: (i) Prismatic borax which is the common form and is the
decahydrate form, Na2
B4
O7
·10H2
O. It is obtained by crystallising the solution at ordinary
temperature. It is less soluble in hot water. (ii) Octahedralform, which is the pentahydrate, Na2
B4
O7
·5H2
O, is obtained bycrystallising solution at 600
C. This is jeweller’s borax. (ii) Borax glass is the
anhydrous form, Na2
B4
O7
. It is obtained by heating the common from above its melting point. It is
a colourless glassymass and is not stable in moist air as it absorbs moisture and is graduallyconverted
into decahydrate form.
Basic nature:The solution ofborax is alkaline in nature. This is due to its hydrolysis (salt of strong alkali
and weak acid). Na2
B4
O7
+ 7H2
O 2NaOH + 4H3
BO3
strong alkali weak acid
Action of heat:
On heating, borax first swells up due to elimination ofwater molecules. On further heating, it melts
to a liquid which then solidifies to a transparent glassy mass.
Na2
B4
O7
·10H2
O
O
H
10
Heat
2




Anhydrous
7
4
2 O
B
Na 
 
 C
7400
meta
Sodium
2
NaBO
2 +
Boric
3
2O
B


 


 

mass
Glassy
anhydride
borate
When hot glassymass is brought in contact with a coloured salt and heated again in the flame, B2
O3
displaces the volatile oxides and combines with basic oxides to form metaborates.
Metaborates of basic radicals have characterstic colours.
CuSO4
+ B2
O3
 CuO·B2
O3
+ SO3

Cu(BO2
)2
Blue
This is the chemistry of borax bead test.
(iv) Various compounds of boron can be obtained from borax.

(v) Aqueous solution of borax acts as a buffer because it contains weak acid and its salt with
strong base.
Na2
B4
O7
+ 7H2
O  2Na[B(OH)4
] + 2H3
BO3
Structure of borax:
Borax can be represented as Na2
[B4
O5
(OH)4
]·8H2
O. It has two tetrahedral and two trianglular
units joined together as shown in the following figure.
Uses: Borax is used:
(i) for the borax bead test in qualitative analysis.
(ii) as a flux. [This is based on its property to dissolve many metal oxides to form borates of
low melting points.]
(iii) as an antiseptic.
(iv) in water softening as it forms insoluble calcium and magnesium borates (CaB4
O7
, MgB4
O7
).
(v) in the manufacture of enamels and glazes for pottery and tiles.
(vi) in making optical glasses and also borosilicate glass which is very resistant to heat and
shock.
(vii) in leather industry for cleaning hides and skins.
(viii) for impregnating match-sticks to prevent after glow.
(ix) for stiffening of candles.
3. Orthoboric acid, H3
BO3
Preparation : (i) From borax:
A hot concentrated solution of borax is treated with calculated quantityof concentrated sulphuric
acid. When the solution is cooled, crystals of boric acid are obtained.
Na2
B4
O7
+ H2
SO4
+ 5H2
O  Na2
SO4
+ 4H3
BO3
(ii) From colemanite:
A large quantity of boric aicd is made from colemanite mineral. The mineralis powdered and mixed
with boiling water. Sulphur dioxide is circulated through the suspension when boric acid is formed.
Ca2
B6
O11
+ 2SO2
+ 11H2
O  2Ca(HSO3
)2
+ 6H3
BO3
on cooling, boric and crystalline out.
(iii) From Tuscany soffioni:
Boric acid occurs in the jets ofsteam called soffioni issuing from the ground in the volcanic regions
of Tuscany. The jets are caught in large tanks of water. The resulting liquid is concentrated by
steam. On allowing the hot solution to cool, crystals of boric acid separate out.
Properties:(i) It forms soft , white, needle like crystals having a soapy touch.
(ii) It is less soluble in cold water but more soluble in hot water. It is steam volatile.
(iii) It is a weak acid and ionises mainly as monobasic acid.
H3
BO3
+ H2
O  H3
O+
+ H2
BO3
–
(iv) When heated at 1000
C, it loses water and converted into metaboric acid.
H3
BO3
1000
C
 

 HBO2
+ H2
O
When metaboric acid is heated at 1600
C, tetraboric acid results.

