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Chemistry · Practice book · Senior 6 · Unit 2 of 14

Extraction of metals

Senior 6 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 2 Extraction of metals

Key unit competence: Relate the properties of metals to their methods of extraction.

Key ideas

  • The method of extraction depends on the reactivity of the metal: very reactive metals (K, Na, Ca, Mg, Al) are extracted by electrolysis of molten compounds; moderately reactive metals (Zn, Fe, Sn, Pb) by reduction of the oxide with carbon or carbon monoxide; unreactive metals (Cu, Ag, Au) by roasting or are found native.
  • General steps: mining → concentration of the ore (froth flotation for sulfides, gravity separation for dense ores like cassiterite, magnetic separation for wolframite) → roasting (sulfide to oxide) or calcination (carbonate to oxide) → reduction → refining.
  • Iron (blast furnace): ore haematite Fe2O3, coke, limestone, hot air. C + O2 → CO2; CO2 + C → 2CO; Fe2O3 + 3CO → 2Fe + 3CO2. Limestone removes silica: CaCO3 → CaO + CO2; CaO + SiO2 → CaSiO3 (slag). Pig iron (about 4 % C) is converted to steel by blowing oxygen.
  • Copper: chalcopyrite CuFeS2 → froth flotation → roasting → smelting with silica (iron removed as slag) → Cu2S + O2 → 2Cu + SO2 (blister copper) → electrolytic refining. Zinc: zinc blende ZnS roasted to ZnO, then reduced by carbon or by electrolysis of ZnSO4.
  • Rwanda's ores: tin from cassiterite SnO2 (reduced by carbon: SnO2 + 2C → Sn + 2CO); tungsten (wolfram) from wolframite (Fe,Mn)WO4 → WO3, reduced by hydrogen: WO3 + 3H2 → W + 3H2O; tantalum from coltan → K2TaF7, reduced by sodium: K2TaF7 + 5Na → Ta + 2KF + 5NaF. Sodium: Downs cell (molten NaCl + CaCl2); aluminium: bauxite purified (Bayer), electrolysed in molten cryolite.
  • Uses: Cu (wires, pipes, brass), Fe (steel), Sn (solder, tin plating), Zn (galvanising, batteries, brass), W (bulb filaments, drill tips: highest melting metal), Ta (capacitors in phones and laptops). Dangers: SO2 and acid rain, dust, land degradation, water pollution, collapse of pits; prevention: scrubbers, land restoration, protective equipment, safe tailings storage.

Common misconceptions

  • "Any metal can be extracted by heating its oxide with carbon." No: Metals above carbon in reactivity (Na, Mg, Al) cannot be reduced by carbon; they need electrolysis.
  • "In the blast furnace carbon reduces iron(III) oxide directly." No: The main reducing agent is carbon monoxide, formed from coke and carbon dioxide.
  • "Limestone is added to the blast furnace as a fuel." No: Limestone removes silica impurities as calcium silicate slag.

Worked examples

Example 1. Describe the extraction of iron in the blast furnace, with equations.

Solution

Raw materials: haematite (Fe2O3), coke (C), limestone (CaCO3) and hot air.

Coke burns: C + O2 → CO2; then CO2 + C → 2CO.

Reduction: Fe2O3 + 3CO → 2Fe + 3CO2; molten iron sinks.

Limestone decomposes: CaCO3 → CaO + CO2; CaO + SiO2 → CaSiO3 (slag floats on the iron).

Products: pig iron, slag (road making), waste gases.

Example 2. Explain how cassiterite from a Rwandan mine is turned into pure tin.

Solution

Crush the ore; wash with water on shaking tables: dense cassiterite settles, light sand is washed away (gravity separation).

Remove magnetic impurities (e.g. wolframite) with a magnet; roast to remove sulfur and arsenic.

Smelt with carbon (coke or charcoal): SnO2 + 2C → Sn + 2CO.

Refine the molten tin by liquation or electrolysis.

Practice questions

  1. Explain why gold is found native.

    Show answer

    Gold is very unreactive, so it does not combine with oxygen or sulfur.

