Inorganic Chemistry

Chapter 2 · Chemistry · 6 questions in the paper · 11 min read

Inorganic Chemistry

What the syllabus asks for 3
  • Non-metals: hydrogen, oxygen, ozone, water, nitrogen and its compounds, halogens, carbon, phosphorus, sulphur, noble gases and environmental pollution
  • Metals: metallurgical principles, alkali metals, alkaline earth metals, coinage metals (copper, silver, gold)
  • Extraction of metals: zinc, mercury, and iron compounds

Inorganic chemistry carries about 6 of 22 questions (our estimate). Unlike physical chemistry it is mostly factual: an industrial process and its catalyst, the product of a named reaction, an ore and its extraction, the formula of a compound in daily use. Section A items are one-fact recall; Section B items add a short stoichiometric step. Learn each topic as name – formula – process – use.

What the paper asks from this chapter
  • Preparation, properties and tests: H₂, O₂, O₃, H₂O₂, water hardness, NH₃, HNO₃, Cl₂, HCl, bleaching powder, SO₂, H₂SO₄.
  • Industrial processes with catalysts: Haber (Fe), Ostwald (Pt), Contact (V₂O₅), Solvay (NaCl, NH₃, CO₂), Castner–Kellner.
  • Ores and extraction: bauxite, haematite, zinc blende, cinnabar, copper pyrites; roasting, smelting, Hall–Héroult.
  • s-block trends (flame colours, carbonate stability, solubility), Plaster of Paris, cement, coinage metals.
  • Pollution: SO₂, NOₓ, CFCs, CO, greenhouse gases; noble-gas uses.

2.1 Non-metals

Hydrogen. Lab: Zn + dil. H₂SO₄ → ZnSO₄ + H₂. Industrial: steam reforming CH₄ + H₂O → CO + 3H₂, water-gas shift. Isotopes protium, deuterium, tritium; hydrides (ionic, covalent, interstitial); uses: Haber, hydrogenation of oils, fuel cell. Oxygen: from KClO₃/MnO₂ (2KClO₃ → 2KCl + 3O₂), fractional distillation of liquid air; supports combustion; oxides acidic, basic, amphoteric (Al₂O₃, ZnO), neutral (CO, N₂O). Ozone O₃ (bent, 117°): made by silent electric discharge on O₂, absorbs UV-B in the stratosphere; oxidising agent (turns KI to I₂, bleaches). Water: polar, hydrogen-bonded, maximum density at 4 °C; hardness: temporary (Ca/Mg bicarbonates; removed by boiling or Clark's lime process), permanent (chlorides and sulphates of Ca, Mg; removed by washing soda Na₂CO₃, permutit/zeolite, ion exchange). H₂O₂: oxidising and reducing; bleach, antiseptic; decomposes to H₂O + O₂.

Nitrogen. N₂ is inert (N≡N, 945 kJ mol⁻¹). NH₃: lab NH₄Cl + Ca(OH)₂ (dry with CaO, not H₂SO₄/CaCl₂); Haber process N₂ + 3H₂ ⇌ 2NH₃ (\Delta H=-92 kJ), Fe catalyst with Mo promoter, 450 °C, 200 atm; pyramidal, basic, very soluble. HNO₃ by Ostwald: 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt–Rh gauze, 800 °C); 2NO + O₂ → 2NO₂; 3NO₂ + H₂O → 2HNO₃ + NO. Cu + conc. HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O; Cu + dilute HNO₃ → NO; very dilute with Zn/Mg gives N₂O/NH₄⁺; Fe, Al passivated by conc. HNO₃; aqua regia (3 HCl : 1 HNO₃) dissolves Au, Pt; brown-ring test for nitrate. Oxides N₂O (laughing gas), NO, N₂O₃, NO₂ (brown), N₂O₄, N₂O₅.

Halogens. Cl₂ (MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O; Deacon; electrolysis of brine); bleaching by oxidation (moist); Cl₂ + cold dil. NaOH → NaCl + NaClO; with hot conc. NaOH → NaCl + NaClO₃. Bleaching powder CaOCl₂ from Ca(OH)₂ + Cl₂. HCl (NaCl + conc. H₂SO₄). Reactivity F > Cl > Br > I; F₂ is the most electronegative (4.0) and its bond is weak; Br₂ is the liquid halogen, I₂ sublimes (starch test: blue); acid strength HI > HBr > HCl > HF; HF etches glass (SiF₄). Interhalogens (ICl, BrF₃). Oxoacids HOCl, HClO₂, HClO₃, HClO₄ (strongest).

