General Principles of Isolation of Elements and p-Block Elements (Groups 15–18)
What this domain tests
Metallurgical principles (concentration, roasting and calcination, reduction, refining) and the key chemistry of N, P, O, S, halogens and noble gases, including important compounds and periodic trends.
Core rules
- Concentration: froth flotation for sulphide ores, gravity separation, magnetic separation, leaching (Ag and Au with dilute NaCN in air).
- Roasting heats a sulphide ore strongly in air; calcination heats a carbonate or hydroxide in limited air. Both remove volatile impurities.
- Reduction: carbon, CO, Al (thermite), or electrolysis; Ellingham diagrams compare ΔG values for oxide formation and identify the reducing agent.
- Refining: distillation, liquation, electrolytic refining, zone refining (Si, Ge), vapour phase refining (Mond process for Ni, van Arkel for Ti, Zr).
- Group trends: N₂ is inert because of its high bond enthalpy; the inert pair effect makes +3 and +5 states less stable down Groups 15 and 16; fluorine lacks d orbitals and shows anomalous behaviour.
- Important preparations: HNO₃ by Ostwald's process; H₂SO₄ by the contact process; XeF₂, XeF₄ and XeF₆ by direct combination under controlled conditions.
Worked example
Compare the acid strength of HClO, HClO₂, HClO₃ and HClO₄. All have the same central atom, but the conjugate base stability increases as more oxygen atoms delocalise the negative charge. In ClO₄⁻ the charge is spread over four oxygens, so HClO₄ is the strongest acid; HClO, with no resonance stabilisation of ClO⁻, is the weakest. The order is HClO₄ > HClO₃ > HClO₂ > HClO.
Traps that cost marks
Do not describe roasting as heating a carbonate; that is calcination. XeF₄ is square planar and XeF₆ is a distorted octahedron; XeF₂ is linear. HF is a weak acid but has a high boiling point because of hydrogen bonding. NF₃ resists hydrolysis because nitrogen cannot expand its octet.