Metals in the Earth’s Crust

Specification Point 4.4:
  • Recall that:
    a) most metals are extracted from ores found in the Earth’s crust
    b) unreactive metals are found in the Earth’s crust as the uncombined elements

Extraction of Metals

  • The Earth’s crust contains metals and metal compounds such as gold, copper, iron oxide and aluminium oxide.
  • Useful metals are often chemically combined with other substances forming ores.
  • A metal ore is a rock that contains enough of the metal to make it worthwhile extracting.
  • They have to be extracted from their ores through processes such as electrolysis, using a blast furnace or by reacting with more reactive material.
  • In many cases the ore is an oxide of the metal e.g. the ores of iron and aluminium are both oxides called haematite and bauxite respectively.

Unreactive Metals

  • Unreactive metals do not have to be extracted as they are often found as the uncombined element.
  • This occurs as they do not easily react with other substances due to their chemical stability.
  • Examples include gold and platinum which can both be mined directly from the Earth’s crust.

Redox Reactions & Oxygen

Specification Point 4.5:
  • Explain oxidation as the gain of oxygen and reduction as the loss of oxygen.
  • We have already looked at redox reactions in terms of the transfer of electrons. 
  • We now expand our understanding of oxidation and reduction reactions to include the addition or removal of oxygen.
  • Oxidation is a reaction in which:
    • an element, ion or compound loses electrons.
    • oxygen is added to an element or a compound.
  • Reduction is  a reaction in which:
    • an element, ion or compound gains electrons.
    • oxygen is removed from an element or a compound.

Redox - Oxygen, Edexcel GCSE Chemistry

Iron oxide is reduced as it loses oxygen, while carbon monoxide gains oxygen so is oxidised

Reduction of Ores

Specification Point 4.6:
  • Recall that the extraction of metals involves reduction of ores.
  • Most of the useful metals are found as ores in the Earth’s crust which are first mined and then chemically processed to isolate the metals.
  • Since most ores contain the metal and oxygen chemically combined, the extraction of metals is therefore a reduction process in which the oxygen must be removed. 
  • Reducing the ores produces the desired pure metal.

Methods of Extracting Metals

Specification Point 4.7:
  • Explain why the method used to extract a metal from its ore is related to its position in the reactivity series and the cost of the extraction process, illustrated by:
    a) heating with carbon (including iron)
    b) electrolysis (including aluminium) (knowledge of the blast furnace is not required)

Extraction & the Reactivity Series

  • The position of the metal on the reactivity series determines the method of extraction.
  • Higher placed metals (above carbon) have to be extracted using electrolysis.
  • Lower placed metals can be extracted by heating with carbon which reduces them.

Extraction & Reactivity table, Edexcel GCSE Chemistry

Extraction of Iron 

  • Iron sits below carbon on the reactivity series and so can be extracted by reduction methods.
  • Iron ore, carbon and limestone are mixed together and fed into the top of the blast furnace and hot air is blasted in at the bottom.
  • Carbon monoxide reduces the iron(III) oxide in the ore to form iron, which will melt and collect at the bottom of the furnace, where it is tapped off:

2Fe2O3 + 3CO → 4Fe + 3CO2

  • The limestone is added to remove silica impurities by reacting with them and forming a slag which floats on the molten iron and is removed.

Extraction of Aluminium

  • Aluminium sits above carbon on the reactivity series and cannot be extracted by reduction so electrolysis is used.
  • This process is much more expensive due to the large amounts of electricity required.
  • Aluminium oxide (Al2O3) has a very high melting point so it is first dissolved in molten cryolite producing an electrolyte with a lower melting point at around 1000ºC.
  • Graphite electrodes are used which conduct electricity but do not chemically interfere with the process.
  • Aluminium ions are reduced cathode:

Al3+ + 3e → Al

  • Oxygen ions are oxidised at the anode:

2O2- – 4e→O2

  • Some of the oxygen produced at the anode reacts with the graphite electrode to produce carbon dioxide gas:

C + O2 → CO2

  • This causes the carbon anodes to burn so they must be replaced regularly, adding to the costs of this method of extraction.
  • The molten aluminium is siphoned off as fresh aluminium oxide is added to the cell. 
  • The cell operates at 5-6 volts and with a current of 100,000 amps. 
  • The heat generated by the huge current keeps the electrolyte molten.

Diagram of an electrolytic cell used to extract aluminium from bauxite

Oxidation & The Reactivity Series

Specification Point 4.9:
  • Explain how a metal’s relative resistance to oxidation is related to its position in the reactivity series.
  • The most reactive metals are at the top of the series.
  • They readily lose electrons to form cations and are hence oxidised easily.
  • The opposite occurs for metals placed lower down as they are unreactive and do not easily lose their electrons.
  • The tendency to become oxidised is thus linked to how reactive a metal is and therefore its position on the reactivity series.
  • Metals higher up are therefore less resistant to oxidation than the metals placed lower down which are more resistant to oxidation.

Need help?

Edexcel GCSE Chemistry Notes

Want to aim for a Level 9?

See if you’ve got what it takes. Test yourself with our topic questions.

Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.