Chemistry of Non-metals

This is one of the two heavyweight inorganic chapters — about 4 questions — and it is unashamedly factual: preparations, properties, tests and which reagent does what to whom. The winning strategy is to file each species under three hooks — how it is made, what kind of agent it is (oxidant, reductant, acid, dehydrator…), and its one diagnostic test — and to know the trends that bind families (halogen reactivity falls down the group, HX acidity rises). Every equation below is balanced; in the examination hall, write the same way: charge and atoms both counted.

13.1 Atomic, molecular and nascent hydrogen; isotopes; heavy water; types of oxides

Ordinary hydrogen is the molecular diatomic gas. Two more reactive forms are exam staples: nascent hydrogen [H], the just-born atoms liberated in situ by a reaction such as Zn + 2HCl → ZnCl₂ + 2[H], which reduce things (e.g. acidified KMnO₄, Fe³⁺ salts, KClO₃) that bubbling molecular H₂ through the same solution leaves untouched; and atomic hydrogen, made by passing H₂ through an electric arc — its atoms recombine on a surface releasing so much heat (≈ 4000 °C, the atomic hydrogen torch for welding refractory metals) and it is the strongest reducing form of all: reducing power: atomic H > nascent [H] > molecular H₂. (Modern chemistry treats "nascent" effects as adsorbed hydrogen atoms/electron transfer at the metal surface — the exam keeps the classical name and ranking.)

Isotope	Nucleus	Abundance/behaviour	Uses
Protium ¹H	1 p	99.985 %	—
Deuterium ²H (D)	1 p + 1 n	0.015 %; stable	D₂O, tracers, NMR solvents, fusion fuel
Tritium ³H (T)	1 p + 2 n	radioactive β⁻, t½ 12.3 y	luminous signs, biological labelling (Ch. 12)

Heavy water, D₂O, is obtained in practice by prolonged electrolysis of ordinary water (containing alkali) — H₂ is evolved preferentially, concentrating D₂O behind — or by the Girdler-sulfide exchange industrially. Its molecules are heavier, so its constants crawl up (m.p. 3.8 °C, b.p. 101.4 °C, density 1.106 g cm⁻³, maximum density at 11.6 °C) and its reactions run slower; organisms tolerate only dilution (high fractions are toxic because enzyme kinetics and mitotic spindles depend on those very rates). Its headline job is the neutron moderator in CANDU-type reactors, where its low neutron absorption lets natural (unenriched) uranium burn.

Types of oxides (memory scheme: the partner's character decides):

Type	Examples	Behaviour/test
Acidic (mostly non-metals, high oxidation states)	CO₂, SO₂, SO₃, NO₂, P₂O₅, CrO₃	turn moist litmus red; +base → salt + water; many are acid anhydrides
Basic (metals)	Na₂O, CaO, MgO, CuO	+acid → salt + water
Amphoteric	Al₂O₃, ZnO, PbO, SnO	react with both acid and alkali
Neutral	CO, NO, N₂O, H₂O	neither acidic nor basic to litmus
Peroxides (O = −1)	Na₂O₂, BaO₂, H₂O₂	give H₂O₂ with dilute acid
Superoxides (O = −½)	KO₂, RbO₂	large alkali metals in excess O₂; used to regenerate O₂ in spacecraft/submarines
Mixed	Fe₃O₄ (FeO·Fe₂O₃), Pb₃O₄ (2PbO·PbO₂)	two formal oxides in one lattice

13.2 Ozone: preparation, structure and tests; depletion of the ozone layer

Preparation. A slow, silent (sparkless) electric discharge through dry oxygen in an ozonizer (Siemens/Brodie) gives up to ~10 % of the pale-blue, pungent gas:

3O₂ ⇌ 2O₃ (ΔH = +285 kJ — endothermic; heat destroys the product, hence the silent, cold discharge)

Pure ozone is condensed out as a deep-blue, dangerously explosive liquid. Structure: the molecule is bent (bond angle 116.8°, O–O 127 pm — between single (148) and double (121)), a resonance hybrid of two forms with an sp² central oxygen carrying a lone pair; the delocalized π system makes both bonds equal (bond order 1.5). Properties: a powerful oxidant — stronger than O₂, because it sheds an atom of "nascent oxygen", O₃ → O₂ + [O]: it oxidizes I⁻ to I₂ (starch–iodide paper turns blue — the standard test), black PbS to white PbSO₄, and mercury suffers tailing (oxidized to Hg₂O, it loses its meniscus and sticks to glass — the classic distinguishing test); it bleaches dyes and disinfects drinking water without chlorine's taste. Its quantity is estimated iodometrically (O₃ + 2KI + H₂O → I₂ + O₂ + 2KOH).

