Overview
This lecture explains how metals are extracted from ores, detailing two primary methods—electrolysis and reduction by carbon—focusing on aluminum and iron, and introduces necessary chemical equations, terms, and processes.
Metals in Nature
- Most metals are found in the Earth's crust as compounds, not as pure elements.
- Metals combine with other elements to form compounds like oxides or sulfides.
- Examples: Aluminum as aluminum oxide (Al₂O₃), iron as iron(III) oxide (Fe₂O₃), zinc as zinc sulfide (ZnS).
- Only gold (Au) and silver (Ag) are found naturally as pure elements.
Metal Ores and Extraction Methods
- "Ore" refers to the raw material (compound) used to extract metals.
- Main ores: Aluminum from bauxite (aluminum oxide, Al₂O₃); iron from hematite (iron(III) oxide, Fe₂O₃).
- Two key extraction methods: Electrolysis (for reactive metals above carbon in the reactivity series) and reduction by carbon (for less reactive metals below carbon).
Reactivity Series and Choice of Method
- The reactivity series ranks metals by their tendency to react with oxygen.
- Metals above carbon: extract by electrolysis (e.g., aluminum).
- Metals below carbon: extract by reduction with carbon (e.g., iron).
- Reactivity series is derived from the electrochemical series by inserting carbon before zinc.
Extraction of Aluminum (Electrolysis)
- Raw material: Bauxite (Al₂O₃), mixed with cryolite to lower melting point and increase electrical conductivity.
- Electrolysis involves a carbon-lined container (cathode) and carbon anodes.
- At cathode: Al³⁺ + 3e⁻ → Al (aluminum metal forms).
- At anode: 2O²⁻ → O₂ + 4e⁻ (oxygen gas forms).
- Carbon anodes must be replaced regularly as they react with O₂ to form CO₂ and get consumed.
Extraction of Iron (Reduction by Carbon/Blast Furnace)
- Raw materials: Hematite (Fe₂O₃), coke (carbon), and limestone (CaCO₃).
- Steps:
- Coke burns in air to form CO₂, increasing furnace temperature.
- Coke reacts with CO₂ to form CO.
- Fe₂O₃ reduced by CO or C to iron: Fe₂O₃ + 3CO → 2Fe + 3CO₂.
- Limestone decomposes to CaO, which reacts with SiO₂ impurities to form slag (CaSiO₃).
- Molten iron and slag are collected separately at the furnace base.
Other Extraction Method: Using More Reactive Metals
- A more reactive metal can displace a less reactive one from its compound (e.g., Mg reduces CuO to Cu), but this is less common in SPM exams.
Key Terms & Definitions
- Ore — Raw material from which metals are extracted.
- Electrolysis — Extraction method using electricity to break down compounds.
- Reduction by Carbon — Process where carbon removes oxygen from metal oxides.
- Reactivity Series — Table ranking metals by reactivity with oxygen.
- Blast Furnace — Industrial apparatus for extracting iron.
- Cryolite — Compound added to bauxite to lower its melting point.
- Slag — Waste product formed from impurities during iron extraction.
Action Items / Next Steps
- Review the functions of cryolite and steps in electrolysis and blast furnace processes.
- Practice writing and balancing extraction equations for aluminum and iron.
- Learn to identify when to use electrolysis or reduction by carbon based on the reactivity series.