Overview
This lecture reviews the extraction methods of metals, focusing on aluminium and iron, and explains how a metal’s reactivity determines the extraction process used.
Metal Extraction Basics
- Extraction of metals from ores is a reduction process, involving a decrease in oxidation state.
- The method of extraction depends on the metal’s position in the reactivity series.
- Highly reactive metals (top of series) require strong reduction methods like electrolysis.
- Least reactive metals (bottom of series) can often be found as native elements and may be extracted by simple heating.
- Metals in the middle of the series are commonly extracted by reduction with carbon or carbon monoxide.
The Reactivity Series
- Reactivity increases up the series: potassium (most reactive) at the top, gold (least reactive) at the bottom.
- Hydrogen is used as a reference point within the reactivity series.
- The stability of metal ores increases with metal reactivity, requiring more energy for extraction.
Extraction of Aluminium
- Ore used: Aluminium oxide (Al₂O₃), also called bauxite.
- Aluminium is highly reactive; its extraction method is electrolysis.
- Steps: Purify aluminium oxide; dissolve in molten cryolite (Na₃AlF₆) to lower melting point for electrolysis.
- Electrolysis uses graphite electrodes; aluminium forms at the cathode, oxygen at the anode.
- Oxygen reacts with graphite anodes, forming CO₂, which wears down the anodes over time.
Extraction of Iron
- Ore used: Iron(III) oxide (Fe₂O₃), known as hematite.
- Iron is less reactive; extracted by reduction with carbon monoxide in a blast furnace.
- Mixture of iron ore, limestone (CaCO₃), and coke (carbon) is added to the furnace.
- Reactions:
- C + O₂ → CO₂ (exothermic)
- CO₂ + C → 2CO (endothermic)
- Fe₂O₃ + 3CO → 2Fe + 3CO₂
- Limestone decomposes to CaO, which reacts with silica impurities (SiO₂) to form slag (CaSiO₃), which is separated from the molten iron.
Key Terms & Definitions
- Ore — a naturally occurring mineral from which a metal can be profitably extracted.
- Reduction — gain of electrons or decrease in oxidation state.
- Electrolysis — chemical decomposition produced by passing an electric current through a liquid or solution containing ions.
- Reactivity series — a list of metals ranked by their reactivity.
- Blast furnace — a large structure used for extracting iron from ore using high temperatures and chemical reduction.
Action Items / Next Steps
- Revise the steps and equations involved in aluminium and iron extraction.
- Memorize positions of common metals in the reactivity series.
- Practice explaining why different extraction methods are chosen for different metals.