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5.3 Extracting Metals

GCSE Chemistry · AQA · Chapter 5: Chemical Changes

A free GCSE Chemistry lesson for AQA (8462) 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, and explain when hydrogen is used instead of carbon
  • Calculate the percentage of metal in an ore and the mass of metal that can be extracted

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 (Chapter 6).

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 with hydrogen

Hydrogen can also remove oxygen from some metal oxides. It is used instead of carbon when carbon would spoil the metal: carbon can combine with some metals, such as molybdenum, to form hard carbides, so the metal would not be pure. Hydrogen gives a very pure metal, and the other product, steam, simply escapes.

molybdenum(VI) oxide + hydrogen → molybdenum + water MoO₃(s) + 3H₂(g) → Mo(s) + 3H₂O(g)

Steel-makers are now testing hydrogen in place of coke to reduce iron ore. The waste product is water instead of carbon dioxide, so this could cut greenhouse gas emissions – but making enough hydrogen needs a lot of (ideally renewable) electricity.

Exam tip: In MoO₃ + 3H₂ → Mo + 3H₂O, molybdenum oxide loses oxygen (reduced) and hydrogen gains oxygen (oxidised).

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. Low-grade copper ores can also be processed using plants or bacteria (Chapter 15).

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 80% Fe₂O₃ by mass. What mass of iron can be extracted from 1000 kg of ore?
mass of Fe₂O₃ = 0.80 × 1000 = 800 kg
mass of Fe = 0.70 × 800 = 560 kg

Answer: 560 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 gold 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.)

Answer: it is very unreactive

Working: Gold is very unreactive, so it does not combine with oxygen or other elements. It is found native.

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

Answer: aluminium

Working: Aluminium is more reactive than carbon, so carbon cannot remove oxygen from aluminium oxide. It is extracted by electrolysis.

Question 3. In the reaction 2CuO + C → 2Cu + CO₂, name the substance that is reduced.

Answer: copper oxide

Working: Copper oxide loses oxygen to become copper, so copper oxide is reduced. Carbon gains oxygen, so it is oxidised.

Related lessons

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This lesson is section 5.3 (AGCCH0503) of our free GCSE Chemistry 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 Chemistry course.

 

 

 

 

 

 

 

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