METALS (EXTRACTION AND PROPERTIES) 1/3 free
20.0 Metals (Extraction and Properties) – Complete Study Sheet

20.0 METALS (EXTRACTION AND PROPERTIES)

Metals have shaped human civilization – from the Bronze Age to the Iron Age to the modern age of aluminium and alloys. This chapter explores how we extract metals from their ores, focusing on the relationship between reactivity and extraction method. We examine the extraction of sodium, aluminium, iron, copper, zinc, and lead, and conclude with the properties and uses of important alloys like steel, brass, and bronze.

20.1 METHODS OF EXTRACTION

The method used to extract a metal from its ore depends on its position in the reactivity series.

20.1.1 The Reactivity Series

Metals are arranged in order of their reactivity (most reactive first):

  • Potassium (K) – most reactive
  • Sodium (Na)
  • Calcium (Ca)
  • Magnesium (Mg)
  • Aluminium (Al)
  • Zinc (Zn)
  • Iron (Fe)
  • Tin (Sn)
  • Lead (Pb)
  • Copper (Cu)
  • Silver (Ag) – least reactive (found native)
  • Gold (Au) – least reactive (found native)

20.1.2 Extraction by Electrolysis

Metals above carbon in the reactivity series (K, Na, Ca, Mg, Al) are extracted by electrolysis of their molten compounds. These metals are too reactive to be reduced by carbon; they form very stable compounds that require large amounts of energy to break down.

Principle: Passing an electric current through the molten (melted) ore decomposes it into the metal and non-metal.

Examples: Sodium (Down's cell), Aluminium (Hall-Héroult process), Magnesium (from molten MgCl₂).

20.1.3 Extraction by Reduction with Carbon

Metals below carbon in the reactivity series (Zn, Fe, Sn, Pb) are extracted by reduction with carbon (coke) in a furnace. Carbon is cheaper and more energy-efficient than electrolysis.

Principle: The metal oxide is heated with carbon (coke), which removes the oxygen, producing the metal and carbon dioxide (or monoxide).

Metal oxide + Carbon → Metal + Carbon dioxide

Example: 2Fe₂O₃ + 3C → 4Fe + 3CO₂ (or with CO as reducing agent).

20.1.4 Native Metals

Metals very low in the reactivity series (Cu, Ag, Au) can sometimes be found in their native (free) state in the Earth's crust. They are unreactive and do not easily form compounds. Copper is sometimes found native, but is usually extracted from its ores.

20.2 SODIUM – THE DOWN'S CELL

Sodium is extracted by the electrolysis of molten sodium chloride (Down's process).

20.2.1 The Down's Cell

  • Electrolyte: Molten sodium chloride mixed with calcium chloride (to lower the melting point from 800°C to about 600°C, saving energy).
  • Electrodes:
    • Anode: Graphite (carbon) – positive electrode.
    • Cathode: Steel – negative electrode (circular surrounding the anode).
  • Separator: A steel gauze separates the anode and cathode compartments to prevent the products (sodium and chlorine) from recombining.

20.2.2 Reactions

  • At the cathode (-): Na⁺ + e⁻ → Na(l) (sodium metal formed, less dense than electrolyte, floats and is collected).
  • At the anode (+): 2Cl⁻ → Cl₂(g) + 2e⁻ (chlorine gas is produced and collected).

Overall: 2NaCl(l) → 2Na(l) + Cl₂(g)

20.3 ALUMINIUM – THE HALL-HÉROULT PROCESS

Aluminium is extracted from its ore, bauxite (Al₂O₃·xH₂O), by electrolysis.

20.3.1 Purification of Bauxite (Bayer Process)

Bauxite contains impurities like iron(III) oxide and silica. It is purified by:

  1. Digesting with hot concentrated sodium hydroxide, which dissolves aluminium oxide (amphoteric) to form sodium aluminate.
  2. Impurities (red mud) are filtered off.
  3. Aluminium hydroxide is precipitated, then heated to give pure aluminium oxide (alumina, Al₂O₃).

