(b) Halogens — Chlorine (Cl₂) as Representative

Laboratory Preparation of Chlorine

By oxidizing concentrated hydrochloric acid with manganese(IV) oxide (MnO₂) or potassium tetraoxomanganate(VII) (KMnO₄), with gentle heating.

Industrial Preparation — Electrolysis of Brine

Chlorine is produced industrially by the electrolysis of concentrated sodium chloride solution (brine) using a diaphragm cell.

At the anode: \(\ce{2Cl- -> Cl2 + 2e-}\)  |  At the cathode: \(\ce{2H+ + 2e- -> H2}\)

Properties of Chlorine

• Greenish-yellow gas with a pungent, choking smell.
• Denser than air — collected by downward delivery (upward displacement of air).
• Soluble in water, forming chlorine water: \(\ce{Cl2 + H2O -> HCl + HOCl}\)
• Powerful oxidizing agent and bleaching agent (in the presence of moisture).

Uses of Chlorine

• Water sterilization: Kills bacteria in drinking water and swimming pools.
• Bleaching: Used in textile and paper industries.
• Manufacture of HCl: \(\ce{H2 + Cl2 -> 2HCl}\)
• Manufacture of plastics (PVC): Chloroethene → polyvinyl chloride.
• Insecticides: e.g., DDT, BHC.

Hydrogen Chloride (HCl) & Hydrochloric Acid

Preparation of HCl gas: By heating sodium chloride with concentrated tetraoxosulphate(VI) acid.

HCl gas is colourless, fumes in moist air, highly soluble in water (forming hydrochloric acid), and turns damp blue litmus paper red.

Test for Chlorides (Cl⁻)

(c) Oxygen and Sulphur

(i) Oxygen — O₂

Laboratory Preparation

By the thermal decomposition of potassium trioxochlorate(V) (KClO₃) using manganese(IV) oxide (MnO₂) as a catalyst at ~300°C.

Alternatively, by decomposing hydrogen peroxide with MnO₂: \(\ce{2H2O2(aq) ->[\text{MnO2}] 2H2O(l) + O2(g)}\)

Commercial Production from Liquid Air

Air is liquefied by repeated compression and cooling. The liquid air is fractionally distilled. Nitrogen (b.p. -196°C) boils off first, leaving liquid oxygen (b.p. -183°C).

Classification of Oxides

Ozone (O₃) — Allotrope of Oxygen

Trioxygen (ozone) is a pale blue gas with a sharp smell. It is an allotrope of oxygen. In the stratosphere, ozone absorbs harmful UV radiation, protecting life on Earth. The ozone layer is depleted by CFCs (chlorofluorocarbons).

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(ii) Sulphur

Allotropes of Sulphur

• Rhombic (α-sulphur): Stable at room temperature; yellow, crystalline; melting point ~113°C.
• Monoclinic (β-sulphur): Stable above 96°C; needle-shaped crystals; melting point ~119°C.
• Plastic (amorphous) sulphur: Formed by pouring molten sulphur into cold water; elastic and rubbery.

Uses: Vulcanization of rubber, manufacture of H₂SO₄, gunpowder, fungicides, and matches.

Sulphur(IV) Oxide — SO₂

Preparation: By heating sodium trioxosulphate(IV) with dilute HCl or by burning sulphur in air.

Properties: Colourless gas with a choking smell; acidic oxide; turns acidified K₂Cr₂O₇ from orange to green (reducing agent). Reacts with alkalis: \(\ce{SO2 + 2NaOH -> Na2SO3 + H2O}\)

Tetraoxosulphate(VI) Acid — H₂SO₄ (Contact Process)

Commercial Preparation (Contact Process):

1. Sulphur is burned in dry air: \(\ce{S + O2 -> SO2}\)
2. SO₂ is oxidized to SO₃ using V₂O₅ catalyst at 450°C: \(\ce{2SO2 + O2 <=>[V2O5][450°C] 2SO3}\)
3. SO₃ is absorbed in concentrated H₂SO₄ to form oleum, then diluted: \(\ce{SO3 + H2SO4 -> H2S2O7}\) (oleum); \(\ce{H2S2O7 + H2O -> 2H2SO4}\)

Properties of H₂SO₄: As a dilute acid — reacts with metals above hydrogen; as an oxidizing agent — oxidizes Cu to CuSO₄; as a dehydrating agent — chars sugar and removes water from organic compounds.