The ozone layer (stratosphere, 20–35 km) absorbs 97–99 % of the Sun's medium UV (200–315 nm), shielding DNA from its mutagen. Depletion: chlorofluorocarbons (CFCs — old refrigerants/aerosols) survive to that altitude, where UV frees chlorine atoms that catalytically destroy ozone:

Cl• + O₃ → ClO• + O₂, and ClO• + O → Cl• + O₂ (the Cl• is regenerated: one atom kills ≈ 10⁵ O₃ molecules)

with the Antarctic spring "ozone hole" worsened by polar stratospheric clouds. The Montreal Protocol (1987) phased CFCs out worldwide — atmospheric chlorine is falling and the layer is on a slow (mid-century) mend, the most successful environmental treaty to date; NO from jets and N₂O are also catalytic offenders. Consequences of thinning: more UV-B → skin cancer, cataracts, phytoplankton and crop damage. (Do not confuse good stratospheric ozone with bad ground-level ozone, a photochemical-smog pollutant — Ch. 21.)

13.3 Ammonia, phosphine and nitric acid

Ammonia NH₃. Laboratory: heat NH₄Cl with slaked lime, 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃↑ + 2H₂O, and dry over quicklime (never conc. H₂SO₄ or CaCl₂ — both absorb it). Industrial: Haber, N₂ + 3H₂ ⇌ 2NH₃ at 450 °C, 200 atm, Fe/Mo (Ch. 7's compromise). A colourless, pungent gas, fantastically soluble in water (the fountain experiment: ~700 volumes in 1), giving the weak base NH₄OH. Its trigonal- pyramidal molecule (sp³ N, lone pair at the apex) explains both faces: it is a base (turns litmus blue; dense white fumes of NH₄Cl with HCl vapour — a field test), and a ligand, donating the pair to metal ions: pale-blue Cu(OH)₂ dissolves in excess NH₃ to the deep-blue tetraammine [Cu(NH₃)₄]²⁺ (Schweizer's reagent chemistry, Ch. 14). Tests: Nessler's reagent (alkaline K₂[HgI₄]) gives a brown colour/precipitate with even traces of NH₃/NH₄⁺. Uses: fertilizers (urea, ammonium salts), HNO₃ manufacture, refrigeration, and (aqueous) as a laboratory base and complexing agent.

Phosphine PH₃. From calcium phosphide and water, Ca₃P₂ + 6H₂O → 3Ca(OH)₂ + 2PH₃↑ (companion: white phosphorus boiled with NaOH gives PH₃ + sodium hypophosphite). A colourless, rotten-fish/garlic-smelling, very poisonous gas, sparingly soluble, only feebly basic (PH₄⁺ salts form only with strong acids like HI; basicity NH₃ ≫ PH₃ because the lone pair sits in a diffuse 3s-ish orbital on the bigger atom) and a stronger reductant than NH₃. Impure PH₃ (with P₂H₄ vapour) ignites spontaneously in air — the basis of Holmes' signals (sea flares: Ca₃P₂ + CaC₂ cannisters) and of marsh "will-o'-the-wisp" tales.

Nitric acid HNO₃. Laboratory: distil KNO₃/NaNO₃ with conc. H₂SO₄ in an all-glass retort (KNO₃ + H₂SO₄ → KHSO₄ + HNO₃↑). Industrial: Ostwald's process — 4NH₃ + 5O₂ →(Pt–Rh gauze, ~800 °C) 4NO + 6H₂O; 2NO + O₂ → 2NO₂; 2NO₂ + H₂O → HNO₃ + HNO₂ (practically 3NO₂ + H₂O → 2HNO₃ + NO, the NO recycled). Pure HNO₃ is colourless but commercial acid is yellow-brown from dissolved NO₂ (made by its photodecomposition — store shaded). It is both a strong acid and a fierce oxidizing agent whose reduction product tracks concentration:

Reaction	Product gas	Notes
Cu + dilute HNO₃: 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO↑ + 4H₂O	NO (browns in air)	the standard equation
Cu + conc. HNO₃: Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂↑ + 2H₂O	NO₂ (brown)
Zn + very dilute/cold HNO₃	even NH₄NO₃ (N reduced to −3)	deeper reduction with active metals
Fe, Al, Cr + conc. HNO₃	no attack — passivity	dense oxide film; hence conc. HNO₃ ships in aluminium
Non-metals + hot conc.: C → CO₂, S → H₂SO₄, P → H₃PO₄, I₂ → HIO₃	—	HNO₃ plays pure oxidant
Aqua regia: HNO₃ + 3HCl (1 : 3)	dissolves Au, Pt	nascent Cl₂ and NOCl do the work

Test: the brown ring — FeSO₄ solution + conc. H₂SO₄ down the side gives a brown ring of [Fe(NO)]SO₄ at the junction (NO₃⁻/NO₂⁻ confirm; Ch. 22).

13.4 Chlorine, bromine and iodine; HCl, HBr and HI

The group-17 elements are diatomic, coloured, and hangrier for electrons at the top: oxidizing power F₂ > Cl₂ > Br₂ > I₂, so the higher halogen displaces the lower from its salts (Cl₂ + 2KBr → 2KCl + Br₂; Br₂ will not displace Cl⁻ in return). Laboratory Cl₂: warm MnO₂ with conc. HCl (MnO₂ + 4HCl → MnCl₂ + Cl₂↑ + 2H₂O, similarly KMnO₄ acts in the cold); industry gets it from brine electrolysis (Ch. 10) and historically by Deacon's 4HCl + O₂ → 2Cl₂ + 2H₂O (over CuCl₂ catalyst). Chlorine water bleaches by oxidation (via HOCl) and its bleaching is permanent (contrast SO₂ below); with cold dilute NaOH it gives NaCl + NaClO (bleach liquor), with hot concentrated, NaCl + NaClO₃; over slaked lime it makes bleaching powder, Ca(OCl)Cl. Bromine (red-brown fuming liquid) is won from sea-water/brines by chlorine displacement; iodine (violet vapour, sublimes, starch turns deep blue) from seaweed ash and Chilean nitrate deposits; both dissolve in KI solution (as I₃⁻) and in organic solvents with characteristic colours (Br₂ orange in CCl₄; I₂ violet in CCl₄, brown in water/ethanol — a living qualitative test).

Hydrogen halides are made by displacing from salts with a non-volatile acid — with a crucial exception:

HX	Preparation	Acid strength in water	Signature
HF	CaF₂ + conc. H₂SO₄ (in lead/Pt, not glass!)	weak (strong H–F bond, H-bonding)	etches glass: SiO₂ + 4HF → SiF₄↑ + 2H₂O — used for marking glassware, fluoridation
HCl	NaCl + conc. H₂SO₄; direct synthesis H₂ + Cl₂ (industry)	strong	white fumes with NH₃; AgCl white ppt, soluble in NH₄OH
HBr	not conc. H₂SO₄ (it oxidizes HBr → Br₂); use H₃PO₄, or PBr₃ + water	stronger	reducing agent
HI	H₃PO₄ route or PI₃ hydrolysis; conc. H₂SO₄ would give I₂	strongest	strongest reductant of the four; even reduces conc. H₂SO₄ to H₂S/S

Trends in one breath: bond strength HF > HCl > HBr > HI, so thermal stability falls and acidity and reducing power rise down the group; silver halide tests run AgCl (white, NH₄OH-soluble) → AgBr (pale yellow, sparingly soluble) → AgI (yellow, insoluble) as Ksp falls (Ch. 7 patterns again).

13.5 Allotropes of carbon and carbon monoxide; H₂S, SO₂ and sulphuric acid

Allotropes. Diamond: every C sp³-bonded in a 3-D tetrahedral lattice — the hardest natural substance, electrical insulator, superb heat conductor, transparent, very high melting. Graphite: sp² sheets (each C bonded to three, the fourth electron delocalized) held by weak forces — soft, slippery (pencil "lead", lubricant), conducts electricity along the planes (electrodes, brushes). Fullerenes (C₆₀ buckminsterfullerene, soccer-ball cages), graphene (a single graphite sheet — Physics Ch. 22), nanotubes, and the amorphous family (coke, charcoal — adsorptive, gas masks; lampblack/carbon black — inks, tyres). Whichever the costume, the chemistry (combustion to CO₂ 393.5 kJ per mole for graphite) is carbon's.