H2
B4
O7
 2B2
O3
+ H2
O
(v) A mixture ofethyl alcohol with boric acid burns withgreed edged flame due to the formation
of volatile ethyl borate.
H3
BO3
+ 3C2
H5
OH  B(OC2
H5
)3
+ 3H2
O
Ethyl borate
Uses: Orthoboric acid is used:
(i) as an antiseptic and eye wash under the name ‘boric lotion’.
(ii) in the manufacture of enamels and glazes for pottery.
(iii) as food preservative.
(iv) in glass industry.
4. Halides of boron :
Boron combines with halogens anf forms the halides of type BX3
, (X = F, Cl, Br, I).
Except BF3
, other trihalides can be prepared by the treatment of halogens on a mixture of B2
O3
and carbon at high temperature.
B2
O3
+ 3C + 3X2
 2BX3
+ 3CO
(X2
= Cl2
, Br2
, I2
)
BF3
may be obtained by heating CaF2
with concentrated sulphuric acid and boric anhydride.
3CaF2
+ B2
O3
+ 3H2
SO4
 2BF3
+ 3CaSO4
+ 3H2
O
Boron trihalides are also obtained by direct combination of boron and halogens under suitable
conditions.
2B + 3X2
 3BX3
Properties:
(i) These are covalent in nature due to small size and high charge density on B3+
ion.
(ii) These are non-electrolyses as in liquid state do not conduct electricity.
(iii) The boiling points are verylow. The boiling points increase as the atomic number ofhalogens
increases. BF3
is a colourless gas, BCl3
is a colourless fuming liquid (b. pt. = 130
C), BBr3
is also a colourless fuming liquid (b. pt. = 900
C) while BI3
is a white fusible solid (m. pt.
3100
C)
(iv) The trihalides are electrondeficient compounds. Boron atomacquires sixelectrons onaccount
of three B – X bonds, i.e., 2 electrons short of an octet. Thus, the boron atom in BX3
molecule can
accept two more electrons, i.e., an electron pair from the donor atoms like N, P, O, S, F etc., in
NH3
, PH3
, H2
O, H2
S, HF, F–
, etc., respectively to form addition compounds (donor-acceptor
compounds).
H3
N : + BF3
 [H3
N  BF3
]
Donor Acceptor
(Lewis base) (Lewis acid)
The relative Lewis acid character of boron trihalides is found to follow the following order.
BI3
> BBr3
> BCl3
> BF3
but the expected order on the basis ofelectronegativityofthe halogens (electronegativityofhalogens
decreases from F to I should be,
BF3
> BCl3
> BBr3
> BI3
This anomaly is expalined on the basis of the relative tendency of the halogen atomto back donate
its unutilised electrons to vacant p-orbital of boron atom. In BF3
, boron has a vacant 2p orbital and
each fluorine has fully filled unutilised 2p-orbitals. Fluorine transfers two electrons to vacant 2p-
orbital of boron, thus forming p-p bond.

This type ofbond has some double bond character and is known as dative or back bonding. All the
three bond lengths are same. It is possible when double bond is delocalized. The delocalization
may be represented as:
  -
This bond reduces the electron deficieny of boron atom hence its Lewis acid character decreases.
The tendency to form back bonding its maximum in BF3
and decreases from BF3
to BI3
. Thus,
BCl3
, BBr3
and BI3
are stronger Lewis acids then BF3
.
(v) All boron trihalides, except boron trifluoride, are hydrolysed to boric acid.
BCl3
+ 3H2
O  H3
BO3
+ 3HCl
The degree ofhydrolysis increases from BCl3
to BI3
. Due to resistance of BF3
to hydrolysis and its
tendency to act as Lewis acid, BF3
is used as a catalyst in organic reaction.
5. Hydrides of Boron:
Boron forms a number of hydrides. These are called boranes by analogy with alkanes.
These belong to one of the two series, viz., Bn
Hn+4
and Bn
Hn+6
. The members of Bn
Hn+6
are less
stable.
Bn
Hn+4
Bn
Hn+6
B2
H6
B4
H10
B5
H9
B5
H11
B6
H10
B6
H12
B10
H14
B9
H15
The simplest boron hydride, BH3
, inunknown. The most important hydride is dibornae(B2
H6
) which
has been extensibely studied.
Diborane , B2
H6
: It is prepared:
(i) By the action of lithium aluminium hydride on boron trichloride in the presnce of ether.
4BCl3
+ 3LiAlH4
Ether
 

