Oxygen Family (Group 16): Ozone, H₂O₂ & H₂SO₄ (Contact Process) Why Group 16 matters for NEET Group 16 (oxygen family) runs from oxygen to polonium. It gives us life-supporting O₂ and O₃, the versatile oxidant H₂O₂, and industry’s king acid H₂SO₄. Questions often test: trends, allotropes, ozone chemistry, redox of H₂O₂, and the Contact Process conditions. Oxygen family (Group 16) chalcogens Elements O, S, Se, Te, Po; valence shell ns2 np4; show −2 (O), +2/+4/+6 (heavier) oxidation states. Triatomic oxygen (O3), a bent, resonance-stabilized, powerful oxidant; protects from UV-B in stratosphere. Ozone H2O2; contains a peroxide (−O−O−) linkage; both oxidizing and reducing depending on reactant/pH. Hydrogen peroxide dihydrogen peroxide −O−O− functional group with weak O−O bond; key to H2O2 reactivity. Peroxide group Different structural forms of sulphur: rhombic (α-S8), monoclinic (β-S8), plastic S, etc. Sulphur allotrope Sulphur dioxide SO2; bent molecule; acts as both oxidizing and reducing agent; important intermediate to H2SO4. SO3; formed catalytically from SO2 + O2; absorbed into H2SO4 to make oleum. Sulphur trioxide Industrial manufacture of H2SO4 via catalytic oxidation of SO2 to SO3 over V2O5 at ~720 K, 1–2 atm. Contact process Oleum H2S2O7 (fuming sulphuric acid); made by absorbing SO3 in H2SO4; then diluted to H2SO4. Vulcanization Cross-linking rubber using sulphur; improves strength and elasticity. Substance that removes water from other compounds (e.g., conc. H2SO4 charring sugar). Dehydrating agent Oxidative cleavage of C=C using ozone (covered in organic; cross-link here for context). Ozonolysis Key terms at a glance Electronic configuration and periodic trends All Group 16 elements have valence configuration ns2 np4. Down the group: atomic radius increases, ionization enthalpy decreases, electronegativity decreases, and metallic character increases (O, S, Se are non-metals; Te is metalloid; Po is metallic). Oxidation states: oxygen is dominantly −2; sulphur commonly shows +2, +4, +6 (and −2 in sulphides). Multiple bonds become less stable down the group; catenation (S−S) is significant for sulphur. Atomic radius Increases: O < S < Se < Te < Po More shells down group First ionization enthalpy (IE1) Decreases: O > S > Se > Te > Po Valence e− farther from nucleus Electronegativity (Pauling) Decreases: O > S > Se > Te > Po O is most electronegative in group Metallic character Increases: O (non-metal) → Po (metal) Po shows metallic behaviour Common oxidation states O: −2; S: −2, +2, +4, +6; Se/Te: +4, +6; Po: +2, +4 Higher states stabilized by d-orbitals and size Melting point (general) Increases down to Te, then Po variable Structure and bonding effects Property Trend/order (O → Po) Comment Group 16 qualitative trends gpt-image-2 2026-05-26T17:04:57.776Z Trend sketch: atomic radius ↑, IE and EN ↓, metallic character ↑ from O to Po. Periodic trend infographic for Group 16 (O, S, Se, Te, Po). Arrows for radius (upward), ionization enthalpy and electronegativity (downward), metallic character (upward). Clean vector style, each element labeled, neutral palette, no dense text. Allotropy of sulphur — rhombic, monoclinic, and plastic Sulphur shows rich allotropy. The most common crystalline forms are rhombic (α-sulphur) and monoclinic (β-sulphur). Both contain crown-shaped S8 rings but pack differently. • Rhombic sulphur (α-S8) is stable below about 96 ° C. • Monoclinic sulphur (β-S8) is stable between about 96–119 ° C; on cooling, it slowly reverts to rhombic. • Sudden quenching of molten sulphur in cold water forms plastic sulphur (amorphous, elastic initially) that gradually becomes crystalline. In vapour at high temperature, smaller species like S2 (paramagnetic) and S6 can exist. Crown-shaped S8 ring and how rhombic vs monoclinic crystals pack. Panel diagram: left shows S8 crown ring with labeled S–S bond lengths; middle shows rhombic sulphur crystal lattice depiction; right shows monoclinic lattice. Include temperature stability labels (< 96 C rhombic; 96– 119 C monoclinic). Vector chemistry style. 