    Common mistake: Saying gold is too heavy to react.

  2. Write the equation for the reduction of zinc oxide with carbon.

    Show answer

    ZnO + C → Zn + CO

    Common mistake: Leaving the equation unbalanced.

  3. State the role of cryolite in aluminium extraction.

    Show answer

    It dissolves alumina and lowers the operating temperature (about 950 °C), saving energy.

    Common mistake: Saying cryolite is the ore.

  4. Name the products at each electrode of the Downs cell.

    Show answer

    Cathode: sodium; anode: chlorine.

    Common mistake: Swapping the products.

  5. Give two uses of tin.

    Show answer

    Solder, tin plating of steel cans, bronze, pewter (any two).

    Common mistake: Listing 'electric bulb filaments'.

  6. Why is coltan important to the world economy?

    Show answer

    Its tantalum is used in capacitors for phones, laptops and medical devices.

    Common mistake: Saying it is used as a fuel.

  7. Give two environmental dangers of mining.

    Show answer

    Land degradation/erosion, water pollution from tailings, dust, SO2 and acid rain, loss of habitat (any two).

    Common mistake: Listing only 'noise'.

  8. What is the difference between roasting and calcination?

    Show answer

    Roasting heats an ore in air (sulfide → oxide); calcination heats it with little air (carbonate → oxide + CO2).

    Common mistake: Saying they are the same.

  9. Why is pig iron brittle?

    Show answer

    It contains about 4 % carbon and other impurities that make it hard and brittle.

    Common mistake: Saying it is pure iron.

  10. Write the equation for the reduction of K2TaF7 by sodium.

    Show answer

    K2TaF7 + 5Na → Ta + 2KF + 5NaF

    Common mistake: Using carbon as reducing agent.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Calcium chloride is added to sodium chloride in the Downs cell to:a. reactivity
    2. Sodium is extracted by:b. lower the melting point
    3. The method used to extract a metal depends mainly on its:c. electrolysis of molten sodium chloride
    4. Cassiterite is concentrated mainly by:d. gravity separation using its high density
    Show answer

    1 → b (lower the melting point) · 2 → c (electrolysis of molten sodium chloride) · 3 → a (reactivity) · 4 → d (gravity separation using its high density)

    • Saves energy.
    • Downs cell.
    • More reactive metals are harder to reduce.
    • It is much denser than sand.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    copper and zinc  ·  sulfide ores  ·  carbon dioxide  ·  carbon monoxide
    1. The main reducing agent in the blast furnace is .
    2. Brass is an alloy of .
    3. Froth flotation is used to concentrate .
    Show answer

    1: carbon monoxide · 2: copper and zinc · 3: sulfide ores

    • Fe2O3 + 3CO → 2Fe + 3CO2.
    • Bronze is copper and tin.
    • Sulfide particles stick to bubbles.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Aluminium oxide for electrolysis is obtained from:a. bauxite
    2. Tantalum is mainly used to make:b. cassiterite
    3. The main ore of iron used in the blast furnace is:c. capacitors in phones and computers
    4. The ore of tin found in Rwanda is:d. haematite
    Show answer

    1 → a (bauxite) · 2 → c (capacitors in phones and computers) · 3 → d (haematite) · 4 → b (cassiterite)

    • Purified by the Bayer process.
    • It stores charge well in tiny volumes.
    • Fe2O3.
    • SnO2.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    an oxide and sulfur dioxide  ·  hydrogen  ·  has a very high melting point  ·  is a liquid
    1. Tungsten (wolfram) is obtained by reducing tungsten(VI) oxide with .
    2. Tungsten is used in bulb filaments because it .
    3. Roasting a sulfide ore in air converts it to .
    Show answer

    1: hydrogen · 2: has a very high melting point · 3: an oxide and sulfur dioxide

    • Carbon would form tungsten carbide.
    • Melting point about 3400 °C.
    • e.g. 2ZnS + 3O2 → 2ZnO + 2SO2.

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