 2B2
H6
+ 3LiCl + 3AlCl3
(ii) By passing silent electric discharge at low pressure through a mixture of boron trichloride
or tribromide and excess of hydrogen.
2BCl3
+ 6H2
Silent electric
disch e
 


arg
B2
H6
+ 6HCl
(iii) By reacting lithiumhydride with boron trifluoride
8BF3
+ 6LiH  B2
H6
+ 6LiBF4
Properties:
(i) It is a colourless gas which is stable at low temperature in the absence of moisture and grease.At
higher temperatures in the absence ofmoisture and grease. At higher temperatures, it changes to
higher boranes and at red heat it decomposes to boron and hydrogen. It has disagreeable odour
and causes headache.
B2
H6
Red heat
 

 2B + 3H2
(ii) It burns in oxygen. The reaction is highly exothermic.
B2
H6
+ 3O2
 B2
O3
+ 3H2
O + heat
(iii) It readily reacts with water liberating hydrogen.
B2
H6
+ 6H2
O  2H3
BO3
+ 6H2
(iv) It reacts with strong alkalies to form metaborates and hydrogen.
B2
H6
+ 2KOH + 2H2
O  2KBO2
+ 6H2
(v) It reacts with chlorine forming boron trichloride.
B2
H6
+ 3Cl2
 2BCl3
+ 3HCl

(vi) In presence ofanhydrous aluminium chloride, it reacts with dry HCl.
B2
H6
+ HCl  B2
H5
Cl + H2
Chlorodiborane
(vii) Lithium borohydride is formed when dibornae reacts with LiH in presence of ether.
2LiH + B2
H6
Ether
 

 2LiBH4
(viii) It reacts with carbon monoxide under pressure to form carbonyl, BH3
CO.
B2
H6
+ 2CO  2BH3
CO
(ix) It reacts with sodium amalgam forming an addition compound.
B2
H6
+ 2Na (amalgam)  B2
H6
·Na2
(x) At low temperatures, an addition product, B2
H6
·2NH3
, is obtained with ammonia.
B2
H6
+ 2NH3
Low temp.
 

 B2
H6
·2NH3
When the addition product is heated at 2000
C, a volatile compound borazole or inorganic benzene
is formed.
2B2
H6
·2NH3
 2B3
N3
·H6
+ 12 H2
Borazole
Borazole has a ring structure like benzene.
Uses: The important uses of diborane are:
(i) as a catalyst in polymerization reactions.
(ii) as a reducing agent in organic reactions
(iii) for making high energy fuels and propellants
(iv) for preparing ydrcarbons, alcohols, ketones and acids through hydroboration method.
Structure:Diborane is an example of electron deficient compound. Boron atomhas three half filled orbitals
in excited state, i.e., it can link with three hydrogen atoms. Thus, while each boron atom in diborane
can link to itself three hydrogen atoms, there are no electrons left to form a bond between two
boron atoms as shown below:
Diethyl in 1921 proposed a bridge structure for diborane. Four hydrogen atoms, two on the left
and two on the right, known as terminal hydrogens and two boron atoms lie in the same plane. Two
hydrogen atoms forming bridges, one aboveand other below, lie in a plane

perpendicular to the rest of molecule. This structure shows that there are two types of hydrogen
atoms – Terminal and bridging. 4–terminalhydrogen atoms can easilybe replaced bymethylgroups
but when two bridging hydrogen atoms are attacked, the molecule isruptured.
According to molecular orbital theory, each of the two boron atoms is sp3
hybrid state. Of t h e
four hybrid orbitals, three have one electron each while the fourth is empty. Two ofthe four orbitals
of each of the boron atomoverlap with two terminalhydrogen atoms forming two normal B–H
bonds.One of the remaining hybrid orbital (either filled or empty) of one of the boron atoms, 1s
orbital of hydrogen atom (bridge atom and one of the hybrid orbitals of the other boron atom
overlap to forma delocalized orbital covering the three nuclei with a pair of electrons. Such a bond
is known as three centre two electron bonds.

