Aluminium Study Guide

📖 Core Concepts Aluminium (Al, Z = 13) – post‑transition metal, group 13 (boron group). Electron configuration: $[{\rm Ne}]\,3s^{2}\,3p^{1}$ → 13 valence electrons, electronegativity 1.61 (Pauling). Stable isotope: $^{27}{\rm Al}$ (the only naturally occurring isotope). Common oxidation state: +3; Al³⁺ is small, highly charged → strong polarising power → many Al compounds show covalent character. Amphoteric behavior: Al(OH)₃ dissolves in both acids (→ Al³⁺) and bases (→ $[{\rm Al(OH)}{4}]^{-}$). Crystal structure: face‑centered cubic (fcc) metal; density 2.70 g cm⁻³, melting point 660.3 °C. Passivation: thin Al₂O₃ film forms instantly, protecting bulk metal from further corrosion. 📌 Must Remember Atomic weight: 26.981538 ± 0.0000003 u. Density: 2.70 g cm⁻³ (≈ 1/3 that of steel). Electrical conductivity: ≈ 61 % of copper by cross‑section. Primary oxidation state: +3 (almost exclusive). Key inorganic compounds: AlCl₃ (Lewis acid, polymeric < 192.4 °C), Al₂O₃ (corundum, Mohs 9, $T{\rm melt}=2045 °C$), AlN, Al₂S₃, AlP, AlAs, AlSb (III‑V semiconductors). Major production steps: Bayer process → Al₂O₃ → Hall–Héroult electrolysis → metallic Al. Energy demand: 13–15 kWh kg⁻¹ Al (≈ 7 kg oil‑equivalent per kg Al). Recycling energy: only 5 % of primary production energy. Environmental greenhouse gases: CF₄ and C₂F₆ (perfluorocarbons) from Hall–Héroult. 🔄 Key Processes Bayer Process (Alumina Production) Grind bauxite → mix with hot NaOH. Form soluble $[{\rm Al(OH)}{4}]^{-}$; insoluble impurities settle. Cool → seed → precipitate Al(OH)₃. Filter & calcine → $ {\rm Al2O3}$ + $ {\rm H2O}$. Hall–Héroult Electrolysis Dissolve $ {\rm Al2O3}$ in molten cryolite (${\rm Na3AlF6}$) @ 940–970 °C. Apply DC: $ {\rm Al^{3+} + 3e^- \rightarrow Al(l)}$ at cathode (metal sinks). Carbon anode oxidises: $ {\rm C + O^{2-} \rightarrow CO2 + 4e^-}$ (consumed 0.4–0.5 kg C kg⁻¹ Al). Recycling Loop Collect scrap → clean → melt in electric furnace → cast ingots → feed Hall–Héroult or directly fabricate products. 🔍 Key Comparisons Aluminium vs. Copper (conductors) → Al: lighter, ½ weight per same current, 61 % conductivity; Cu: higher conductivity, heavier. AlCl₃ (polymeric) vs. Al₂Cl₆ (dimer) → Below 192.4 °C AlCl₃ forms layered polymer; melt → dimeric Al₂Cl₆. Aluminium oxide (Al₂O₃) vs. Aluminium hydroxide (Al(OH)₃) → Al₂O₃: hard (Mohs 9), refractory, insoluble; Al(OH)₃: amphoteric, soluble in acids/bases. Primary Al vs. Recycled Al → Energy: 13–15 kWh kg⁻¹ vs. 0.6–0.8 kWh kg⁻¹ (≈ 95 % energy saving). ⚠️ Common Misunderstandings “Aluminium is non‑reactive” – true only because of the protective Al₂O₃ film; once removed, Al reacts vigorously (e.g., with halogens, hot water). “All Al compounds are ionic” – Al³⁺’s high charge density gives many compounds significant covalent character. “Aluminium causes Alzheimer’s” – no credible scientific evidence as of 2018; the claim is unsupported. “Recycling eliminates all waste” – some dross (1 % after modern melters) remains; proper treatment is required. 🧠 Mental Models / Intuition “Tiny, highly charged → polarising” – picture Al³⁺ as a tiny magnet pulling electron density toward itself, turning otherwise ionic bonds into covalent‑ish ones. “Aluminium’s life cycle = light → heavy → light again” – mined as heavy bauxite → refined to lightweight metal → recycled back to light scrap, emphasizing the energy‑saving loop. “Passivation = self‑healing armor” – the instant Al₂O₃ layer behaves like a skin that seals wounds; it only breaks under strong acid/base attack. 🚩 Exceptions & Edge Cases AlCl₃ polymeric vs. monomeric – polymeric only below 192.4 °C; above, it converts to Al₂Cl₆. Aluminium corrosion – normally protected, but in acidic or alkaline environments the amphoteric oxide dissolves, exposing bare metal. Biological availability – neutral pH precipitates Al(OH)₃ (unavailable), but acidic foods or citrate increase soluble Al³⁺ absorption. 📍 When to Use Which Choose Al₂O₃ when you need a hard, refractory, or abrasive material (e.g., grinding wheels). Select AlCl₃ as a Lewis acid for Friedel–Crafts alkylation/acylation; ensure temperature < 192 °C to keep polymeric form if required. Use LiAlH₄ for strong reductions (e.g., converting esters → alcohols) rather than NaBH₄, which is milder. Opt for recycled Al in product design when energy cost or carbon footprint is a priority; otherwise, primary Al may be chosen for ultra‑high purity (≥ 99.99 %). 👀 Patterns to Recognize Amphoteric oxide/hydroxide pattern: compounds that dissolve in both acids and bases → likely Al(OH)₃ or Al₂O₃. III‑V semiconductor series: AlN, AlP, AlAs, AlSb – all share similar crystal structures and are used in optoelectronics. Energy‑intensive step flag: any mention of cryolite melt, high‑temperature electrolysis → expect large electricity consumption and PFC emissions. 🗂️ Exam Traps “Aluminium’s melting point is 660 °C” vs. “boiling point 2 470 °C.” – students may confuse the two; remember 660 °C is melt, 2 470 °C is boil. “Aluminium is the most abundant metal in the crust” – true, but not the most abundant element overall (O and Si are higher). “All aluminium compounds are ionic.” – trap: Al³⁺ polarises anions → many show covalent character (e.g., AlCl₃). “Recycling uses 5 % of primary energy.” – some may misread as 5 % of total energy; the correct statement is 5 % of the energy required for primary production. “Aluminium production emits CO₂ only.” – false; the dominant greenhouse gases are CF₄ and C₂F₆ (perfluorocarbons). --- Use this guide to skim key facts, visualize processes, and dodge common pitfalls before your exam.