BORAX
Na2B4O7.10H2O
Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is
an important boron compound, a mineral, and a salt of boric acid. It is usually a white
powder consisting of soft colourless crystals that dissolve easily in water.
Its actual formula is Na2[B4O5(OH)4].8H2O
It is obtained from the mineral colemanite by boiling it with a solution of Na2CO3 .
Ca2B6O11 + 2Na2CO3 ———→ Na2B4O7 + 2CaCO3 + 2NaBO2
Colemanite
Borax
Sod meta borate
Properties :
(a) Its aqueous solution is alkaline due to hydrolysis,
Na2 B4O7 + 7H2O ——→ 2NaOH+4 H3BO3 .
(b) On heating borax loses its water of crystallization and swells up to form a fluffy
mass. On further heating, it melts to give a clear liquid which solidifies to a
transparent glassy bead consisting of sodium metaborate ( NaBO2 ) and boric
anhydride ( B2O3 ),
Na2B4O7. 10H2O ———→ Na2B4O7 ———→ 2NaBO2 + B2O3
-10H2O
Boraxbead
This glassy bead gives different coloured beads with ions such as Ni 2+ ,Co 2+ ,Cr 3+
etc.The salts containing these ions are heated to form oxides which form coloured
meta borates with boron oxide.This test is called borax bead test
CuO + B2O3 → Cu(BO2)2
Cupric meta borate(blue)
Uses:
1. For borax bead test
2. anteseptic in nature
3. in making optical glass
BORIC ACID
H3BO3
Boric acid is also called as orthoboric acid H3BO3.
Preparation of Boric Acid
Boric acid is obtained by the action of hydrochloric acid or sulphuric acid on
borax. On cooling the reaction mixture, white flakes of boric acid are obtained.
Boric acid may also be obtained by the hydrolysis of most boron compounds like
nitrides, sulphides, halides etc.
Properties of boric acid
Boric acid is a white crystalline substance, soft and soapy to touch.
Boric acid is moderately soluble in cold water, but fairly soluble in hot water.
On heating boric acid decomposes to form metaboric acid at 375 K, tetraboric
acid at 435 K and boron trioxide at red heat.
Boric acid is a very weak monobasic acid
Structure of boric acid
Boric acid contains planar BO33-units bound together through hydrogen bonds
forming a trigonal planar layer structure. Hydrogen atoms act as bridge between
two oxygen atoms of different BO33- units
Uses of boric acid:
As a food preservative in food industry.
In medicines as eyewash.
For making borosilicate glass.
DIBORANE
simplest stable boron hydride is diborane (B2H6).
Preparation of Diborane:
By passing silent electrical discharge through BCl3 and H2
By the reaction of BF3 on LiH at 450K
2BF3 + 6LiH → B2H6 + 6LiF
This type of overlapping produces a three centered-two electron bond. Thus in
diborane molecule two such bonds also called as banana bonds are formed. The two
boron atoms and the four terminal hydrogen atoms lie in one plane while, the two
bridging hydrogen atoms lie symmetrically above and below this plane.
It undergoes spontaneous combustion in air due to strong affinity of boron with
oxygen forming boric anhydride.
It gets hydrolysed easily by water, liberating dihydrogen gas
Uses of diborane
As a rocket fuel .
As a catalyst in polymerisation reactions.
Reducing agent in organic reactions.
SILICONES
silicones are polymers of silicon having Si -O- Si linkage with general formula
(R2SiO)n
Silicones are inert, synthetic compounds with a wide variety of forms and uses.
Typically heat-resistant and rubber-like, they are commonly used in medical
applications, adhesives, lubricants, and insulation.
When alkyl halide is heated with Si in presence of Cu powder it gives R2SiCl2
2RCl +Si → R2SiCl2
It upon hydrolysis gives R2Si (OH)2 which upon condensation ploymerisation gives
silicone
SILICATES
Silicates are the metal derivatives of silicic acid (H4SiO4). The basic of all silicates is
the anion. In SiO4–4 anion, In SiO4–4 Si is sp3hybridized, and it forms four covalent
bonds with four negatively charged oxygen atoms. SiO4–4 anion has a tetrahedral
shape.
TYPES OF SILICATES:
1.Ortho silicates: having basic unit as SiO4 4- for eg Zircon ZrSiO4
2. Pyro silicates: two silicon tetrahedra share one corner ,basic unit is Si2O7 6- for eg
Thortevetite Sc2Si2O7
3.Cyclic silicates: two oxygens are shared ,Si3O9 6- eg wollastonite Ca3Si3O9
4.Chain silicates: two oxygens are shared of each silica tetrahedral unit (SiO3)n 2- eg
spodumene LiAl(SiO3)2
5. Sheet silicates : three oxygens are shared of each unit, Si2O5 2- for eg kaolinite
Al(OH)4Si2O5
6. Three dimensional silicates: all the 4 oxygens are shared of each unit.general
formula is (SiO2)n eg quartz
ZEOLITES:
Zeolites are alumino silicates having general formula Mx/n (AlO2)x(SiO2)y.zH2O
Zeoltes are porous due to network framework structure.Zeolites are used as catalyst in
petrochemical industries for eg ZSM-5 is used to convert alcohols into gasoline.