Formation of three centre bond (B–H–B)
Similar overlapping occurs in one hydrogen atom(bridging and fourth hybrid orbitalof each boron
atom. Thus, the formation of diborane molecule can be depicted as shown in the following figure.
Structure of diborane
On account of repulsion between the two hydrogen nuclei, the delocalized orbitals of bridges are
drifted away fromeach other giving the shape ofa banana. The three centre two electron bonds are
also known as banana bonds.
ALUMINIUM
Occurence:Aluminium is a third most abundant element forming 8.3% of earth’s crust. It is a constituent
of clay, slate and many types ofsilicate rocks. It is found only in the combined state, the important
minerals are:
(i) Oxides:(a) Corundum, ruby, sapphire, emerald, Al2
O3
.
(b) Diaspore, Al2
O3
·H2
O
(c) Gibbsite, Al2
O3
·3H2
O.
(d) Bauxite, Al2
O3
·2H2
O, it is the chief ore ofaluminium fromwhich extraction of aluminium is
made. It is usually associated with varying amounts of ferric oxide and silica.
(ii)Fluoride: Cryolite, Na3
AlF6
. It is the second important ore of aluminium.
(iii) Basic Sulphate:
Alunite or alum stone, K2
SO4
·Al2
(SO4
)3
·4Al(OH)3
(iv) Basic Phosphate:
Turquoise, AlPO4
·Al(OH)3
·H2
O. It is usually blue coloured due to presence of copper phosphate.
(v) Silicates: Felspar, KAlSi3
O8
, kaolin, porcelain, mica, china clay, slate, etc. Al2
O3
·2SiO2
·2H2
O
Extraction: Aluminium is mainly isolated from bauxite ore which is generally contaminated with ferric
oxide and silica. The removal of ferric oxide and silica from bauxite ore is essential before it is
subjected to electrolysis because it is rather difficult to remove iron and silicon from aluminium
metal, the presence of these elements makes the aluminium metal brittle and liable to corrosion.
Thus, the extraction of aluminium from bauxite ore involves the following three steps. m
(i) Purification of bauxite ore, i.e. removal of ferric oxide and silica.
(ii) Electrolytic reduction ofAl2
O3
.
(iii) Electrolytic purificationofaluminium.

Purification of bauxite ore:
The following methods are used for purifying the bauxite ore.
(a)Baeyer’s process:
This process is mainly applied to bauxite ore containing ferric oxide as chief impurity. The colourof
such ore is usually red and hence called red bauxite. The powdered ore is first roasted at a low
temperature as to convert any ferrous oxide, if present, into ferric oxide. It is then digested with a
concentrated solution of sodium hydroxide (density 1.45 g mL–1
) in an autoclave under pressure at
1500
C for several hours. The aluminium oxide dissolves in caustic soda forming soluble sodium
meta aluminate and settle down. These are removed by filteration.
Al2
O3
·2H2
O + 2NaOH  2NaAlO2
+ 3H2
O
Sodium meta aluminate
(soluble)
The precipitate is washed and dried.
The soluble of NaOH is concentrated and used again.
(b)Hall’s process:
Bauxite is fused with sodium carbonate. Al2
O3
combines with sodium carbonate forming sodium
meta aluminate. The fused mass is extracted with water where Fe2
O3
and SiO2
remain as insoluble
in the residue.
Al2
O3
+ Na2
CO3
 2NaAlO2
+ CO2
The solutioncontaining sodiummeta aluminate is warmed to 50-600
C andcarbondioxideis circulated
through it. Al(OH)3
separates out as precipitates.
2NaAlO2
+ CO2
+ 3H2
O  2Al(OH)3
+ Na2
CO3
The precipitate is filtered, washed and dried.
The solution of Na2
CO3
is concentrated and used again.
(c) Serpeck’s process:
This process is used when silica is present in considerable amounts in bauxite ore. The ore is mixed
with coke and heated at 18000
C in presence of nitrogen, whereAIN (Aluminiumnitride) is formed.
Al2
O3
+ 3C + N2
 2AIN + 3CO
Silica is reduced to silicon which violatilises offat this temperature.
SiO2
+ 2C  Si + 2CO
AIN is hydrolysed with water into aluminium hydroxide.
This process has one distinct advantage that ammonia is obtained as a valuable by product.
Calcination of aluminium hydroxide:
The aluminium hydroxidem precipitate obtained in the above processes is calcined at 15000
C in a
rotary kiln to obtain pure alumina (Al2
O3
)
2Al(OH)3
15000
C
 
 Al2
O3
+ 3H2
O
Electrolytic reduction of pure alumina:
The electrolysisofpure alumina facestwo difficulties: (i) Purealumina is a bad conductor ofelectricity,
(ii) The fusion temperature of pure alumina is about 20000
C and at this temperature when the
electrolysis is carried of fused mass, the metal formed vapourises as the boiling point of aluminium
is 18000
C. The above difficulties are overcome by using a mixture containing alumina, cryolite
(Na3
AlF6
) and fluorspar (CaF2
) in the ratio of 20:24:20. The fusion temperature of this mixture is
9000
C and it is a good conductor of electricity. The electrolysis is carried out in an iron box
lined inside with gas carbon which acts as cathode. The anode consists of carbon rods which dip
in the fused mixture of the electrolyte from above. The fused electrolyte is covered with a layer of
coke.