aDiamond - cubic sp3 network, hardest natural substancebGraphite - layers of sp2 hexagons, conducts electricitycLonsdaleite - hexagonal diamond, found in meteoritesdC60 Buckminsterfullerene - 12 pentagons, 20 hexagonseC540 fullerene - a larger cagefC70 fullerene - rugby-ball shapedgAmorphous carbon - no long-range order (soot, charcoal)hCarbon nanotube - a graphene sheet rolled into a tube

Carbon monoxide CO. Laboratory: dehydrate formic (or oxalic) acid with hot conc. H₂SO₄: HCOOH → CO↑ + H₂O. Industrial: producer gas (air over red-hot coke: CO + N₂) and water gas (steam over coke: CO + H₂, endothermic — alternated with air blasts). Colourless, odourless, almost insidiously poisonous: it binds haemoglobin (carboxyhaemoglobin) some 200–250 times more avidly than O₂ — faulty room heaters kill silently, and cherry-red blood is the post-mortem clue. Chemically a neutral oxide (yet it reduces Fe₂O₃ in the blast furnace, Ch. 14), burns with a blue flame (2CO + O₂ → 2CO₂), and is a strong ligand (Ni + 4CO ⇌ Ni(CO)₄, the Mond refining step).

Hydrogen sulphide H₂S. Kipp's apparatus, FeS + dil. H₂SO₄ → FeSO₄ + H₂S↑ (never conc. acid or HNO₃ — they oxidize it). Rotten-egg gas, denser than air, very poisonous (and treacherously paralyses the sense of smell). A weak dibasic acid (→ HS⁻ → S²⁻) and a reductant (decolorizes Br₂ water and acidified KMnO₄, depositing sulfur). Its kingdom is analysis: with controlled [S²⁻] (acidic vs alkaline medium, the common-ion machinery of Ch. 7) it precipitates metal sulphides group by group (Ch. 22). Test: lead acetate paper turns black (PbS).

Sulphur dioxide SO₂. Made by burning sulfur, roasting sulphides (4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂), or Cu + hot conc. H₂SO₄. Pungent, denser than air, very soluble; an acidic oxide (H₂SO₃ in water; salts sulphites/bisulphites) — a major acid-rain precursor. It is a reducing agent (turns acidified K₂Cr₂O₇ paper green, Cr³⁺ — its distinguishing test; decolorizes KMnO₄) and its bleaching, by reduction of coloured matter to colourless, is temporary (air slowly re-oxidizes the fabric — the eternal pair with chlorine's permanent oxidative bleach).

Sulphuric acid H₂SO₄ — the "king of chemicals". Contact process: purify SO₂; 2SO₂ + O₂ ⇌ 2SO₃ over V₂O₅ at ~450 °C and modest pressure (exothermic: low T favoured but rate demands compromise, Ch. 7); absorb SO₃ not in water (violent mist) but in conc. H₂SO₄ to give oleum H₂S₂O₇, then dilute to order. Properties to file in threes: (1) strong dibasic acid (first proton complete); (2) dehydrating agent when concentrated — chars sugar to black carbon (C₁₂H₂₂O₁₁ → 12C + 11H₂O), dries gases (not NH₃, HBr, H₂S), and demands respect on dilution — always acid into water, the mixing is fiercely exothermic; (3) oxidizing agent when hot and concentrated (Cu + 2H₂SO₄ → CuSO₄ + SO₂↑ + 2H₂O; C → CO₂ + SO₂). Tests: BaCl₂ gives a white precipitate of BaSO₄ insoluble in acids. Industrial fingerprints: fertilizers ((NH₄)₂SO₄, superphosphate), detergents, petroleum refining, lead-acid batteries (density of the acid even gauges their charge, Ch. 10).

Formula and fact sheet

Topic	Must-know
Hydrogen forms	atomic > nascent > molecular (reducing); D₂O: electrolysis, moderator
Oxides	CO, NO, N₂O neutral; Al₂O₃, ZnO amphoteric; Na₂O₂ peroxide; KO₂ superoxide
Ozone	silent discharge; bent 116.8°, resonance; tests: starch–iodide blue, Hg tailing; CFC Cl• eats it
NH₃/PH₃	Haber Fe/Mo; Nessler brown; [Cu(NH₃)₄]²⁺; PH₃ feebly basic, Ca₃P₂ route, poisonous
HNO₃	Ostwald Pt gauze; Cu dil→NO, conc→NO₂; passivity Fe/Al; brown ring; aqua regia 1:3
Halogens/HX	F>Cl>Br>I oxidizing; acidity and reducing power rise HF ≪ HCl, then HBr, then HI; HF etches glass; AgX colours/solubility
Carbon/CO	diamond sp³ insulator; graphite sp² conductor; CO: formic acid route, Hb poison, blue flame
Sulphur family	H₂S: Kipp, Pb acetate black, group reagent; SO₂: dichromate green, temporary bleach; H₂SO₄: Contact V₂O₅/oleum, dehydrating + oxidizing (hot conc.)