Carbon. Allotropes: diamond (sp³, hardest, insulator), graphite (sp² layers, conducts, lubricant), fullerene C₆₀. CO (poisonous, binds haemoglobin; producer gas), CO₂ (greenhouse gas; dry ice), carbides, silica. Phosphorus: white P₄ (reactive, stored under water), red (chain); PH₃; P₄O₁₀ (dehydrating agent); H₃PO₃ (P +3, dibasic), H₃PO₄ (tribasic); superphosphate fertiliser. Sulphur: S₈ allotropes (rhombic, monoclinic). SO₂: S + O₂ or roasting sulphides; bleaches by reduction (temporary); reduces K₂Cr₂O₇ (orange → green). Contact process: 2SO₂ + O₂ ⇌ 2SO₃ (V₂O₅, 450 °C, 2 atm); SO₃ is absorbed in 98 % H₂SO₄ to give oleum H₂S₂O₇, then diluted (water is not used directly: acid mist). H₂SO₄ is dehydrating, oxidising (Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O), non-volatile. H₂S (rotten egg; precipitates black PbS). Noble gases: He (balloons, deep-sea diving, non-inflammable), Ne (signs), Ar (welding, bulbs), Kr, Xe (XeF₂, XeF₄, XeF₆ — first compound by Bartlett), Rn (radioactive).

Pollution. SO₂ and NOₓ cause acid rain (pH < 5.6); CO₂, CH₄, N₂O, CFCs cause the greenhouse effect; CFCs (Cl atoms catalyse O₃ breakdown: Cl + O₃ → ClO + O₂) deplete the ozone layer; CO from incomplete combustion; photochemical smog (NOₓ + hydrocarbons + sunlight → ozone, PAN); particulates; water pollution (eutrophication by phosphates and nitrates; BOD/COD); catalytic converters (Pt–Rh).

2.2 Metals

Metallurgical principles. Ore → concentration (gravity separation, froth flotation for sulphides, magnetic separation, leaching: bauxite with NaOH) → roasting (sulphide in air → oxide + SO₂) or calcination (carbonate, hydrate heated without air) → reduction (smelting with carbon, thermite with Al, electrolysis for reactive metals) → refining (electrolytic, zone, van Arkel, distillation). Flux (CaO for SiO₂ → slag CaSiO₃). Ellingham diagrams: a metal whose oxide has a lower line reduces the oxide above it. Common ores: bauxite Al₂O₃·2H₂O; haematite Fe₂O₃, magnetite Fe₃O₄; zinc blende ZnS; calamine ZnCO₃; cinnabar HgS; copper pyrites CuFeS₂; galena PbS; cryolite Na₃AlF₆; rock salt NaCl; carnallite KCl·MgCl₂·6H₂O; gypsum CaSO₄·2H₂O; dolomite CaCO₃·MgCO₃.

Alkali metals (Li–Cs). ns¹, strong reducing agents, flame colours: Li crimson, Na golden yellow, K lilac, Rb red-violet, Cs blue. Li is the strongest reducing agent in water (highest hydration energy), Cs the most reactive. Na and K stored under kerosene. NaOH (Castner–Kellner mercury-cell electrolysis of brine; Cl₂ at the anode); Na₂CO₃ by Solvay process (NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl; 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O; NH₃ is recovered with Ca(OH)₂; K₂CO₃ cannot be made this way because KHCO₃ is too soluble); washing soda Na₂CO₃·10H₂O; baking soda NaHCO₃. Li compounds resemble Mg (diagonal).

Alkaline earth metals. ns²; flame colours Ca brick red, Sr crimson, Ba apple green (Be, Mg none). Solubility of sulphates and thermal stability of carbonates increase down the group (BaCO₃ > CaCO₃ > MgCO₃ > BeCO₃); hydroxides' solubility increases down (Ba(OH)₂ most soluble). CaO (quicklime, from limestone), Ca(OH)₂ (lime water; turns milky with CO₂, clear with excess). Plaster of Paris CaSO₄·½H₂O from gypsum at 120 °C; sets by taking up water to gypsum. Cement: calcium silicates and aluminates (lime, silica, alumina, iron oxide; gypsum added to slow setting). Mg is in chlorophyll; Epsom salt MgSO₄·7H₂O.

