Tamilnadu Samacheer Kalvi 12th Chemistry Notes Chapter 3 p-Block Elements-II Notes
Group-15 elements: About 78% of earth atmosphere contains dinitrogen gas. The other elements arsenic, antimony and bismuth are present as sulphides and are not very abundant.
- Nitrogen, the principle gas of atmosphere (78% by volume) is separated industrially from liquid air by fractional distillation.
- Nitrogen gas is rather inert.
- It can form complexes with metal.
Uses of nitrogen:
- Nitrogen is used for the manufacture of ammonia, nitric acid and calcium cyanamide etc.
- Liquid nitrogen is used for producing low temperature required in cryosurgery, and in biological preservation.
- Ammonia is formed by the hydrolysis of urea.
- It is a pungent smelling gas and it is lighter than air.
- It is highly associated through strong hydrogen bonding.
- Ammonia act as a reducing agent.
- Nitric acid is prepared by heating equal amounts of potassium or sodium nitrate with concentrated H2SO4.
- Nitric acid act as an oxidising agent.
- Some metals such as Al, Fe, Co, Ni and Cr are rendered passive in concentrated nitric acid due to the formation of a layer of their oxides on the metal surface.
Uses of nitric acid:
- Nitric acid is used as a oxidising agent and in the preparation of aquaregia.
- Salts of nitric acid are used in photography (AgNO3) and gunpowder for firearms. (NaNO3)
Allotopic forms of phosphorus: Three forms namely,
- White phosphorous
- Red phosphorous
- Black phosphorous
Uses of phosphorous:
- The red phosphorus is used in the match boxes.
- It is also used for the production of certain alloys such as phosphor bronze.
- Phosphine is the most important hydride of phosphorous.
- It is prepared by action of sodium hydroxide with white phosphorous.
- It is poisonous gas with rotten fish smell.
Uses of Phosphine: Phosphine is used for producing smoke screen as it gives large smoke. In a ship, a pierced container with a mixture of calcium carbide and calcium phosphide, liberates phosphine and acetylene when thrown into sea. The liberated phosphine catches fire and ignites acetylene. These burning gases serves as a signal to the approaching ships. This is known as Holmes signal.
- When a slow stream of chlorine is passed over white phosphorous, PCl3 is formed.
- PCl3 is used as a chlorinating agent and for the preparation of H3PO3.
- hen PCl3 is treated with excess chlorine, PCl5 is obtained.
- PCl5 is chorinating agent and is useful for replacing hydroxyl groups by chlorine atom.
- Elements belonging group-16 are called chalgogens or ore forming elements.
- First element oxygen, the most abundant element, exists in both as dioxygen in air and in combined form as oxides.
- The other elements of this groups are, Sulphur, Selenium, Tellurium and Pollonium.
- The atmosphere and water contain 23% and 83% by mass of oxygen respectively.
- The decomposition of hydrogen peroxide in the presence of catalyst produce oxygen.
- Under ordinary condition oxygen exists as a diatomic gas.
- Oxygen exists in two allotropic forms namely dioxygen (O2) and ozone (O3).
Uses of oxygen:
- Oxygen is one of the essential component for the survival of living organisms.
- It is used in welding (oxyacetylene welding)
- Liquid oxygen is used as fuel in rockets etc…
Allotrophic forms of sulphur: The crystallline allotropic forms are,
- Rhombic sulphur
- Monoclinic suphur
Amorphous allotropic forms are,
- Plastic sulphur
- Milk of sulphur
- Colloidal sulphur
- Sulphur dioxide is prepared in the laboratory treating a metal or metal sulphite with sulphuric acid.
- Sulphur dioxide gas is formed in volcanic eruptions.
- It is an acidic oxide.
- It can be liquefied at 2.5 atomospheric pressure and 288 K temperature.
Uses of Sulphur dioxide:
- Sulphur dioxide is used in bleaching hair, silk, wool etc…
- It can be used for disinfecting crops and plants in agriculture.
- Sulphuric acid can be manufactured by lead chamber process or contact process.
- Pure sulphuric acid is a colourless,viscous liquid.
- It is highly reactive and it can act as dehydrating agent.
Uses of Sulphuric acid:
- Sulphuric acid is used in the manufacture of fertilisers, ammonium sulphate and super phosphates and other chemicals such as hydrochloric acid, nitric acid etc…
- It is used as a drying agent and also used in the preparation of pigments, explosives etc..
- The halogens are present in combined form as they are highly reactive.
- They are also called as salt producers.
- The elements present in group-17 are, Fluorine, Chlorine, Bromine, Iodine and Astatine.
- The main source of chlorine is sodiumchloride from sea water.
- Chlorine is highly reactive.
- Chlorine is manufactured by the electrolysis of brine in electrolytic process or by oxidation of HCl by air in Deacon’s process.
- It is a greenish yellow gas with pungent irritating odour.
- Bleaching powder is produced by passing chlorine gas through dry slaked time.
Uses of Chlorine:
- Purification of drinking water.
- Bleaching of cotton textiles, paper and rayon.
- It is used in extraction of gold and platinum.
- It is prepared by the action,of sodium chloride and concentrated sulphuric acid.
- When three parts of cone. HCl and one part of cone. HNO3 are mixed aquaregia (Royal water) is obtained.
Uses of Hydrochloric acid:
- Hydrochloric acid is used for the manufacture of chlorine, ammonium chloride, glucose from com starch etc.,
- It is used in the extraction of glue from bone and also for purification of bone black.
- Direct combination is a useful means of preparing hydrogen chloride. The reaction between hydrogen and fluorine is violent while the reaction between hydrogen and bromine or hydrogen and iodine are reversible and don’t produce pure forms.
- The hydrogen halides are extremely soluble in water due to the ionisation.
- Solution of hydrogen halides are acidic and known as hydrohalic acids.
Interhalogen compounds: Each halogen combines with other halogen to form a series of compounds called interhalogen compounds.
- All the noble gases occur in the atmosphere.
- Elements belongs to group-18 are Helium, Neon, Argon, Krypton, Xenon and Radon.
- The first ionisation energy decreases from helium to radon.
- They are extremely stable and have a small tendency to gain or lose electrons.
- Noble gases are monoatomic.
Uses of Helium:
- Helium and oxygen mixture is used by divers in place of air oxygen mixture. This prevents the painful dangerous condition called bends.
- Helium is used to provide inert atmosphere in electric arc welding of metals.
- Helium has lowest boiling point hence used in cryogenics (low temperature science).
- It is much less denser than air and hence used for filling air balloons.
Uses of Neon: Neon is used in advertisement as neon sign and the brilliant red glow is caused by passing electric current through neon gas under low pressure.
Uses of Argon: Argon prevents the oxidation of hot filament and prolongs the life in filament bulbs
Uses of Krypton: Krypton is used in fluorescent bulbs, flash bulbs etc… Lamps filed with krypton are used in airports as approaching lights as they can penetrate through dense fog.
Uses of Xenon: Xenon is used in fluorescent bulbs, flash bulbs and lasers. Xenon emits an intense light in discharge tubes instantly. Due to this it is used in high speed electronic flash bulbs used by photographers.
Uses of Radon: Radon is radioactive and used as a source of gamma rays.Radon gas is sealed as small capsules and implanted in the body to destroy malignant i.e. cancer growth
|Reactivity of hydrogen||Decreases from fluorine to iodine|
|Stability||Decreases from HF to HI|
|Volatility of the hydrides||HF < HI < HBr < HCl|
|Thermal stability||HF > HI > HBr > HCl|
|Boiling point||HCl < HBr < HI|
|Acid strength||Increases from HF to HI|