The current passed through the cell serves two purposes– (i) heating of the electrolyte–The
temperature ofthe cellis automaticallymaintained at 900–9500
C, (ii) electrolysis. Onpassing current
current, aluminium is discharged at cathode. Aluminiumbeing heavier than electrolyte sinks to
the bottomand is atpped out periodically from tapping hole. Oxygen is liberated at anode. It attacks
carbon rods forming CO and CO2
. The process is continuous. When the concentration of the
electrolyte decreases, the resistance of the cell increases. This is indicated by the glowing of a lamp
placed in parallel. AT this stage more of alumina is added.
The exact mechanism of the electrolysis is not yet known. Two concepts have been proposed.
First concept:
AlF3
Al3+
+ 3F–
Al3+
ions are discharged at cathode and F–
ions at anode.
Al3+
+ 3e  Al (at cathode)
2F–
 F2
+ 2e (at anode)
The liberated fluorine reacts with alumina to formAlF3
and O2
. The oxygen attacks the carbon
anodes to form CO and CO2
.
Al2
O3
+ 3F2
 2AlF3
+ 3/2 O2
2C + O2
 2CO
C + O2
 CO2
Anodes are replaced frequently.
Second concept:
Alumina (Al2
O3
) ionises as
Al2
O3
Al3+
+ AlO3
3–
Cathode Anode
Al3+
+ 3e  Al(at cathode)
At anode AlO3
–
is oxidised.
4AlO3
3–
 2Al2
O3
+ 3O2
+ 12 e (at anode)
Thus, the overall chemical reaction taking place during electrolysis is,
2Al2
O3
 4Al + 3O2
Aluminium of 99.8% purity is obatined from this process.
Refining of aluminium by Hoope’s electrolytic method:
Aluminiumis further purified by Hoope’s process. The electrolytic cellconsists of an ironbox lined
inside with carbon. The cell consists of three layers which differ in specific gravities. The upper
layer is of pure aluminium which acts as cathode. The middle layer consists of a mixture of the
fluorides ofAl, Ba and Na. The lowest layer consists ofimpure aluminium which acts as anode. The
middle layer works as electrolyte.

The graphite rods dipped in pure aluminium and Cu-Al alloy rod at the bottom in the impure
aluminiumwork as conductors. One electrolysis, aluminiumis deposited at cathode from the middle
layer and an equminium is taken up by the middle layer from the bottomlayer (impure aluminium).
Therefore, aluminium is transferred form bottom to the top layer through middle layer while the
impurities are left behind. Aluminium thus obtained is 99.98% pure.
Physical Properties:
(a) Aluminium is a bluish white lustre metal. The lustre is destroyed on long exposure to air
due to formation of a thin film of oxide on the surface.
(b) The densityof aluminium is 2.7 g mL-1
(light metal). It is malleable and ductile.
(c) It is a good conductor of heat and electricity.
(d) It melts at 660o
C and boils at 1800o
C.
(e) It can be welded and cast but difficult to solder.

FLOWSHEET FORTHE EXTRACTION OFALUMINIUM
Aluminiumore, Al2
O3
·2H2
O(Bauxite)

REFINING OF BAUXITE
(a) Baeyer’s Process:
Bauxite ore Roasted
 
 as to convert FeO into Fe2
O3
Roasted ore + NaOH
solution
150
80
0
C
atm
 


.
NaAlO2
Hydrolysis
in presence
of little Al OH
 


( )3
Al OH
ppt
( )
.
3 + NaOH
(b) Hall’s Process:
Bauxite ore + Na2
CO3
Fused
 

 NaAlO2
 Extracted with water
Solution
 Warmed 50–600
C
CO2
is circulated

Al OH
ppt
( )
.
3 + Na2
CO3
(c) Serpeck’s Process:
Bauxite ore
nitrogen
+ Coke 18000
C
 
 AIN H O
2
 

 Al OH
ppt
( )
.
3 + NH
by product
3


CALCINATION
Al(OH)3
15000
C
 
 Al2
O3
Anhydrous alumina

ELECTROLYTIC REDUCTION
Electrolyte Al2
O3
dissolved in Na3
AlF6
and CaF2
Cathode–Carbon lining Anode– Graphite rods
Al2
O3
Electrolysis
C
 


9500
Al
pure
99 8%
. + O2
ELECTROLYTIC REFINING
(Hoope’s process) Pure Al (99.98% pure)
Chemical Properties:
(a) Action of air: Aluminium is not affected by dry air but in moist air a thin film of oxide is formed
over its surface. It burns in oxygen producing brilliant light.
4Al + 3O2
 2Al2
O3
The reaction is highly exothermic and the heat evolved is used in the thermite process for the
reduction of oxides of Cr, Fe, Mn, etc.
(b) Action of water: Pure aluminium is not affected by pure water. The impure aluminium is readily
corroded by water containing salts (sea water). Aluminium decomposes boiling water evolving
hydrogen.