20.3.2 Electrolysis (Hall-Héroult Process)

  • Electrolyte: Alumina (Al₂O₃) is dissolved in molten cryolite (Na₃AlF₆). Cryolite lowers the melting point of alumina from over 2000°C to about 950°C, saving energy and cost.
  • Electrodes:
    • Cathode: The carbon lining of the steel tank (negative).
    • Anode: Large carbon blocks suspended in the electrolyte (positive).
  • Reactions:
    • At the cathode (-): Al³⁺ + 3e⁻ → Al(l) (molten aluminium collects at the bottom and is tapped off).
    • At the anode (+): 2O²⁻ → O₂(g) + 4e⁻. However, the oxygen produced at high temperature reacts with the carbon anodes: C + O₂ → CO₂. This means the anodes are gradually consumed and must be replaced regularly.

Overall: 2Al₂O₃(l) + 3C(s) → 4Al(l) + 3CO₂(g) (simplified, including anode consumption).

20.4 IRON – THE BLAST FURNACE

Iron is extracted from its ore, usually haematite (Fe₂O₃), by reduction with carbon (coke) in a blast furnace.

20.4.1 Raw Materials

  • Iron ore: Haematite (Fe₂O₃) or magnetite (Fe₃O₄).
  • Coke: Almost pure carbon (acts as fuel and reducing agent).
  • Limestone: Calcium carbonate (CaCO₃) – acts as a flux to remove impurities.
  • Hot air: Blasted in at the bottom to provide oxygen for combustion.

20.4.2 Reactions in the Blast Furnace

  1. Combustion: Coke burns in hot air to produce carbon dioxide and heat.
    C(s) + O₂(g) → CO₂(g) (exothermic)
  2. Formation of reducing agent: Carbon dioxide reacts with more coke to form carbon monoxide.
    CO₂(g) + C(s) → 2CO(g) (endothermic)
  3. Reduction of iron ore: Carbon monoxide reduces the iron(III) oxide to iron.
    Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g) (main reaction)
    Some direct reduction by carbon also occurs: 2Fe₂O₃ + 3C → 4Fe + 3CO₂
  4. Removal of impurities (slag formation): Limestone decomposes: CaCO₃ → CaO + CO₂. Calcium oxide (basic) reacts with silica (sand, SiO₂, acidic impurity) to form calcium silicate (slag).
    CaO(s) + SiO₂(s) → CaSiO₃(l)

Products:

  • Molten iron: Tapped from the bottom. This is called pig iron (contains about 4% carbon and other impurities). It is brittle and used to make cast iron or steel.
  • Slag: Molten calcium silicate (floats on iron) – tapped off and used for road building, cement manufacture.
  • Waste gases: Mainly N₂, CO, CO₂ – cleaned and used to preheat the air.

20.5 COPPER, ZINC, AND LEAD EXTRACTION

20.5.1 Copper Extraction

Copper is often found as copper pyrites (CuFeS₂) or copper oxide ores. The process involves several steps.

  1. Concentration: Ore is crushed and ground. Froth flotation is used to separate copper minerals from gangue.
  2. Roasting: Sulfide ores are heated in air to produce copper oxide:
    2CuFeS₂ + 4O₂ → Cu₂S + 2FeO + 3SO₂ (simplified; actually produces a mixture called "matte").
  3. Smelting: The matte is heated with silica to remove iron as slag (iron silicate). Copper sulfide remains.
  4. Conversion: Copper sulfide is heated with air to produce blister copper (about 98% pure):
    Cu₂S + O₂ → 2Cu + SO₂.
  5. Electrolytic refining: Blister copper is cast into anodes and purified by electrolysis (as in Chapter 19) to obtain 99.99% pure copper for electrical use.

20.5.2 Zinc Extraction

Zinc is extracted from its ore, usually zinc blende (sphalerite, ZnS).

  1. Concentration: Froth flotation.
  2. Roasting: Zinc sulfide is heated in air to produce zinc oxide:
    2ZnS + 3O₂ → 2ZnO + 2SO₂ (SO₂ used to make sulfuric acid).
  3. Reduction: Zinc oxide is mixed with coke and heated to about 1200°C. Carbon monoxide reduces the oxide:
    ZnO + C → Zn + CO (and ZnO + CO → Zn + CO₂).
  4. Distillation: Zinc is volatile (boils at 907°C). It is distilled off and condensed as pure zinc (zinc vapor is quickly cooled to avoid re-oxidation).

Electrolytic method is also used: zinc oxide is dissolved in sulfuric acid, purified, and electrolyzed to deposit pure zinc.