Rapid-revision tables and high-yield facts

Item	Value
Nascent hydrogen	newly born atoms, extra reactive
Oxide families	acidic / basic / amphoteric / neutral
Ozone making	silent electric discharge
Ozone geometry	bent, ≈ 117°, bond order 1.5
Ozone depletion	Cl· catalytic, ~10⁵ O₃ per Cl
Montreal Protocol	1987
Aqua regia	3 HCl : 1 HNO₃
Brown ring	[Fe(H₂O)₅NO]²⁺
HX acid order	HI > HBr > HCl ≫ HF
Contact catalyst	V₂O₅; oleum H₂S₂O₇

Chapter summary

• Hydrogen's personas scale in reducing violence (H₂ < [H] < atomic H); its isotopes give heavy water (a moderator) and tritium (a tracer).
• Oxides sort by character — acidic, basic, amphoteric, neutral, peroxide, superoxide, mixed — and ozone is oxygen's bent, resonating, oxidizing allotrope, whose stratospheric veil CFC chemistry taught us to defend.
• Ammonia is base and ligand in one lone pair; phosphine its feebler, fouler sibling; nitric acid an oxidizing acid whose product gas advertises its concentration.
• Halogens weaken downwards as oxidants while their hydrides strengthen as acids and reductants; HF alone is weak — and alone eats glass.
• Carbon reinvents itself from diamond to graphene; CO poisons quietly and refines metals; the sulphur trio H₂S/SO₂/H₂SO₄ escalates from analytical precipitant to the dehydrating, oxidizing king of chemicals.

Practice questions

Set A — Recall (25 questions)

1. Nascent hydrogen is more reactive than ordinary (molecular) hydrogen because it isaionic H⁺ in solutionbmixed with oxygencat higher pressuredavailable as atoms/at the surface at the moment of formation
2. Heavy water isaD₂O (deuterium oxide)bH₂O with dissolved saltsctritium waterdwater at 4 °C only
3. The isotopic composition of tritium nucleus isa1 proton, 0 neutronb1 proton, 1 neutronc1 proton, 2 neutronsd2 protons, 1 neutron
4. Which of the following oxides is neutral?aCO₂bCOcSO₂dCaO
5. ZnO and Al₂O₃ are examples ofaamphoteric oxidesbacidic oxidescneutral oxidesdperoxides
6. Ozone is prepared in the laboratory by passing through oxygen aaspark discharge loudlybbeam of α-rays onlyccurrent of steamdsilent electric discharge
7. The "tailing of mercury" test is used forachlorinebozonecsulphur dioxidedammonia
8. The depletion of the ozone layer is mainly caused by free radicals ofamethane directly fragmenting O₃bcarbon dioxidecchlorine from CFCsdwater vapour
9. In Haber's process the catalyst used isaplatinum gauzebiron with molybdenum promotercvanadium pentoxidednickel
10. Nessler's reagent gives a brown colour/precipitate withaammonia/ammonium saltsbCO₂cH₂Sdchlorine
11. The deep blue colour when excess ammonia is added to copper sulphate solution is due toaCuObNH₄⁺ itself colouredcCu metald[Cu(NH₃)₄]²⁺ complex ion
12. Phosphine is prepared in the laboratory by treating calcium phosphide withaconc. HNO₃bNaOH strong only heating P redcwaterdoxygen
13. In Ostwald's process, ammonia is oxidized to NO over a catalyst ofaplatinum–rhodium gauzebV₂O₅cFe/ModCuCl₂
14. Copper with dilute nitric acid gives mainly the gasaNO₂bN₂OcNOdH₂
15. Iron becomes passive in concentrated HNO₃ because of the formation ofaferrous nitrateba protective oxide film on the surfacechydrogen layerdmagnetic iron only by cooling
16. Aqua regia is a mixture (by volume) of concentratedaHCl : HNO₃ = 1 : 3bHCl : H₂SO₄ = 3 : 1cH₂SO₄ : HNO₃ = 1 : 1dHNO₃ : HCl = 1 : 3
17. The strongest acid among the hydrogen halides isaHFbHClcHIdHBr
18. Hydrofluoric acid is stored in wax/plastic bottles (not glass) because itais very volatilebattacks glass forming SiF₄cis a strong oxidantdfreezes glass joints always
19. The allotrope of carbon that conducts electricity isagraphitebdiamondccharcoal ideally puredcoke metal-like entirely
20. Carbon monoxide is poisonous because itaburns the lungsbstops diffusion by weightcis acidicdbinds haemoglobin much more strongly than oxygen (carboxyhaemoglobin)
21. Water gas is a mixture ofaCO + N₂bCO + H₂cCO₂ + H₂dH₂ + O₂
22. Lead acetate paper turning black is a test foraSO₂bNH₃cCl₂dH₂S
23. Sulphur dioxide turns acidified potassium dichromate solutionabluebcolourless eternalcgreendpink
24. In the Contact process for H₂SO₄, SO₃ is absorbed in concentrated sulphuric acid to formaoleum (H₂S₂O₇)bpersulphuric acidcsulphurous aciddthiosulphuric acid
25. The bleaching action of SO₂ differs from that of Cl₂ in beingastrongerbonly for wool bannedctemporary (reductive) whereas chlorine's is permanent (oxidative)dheat-free always

