Fundamentals of Applied Chemistry

(a) Urea

Urea, CO(NH₂)₂, contains about 46% nitrogen, so it is a nitrogenous fertilizer. Superphosphate supplies phosphorus, potash supplies potassium, and lime is used to correct soil acidity.

Polymer

A polymer is a very large molecule (macromolecule) of high molecular mass formed by the repeated linking of a large number of small, simple molecules called monomers. The process of joining these monomers is called polymerisation. For example, many molecules of ethene ($\mathrm{CH_{2}=CH_{2}}$) join to form polythene.

Homopolymer

A homopolymer is a polymer formed from only one type of monomer unit. The repeating unit is identical throughout the chain.

Example: Polythene, formed by the polymerisation of ethene monomer units only.

$n\,\mathrm{CH_{2}=CH_{2}} \longrightarrow \mathrm{-(CH_{2}-CH_{2})_n-}$

Copolymer

A copolymer is a polymer formed by the polymerisation of two or more different types of monomer units. The different units arrange themselves in the chain, giving the polymer new properties.

Example: Nylon-6,6, formed by the condensation of hexamethylenediamine and adipic acid (two different monomers).

$n\,\mathrm{H_{2}N-(CH_{2})_{6}-NH_{2}} + n\,\mathrm{HOOC-(CH_{2})_{4}-COOH} \longrightarrow \mathrm{[-NH-(CH_{2})_{6}-NH-CO-(CH_{2})_{4}-CO-]_n} + 2n\,\mathrm{H_{2}O}$

Two important uses of synthetic polymers in daily life

1. Polythene is used for making carry bags, buckets, water pipes and packaging materials because it is light, strong and chemically inert.
2. Nylon is used for making ropes, fishing nets, parachutes and clothing because of its high tensile strength and resistance to wear.

Other examples: PVC is used for electrical insulation and water pipes; bakelite is used for making electrical switches and handles of utensils.

Soap vs detergent

Saponification reaction (soap preparation): a fat/oil (triglyceride) is boiled with NaOH, giving soap and glycerol.

$C_3H_5(OOCR)_3 + 3NaOH \rightarrow 3RCOONa + C_3H_5(OH)_3$ $\text{(fat/oil)} \quad \text{(alkali)} \quad \text{(soap)} \quad \text{(glycerol)}$

For example, with glyceryl stearate:

$C_3H_5(OOC C_{17}H_{35})_3 + 3NaOH \rightarrow 3C_{17}H_{35}COONa + C_3H_5(OH)_3$

The soap is salted out using common salt and purified.

Chemical fertilizers are inorganic, synthetically manufactured or mined salts that supply one or more essential plant nutrients (N, P, K and sometimes secondary/micronutrients) in concentrated, water-soluble form. They are applied to soil or sprayed on plants to increase crop yield. They are also called mineral fertilizers or artificial fertilizers.

Classification of chemical fertilizers

1. Nitrogenous fertilizers — supply nitrogen, mainly as ammonium (NH₄⁺) or nitrate (NO₃⁻) ions. They promote vegetative growth, giving green, leafy, healthy plants. Example: Urea, CO(NH₂)₂ — contains about 46% N, the highest among solid nitrogenous fertilizers. (Others: ammonium sulphate, calcium ammonium nitrate.)

2. Phosphatic fertilizers — supply phosphorus in available form (H₂PO₄⁻/HPO₄²⁻). They promote root development, flowering, fruiting and seed formation. Example: Superphosphate of lime, Ca(H₂PO₄)₂·CaSO₄·2H₂O — contains about 16–20% P₂O₅. (Others: triple superphosphate, bone meal.)

3. Potassic fertilizers — supply potassium as K⁺. They improve stem strength, disease resistance, and the quality and size of fruits and grains. Example: Muriate of potash (potassium chloride), KCl — contains about 60% K₂O. (Others: potassium sulphate, K₂SO₄.)

Role of NPK in plant growth

• Nitrogen (N): A constituent of proteins, nucleic acids, chlorophyll and many enzymes. It is essential for cell division, vegetative growth and the green colour of leaves. Deficiency causes stunted growth and yellowing (chlorosis) of leaves.
• Phosphorus (P): A constituent of nucleic acids (DNA, RNA), ATP, phospholipids and coenzymes. It is needed for root development, energy transfer, cell division, flowering and seed/fruit formation. Deficiency causes poor root growth and delayed maturity.
• Potassium (K): Not a structural element of organic molecules, but it acts as an enzyme activator and regulates stomatal opening, water balance and osmotic pressure. It increases stem rigidity, resistance to pests and diseases, and improves the quality of fruits and grains. Deficiency causes weak stems and marginal leaf scorching.

Thus NPK are the three primary macronutrients, and their balanced supply in the correct ratio is necessary for healthy, high-yielding crops.

Harmful effects of excessive use of chemical fertilizers

1. Soil degradation: Continuous heavy use kills or reduces beneficial soil microorganisms (nitrogen-fixing bacteria, earthworms), destroys soil structure and humus, and causes hardening, salinity and reduced fertility. Excess nitrogen also makes the soil acidic, lowering its pH.
2. Water pollution (eutrophication): Soluble nitrates and phosphates are leached by rain into ponds, lakes and rivers. They act as nutrients for algae, causing excessive algal blooms; when these die and decompose, dissolved oxygen falls, killing fish and other aquatic life. Nitrate-contaminated drinking water also causes methaemoglobinaemia (blue baby syndrome) in infants.
3. Bioaccumulation and health hazards: Non-biodegradable fertilizer residues and heavy-metal impurities (Cd, Pb, As) enter crops and pass into the food chain, causing toxicity in humans and animals. Excess nitrate in vegetables is converted to carcinogenic nitrosamines in the body.

Eco-friendly alternatives

1. Organic manures and compost: Farmyard manure, green manure, compost and vermicompost supply nutrients slowly, add humus, improve soil texture and water-holding capacity, and support beneficial microbes.
2. Biofertilizers: Rhizobium, Azotobacter, Azospirillum (nitrogen fixers) and phosphate-solubilising bacteria (PSB) or mycorrhiza enrich soil nutrients biologically without polluting soil or water. Crop rotation with legumes and use of neem-based organic pesticides are other eco-friendly practices.