CBSE Class 12-science Answered

1. Potash alum

Formula of potash alum

It is the chemical compound of potassium double sulphate of aluminum. Its chemical formula is KAl(SO₄)₂and it is commonly found in its dodecahydrate form as KAl(SO₄)₂·12(H₂O).


Preparation of potash alum

Aluminum metal is treated with hot aqueous KOH solution. Aluminium dissolves as potassium aluminate, KAl(OH)4, salt.

2Al_(s) + 2KOH_(aq) + 6H₂O_(l) → 2KAl(OH)_{4 (aq)} + 3H₂

Potassium aluminate solution on treatment with dil. Sulphuric acid first gives precipitate Al(OH)₃, which dissolves on addition of small excess of H₂SO₄ and heating.

2KOH_(aq) + H₂SO_4(aq) →  2Al(OH)_{3 (s)} + K₂SO_4(aq) + 2H₂O_(l)

2Al(OH)_{3 (s)} + 3 H₂SO_4(aq) → Al₂(SO₄)_3(aq) +6H₂O_(l)

The resulting solution is concentrated to near saturation and cooled. On cooling crystals of potash alum crystallize out.

K₂SO_4(aq) + Al₂(SO₄)_3(aq) + 24H₂O_(l)  →  K₂SO₄.Al₂(SO₄)₃. 24H₂O_(s)

Uses of potash alum

Chemical

in tanning of leather to prepare the hide

as a mordant for dyes

in the clarifying of turbid liquids, including post-storm treatment of lakes to precipitate contaminants

as a fire retardant in textile products

(historically) as a hardener for photographic emulsions (films and papers), usually as part of the fixer. Modern alternatives are superior.

Medicinal

as an astringent/styptic and antiseptic.

as a natural deodorant by inhibiting the growth of the bacteria responsible for body odor.

after shaving to prepare the skin

to reduce bleeding in minor cuts and abrasions, nosebleeds, and hemorrhoids.

Traditional

In Ayurveda, where it is called phitkari or saurashtri.

In traditional Chinese medicine it is called ming fan.

2. Silicones

Structure of Silicones

Silicones are polymers that include silicon together with carbon, hydrogen, oxygen, and sometimes other elements.

These are group of organosilicon polymers. The repeating units is

These are prepared by polymerization of alkyl or aryl substituted silicon chlorides, R _n SiCl _{(4 − n),}

Preparation of Silicons

Alkyl or aryl substituted silicon chlorides, R _n SiCl _{(4 − n),} are the starting materials for the manufacture of silicones. [R is alkyl or aryl group]

The chain length of the polymer can be controlled by adding (CH ₃ ) ₃ SiCl.

Uses of Silicones

In making aquariums, manufacturers now commonly use 100% silicone sealant to join glass plates.

In the automotive field, silicone grease is typically used as a lubricant for brake components since it is stable at high temperatures, is not water-soluble, and is far less likely than other lubricants to foul.

Silicone films can be applied to such silica-based substrates as glass to form a covalently bonded hydrophobic coating.