Answers and explanations — Set A

1. (d) — atoms not yet paired into H₂ (and surface-adsorbed H) reduce directly; molecular H₂ must first break its bond.

2. (a) — deuterium oxide; tritium water would be T₂O (a biohazard, option (c) is a real but different thing).

3. (c) — mass 3 with the lone charge of hydrogen.

4. (b) — CO is neutral (also NO, N₂O, H₂O). (a), (c) acidic; (d) basic.

5. (a) — dual behaviour with acids and alkalis.

6. (d) — sparkless discharge: the product is endothermic and easily destroyed by heat/sparks.

7. (b) — O₃ oxidizes the mercury surface (Hg₂O), which then wets and sticks to glass.

8. (c) — Cl• (and Br•) radicals from photolysed CFCs/halons, recycled ~10⁵ times each.

9. (b) — finely divided Fe promoted with Mo (modern plants use K₂O/Al₂O₃ promoters — the Fe/Mo formulation is the syllabus classic).

10. (a) — K₂[HgI₄]/KOH gives brown iodide of Millon's base with NH₃ traces (water analysis standard).

11. (d) — tetraamminecopper(II): the hydroxide precipitate first formed dissolves into the complex.

12. (c) — Ca₃P₂ + 6H₂O → 3Ca(OH)₂ + 2PH₃.

13. (a) — Pt–Rh gauze at ~800 °C, millisecond contact.

14. (c) — 3Cu + 8HNO₃(dil) → 3Cu(NO₃)₂ + 2NO + 4H₂O (the NO browns at the mouth, NO₂).

15. (b) — an impervious Fe₃O₄-type film; scratch it under the acid and attack restarts.

16. (d) — 1 volume conc. HNO₃ to 3 volumes conc. HCl (freshly mixed).

17. (c) — weakest H–X bond, easiest proton release; HF is (famously) the weak one.

18. (b) — it converts silica to gaseous SiF₄/soluble H₂SiF₆, etching the bottle through.

19. (a) — graphite's delocalized π electrons travel the sheets; diamond's are all σ-locked.

20. (d) — carboxyhaemoglobin locks up the O₂ seats (affinity ~250 : 1).

21. (b) — from steam over coke; producer gas (air) is CO + N₂, option (a).

22. (d) — black PbS; the matching sulphide-analytical chemistry is Chapter 22.

23. (c) — Cr(VI) orange → Cr(III) green: the signature of a reducer.

24. (a) — absorbing into water directly makes a stubborn acid mist; oleum is then diluted to strength.

25. (c) — reduction to a colourless form that air re-oxidizes (straw hats re-yellow), while chlorine destroys chromophores permanently.