Coinage metals (Cu, Ag, Au). Cu: reddish, ore CuFeS₂; extraction: concentrate by froth flotation, roast, smelt in a reverberatory furnace to matte (Cu₂S + FeS), bessemerise to blister copper, refine electrolytically (acidified CuSO₄, pure Cu cathode). Alloys: brass (Cu–Zn), bronze (Cu–Sn). CuSO₄·5H₂O (blue vitriol, turns white on heating; Bordeaux mixture). Ag: cyanide process (Ag₂S + 4NaCN → 2Na[Ag(CN)₂] + Na₂S; then Zn displaces Ag); AgNO₃ (lunar caustic); AgBr in photography (hypo dissolves unreacted AgBr). Au: cyanide leaching, dissolves in aqua regia; carat (24 = pure).

2.3 Extraction of metals: zinc, mercury, iron compounds

Zinc: ore blende ZnS; froth flotation; roasting 2ZnS + 3O₂ → 2ZnO + 2SO₂ (the SO₂ makes H₂SO₄); reduction ZnO + C → Zn + CO at about 1400 °C and the zinc distils off; refined by electrolysis or distillation; galvanising protects iron. Mercury: cinnabar HgS roasted in air: HgS + O₂ → Hg + SO₂ (self-reduction), the vapour condensed; liquid metal, amalgams (Na-amalgam, dental), toxic (Minamata). Iron: ores haematite, magnetite; blast furnace: coke burns to CO; Fe₂O₃ + 3CO → 2Fe + 3CO₂ in the upper zone; CaCO₃ → CaO, CaO + SiO₂ → CaSiO₃ (slag); pig iron (3–4 % C) → cast iron, wrought iron (purest), steel (0.1–1.5 % C; stainless steel Fe–Cr–Ni). Compounds: FeSO₄·7H₂O (green vitriol, pale green; on heating gives Fe₂O₃, SO₂, SO₃), Mohr's salt FeSO₄·(NH₄)₂SO₄·6H₂O (a stable double salt used as a primary standard in redox titrations), FeCl₃ (anhydrous by Cl₂ on Fe; test with KCNS gives blood-red; K₄[Fe(CN)₆] gives Prussian blue), Fe₂O₃ (rust), Fe₃O₄.

Inorganic traps

SO₂ bleaches by reduction (colour returns), Cl₂ by oxidation (permanent). Concentrated H₂SO₄ must not be used to dry NH₃. Thermal stability of Group 2 carbonates rises down the group; for Group 1 it also rises, with Li₂CO₃ an exception. Cu does not displace hydrogen from dilute acids; it reacts with oxidising acids. The Contact process absorbs SO₃ in H₂SO₄, not water.

Formula and fact sheet

Process / compound Key fact
Haber N₂ + 3H₂ ⇌ 2NH₃, Fe/Mo
Ostwald NH₃ → NO, Pt–Rh
Contact SO₂ → SO₃, V₂O₅
Solvay Na₂CO₃ from NaCl, NH₃, CO₂
Bleaching powder CaOCl₂
Plaster of Paris CaSO₄·½H₂O
Mohr's salt FeSO₄·(NH₄)₂SO₄·6H₂O
Blast furnace slag CaSiO₃
ZnS / HgS / bauxite zinc blende / cinnabar / Al₂O₃·2H₂O
Flame Na yellow, K lilac, Ca brick red, Sr crimson, Ba apple green

Chapter summary

  • Each industrial process has one catalyst: Fe (Haber), Pt (Ostwald), V₂O₅ (Contact).
  • Know the oxidation products of copper with dilute versus concentrated nitric acid.
  • Roast sulphides, calcine carbonates, smelt oxides; slag is CaSiO₃.
  • Group 2 carbonates and sulphates become more stable / less soluble down the group.
  • SO₂ and NOₓ give acid rain; CFCs destroy ozone; CO₂ and CH₄ warm the planet.

Practice questions

Set A — Recall (14 questions)

  1. The gas evolved when zinc reacts with dilute sulphuric acid isahydrogenboxygencsulphur dioxidedhydrogen sulphide
  2. The ozone layer protects life on Earth by absorbingainfrared radiationbultraviolet radiationcvisible lightdradio waves
  3. The catalyst used in the Haber process isaPtbV₂O₅cFedNi
  4. The catalyst in the Contact process isaFebPt gauzecMnO₂dV₂O₅
  5. In the Ostwald process ammonia is first oxidised toaNObN₂OcNO₂dN₂
  6. Bleaching powder isaCa(OH)₂bCaOCl₂cCaCl₂dCa(ClO₃)₂
  7. Plaster of Paris isaCaSO₄·2H₂ObCaSO₄cCaSO₄·½H₂OdCaSO₄·H₂O
  8. The principal ore of aluminium isacryolitebmagnetitecgalenadbauxite
  9. The noble gas used to fill weather balloons and in deep-sea diving mixtures isaHebArcNedXe
  10. The Solvay process is used to manufactureaNaOHbNa₂CO₃cNaHCO₃ onlydNaCl
  11. The chief ore of zinc isacinnabarbhaematiteczinc blendedbauxite
  12. The ore of mercury isagalenabzinc blendecbauxitedcinnabar
  13. The gas chiefly responsible for acid rain isaSO₂bN₂cO₂dAr
  14. A good electrical conductor among the allotropes of carbon isadiamondbgraphitecsilicadglass
Answers and explanations — Set A

1. (a) — Zn + H₂SO₄ → ZnSO₄ + H₂.

