Metallurgy

Chapter in a nutshell: how elements split into metals / non-metals, how a metal's place in the activity series decides its reactions and how it is extracted, plus corrosion, the extraction of aluminium and common alloys.

1. Metals vs Non-metals — Definitions

• Metal: an element that forms positive ions (cations) by losing electrons → electropositive. e.g. Na − e⁻ → Na⁺, Mg − 2e⁻ → Mg²⁺, Al − 3e⁻ → Al³⁺.
• Non-metal: an element that forms negative ions (anions) by gaining electrons → electronegative. e.g. Cl + e⁻ → Cl⁻, O + 2e⁻ → O²⁻, N + 3e⁻ → N³⁻.
• Number of electrons lost/gained = valency. (Of 118 elements, only ~22 are non-metals. H is a non-metal that forms H⁺.)

2. Comparison of Metals and Non-metals

3. Position in the Periodic Table

• Alkali metals (Group IA): Li, Na, K, Rb, Cs, Fr — 1 valence e⁻, valency 1.
• Alkaline-earth metals (Group IIA): Be, Mg, Ca, Sr, Ba, Ra — 2 valence e⁻, valency 2.
• Metals fill the left + middle; non-metals the right; a zig-zag line of metalloids (B, Si, Ge, As, Sb, Te) separates them. Only one non-metal — H — sits on the left.

4. Alkali Metals (IA) — key features

1. Very reactive → never found free; stored under kerosene.
2. Soft (cut with a knife), low M.P/B.P; ionic salts (except some Li salts).
3. Reactivity increases down the group; strong reducing agents (low ionisation energy).
4. Flame colours: Li crimson red, Na golden yellow, K lilac/pale violet.

• Action of water: 2M + 2H₂O → 2MOH + H₂↑
• Action of acid: 2M + 2HCl → 2MCl + H₂↑

5. Alkaline-Earth Metals (IIA) — key features

• Less reactive than alkali metals, harder, higher M.P; form +2 ions; basic oxides/hydroxides.
• Action of water: M + 2H₂O → M(OH)₂ + H₂↑
• Action of acid: M + H₂SO₄ → MSO₄ + H₂↑
• Flame colours: Ca brick red, Sr crimson, Ba apple green (Be, Mg give none).

6. Nature of Oxides

7. The Activity (Reactivity) Series

• Electropositive character & reducing power decrease down the series.
• Action of water: K, Na react with cold water; Ca with cold water (no ignition); Mg with hot water/steam; Al, Zn, Fe only with steam (e.g. 3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂); Cu and below — no reaction.
• Action of dilute acid: metals above H displace hydrogen (e.g. Zn + 2HCl → ZnCl₂ + H₂); metals below H (Cu, Hg, Ag, Au) do not.
• Displacement: a higher metal displaces a lower one from its salt solution.

8. Reduction of Metal Oxides

• Oxides of K, Na, Ca, Mg, Al cannot be reduced by C, CO or H₂ → extracted by electrolysis.
• Oxides below Al (Zn, Fe, Pb, Cu) can be reduced by C / CO / H₂:

• Oxides of Hg, Ag decompose on heating alone (2HgO → 2Hg + O₂).

9. Action of Heat on Metallic Compounds

10. Corrosion

• Corrosion: slow eating away of a metal surface by air, moisture or chemicals.
• Rusting = corrosion of iron: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O (brown, flaky → keeps exposing fresh iron). Needs both air (O₂) and water.
• Other corrosion: Pb → basic lead carbonate (white); Cu → green basic copper carbonate; Ag → black Ag₂S (tarnish).
• Faster when: salt/electrolyte present (sea air), pollutants (CO₂, NO₂), reactive metal, impurities.
• Prevention: (i) Barrier — oiling, painting, electroplating; (ii) Galvanisation — coating iron with zinc; (iii) Sacrificial / cathodic protection — attach a more reactive metal (Zn, Mg) that corrodes instead (used for pipelines, ships).
• Advantage: Al and Zn form a sticky, impervious oxide layer that protects the metal beneath.

11. Occurrence & Ores

• Mineral: any natural compound of a metal. Ore: a mineral from which the metal is extracted profitably.
• Gangue: earthy impurities (sand, silica). Flux: added to remove gangue → flux + gangue → slag (fusible). Smelting: reduction of roasted oxide using a flux.

12. Steps of Extraction (Metallurgy)

• Concentration (dressing): gravity separation, froth flotation (sulphide ores), magnetic separation (e.g. iron ores), chemical/leaching.
• Roasting: heat ore strongly in air (sulphide → oxide, e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂).
• Calcination: heat in limited/no air (carbonate/hydrated → oxide, e.g. ZnCO₃ → ZnO + CO₂).
• Reduction: oxide → metal (carbon reduction or electrolytic reduction for reactive metals).

13. Extraction of Aluminium (from Bauxite)

• Electrolyte: molten Al₂O₃ dissolved in molten cryolite (Na₃AlF₆) + fluorspar (CaF₂) — these lower the melting point (~2050 °C → ~950 °C) and increase conductivity.
• Cathode: carbon (graphite) lining → Al³⁺ + 3e⁻ → Al (molten Al collects at the bottom).
• Anode: carbon blocks → 2O²⁻ → O₂ + 4e⁻; the O₂ burns the carbon anode to CO₂, so anodes are replaced periodically.

14. Common Alloys

Quick recall: Activity series order; metals above Al → electrolysis, below Al → C/CO/H₂. Rusting needs air + water. Aluminium: bauxite → Baeyer's → electrolysis in cryolite (cathode Al, anode O₂). Roasting = in air (sulphides); calcination = without air (carbonates).