Metals in the Earth’s Crust
Specification Point 4.4:
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 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.
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 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.
Edexcel GCSE Chemistry Notes
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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.
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