20.5.3 Lead Extraction

Lead is mainly extracted from galena (PbS).

  1. Concentration: Froth flotation.
  2. Roasting in air: Partially roasted to produce lead oxide and some lead sulfate:
    2PbS + 3O₂ → 2PbO + 2SO₂
  3. Reduction in a furnace: The mixture of PbO, PbS, and PbSO₄ is heated with coke and air. Reactions include:
    • 2PbO + PbS → 3Pb + SO₂ (self-reduction)
    • PbO + C → Pb + CO
  4. Molten lead is tapped off. It may be further refined by electrolysis.

20.6 ALLOYS AND THEIR USES

An alloy is a mixture of a metal with one or more other elements (metals or non-metals). Alloys are designed to have properties superior to the pure metal, such as greater strength, hardness, corrosion resistance, or lower melting point.

20.6.1 Why Alloys are Stronger

In a pure metal, atoms are arranged in regular layers that can slide over each other (making the metal malleable and ductile). In an alloy, atoms of different sizes disrupt the regular arrangement, making it harder for layers to slide. This increases strength and hardness.

20.6.2 Important Alloys

  • Steel (Iron + Carbon, plus other metals):
    • Composition: Iron with 0.1–2.1% carbon. Other metals (e.g., chromium, nickel, manganese) can be added for specific properties.
    • Properties: Hard, strong, durable. Properties vary with carbon content and alloying elements.
    • Uses: Construction (girders, beams), tools, vehicles, machinery.
    • Stainless steel: Contains chromium and nickel, corrosion-resistant – used in cutlery, medical instruments.
  • Brass (Copper + Zinc):
    • Composition: Typically 60-80% copper, 20-40% zinc.
    • Properties: Harder than copper, corrosion-resistant, malleable, gold-like appearance.
    • Uses: Musical instruments, decorative items, plumbing fittings, cartridge cases.
  • Bronze (Copper + Tin):
    • Composition: Typically 80-90% copper, 10-20% tin.
    • Properties: Hard and tough, corrosion-resistant (especially in seawater), good casting properties.
    • Uses: Statues, medals, ship propellers, bearings, bells.
  • Other alloys:
    • Duralumin (Aluminium + Copper + Magnesium): Lightweight, strong – used in aircraft.
    • Solder (Lead + Tin): Low melting point – used for joining electrical components.
    • Pewter (Tin + Antimony + Copper): Decorative tableware.
    • Amalgam (Mercury + other metals): Used in dental fillings.

20.7 SUMMARY TABLE: EXTRACTION METHODS

MetalReactivityExtraction MethodKey Process/Ore
SodiumVery highElectrolysis of molten NaClDown's cell
AluminiumHighElectrolysis of Al₂O₃ in cryoliteHall-Héroult (bauxite)
ZincMediumReduction with carbon (roast then smelt)Zinc blende (ZnS)
IronMediumReduction with carbon (CO)Blast furnace (haematite)
LeadLowReduction with carbonGalena (PbS)
CopperLowRoasting, smelting, then electrolytic refiningCopper pyrites (CuFeS₂)

✍️ COMPREHENSIVE PRACTICE QUESTIONS

Section A: Short Answer & Definitions

  1. List the reactivity series from most reactive to least reactive (at least 8 metals).
  2. Explain why metals above carbon are extracted by electrolysis, while metals below carbon can be extracted by reduction with carbon.
  3. Describe the Down's cell for sodium extraction. What are the anode, cathode, and electrolyte?
  4. Write the half-equations for the reactions in the Down's cell.
  5. Why is cryolite used in the Hall-Héroult process for aluminium extraction?
  6. What happens to the carbon anodes in the Hall-Héroult process? Why?
  7. Name the raw materials fed into a blast furnace for iron extraction.
  8. Write the main chemical equations for the reactions in the blast furnace:
    • a) Combustion of coke
    • b) Formation of reducing agent
    • c) Reduction of iron ore
    • d) Removal of impurities (slag formation)
  9. What is pig iron? How is it different from steel?
  10. Describe briefly how copper is extracted from its sulfide ore and then refined.
  11. What is an alloy? Give two examples of alloys and their constituent metals.
  12. Why are alloys generally stronger than pure metals?