2. (b) — O₃ absorbs UV-B and UV-C.

3. (c) — iron with molybdenum as promoter. V₂O₅ is for the Contact process.

4. (d) — V₂O₅ at 450 °C.

5. (a) — 4NH₃ + 5O₂ → 4NO + 6H₂O over Pt–Rh.

6. (b) — CaOCl₂ (Ca(OCl)Cl).

7. (c) — gypsum \text{CaSO}_4\cdot2\text{H}_2\text{O} loses 1.5 H₂O at 120 °C.

8. (d) — bauxite Al₂O₃·2H₂O; cryolite is only the flux in electrolysis.

9. (a) — helium is non-inflammable and has low density.

10. (b) — sodium carbonate.

11. (c) — ZnS.

12. (d) — HgS.

13. (a) — SO₂ forms H₂SO₃ and H₂SO₄ in rain.

14. (b) — free electrons in the π layers.

Set B — Understanding (8 questions)

  1. A brick-red flame colour is shown byacalciumbstrontiumcbariumdpotassium
  2. The most thermally stable carbonate isaMgCO₃bBaCO₃cCaCO₃dBeCO₃
  3. Copper reacts with dilute nitric acid to give mainlyaNO₂bN₂OcNOdNH₃
  4. The bleaching action of sulphur dioxide is due toaoxidationbhydrationcdehydrationdreduction
  5. The halogen that is liquid at room temperature isabrominebchlorineciodinedfluorine
  6. The oxidation number of phosphorus in H₃PO₃ isa+5b+3c+1d−3
  7. The slag formed in the blast furnace in the extraction of iron isaFeSiO₃bCa₃(PO₄)₂cCaSiO₃dCaCO₃
  8. Permanent hardness of water is due toabicarbonates of calcium and magnesiumbsodium chloridecsodium sulphatedchlorides and sulphates of calcium and magnesium
Answers and explanations — Set B

1. (a) — Ca brick red; Sr crimson; Ba apple green; K lilac.

2. (b) — stability rises down the group.

3. (c) — dilute: 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O; concentrated gives NO₂.

4. (d) — SO₂ + H₂O reduces coloured matter; the colour returns on standing.

5. (a) — Br₂ (mercury is the liquid metal).

6. (b) — 3+x-6=0.

7. (c) — CaO + SiO₂ → CaSiO₃.

8. (d) — bicarbonates cause temporary hardness.

Set C — Application (6 questions)

  1. 245 g of KClO₃ (M = 122.5) is decomposed completely. The volume of oxygen at STP isa22.4 Lb44.8 Lc67.2 Ld89.6 L
  2. The mass of CaO obtained by heating 100 g of CaCO₃ isa56 gb44 gc100 gd28 g
  3. Roasting 97 g of ZnS (M ≈ 97) in excess air gives SO₂ at STP ofa11.2 Lb44.8 Lc22.4 Ld33.6 L
  4. 28 g of nitrogen reacts completely with hydrogen in the Haber process. The mass of NH₃ formed isa17 gb51 gc68 gd34 g
  5. The percentage of iron in Fe₂O₃ (Fe = 56, O = 16) isa56b70c77.7d72.4
  6. The volume of 0.1 M HCl needed to neutralise 1.06 g of Na₂CO₃ (M = 106) completely isa200 mLb100 mLc50 mLd400 mL
Answers and explanations — Set C

1. (c) — 2 mol KClO₃ → 3 mol O₂: 3\times22.4=67.2 L.

2. (a) — CaCO₃ → CaO + CO₂: 100 g gives 56 g.

3. (c) — 2ZnS → 2SO₂: 1 mol gives 1 mol.

4. (d) — 1 mol N₂ → 2 mol NH₃ = 34 g.

5. (b) — \frac{112}{160}=70\%.

6. (a) — 0.01 mol Na₂CO₃ needs 0.02 mol HCl = 200 mL.