2Al + 6H2
O  2Al(OH)3
+ 3H2

(c) Action of acids: The oxidation potential of aluminium is 1.66 volts thus, it is strongly electropositive,
very reactive and a powerful reducing agent. It dissolves in HCl(dil. and conc.) and dilute sulphuric
acid, evolving hydrogen.
2Al + 6HCl  2AlCl3
+ 3H2

2Al + 3H2
SO4
 Al2
(SO4
)3
+ 3H2

The reactionwith dil. H2
SO4
isveryslow probablyonaccount ofthe insolubilityofthe oxide filmin this acid.
Hot concentrated sulphuric aicd dissolves Al with evolution of SO2
.
2Al+6H2
SO4
 Al2
(SO4
)3
+ 3SO2
+ 6H2
O
Dilute and concentrated HNO3
has no effect onAl, i.e., Al is rendered passive by nitric acid. This
is due to surface oxidation and formation of a thin film ofoxide on its surface. Organic acids have
little action onAl at ordinary temperatures.
(d) Action of alkalies:
Aluminium is attacked by caustic alkalies with the evolution of hydrogen.
2Al +
2 2 2
NaOH H O
solution

 
  
  
2 2
NaAlO
Sodium meta
alu ate so le
min ( lub )
+ 3H2
­
2Al + 6NaOH Fused
 


2 3 3
Na AlO
Sodium alu ate
min
+ 3H2
­
(e) Action of non-metals:
Besides oxygen, aluminium reacts with non-metals directly to form corresponding compounds.
Aluminium when heated in the atmosphere of nitrogen, forms aluminium nitride.
2Al + N2
 2AIN
Aluminium powder when fused with sulphur forms aluminium sulphide.
2Al + 3S  Al2
S3
Finely powdered heated aluminium combines with halogens to form corresponding halides.
2Al + 3X2
 2AlX3
(X2
= F2
, Cl2
, Br2
, I2
)
All these compounds are hydrolysed with water.
AlN + 3H2
O  Al(OH)3
+ NH3
Al2
S3
+ 6H2
O  2Al(OH)3
+ 3H2
S
AlX3
+ 3H2
O  Al(OH)3
+ 3HX
(f) Reducing agent:
It is a good reducing agent and reduces oxides of metals like Cr, Fe, Mn etc.
Cr2
O3
+ 2Al  2Cr + Al2
O3
+ heat
Fe2
O3
+ 2Al  2Fe + Al2
O3
+ heat
3Mn3
O4
+ 8Al  9Mn + 4Al2
O3
+ heat
It reduces oxides of non-metals also
3CO2
+ 4Al  2Al2
O3
+ 3C
3SiO2
+ 4Al  2Al2
O3
+ 3Si
(g) Displacement of other metals:
Being more electropositive it displaces copper, zinc and lead from the solution of their salts.
3ZnSO4
+ 2Al  Al2
(SO4
)3
+ 3Zn
3CuSO4
+ 2Al  Al2
(SO4
)3
+ 3Cu
Uses: (i) Aluminium being cheap and light metalis largely used for making household utensils, trays,
frames, etc. Bodies of autmobiles, aircraft and roofing are made of aluminium sheet.
(ii) Thin foils ofAl are used in wrapping soaps, cigarettes and confectionary.
(iii) Al wire is used in transmission lines and coils for dynamos and motors.
(iv) It is used for making silverypaints for covering iron and other materials.
(v) It is used in thermic process for tyhe extraction of Cr, Mn, etc.