Hydrogen Sulphide — H₂S

Preparation: By the action of dilute HCl on iron(II) sulphide (FeS).

Properties: Colourless gas with a rotten-egg smell; weak dibasic acid; strong reducing agent; precipitating agent — forms coloured sulphide precipitates with metal ions.

Tests Summary for Sulphur Compounds

(d) Nitrogen

Laboratory Preparation of Nitrogen

By heating a mixture of ammonium chloride (NH₄Cl) and sodium nitrite (NaNO₂).

Alternatively, by passing ammonia over heated copper(II) oxide: \(\ce{2NH3 + 3CuO -> 3Cu + 3H2O + N2}\)

Production from Liquid Air

Fractional distillation of liquid air yields nitrogen (b.p. -196°C) which boils off first, leaving oxygen (b.p. -183°C).

Ammonia — NH₃

Laboratory Preparation

By heating an ammonium salt (e.g., NH₄Cl) with a base (e.g., Ca(OH)₂).

Industrial Preparation — Haber Process

Conditions: Iron catalyst (with K₂O/Al₂O₃ promoters), 450°C, 200 atmospheres. The reaction is exothermic and reversible.

Properties of Ammonia

• Colourless gas with a pungent, characteristic smell.
• Less dense than air — collected by downward delivery.
• Highly soluble in water, forming a weak alkaline solution: \(\ce{NH3 + H2O <=> NH4+ + OH-}\)
• Turns damp red litmus paper blue.
• Forms white fumes with HCl gas: \(\ce{NH3 + HCl -> NH4Cl}\)

Test for NH₄⁺ (Ammonium Ion)

Trioxonitrate(V) Acid — HNO₃

Laboratory Preparation: By heating potassium trioxonitrate(V) (KNO₃) with concentrated H₂SO₄.

Properties: Colourless fuming liquid; strong acid; strong oxidizing agent; nitrates decompose on heating.

Test for NO₃⁻ (Nitrate Ion)

Oxides of Nitrogen & The Nitrogen Cycle

The Nitrogen Cycle involves: nitrogen fixation (by lightning and bacteria), nitrification (NH₄⁺→NO₂⁻→NO₃⁻), assimilation by plants, ammonification (decay), and denitrification (NO₃⁻→N₂). It maintains the balance of nitrogen in the biosphere.

(e) Carbon

Allotropes of Carbon

Carbon(IV) Oxide — CO₂

Laboratory Preparation

By the action of dilute HCl on marble chips (CaCO₃) or any trioxocarbonate(IV) salt.

Properties of CO₂

• Colourless, odourless gas.
• Denser than air — collected by upward displacement of air.
• Turns lime water milky: \(\ce{CO2 + Ca(OH)2 -> CaCO3 \downarrow + H2O}\)
• Does not support combustion; used in fire extinguishers.
• Solid CO₂ (dry ice) sublimes at -78°C.

Test for CO₃²⁻ and HCO₃⁻

Carbon(II) Oxide — CO

Laboratory Preparation: By passing CO₂ over heated carbon (charcoal) at high temperature, or by dehydrating methanoic acid (HCOOH) with concentrated H₂SO₄.

Effect on Blood: CO binds with haemoglobin ~250 times more strongly than oxygen, forming carboxyhaemoglobin, reducing oxygen transport — leading to suffocation (carbon monoxide poisoning).

Sources: Incomplete combustion of carbon-containing fuels; charcoal fires; car exhaust fumes; cigarette smoke.

Coal & Destructive Distillation

Products of Destructive Distillation of Coal: Coke (solid residue), coal tar (liquid), ammoniacal liquor, and coal gas (flammable gases).

Products of Destructive Distillation of Wood: Charcoal (solid), pyroligneous acid (liquid — contains acetic acid, methanol), and wood gas.

Coke & Synthetic Gas

Coke: A hard, porous, high-carbon solid obtained from destructive distillation of coal. Used as a smokeless fuel, in metallurgy (blast furnaces), and in producing water gas.

Synthetic Gas (Syngas): A mixture of CO and H₂ produced by passing steam over red-hot coke. Used in the Fischer-Tropsch process to produce hydrocarbons and as a fuel.

📋 Comprehensive Tests Summary — SSCE

Solubility Trend of Group II Sulphates