Extraction of Metals — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

Section 1 of 12Ores and extraction methods

• Ore
  • a rock containing enough metal to make extraction worthwhile.
• Oxide ores
  • extraction is reduction because oxygen is removed.
  • Examples
    • hematite for iron and bauxite for aluminium.
  • Methods
    • electrolysis, a blast furnace or reaction with a more reactive substance.
• Native metals
  • unreactive metals such as gold and platinum are found uncombined.
• Reason
  • they are chemically stable.
• Reactivity series link
  • metals above carbon need electrolysis, metals below carbon can be heated with carbon.

Section 2 of 12Blast furnace raw materials

• Raw materials
  • hematite, coke (impure carbon) and limestone are fed in at the top.
• Hot air
  • blown in at the bottom.
  • Output
    • iron is produced, at around 10 000 tonnes per day in modern furnaces.

Section 3 of 12Reactions in the blast furnace

1. Coke burns in the hot airC + O₂ → CO₂, an exothermic reaction that heats the furnace.
2. More coke reduces carbon dioxideCO₂ + C → 2CO.
3. Carbon monoxide reduces iron(III) oxideFe₂O₃ + 3CO → 2Fe + 3CO₂, and the molten iron collects at the bottom.

• Tapping
  • molten iron is tapped off.

Section 4 of 12Role of limestone and slag

1. Calcium carbonate decomposesCaCO₃ → CaO + CO₂.
2. Calcium oxide reacts with silicon dioxide impurity: CaO + SiO₂ → CaSiO₃.
3. Calcium silicate melts and floats as slag on the iron and is tapped off separately.

• Note
  • all five equations must be learned.

Section 5 of 12Aluminium: why electrolysis

• Position
  • aluminium is above carbon in the series, so carbon cannot reduce it.
• Ore
  • bauxite, purified to give aluminium oxide, Al₂O₃.

Section 6 of 12Electrolysis cell for aluminium

• Cryolite
  • aluminium oxide is dissolved in molten cryolite.
• Why
  • aluminium oxide melts above 2000 °C, which would use a lot of energy.
  • Effect
    • the mixture melts at a lower temperature without interfering.
• Cell
  • a steel cell lined with graphite, which is the negative electrode, with graphite blocks as positive electrodes.

Section 7 of 12Electrode reactions

Electrode	Process	Half equation
Cathode (negative)	Aluminium ions gain electrons (reduction)	Al3+ + 3e- → Al
Anode (positive)	Oxide ions lose electrons (oxidation)	2O2- → O2 + 4e-

• Overall
  • 2Al₂O₃ → 4Al + 3O₂.
• Product
  • molten aluminium collects at the bottom and is siphoned off, with fresh oxide added.
• Anode wear
  • oxygen reacts with the graphite anode: C + O₂ → CO₂.
• Replacement
  • anodes wear away and must be replaced regularly.
  • Cost
    • a large amount of electricity is needed, a major expense.

Section 8 of 12Why these furnace features matter

• Hot air
  • supplies oxygen to burn coke and keeps the furnace hot.
• Coke
  • acts as the fuel and as the source of the reducing agent carbon monoxide.
  • Limestone
    • removes the acidic silicon dioxide impurity as slag.
• Slag
  • being less dense than iron, it floats on top, so the two are tapped separately.
• Use of slag
  • it can be used in road building.
  • Iron
    • the product is run off as molten iron.

Section 9 of 12Common exam questions

• Name the ores
  • hematite is iron(III) oxide and bauxite is mainly aluminium oxide.
• Name the reducing agent
  • carbon monoxide in the blast furnace.
  • Why not carbon for aluminium
    • aluminium is more reactive than carbon.
  • Why cryolite
    • it lowers the melting point and saves energy.

Section 10 of 12Worked examples

1. Write equations for the reduction of iron(III) oxide in the blast furnace. [2]2 marks

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2. Explain the role of limestone in the blast furnace. [3]3 marks

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3. Explain why aluminium is extracted by electrolysis rather than by heating with carbon. [2]2 marks

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4. Why is cryolite used in the electrolysis of aluminium oxide? [2]2 marks

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Section 11 of 12Key terms

Term	Meaning
Ore	A rock containing enough metal to be worth extracting.
Blast furnace	A furnace used to extract iron by reduction with carbon and carbon monoxide.
Slag	Molten calcium silicate formed from impurities.
Cryolite	A compound that lowers the melting point of aluminium oxide.
Electrolysis	Decomposition of a molten or dissolved ionic compound by electricity.
Native metal	A metal found uncombined in the Earth's crust.

Section 12 of 12Exam technique

1. Saying carbon monoxide is produced from limestone — it comes from coke and carbon dioxide.
2. Saying graphite anodes last indefinitely — they burn away.
3. Writing oxidation at the cathode — reduction occurs at the cathode.
4. Forgetting that aluminium oxide must be molten or dissolved.