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C5.3 Extracting Metals

GCSE Combined Science · AQA · Chapter C5: Chemical Changes

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: where metals come from, and how the position of a metal in the reactivity series decides how we extract it. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain why gold and other unreactive metals are found as the metal itself
  • Define an ore and explain why what counts as an ore can change
  • Explain how metals less reactive than carbon are extracted by reduction with carbon, and write equations
  • Choose a method of extraction from a metal’s position in the reactivity series
  • (Higher tier) explain extraction as reduction in terms of metal ions gaining electrons

Key words

  • Native metal: A metal found in the Earth’s crust as the uncombined element, e.g. gold.
  • Ore: Rock containing enough of a metal or metal compound to make extracting the metal worthwhile (economical).
  • Extraction: Getting a metal out of its ore.
  • Reduction: Loss of oxygen. Removing oxygen from a metal oxide gives the metal.
  • Coke: A cheap, almost pure form of carbon made from coal, used to reduce metal oxides.
  • Electrolysis: Using electricity to break down a molten or dissolved ionic compound (see C6).

Metals in the Earth’s crust

Unreactive metals such as gold (and some silver and copper) are found native – as the element itself – because they do not react easily with oxygen or water. Gold can be separated from rock by physical methods, such as panning.

Most metals are reactive, so they are found as compounds, often oxides. An ore is rock that contains enough of a metal compound to make extraction economical. If the price of a metal rises, or a cheaper method is found, rock with less metal in it can become an ore.

Concentrating the ore: mined rock is crushed, and the metal compound is separated from the waste rock. For copper ores, the powdered rock is mixed with water and a chemical that makes the copper mineral cling to air bubbles; it floats off as a froth while the waste sinks (froth flotation).

Is it worth mining? That depends on:

  • how much metal the rock contains (its grade)
  • how easy and how expensive it is to extract the metal (energy, chemicals, transport)
  • how much the metal is worth and how much demand there is for it.

These things change. A rise in price, or a cheaper extraction method, can turn low-grade rock that was once waste into a useful ore.

Reduction with carbon

Carbon is more reactive than zinc, iron, tin, lead and copper. When the oxide of one of these metals is heated with carbon, the carbon takes the oxygen. The metal oxide is reduced (loses oxygen) and the carbon is oxidised (gains oxygen).

copper oxide + carbon → copper + carbon dioxide 2CuO(s) + C(s) → 2Cu(s) + CO₂(g)

zinc oxide + carbon → zinc + carbon dioxide 2ZnO(s) + C(s) → 2Zn(s) + CO₂(g)

iron(III) oxide + carbon → iron + carbon dioxide 2Fe₂O₃(s) + 3C(s) → 4Fe(l) + 3CO₂(g)

In industry, iron is made in a blast furnace. Coke burns to make carbon monoxide, which also reduces the iron oxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂.

Exam tip: You can tell which substance is reduced by asking “what lost oxygen?” In 2CuO + C → 2Cu + CO₂, copper oxide loses oxygen, so copper oxide is reduced.

Reduction in terms of electrons (Higher tier)

Metal oxides are ionic. In iron(III) oxide the iron is present as Fe³⁺ ions. When the metal is extracted, each metal ion gains electrons to become a metal atom. Gain of electrons is reduction, so extraction is reduction in both senses: the compound loses oxygen and the metal ions gain electrons.

Fe³⁺ + 3e− → Fe (iron ions reduced)

Cu²⁺ + 2e− → Cu (copper ions reduced)

Zn²⁺ + 2e− → Zn (zinc ions reduced)

Exam tip: Count the electrons from the charge on the ion: a 3+ ion needs 3 electrons to become a neutral atom. Electrons are on the left of a reduction half equation.

Choosing the method

Metals more reactive than carbon (such as aluminium and magnesium) cannot be reduced by carbon, because carbon cannot remove their oxygen. They are extracted by electrolysis of a molten compound. This needs a lot of electrical energy, so these metals cost more to extract.

Reduction with carbon is cheaper, so it is used whenever the metal is below carbon. Metals at the very bottom, such as gold and platinum, need no chemical reaction at all – they are simply separated from the rock.

Calculations with ores

Worked example: percentage of iron in iron(III) oxide

Mr of Fe₂O₃ = (2 × 56) + (3 × 16) = 160
% Fe = (mass of Fe in formula ÷ Mr) × 100 = (112 ÷ 160) × 100 = 70%

Answer: 70% iron by mass

Worked example: mass of iron from an ore

An ore contains 65% Fe₂O₃ by mass. What mass of iron can be extracted from 2000 kg of ore?
mass of Fe₂O₃ = 0.65 × 2000 = 1300 kg
mass of Fe = 0.70 × 1300 = 910 kg

Answer: 910 kg

Common mistakes

  • Saying aluminium can be extracted by heating with carbon. It is above carbon, so electrolysis is needed.
  • Saying the carbon is reduced. Carbon gains oxygen, so carbon is oxidised; the metal oxide is reduced.
  • Defining an ore as “rock with metal in it”. It must contain enough metal to make extraction economical.
  • Forgetting to multiply by the number of metal atoms in the formula when finding a percentage by mass (2 × 56 for Fe₂O₃).

Revision questions

Try these, then check your answers below each one.

Question 1. Why is platinum found in the Earth’s crust as the metal itself? (Choose from: it is very unreactive, it is very dense, it has a high melting point, it is a good conductor of electricity.)

Answer: it is very unreactive

Working: Platinum is so unreactive that it does not combine with oxygen or other elements, so it is found native.

Question 2. Which metal has to be extracted by electrolysis rather than by heating its oxide with carbon? (Choose from: magnesium, iron, zinc, lead.)

Answer: magnesium

Working: Magnesium is more reactive than carbon, so carbon cannot remove oxygen from magnesium oxide. Electrolysis is needed.

Question 3. In the reaction 2PbO + C → 2Pb + CO₂, name the substance that is oxidised.

Answer: carbon

Working: Carbon gains oxygen to form carbon dioxide, so it is oxidised. Lead(II) oxide loses oxygen, so it is reduced.

Related lessons

Continue in the full course

This lesson is section C5.3 (AGCSC2203) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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