Section B: Application & Explanation

  1. Explain why aluminium is more expensive to produce than iron, even though aluminium is more abundant in the Earth's crust.
  2. Why is calcium chloride added to the electrolyte in the Down's cell?
  3. In the blast furnace, why is limestone added along with iron ore and coke?
  4. Explain why the carbon anodes in the aluminium extraction process must be replaced regularly.
  5. Why is copper purified by electrolysis, rather than simply by heating?
  6. Compare the extraction methods of zinc and iron. How are they similar and different?
  7. Why is stainless steel preferred for making cutlery and surgical instruments, rather than ordinary steel?
  8. Explain why bronze was an important alloy in human history (Bronze Age). What properties made it useful?
  9. Why is it not possible to extract sodium by heating its oxide with carbon?
  10. Describe the environmental concerns associated with the extraction of metals (e.g., sulfur dioxide emissions, energy use).

Section C: Fill in the Blanks

  1. Metals above __________ in the reactivity series are extracted by electrolysis.
  2. The Down's cell extracts __________ from molten sodium chloride.
  3. In the Hall-Héroult process, alumina is dissolved in molten __________.
  4. The main reducing agent in the blast furnace is __________ gas.
  5. Limestone decomposes in the blast furnace to form __________ and carbon dioxide.
  6. The impurity removed as slag in the blast furnace is mainly __________.
  7. Copper obtained after smelting is called __________ copper and is about 98% pure.
  8. Brass is an alloy of copper and __________.
  9. Steel contains iron and a small percentage of __________.

Section D: Equations and Calculations

  1. Write balanced equations for:
    • a) The reduction of iron(III) oxide by carbon monoxide.
    • b) The formation of slag (calcium silicate) in the blast furnace.
    • c) The reduction of zinc oxide by carbon.
    • d) The overall reaction for the electrolysis of alumina (considering anode consumption to CO₂).
    • e) The roasting of zinc sulfide.
    • f) The self-reduction reaction in lead extraction: PbO + PbS → ?
  2. Calculate the mass of aluminium that can be extracted from 1.0 tonne of pure alumina (Al₂O₃). (Al=27, O=16)
  3. What mass of iron can be obtained from 100 tonnes of haematite (Fe₂O₃) that is 80% pure? (Fe=56, O=16)
  4. In the Down's cell, if a current of 10,000 A is passed for 24 hours, what mass of sodium is produced? (Na=23, F=96500 C/mol)
  5. Calculate the volume of sulfur dioxide (at RTP) produced when 1.0 tonne of zinc blende (ZnS, assuming pure) is roasted. (Zn=65, S=32, molar volume = 24 dm³/mol at RTP)

Section E: Challenge / Multi-Concept

  1. Draw a labeled diagram of a blast furnace. Explain the temperature gradient and the chemical reactions occurring in different zones.
  2. Compare and contrast the extraction of aluminium and sodium. Both use electrolysis, but what are the key differences in the processes?
  3. Explain why recycling aluminium is much more energy-efficient than extracting it from bauxite. Why is recycling important for sustainability?
  4. A sample of an alloy is found to contain copper and zinc. Describe a simple chemical test to confirm the presence of both metals.
  5. Connect the concepts: How does the reactivity series determine not only the extraction method but also the uses of metals (e.g., why gold is found native, why sodium is stored under oil)?
  6. Why is iron alloyed with carbon and other metals to make different types of steel? How do the properties change with carbon content?