Set B — Understanding (15 questions)

1. Ozone acts as a stronger oxidizing agent than oxygen mainly because itais heavierbis ioniccreadily yields a reactive oxygen atom, O₃ → O₂ + [O]ddissolves less
2. The two O–O bonds in ozone are of equal length because the molecule isalinearban ion pair fixedchydrogen bondedda resonance hybrid with delocalized π electrons
3. Ammonia gas is dried by passing over quicklime (CaO) and not over conc. H₂SO₄ or CaCl₂ because ammoniaareacts with H₂SO₄ and forms a complex with CaCl₂ (CaCl₂·8NH₃)bis acidiccis too lightddissolves in lime fully
4. Basicity falls drastically from NH₃ to PH₃ chiefly because the lone pair on phosphorusais absentboccupies a larger, more diffuse orbital, less available for donationcis shared with metals onlydis used in π bonding with H strongly
5. A bottle of concentrated nitric acid turns yellow-brown on standing in light because it slowly decomposes to giveaN₂bO₃cNH₃dNO₂ which dissolves in the acid
6. Chlorine displaces bromine from KBr solution, but bromine cannot displace chlorine from KCl. This follows the order ofaatomic massesboxidizing power/electron affinity: Cl₂ > Br₂csolubility fixed ratiosdboiling points descending helps only
7. HBr cannot be prepared by warming NaBr with conc. H₂SO₄, the way HCl is made from NaCl, because conc. H₂SO₄adoes not react with NaBrbis too dilutecoxidizes the HBr formed to Br₂dabsorbs HBr as base
8. Diamond is hard and an insulator, graphite soft and a conductor, because in diamond each carbon isasp³-bonded in a 3-D network with all electrons localizedbsp² in sheetscionizeddlarger in size atoms differing
9. Charcoal decolourizes sugar solutions (and is used in gas masks) mainly throughareduction of the colourbadsorption on its huge porous surfacecoxidation to CO₂ fastdacid neutralization
10. Sulphur dioxide bleaches moist litmus first red then colourless while CO₂ only turns it red; the additional action of SO₂ is itsareducing (bleaching) propertybbasicitycgreater acidity so strong it destroys paper alwaysdprecipitation of dye
11. Why is sulphuric acid added to water, and never the reverse, when diluting?aconvention onlybthe acid would float otherwise inertlycwater evaporates the acid's power chemicallyddilution is highly exothermic; much water absorbs the heat safely
12. When conc. H₂SO₄ is dropped on sugar, it turns black. This demonstrates the acid's role as aareducing agentbcatalyst of burning by aircdehydrating agent (removes H and O as water, leaving carbon)dpigment itself
13. Hydrogen peroxide and ozone both liberate iodine from KI solution, yet H₂O₂ can also remove the iodine colour in presence of... — in the syllabus frame, an agent acting both as oxidant and reductant is exemplified also byaSO₂ (reduces dichromate, oxidized by halogens; oxidized state +4 mid-way)bconc. HNO₃ only ever oxidant strictcCl₂ in water cold alkali only reductantdCO₂
14. Mercury kept in ozonized air loses its shining mobility; the same mercury recovers on shaking with water. The skin formed is ofaHgO red oxide thickbHgScamalgamdHg₂O (mercurous oxide film)
15. Kerosene stoves in sealed winter rooms kill through CO rather than CO₂ mainly because, mole for mole, CO is dangerous at concentrations where CO₂ is merely stuffy, owing to itsacolourbchemical hijacking of haemoglobin at tiny partial pressurescgreater density at ceiling level alwaysdflammability indoors specifically

Answers and explanations — Set B

1. (c) — the third atom is weakly held (ΔH of O₃ formation is positive: it "wants" to let go).

2. (d) — bond order 1.5 in both links; the bent shape comes from the central lone pair.

3. (a) — NH₃ is a base (makes (NH₄)₂SO₄ with the acid) and a ligand (absorbed by CaCl₂ as ammine): both classic driers eat the product.

4. (b) — poorer overlap/donation from the bigger, more s-like lone pair (bond angle also shrinks: PH₃ 93.6° vs NH₃ 107°).

5. (d) — 4HNO₃ → 4NO₂ + O₂ + 2H₂O in sunlight: brown NO₂ stays dissolved (blow air through warm acid to bleach it back).

6. (b) — the halogen pecking order F > Cl > Br > I is an electron-affinity/E° ladder (Ch. 10 series logic).

7. (c) — hot conc. acid takes HBr up to red Br₂ vapour (you collect brown mist, not the hydracid): switch to syrupy H₃PO₄.

8. (a) — the network locks every electron into σ bonds, hard and insulating; graphite's leftover π system gives slip and conduction.

9. (b) — surface adsorption (Physics Ch. 22's area arithmetic at household scale); filtration/regeneration by steam confirms it is physical capture.