(vi) Since it is not attacked by nitric acid, it is used in chemical plants and for transporting nitric
acid.
(vii) Because of its lightness, good conductivityand resistance to corrosion, it is used for making
alloys which find applications in industries and arts.
(viii) Aluminium amalgamis used as a reducing agent.
(ix) Aluminium powder is used in fire works, flash light powder and in thermite welding.
Compounds of aluminium
1. Aluminium Oxide or Alumina, Al2
O3
It occurs in nature as colourless corundum and tinted with metallic oxides as ruby (red), sapphire
(blue), amethyst (violet), emery (green), etc. These coloured oxides are precious stones. Hydrated
oxide (Al2
O3
.2H2
O) occurs as Bauxite.
It is prepared by igniting aluminiumhydroxide, aluminium sulphate or ammoniumalum.
2Al(OH)3
Al2
O3
+ 3H2
O
Al2
(SO4
)3
AL2
O3
+ 3SO3
(NH4
)2
SO4
.Al2
(SO4
)3
.24H2
O Al2
O3
+ 2NH3
+ 4SO3
+ 25H2
O
It is obtained in crystalline formbystrongly heating a mixture ofaluminiumfluoride and boric oxide.
2AlF3
+ B2
O3
Al2
O3
+ 2BF3
It is a white solid, insoluble in water. It is a stable and unreactive substance. It begins to volatilise at
1750o
C. It boils at 2250o
C.
It shows amphoteric nature. When it is fused with oxides of chromium, iron and cobalt, synthetic
semi-precious stones are obtained.
It is widelyused for making bauxite bricks which are used for lining furnaces. Fused alumina is used
as refractory material. When heated in an electric arc at 3000o
C, a hard powder known as alundun
is obtained which is used as abrasive. With lime, it is used as bauxitecement which is not affected
by sea water. It is used in chromatography, in extraction of aluminium and in preparing precious
stones.
2. Aluminium chloride, AlCl3
(i) Anhydrous aluminium chloride:
It is prepared by passing dry HClgas or chlorine gas over heated aluminium turnings in absence of
air. The vapours of aluminium chloride are condensed when solid anhydrousaluminiumchloride is
obtained.
2Al + 6HCl 2AlCl3
+ 3H2
2Al + 3Cl2
2AlCl3
It can also be obtained by heating a mixture of alumina and caubon in an atmosphere of chlorine.
Al2
O3
+ 3C + 3Cl2
10000
C
 
 2AlCl3
+ 3CO
Vapours
BCooled
Solid anhydrous aluminiumchloride
(ii) Hydrated aluminium chloride:
AlCl3
.6H2
O, is formed when aluminium metal or aluminium hydroxide is dissolved in dilute
hydrochloric acid.
2Al + 6HCl AlCl3
+ 3H2
Al(OH)3
+ 3HCl AlCl3
+ 3H2
O
HCl gas is circulated through the solution to obtain crystals ofhydrated aluminium chloride.
Properties:(a)Anhydrous aluminiumchloride is a white solid. It is deliquescent and fumes in air. On heating
it sublimes a 180o
C and the vapour density corresponds to the formula Al2
Cl6
. It is covalent when
anhydrous as it does not conduct current in fused state. It is soluble in organic solvents such as

