Here we are providing the short notes on Chemistry which will be help to crack science part in may examinations like SSC, Railway and other competitive exams.
CHEMICALS IN AGRICULTURE
Fertilizers
- Urea is the best fertilizer as it leaves only carbon dioxide after ammonia, has been assimilated by plants.
- It has 46.6% mitrogen and it does not alter the pH of the soil.
- Mixture of Ca(CN)2 and C is known as nitrolim. Commerically, calcium nitrate is known as Norvegian salt petre.
- The mixture of nitrogenous, phosphatic and potash fertilizers in suitable amounts, is called NPK fertilizers.
Pesticides
Pesticides are the chemicals which are applied to crops, e.g. DDT and malathion.
Difethialone
Vitamin K has been suggested and +successfully used, as antidote for pets or humans accidentally or intentionally exposed to anticoagulant poisons.
Chemicals in medicines
Analgesics (Pain relievers)
These reduce pain. Aspirin and paracetamol are non-narcotic analgesics. Aspirin reduces fever, prevents platelet coagulation.
Narcotic analgesics are chiefly used for the relief of post operative pain, cardiac pain and pains of terminal cancer and in child birth.
Polymerization
- Polymers are defined as high molecular mass macromolecules, which consist of repeating structural units derived from the corresponding monomers.
- Polymers occur in nature also. Cotton, for example, is a polymer called cellulose. Cellulose is made up of a large number of glucose units.
On the basis of intermolecular forces Polymers are classified as:
- Elastomers- rubber, buna-S, buna-N, neoprene etc.
- Fibres – polyamides (nylon 6, 6), polyesters (Terylene), etc.
- Thermoplastic polymers – Such plastic which gets deformed easily on heating and can be bent easily are known as thermoplastics. Polythene and PVC, Polythene, Polystyrene, Polyvinyls, etc.
- Thermosetting Polymers – some plastics which when moulded once, can not be softened by heating. These are called thermosetting plastics.eg: bakelite, melamine etc.
Few important polymers are:
(a)Polythene
(i)Low density polythene-polymerisation of ethene under high pressure in the presence of traces of dioxygen or a peroxide initiator (catalyst).
(ii)High density Polythene – polymerisation of ethene in the presence of a catalyst such as triethylaluminium and titanium tetrachloride (Ziegler-Natta catalyst).
(b)Polytetrafluoroethene (Teflon)- Teflon is manufactured by heating tetrafluoroethene with a free radical or persulphate catalyst at high pressures.
(c)Polyacrylonitrile – polymer of acrylonitrile in presence of a peroxide catalyst.
Condensation Polymerisation
(a)Polyamides – possess amide linkages
(i) Nylon 6, 6 – prepared by the condensation polymerization of hexamethylenediamine with adipic acid under high pressure and at high temperature
- ii) Nylon 6 – obtained by heating caprolactum with water at a high temperature.
(b)Polyesters – polycondensation products of dicarboxylic acids and diols. Polyester is another synthetic fibre. Fabric made from this fibre does not get wrinkled easily. It remains crisp and is easy to wash. So, it is quite suitable for making dress material.
Eg: Terylene is the best known example of polyesters. It is prepared by ethylene glycol and terephthalic acid.It can be drawn into very fine fibres that can be woven like any other yarn.
(c)Phenol – formaldehyde polymer (Bakelite and related polymers)
Prepared by the condensation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst.
The initial product could be a linear product – Novolac used in paints. Novolac on heating with formaldehyde undergoes cross linking to form an infusible solid mass called bakelite. It is used for making combs, phonograph records, electrical switches and handles of various utensils.
Eg. Melamine – Melamine formaldehyde polymer is formed by the condensation polymerisation of melamine and formaldehyde. Melamine is a versatile material. It resists fire and can tolerate heat better than other plastics. It is used for making floor tiles, kitchenware and fabrics which resist fire. It is used in the manufacture of unbreakable crockery.
Copolymerisation.
Natural rubber – Natural rubber may be considered as a linear polymer of isoprene (2-methyl-1, 3-butadiene) and is also called as cis – 1, 4 – polyisoprene.
Vulcanisation of rubber-This process consists of heating a mixture of raw rubber with sulphur and an appropriate additive at a temperature range between 373 K to 415 K so that rubber gets stiffened.
Synthetic Rubbers –
(i) Neoprene – by the free radical polymerisation of chloroprene.
Rayon – rayon or artificial silk. Although rayon is obtained from a natural source, wood pulp, yet it is a man-made fibre.
Nylon – Nylon is also used for making parachutes and ropes for rock climbing. A nylon thread is actually
