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Silicones or poly siloxanes are organo silicon polymers with general empirical formula (R2SiO). Since their empirical formula is similar to that of ketone (R2CO), they were named silicones.

(b) (i) and (iii)

Completereactions:

All silicones are water repellent, this is due to the presence of organic side groups that surrounds the silicon which makes the molecule looks like an alkane. Therefore silicones are water repellent.

(a) Nitrogen – Tetragens

(d) Fluorine in OF2 – +4

(d) Fischer tropsch Synthesis – HCOOH

(a) A and R are correct and R explains A.

1. ns2 np2

2. covalent

3. Fluorine

4. +3

5. MxBy

6. neutrons

7. Amorphous boron

8. Boron

9. Na2B4O7.10H2O

10. Borax

1. All silicones are water repellent.

2. They are thermal and electrical insulators.

3. Chemically they are inert.

4. Lower silicones are oily liquids whereas higher silicones with long chain structure are waxy solids.

5. The viscosity of silicon oil remains constant and doesn’t change with temperature and they don’t thicken during winter.

Boric acid when heated at 373 K gives metaboric acid (A) and A heated at 413 K it gives tetraboric acid (B). B heated at red hot it gives boric anhydride (C)

1. Linear silicones – They are obtained by the hydrolysis and subsequent condensation of dialkyl or diaryl silicon chlorides.

• Silicone rubbers – These silicones are bridged together by methylene or similar groups.
• Silicone resins – They are obtained by blending silicones with organic resins such as acrylic esters.

2. Cyclic silicones – These are obtained by the hydrolysis of R2SiCl2 .

3. Cross linked silicones – They are obtained by hydrolysis of RSiClr

1. Compound (A) is borax, which is used in the manufacture of opticals.

2. Borax (A) on heating to give borax glass (B). Borax glass on further heating to give sodium metaborate (C).

3. Borax (A) reacts with hydrochloride acid to give boric acid.

1. An element (A) extracted form kemite is boron.

2. Boron reacts with nitrogen at high temperature gives Boron nitride (B)

3. Boron reacts with alkali (NaOH) to gives sodium borate (C).

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(b) stable electronic configuration

The elements of group-18 have completely filled s and p orbitals, hence they are more stable and have least reactivity. Therefore group-18 elements are called inert gases. ns2np6 is the general electronic configuration of group elements.

(a) Pyroxenes – Phenacite

In heavier post-transition metals, the outer selectron(m) have a tendency to remain inert and show reluctance to take part in the bonding (only p-orbitalinvolved in chemical bonding), which is known as inert pair effect.

(c) Pnictogens – ns2np4

1. Sheet silicates:

Silicates which contain (Si2O5)n2n- are called sheet or phyllo silicates. In these, Each [SiO4]4- tetrahedron unit shares three oxygen atoms with others and thus by forming twodimensional sheets. These sheets,silicates form layered structures in which silicate sheets are stacked over each other. The attractive forces between these layers are very week, hence they can be cleaved easily just like graphite.

Example: Talc, Mica etc…

2. Three dimensional silicate: Silicates in which all the oxygen atoms of [SiO4]4- tetrahedra are shared with other tetrahedra to form three-dimensional network are called three dimensional or tecto silicates. They have general formula (SiO2)n .

Examples:

Quartz. These tecto silicates can be converted into Three dimentional aluminosilicates by replacing [SiO4]4-units by [AlO4]5- units.

E.g. Feldspar, Zeolites etc.,