alcohol, ether, benzene, etc. The dimeric formula is retained in non-polar sovents but is broken into
[Al(H2
O)6
]Cl3
on dissolution in water on account of high heat of hydration. The molecular (dimer)
is an autocomplex and is represented as,
(b) Anhydrous aluminiumchloride fumes in moist air due to evolution of HCl.
Al2
Cl6
+ 6H2
O 2Al(OH)3
+ 6HCl
When dissolved inwater, it changes into hydrated aluminiumchloride which is ionic in nature.
Al2
Cl6
+ 12H2
O 2AlCl3
.6H2
O
AlCl3
Al3+
+ 3Cl-
(c) Anhydrous aluminium chloride forms an addition product with ammonia gas.
Al2
Cl6
+ 12NH3
 2[AlCl3
.6NH3
]
(d) The solution of aluminium chloride in water is acidic in nature due to hydrolysis.
AlCl3
+ 3H2
O  Al(OH)3
+ 3HCl
Weak base Strong acid
(e) When ammoniumhydroxide is added to the solution of aluminiumchloride, a gelatinous
precipitate of aluminium hydroxide appears which does not dissolve in excess of NH4
OH.
AlCl3
+ 3NH4
OH  Al(OH)3
+ 3NH4
Cl
(f) When sodium hydroxide is added to the solution of aluminium chloride drop by drop, a
white gelatinous precipitate appears which dissolves in excess of sodiumhydroxide forming
sodiummeta-aluminate.
AlCl3
+ 3NaOH  Al(OH)3
+ 3NaCl
Al(OH)3
+ NaOH  NaAlO2
+ 2H2
O
Uses: (i) It is used as a catalyst in Friedel–Craft’s reaction.
(ii) It isalso used in themanufacture ofgasoline bycracking ofhighboilingfractions ofpetroleum.
(iii) It finds extensive use in the manufacture of dyes, drugs and perfumes.
3. Alums : Formerly, the term alum was used to describe only one double sulphate with 24 molecules of
water ofcrystallisation, potassiumaluminiumsulphate, K2
SO4
·Al2
(SO4
)3
·24H2
O, but now this term
is used for all the double sulphates having the composition,
M2
SO4
·M2
(SO4
)3
·24H2
O
where M stands for molecular basic radicals such as Na+
, K+
, Rb+
, Cs+
, Ag+
, Ti+
, NH4
+
and  for
a trivalent basic radicals such as Al3+
, Cr3+
, Fe3+
, Mn3+
, Co3+
, etc.
Some examples of alums are:
Potash alum K2
SO4
·Al2
(SO4
)3
·24H2
O (Commonlycalled alum)
Ammoniumalum (NH4
)2
SO4
·Al2
(SO4
)3
·24H2
O
Sodiumalum Na2
SO4
·Al2
(SO4
)3
·24H2
O
Chrome alum K2
SO4
·Cr2
(SO4
)3
·24H2
O
Ferric alum (NH4
)2
SO4
·Fe2
(SO4
)3
·24H2
O
Alums are generallyobtained whenhot solutions ofequimolar quantities oftheir constituent sulphates
are mixed and the resulting solution is subjected to crystallisation.
Alums are crystalline compounds. In alums each metal ion is surrounded by six water molecules
and the crystals of alums consist of [M(H2
O)6
]+
, [M(H2
O)6
]3+
and SO4
2–
ions.
Alums are fairly soluble in hot water but less soluble in cold water. The solutions are acidic
and have strigent taste.The solutions show the properties of ions of the constituent salts. The alums
are isomorphous to each other and form mixed crystals.
Each alum has different melting point. Alums lose water of crystallisation when heated. If rapidly
heated to a high temperature,the alum swells up and a porous mass called burnt alumis left behind.

The alums are effective in precipitating colloids, i.e., act as coagulants. The alums have germicide
properties. Alums are thus used in the purification of water, tanning of leather, as modrants in
dyeing and as antiseptics.
Potash alum, K2
SO4
·Al2
(SO4
)3
·24H2
O
It is commonly known as alum.
Preparation:
(i) From bauxite or aluminium sulphate:
Bauxite is boiled with sulphuric acid to formaluminiumsulphate. To this solution calculated quantity
of K2
SO4
is added. The solution is concentrated and cooled. After some time crystals of potash
alum are obtained.
Al2
O3
+ 3H2
SO4
 Al2
(SO4
)3
+ 3H2
O
Al2
(SO4
)3
+ K2
SO4
+ 24H2
O  K2
SO4
·Al2
(SO4
)3
·24H2
O
Potash Alum
(ii) From alumstone or alunite:
Alum stone is treated with dilute sulphuric aicd and the solution is boiled. Acalculated quantity of
K2
SO4
is added to the solution. The solution on cooling yields crystals of potash.
K2
SO4
· Al2
(SO4
)3
· 4A;(OH)3
+ 6H2
SO4
 K2
(SO4
)3
+3Al2
(SO4
)3
+ 12H2
O
alumstone
K2
SO4
· Al2
(SO4
)3
+ 24H2
O  K2
(SO4
)3
·Al2
(SO4
)3
· 24H2
O
Properties:(a)It is a white crystalline compound.
(b) It is soluble in water and its aquoeus solution is acidic due to hydrolysis ofAl2
(SO4
)3
.
(c) On heating it swells up on account of elimination of water molecules.
K2
SO4
·Al2
(SO4
)3
·24H2
O 2000
C
 

 K2
SO4
·Al2
(SO4
)3
+ 24 H2
O
K2
SO4
·Al2
(SO4
)3
Red Heat
 

 K2
SO4
+ Al2
O3
+ 3SO3
(d) Its aqueous solution contains K+
,Al3+
and SO4
2–
ions and their usualtests can be performed.
Uses: It is used:
(i) as a modrant in dyeing and calico printing.
(ii) in sizing ofcheap quality of paper.
(iii) in purification ofwater.
(iv) as antiseptic and in stopping bleeding from cuts.
(v) in leather tanning.
* * * * *