2026-05-26T17:04:57.758Z gpt-image-2 Rhombic (α-S8) Crystalline; S8 crown rings Stable below ~96 ° C Pale yellow crystals; most stable at room T Monoclinic (β-S8) Crystalline; S8 rings (different packing) 96–119 ° C Formed by cooling molten S; reverts to α on standing Plastic sulphur Amorphous; long chain-like S Metastable; slowly crystallizes Made by pouring boiling S into cold water; elastic initially Vapour species S2, S6 (at high T) Gas phase S2 is paramagnetic; demonstrates diversity Form Structure Stability range Notes/uses Allotropes of sulphur — key contrasts Sulphur exists only as S8. Beyond solid S8 allotropes, sulphur also forms S2 (in hot vapour) and other rings like S6; plastic sulphur is an amorphous form. Ozone (O3) — structure, prep, and oxidizing behaviour Structure: Ozone is a bent molecule (bond angle ≈ 117 ). It is a resonance hybrid of two canonical structures, giving equal O–O bond lengths ≈ 1.28 , intermediate between O2 (1.21 ) and an O–O single bond (1.48 ). The delocalization explains its extra stability and reactivity. Preparation: Ozone is formed by silent electric discharge of dry oxygen. The reaction is endothermic. gpt-image-2 2026-05-26T17:04:57.323Z Diagram of ozone: central O with two equivalent O–O bonds; show two resonance structures with curved arrows and a resonance hybrid with partial bonds. Mark bond angle ~117° and bond length 1.28 Å. Clean vector style. Resonance in bent ozone; both O–O bonds identical (bond order 1.5). Endothermic ( H 0); performed in an ozonizer with dry O2. Ozone formation (silent discharge) Oxidizing power: O3 bleaches and oxidizes many species. A classic test is KI–starch paper turning blue as iodide is oxidized to iodine. Ozone also oxidizes lead(II) sulphide (PbS, black) to white lead(II) sulphate (PbSO4). Basis of the KI–starch paper test for ozone. Structure Bent; resonance hybrid; bond order 1.5 Diatomic; O=O double bond Bond length ≈ 1.28 ≈ 1.21 Color/odor Pale blue; pungent Colorless; odorless Oxidizing power Stronger oxidant Strong but less than O3 Biological role Stratospheric UV-B shield Cellular respiration (O2) Feature Ozone (O3) Oxygen (O2) Ozone vs molecular oxygen Atmospheric cross-section: troposphere/stratosphere labeled; show O2 photolysis to O(3P), O + O2 + M -> O3, and UV-B absorption by O3. Include approximate altitude band 20–50 km. Vector infographic, red arrows for light. Stratospheric ozone layer (≈20–50 km) absorbing UV-B; photochemical formation and breakdown. 2026-05-26T17:04:58.521Z gpt-image-2 CFC-based ozone depletion: chlorine-catalyzed O3 destruction. Mechanism panels: 1) hv breaks CF2Cl2 releasing Cl·; 2) Cl· + O3 -> ClO· + O2; 3) ClO· + O -> Cl· + O2; Net: O3 + O -> 2 O2. Show radical dots, curved arrows minimal, labels only species. Clean vector chemistry style. 2026-05-26T17:04:59.925Z gpt-image-2 Ozone is a resonance hybrid with two equivalent O–O bonds, each with bond order 1.5 and bond length ≈ 1.28 . O3 has three O–O single bonds. remember Montreal Protocol (1987) phased out many CFCs to protect the ozone layer. Hydrogen peroxide (H2O2) — structure, redox, preparation, uses Structure: H2O2 is not planar. Think of an open book — the two O–H pages are skewed. It predominantly adopts a gauche (skew) conformation with an O–O–H dihedral angle ≈ 111 (vapour) and ≈ 94 (solid). The weak O–O bond (peroxide) drives its reactivity. Stability and storage: Decomposes to water and oxygen; catalyzed by light and traces of metals. Stored in amber, wax-lined, stabilized bottles. 2026-05-26T17:05:00.140Z 3D ball-stick of H2O2 showing nonplanar skew conformation; mark dihedral 111° (vapour) and 94° (solid). Include −O−O− labeled as peroxide. Clean vector chemistry style. Skew (gauche) ‘open-book’ structure of H2O2 with dihedral angles. gpt-image-2 Disproportionation (as written in many exams) Often written per mole; catalyzed by light/metal ions. Balanced decomposition Laboratory observation: effervescence of O2. Dual redox nature: • As oxidizing agent (acidic): oxidizes iodide to iodine — basis of iodometric titrations. • As reducing agent (towards strong oxidants): reduces metal oxides like Ag2O to Ag, itself forming O2. H2O2 as oxidant (acidic) High-yield: commonly tested redox behaviour. H2O2 as reducing agent H2O2 reduces Ag(I) oxide to Ag(0); H2O2 is oxidized to O2. Preparation via peroxydisulphate route (electrolysis of conc. H2SO4) At the anode during electrolysis of cold, concentrated H2SO4, peroxydisulphate ( S2O8 2− ) forms, which on hydrolysis yields H2O2. This is a classical laboratory/industrial chemistry route (modern bulk industry often uses the anthraquinone process; beyond our scope). Anodic formation of peroxydisulphate in concentrated H2SO4. Hydrolysis step producing hydrogen peroxide. Concentration Name Typical uses Common H2O2 uses by concentration ≈3% Antiseptic solution Wound cleaning, disinfectant ≈6% Hair bleach Bleaching hair and mild fabric bleaching >85% Concentrated (high-test peroxide) Rocket propellant, specialized oxidation processes ≈30% 100-volume H2O2 Releases ~100 volumes of O2 per volume of solution at STP tip “100-volume H2O2” means: 1 volume of solution liberates ~100 volumes of O2 at STP. Many exams take