Ch 6 · Extraction of Metals · O Level Chemistry 5070
Topic 6 of 6 in Metals — O Level Chemistry 5070 — 47 lessons.
Introduction
Most metals occur in the Earth's crust as compounds, usually oxides or sulfides, mixed with rock. An ore is a rock containing enough of a metal compound to make extraction worthwhile. How the metal is extracted depends entirely on its position in the reactivity series, which is why this lesson follows directly from the series itself. You must know the general principle for all metals, the blast furnace for iron, and electrolysis for aluminium.
Key ideas — Reactivity series and ease of extraction
The more reactive a metal, the more strongly it is bonded in its compounds and the harder and more expensive it is to extract.
🧑🏫 Teacher's explanation
Think of metal extraction like a game of 'tug-of-war' over oxygen. In many metal ores, the metal is bonded to oxygen. To get the pure metal, you need to pull that oxygen away. The 'strength' of the bond depends on how reactive the metal is.
📖 Definitions
🔍 Worked example — Easy
A sample of copper(II) oxide (CuO) is heated with carbon (C). Write the balanced chemical equation for the reduction of the metal oxide.
🔍 Worked example — Standard
Explain why aluminium cannot be extracted from its ore, aluminium oxide (Al_2O_3), by heating it with carbon.
🔍 Worked example — Exam-style
In the extraction of aluminium, the carbon anodes react with oxygen produced at the anode. If the current density is such that 120 g of oxygen is produced, calculate the mass of carbon consumed if the reaction is 2C + O_2 arrow 2CO_2. (A_r of C=12, O=16)
⚠️ Common errors
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🎯 Exam question
(a) Name the substance used as a reducing agent in the blast furnace. [1] (b) Write a balanced chemical equation for the reaction between iron(III) oxide and carbon monoxide. [2] (c) Explain why the carbon anodes in the electrolysis of aluminium must be replaced regularly. [2] [5 marks]
Key ideas — The blast furnace
extraction of iron from hematite. The four raw materials are hematite (iron(III) oxide, Fe_2O_3), coke (almost pure carbon), limestone (calcium carbonate, CaCO_3) and hot air. The solids are fed in at the top and hot air is blasted in near the bottom. Four reactions occur.
🧑🏫 Teacher's explanation
Think of a blast furnace like a giant, high-temperature chemical kitchen. To get pure iron out of its ore, we can't just 'wash' it; we have to chemically strip the oxygen away. We use a 'reducing agent'—a substance that is hungry for oxygen—to pull it off the iron oxide.
📖 Definitions
🔍 Worked example — Easy
Calculate the mass of iron produced from 160 g of hematite (Fe_2O_3) if the reaction is Fe_2O_3 arrow 2Fe + 3/2O_2. (Ar: Fe=56, O=16)
🔍 Worked example — Standard
A student calculates the mass of CaO produced from 100 g of CaCO_3. Write the balanced equation and find the mass of CaO. (Ar: Ca=40, C=12, O=16)
🔍 Worked example — Exam-style
In the blast furnace, SiO_2 reacts with CaO to form CaSiO_3. If 0.5 moles of SiO_2 reacts completely, how many moles of CaSiO_3 are formed? (Assume CaO is in excess)
⚠️ Common errors
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🎯 Exam question
Key ideas — Electrolysis of aluminium oxide
Aluminium is above carbon, so its ore bauxite must be purified to aluminium oxide and then electrolysed.
🧑🏫 Teacher's explanation
Think of extracting aluminium like trying to pull a very strong magnet away from a pile of iron filings. Because aluminium is much more reactive than carbon, carbon isn't 'strong' enough to pull the oxygen away from the aluminium in the ore. We need a much more powerful force: electricity.
📖 Definitions
🔍 Worked example — Easy
State the charge of the ions involved in the electrolysis of molten aluminium oxide and write the half-equation for the reaction occurring at the cathode.
🔍 Worked example — Standard
Calculate the mass of aluminium produced if 0.5 moles of electrons are passed through the cell during the reduction of Al^3+ ions.
🔍 Worked example — Exam-style
Explain why the carbon anodes in the aluminium extraction process must be replaced regularly. Include the chemical equation for the reaction involving the anode.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Write an equation for the reaction occurring at the anode. [1] (b) Explain why the carbon anodes must be replaced periodically. [2] (c) Why is it more environmentally friendly to recycle aluminium than to extract it from ore? [1] [4 marks]
Key ideas — Choosing the extraction method
Given any metal, the decision follows one test: is it above or below carbon in the reactivity series?
🧑🏫 Teacher's explanation
Think of the reactivity series as a hierarchy of 'strength' or 'greediness' for oxygen. In chemistry, metals want to bond with oxygen to become stable oxides. To get the pure metal back, we need to 'steal' that oxygen away using a more reactive element.
📖 Definitions
🔍 Worked example — Easy
A metal M is found to be below carbon in the reactivity series. State the most suitable method for extracting the pure metal from its oxide and explain why.
🔍 Worked example — Standard
Aluminium is extracted from bauxite via electrolysis. Why is this method used instead of reduction with carbon?
🔍 Worked example — Exam-style
Calculate the mass of iron produced when 160 g of iron(III) oxide is reduced by carbon monoxide in a blast furnace. Equation: Fe_2O_3 + 3CO arrow 2Fe + 3CO_2. (A_r: Fe=56, O=16, C=12)
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Explain why carbon cannot be used to extract metal X. [1] (b) State the name of the molten compound used in the electrolysis of aluminium to lower its melting point. [1] [2 marks]
Worked example
Question. (a) Explain why zinc can be extracted from zinc oxide by heating with carbon, but aluminium cannot be extracted from aluminium oxide in this way. [2] (b) In the extraction of aluminium, explain why the aluminium oxide is dissolved in molten cryolite and why the anodes need to be replaced regularly. Include the equation for the anode reaction. [4]
🔍 Worked example — Easy
A student wants to extract copper from copper(II) oxide. Given the reactivity series: Mg > Al > Zn > Fe > Pb > Cu > H > C, predict the method and write the balanced equation for the reduction using carbon.
🔍 Worked example — Exam-style
Explain why the following reaction occurs at the anode during aluminium extraction: 2O^2-(l) ightarrow O_2(g) + 4e^-.
⚠️ Common errors
Common mistakes
Practice
Answers Hematite [B1], reduction with carbon in a blast furnace [B1]; bauxite [B1], electrolysis [B1]. Gold is very unreactive (at the bottom of the reactivity series) so it does not combine with other elements [B1]; sodium is very reactive so it always exists as compounds and must be extracted by electrolysis [B1]. C(s) + O_2(g) arrow CO_2(g) [B1]; Fe_2O_3(s) + 3CO(g) arrow 2Fe(l) + 3CO_2(g) [B1 species, B1 balanced]; limestone decomposes to calcium oxide, which reacts with silicon dioxide impurities to form slag [B1]. Cathode: mathrmAl^3+(l) + 3e^- arrow Al(l) [B1]; anode: 2mathrmO^2-(l) arrow O_2(g) + 4e^- [B1]; the anode reaction is oxidation because electrons are lost [B1].