📝 ANSWERS TO SELECTED QUESTIONS

1. K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, Cu, Ag, Au (approx).
2. Above C: too reactive, compounds too stable for C to reduce; need electrolysis. Below C: C can reduce oxides.
3. Down's: molten NaCl + CaCl₂, graphite anode, steel cathode, separated to prevent recombination.
4. Cathode: Na⁺ + e⁻ → Na; Anode: 2Cl⁻ → Cl₂ + 2e⁻.
5. Cryolite lowers melting point of Al₂O₃ from >2000°C to ~950°C, saves energy.
6. Anodes react with O₂ produced: C + O₂ → CO₂, so they burn away.
7. Iron ore (haematite), coke, limestone, hot air.
8. a) C + O₂ → CO₂; b) CO₂ + C → 2CO; c) Fe₂O₃ + 3CO → 2Fe + 3CO₂; d) CaCO₃ → CaO + CO₂, CaO + SiO₂ → CaSiO₃.
9. Pig iron: high C (4%), brittle. Steel: lower C (0.1-1.5%), stronger, more versatile.
10. Cu ore (e.g., CuFeS₂) roasted, smelted to blister copper (~98%), then electrolytically refined to 99.99%.
11. Alloy: mixture of metal with other elements. Brass (Cu+Zn), Bronze (Cu+Sn).
12. Different sized atoms disrupt layers, making it harder for layers to slide.
13. Al extraction requires huge electricity (electrolysis); iron uses coke (cheaper).
14. Lowers melting point of NaCl, saving energy.
15. Limestone decomposes to CaO, which removes SiO₂ impurity as slag (CaSiO₃).
16. Anodes are consumed by reaction with oxygen.
17. Electrolysis gives very high purity (99.99%) needed for electrical conductivity.
18. Both involve roasting sulfide to oxide, then reduction with C. Zn requires distillation (volatile).
19. Stainless steel contains Cr and Ni, forming protective oxide layer, resists corrosion.
20. Bronze is harder than copper, casts well, resists corrosion – ideal for tools, weapons, art.
21. Na too reactive; its oxide very stable, C cannot reduce it.
22. SO₂ emissions (acid rain), CO₂ (greenhouse), high energy use, landscape damage from mining.
23. carbon
24. sodium
25. cryolite
26. carbon monoxide (CO)
27. calcium oxide (CaO)
28. silica (SiO₂)
29. blister
30. zinc
31. carbon
33. a) Fe₂O₃ + 3CO → 2Fe + 3CO₂; b) CaO + SiO₂ → CaSiO₃; c) ZnO + C → Zn + CO; d) 2Al₂O₃ + 3C → 4Al + 3CO₂; e) 2ZnS + 3O₂ → 2ZnO + 2SO₂; f) 2PbO + PbS → 3Pb + SO₂.
34. Molar mass Al₂O₃ = 102 g/mol. Mass Al in 102 g Al₂O₃ = 54 g. So from 1 tonne (10⁶ g) Al₂O₃, mass Al = (54/102) × 10⁶ = 529,412 g = 0.529 tonnes.
35. Mass of pure Fe₂O₃ = 80% of 100 t = 80 t = 80 × 10⁶ g. Molar mass Fe₂O₃ = 160 g/mol. Moles Fe₂O₃ = 80×10⁶/160 = 500,000 mol. Each Fe₂O₃ gives 2 Fe, so moles Fe = 1,000,000 mol. Mass Fe = 1,000,000 × 56 = 56,000,000 g = 56 tonnes.
36. Q = 10,000 × (24×3600) = 10,000 × 86400 = 8.64×10⁸ C. Moles e⁻ = 8.64×10⁸/96500 = 8953 mol e⁻. 1 mol Na per mol e⁻, so mass Na = 8953 × 23 = 205,919 g ≈ 206 kg.
37. Molar mass ZnS = 97 g/mol. Moles ZnS in 1 t (10⁶ g) = 10⁶/97 = 10,309 mol. From 2ZnS + 3O₂ → 2ZnO + 2SO₂, moles SO₂ = moles ZnS = 10,309 mol. Volume SO₂ at RTP = 10,309 × 24 = 247,416 dm³ = 247.4 m³.
39. Both electrolysis of molten compounds. Al: Al₂O₃ in cryolite, anodes consumed. Na: NaCl + CaCl₂, inert anodes, separated products.
40. Recycling Al uses only 5% of energy of extraction from ore; conserves resources, reduces waste, CO₂.
41. Dissolve in acid; test for Cu²⁺ (blue solution, may give precipitate). Test for Zn²⁺ (white precipitate with NaOH, dissolves in excess).
42. Reactivity determines extraction method and how metal is used/stored. Very reactive (Na) stored under oil, found as compounds. Unreactive (Au) found native.
43. Carbon content affects hardness and strength. Low C (mild steel) ductile, high C (tool steel) hard and brittle. Alloying with Cr, Ni, etc. gives specific properties (corrosion resistance, hardness).

These notes are your complete guide to the extraction and properties of metals. Master the link between reactivity and extraction method, understand the details of industrial processes like the blast furnace and Hall-Héroult cell, and appreciate the importance of alloys in modern materials. This knowledge connects geology, chemistry, and engineering. Keep learning.

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