STP molar volume as 22.4 L mol -1 (older convention); modern IUPAC uses 22.7 L mol -1 — follow what your exam uses. H2O2 contains a distinct peroxide (−O−O−) functional group with a weak O−O bond, giving very different chemistry from water. H2O2 is just water with extra oxygen. SO2 quick view — shape and redox character Geometry: SO2 has a bent shape due to a lone pair on sulphur (electron-domain trigonal planar; molecular geometry bent). It is both an oxidizing and a reducing agent. • Reducing behaviour: decolorizes acidified KMnO4 ( MnO4 - reduced; SO2 oxidized). • Oxidizing behaviour: oxidizes H2S to S (SO2 reduced). Industrial sources: burning S or roasting sulphide ores; also produced from hot conc. H2SO4 reacting with certain metals such as Cu. Primary industrial route to SO2 (combustion of sulphur). SO2 from copper and hot concentrated sulphuric acid. Contact Process — manufacture of sulphuric acid (H2SO4) H2SO4 is the world’s most-produced industrial acid. The Contact Process involves: (1) generating SO2, (2) catalytic oxidation to SO3 over V2O5 at ~720 K and 1–2 atm, (3) absorbing SO3 in H2SO4 to form oleum, and (4) controlled dilution of oleum to H2SO4. Direct absorption of SO3 in water is avoided because it forms a fine H2SO4 mist that is hard to absorb safely and efficiently. Contact Process overview Drying tower removes moisture; arsenic scrubbing to protect catalyst Burn S (or roast sulphide ores) to form SO2; dry and purify gases. Equilibrium favored by lower T and higher pressure; optimized for rate/yield Catalytic oxidation of SO2 to SO3 over V2O5. ≈720 K; 1–2 atm; V2O5 catalyst; exothermic equilibrium Dry conditions to prevent acid mist Absorb SO3 in concentrated H2SO4 to form oleum (H2S2O7). Dilute oleum with water to obtain required concentration of H2SO4. Controlled dilution to manage heat release sulphur dioxide sulfur dioxide reactant oxygen dioxygen reactant sulfur trioxide product/intermediate sulphur trioxide oleum disulfuric acid absorbed intermediate final product sulfuric acid sulphuric acid SO2 is catalytically oxidized to SO3, which is then converted to H2SO4 via oleum to avoid dangerous acid mist. 2026-05-26T17:05:00.150Z Contact Process flow: S → SO2 → SO3 (V2O5, 720 K, 1–2 atm) → oleum → H2SO4. Process flow diagram with towers/convertor: burner (S+O2), dust/arsenic purifier, drying tower, catalytic converter (V2O5, 720 K, 1–2 atm), absorber to oleum, dilution to H2SO4. Clean plant PFD style, arrows and labels. gpt-image-2 1. SO2 generation S + O2 → SO2 Excess air; gas purification and drying 2. Catalytic oxidation SO2 + 1/2 O2 ⇌ SO3 V2O5; ≈720 K; 1–2 atm; exothermic 3. Absorption SO3 + H2SO4 → H2S2O7 Avoid water (prevents acid mist) 4. Dilution H2S2O7 + H2O → 2 H2SO4 Controlled addition of water Stage Reaction/operation Key conditions Contact Process steps and conditions Key catalytic step (must-know) H = -197 kJ (exothermic). Formation of oleum by absorbing SO3 into concentrated sulphuric acid. Safe conversion of oleum to sulphuric acid. Stage 1: Combustion of sulphur (or sulphide ores) to SO2. End-to-end Contact Process with gas cleaning, catalytic conversion, absorption to oleum, and dilution. In the Contact Process, SO3 is directly absorbed in water to give H2SO4. Direct hydration forms a dense aerosol (acid mist) that is hard to absorb. SO3 is first absorbed in H2SO4 to make oleum, which is then diluted. Properties and key reactions of H2SO4 Conc. H2SO4 is a strong dehydrating and oxidizing agent (hot, concentrated). It dehydrates carbohydrates to carbon and water; and oxidizes some metals/non-metals while itself reducing to SO2. Dehydration of sucrose Classic ‘black column’ demonstration — shows powerful dehydrating action. Oxidizing behaviour on copper (hot conc. acid) Cu is oxidized; H2SO4 is reduced to SO2. Fertilizers Manufacture of superphosphate, ammonium sulphate Batteries Lead–acid battery electrolyte (≈35–37% H2SO4) Metallurgy Pickling (removing oxide scale) from steel Petroleum refining Alkylation and dehydration steps Chemicals Dyes, detergents, explosives, and many inorganic salts Sector Use H2SO4 — major industrial uses Absolutely memorize: catalyst (V2O5), temperature (~720 K), pressure (1–2 atm), and the oleum route. These get asked repeatedly. neet-alert Sulphur allotropes around 96: “At 96, Rhombic flips” — below 96 C α (rhombic) stable; above, β (monoclinic) stable. Extra: Elemental sulphur — everyday uses Vulcanization of rubber Fungicides Gunpowder component SO2 is amphoteric in redox: it reduces strong oxidants (e.g., decolorizes acidified KMnO4) but also oxidizes H2S to sulphur. SO2 is only a reducing agent.