10. (a) — SO₂ + water gives a reducing medium ("nascent hydrogen" in old books, in truth direct electron transfer hydrating the dye).

11. (d) — with water-on-top the first drops boil in a concentrated layer and spatter acid; acid-into-water keeps every drop met by a cool excess.

12. (c) — the elements of water are torn out of the carbohydrate skeleton, leaving black carbon (dehydration, not burning).

13. (a) — sulfur at +4 can climb to +6 (acting reductant, e.g. to dichromate) or fall to 0/−2 as oxidant (e.g. with H₂S: SO₂ + 2H₂S → 3S + 2H₂O — the volcanic sulfur maker).

14. (d) — a film of Hg₂O makes the metal drag and stick ("tail"); acids/shaking strip it off again.

15. (b) — even 0.1 % CO (1000 ppm) ties up a lethal fraction of haemoglobin over hours, far below CO₂ discomfort levels: chemistry, not suffocation by displacement.

Set C — Application (10 questions)

1. Which gas is obtained when formic acid is warmed with concentrated H₂SO₄?aCO₂bCOcH₂dCH₄
2. A gas turns starch–iodide paper blue and also causes tailing of mercury. The gas isaCl₂ alone onlybSO₂cO₃dNO₂
3. When Cl₂ is passed through cold dilute NaOH, the products areaNaCl + NaOClbNaCl + NaClO₃cNaClO₃ + H₂dNaCl + H₂O only
4. White fumes with a glass rod dipped in conc. HCl held near a gas identify the gas asaCl₂bCO₂cSO₂dNH₃
5. In the brown ring test for nitrate, the brown colour is due to the complexa[Fe(CN)₅NO]²⁻ always nitroprusside confusionbFe₂O₃c[Fe(NO)]SO₄dFe(NO₃)₃ brown salt solid
6. Which element will displace iodine from a KI solution but not displace chlorine from KCl?aBr₂bF₂cI₂dAt never used practical scale
7. Passing H₂S through acidified KMnO₄ solutionaturns it greenbprecipitates PbScevolves Cl₂ddecolorizes it with a yellow-white sulfur deposit
8. A mixture required to dissolve gold is freshly prepared aqua regia; the active species attacking gold includeahot waterbnascent chlorine/NOCl and complexation to [AuCl₄]⁻cfluorine gas releaseddozone
9. Sulphur is burnt in air and the gas passed into water; the solution turns blue litmus red and slowly becomes a stronger acid on keeping in air because it containsaH₂SO₃ which air-oxidizes to H₂SO₄bH₂ScHCl impuritydcarbolic acid formation
10. The catalyst and temperature in the oxidation of SO₂ to SO₃ (Contact process) areaFe/Mo at 450 °CbV₂O₅ at about 450 °CcPt gauze at 800 °CdNi at 200 °C

Answers and explanations — Set C

1. (b) — HCOOH —(conc. H₂SO₄)→ CO + H₂O: the standard laboratory CO generator. (Oxalic acid gives CO + CO₂, separated by KOH.)

2. (c) — the pair of tests together pin ozone (Cl₂ also blues starch–iodide but does not tail mercury and is greenish-yellow — the question's combination is the fingerprint).

3. (a) — cold/dilute: chloride + hypochlorite (bleach); hot/concentrated would give the chlorate of (b).

4. (d) — NH₄Cl smoke-particles: the pocket test for ammonia leaks.

5. (c) — nitrosoferrous sulphate at the acid–water junction; shake and it vanishes (NO escapes). (a) is the nitroprusside used for a different (sulphide) test.

6. (a) — the displacement ladder: Br₂ oxidizes I⁻ but not Cl⁻.

7. (d) — H₂S reduces MnO₄⁻ all the way to Mn²⁺, itself stopping at S. (In neutral solution the Mn stops at brown MnO₂ — medium matters, as Ch. 8's n-factors priced.)

8. (b) — HNO₃ oxidizes the HCl's chloride (NOCl + Cl₂), and [AuCl₄]⁻ complexation drags the equilibrium across (Ch. 14 metallurgy meets this again).

9. (a) — sulphurous acid slowly air-oxidizes (2H₂SO₃ + O₂ → 2H₂SO₄): acid rain chemistry in a beaker.

10. (b) — V₂O₅ (or Pt in older plants), ~450 °C, near-atmospheric to 2 atm: the exothermic-equilibrium compromise of §7.2. (a) is Haber's recipe, (c) Ostwald's.)