C10.3 – Extraction of Metals

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1. Describe the use of carbon in the extraction of some metals from their ores.

Metals below carbon in the reactivity series can be extracted using carbon. Carbon reduces the metal oxides that are extracted when heated. Example:

lead (II) oxide + carbon (coke/coal) —————–>  lead  + carbon monoxide

The carbon in the form of coke (which is coal from which impurities have been removed) has reduced the lead by removing the oxygen from it to produce lead.

Carbon monoxide can also be used as a reducing agent. Example:

zinc oxide + carbon monoxide ————> zinc + carbon dioxide

2. Describe the essential reactions in the extraction of iron in the blast furnace.

 

An ore is a rock from which metals can be extracted. The main ore of iron is hematite, which contains large amounts of iron (III) oxide. It is reduced by carbon to form iron, but this takes place in a huge blast furnace.

The raw materials needed to do this are hematite, coke, limestone and air.

The hematite, coke and limestone are added at the top of the furnace. A strong current of hot air is blown in at the bottom of the furnace. It is hot enough to react with the coke.

 

• At the bottom of the furnace, the coke burns in the hot air blast to form carbon dioxide. This is an exothermic reaction and the heat released keeps the furnace hot.
  C(s) + O₂(g) ———–> CO₂(g)

• The carbon dioxide produced reacts with the coke to form carbon monoxide:
  CO₂(g) + C(s)  ———–>  2CO(g)

• This carbon monoxide reduces the iron (III) oxide to iron:
  Fe₂O₃(s)  + 3CO(g) ———->  2Fe(l)  + 3CO₂(g)
  Most of the iron is produced this way and flows to the bottom of the furnace and is removed from time to time.

• In hotter parts of the furnace, the carbon  itself can directly reduce the iron:
  Fe₂O₃(s) + 3C(g) ———-> 2Fe(l) + 3CO₂(g)

(To memorise this series of equations, just remember that coke turns to carbon-dioxide which turns to carbon monoxide which then reduces the iron).

Why is the limestone added? The hematite contains other impurities like sand (silicon(IV)oxide). The limestone (calcium carbonate-CaCO₃) helps remove these impurities.

• The heat from the furnace decomposes the limestone:
  CaCO₃(s) ———-> CaO(s) + CO₂(g)

• The calcium oxide produced reacts with the silicon (IV) oxide to form a ‘slag’ of calcium silicate. This slag is used as a building material, especially in road building.
  CaO(s) + SiO₂(l) ————> CaSiO₃(l)

 

3. Relate the method of extraction of a metal from its ore to its position in the reactivity series.

The way we extract a metal from its ore depends on the position of the metal in the reactivity series. Carbon is used to reduce oxides of metals below it in the reactivity series. Metals above carbon in the reactivity series cannot be extracted from their oxides by heating with carbon. This is because the metal bonds to oxygen too strongly and the carbon is not reactive enough to remove it. So we have to use electrolysis to extract metals more reactive than carbon, such as aluminium, magnesium and calcium.

 

 

Notes submitted by Lintha

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