Why is the method used to extract a metal decided by its position in the reactivity series?
Relate the method of extracting a metal to its position in the reactivity series, describe the extraction of iron in the blast furnace, and explain reduction by carbon and by electrolysis
A focused answer to the O-Level Chemistry outcome on metal extraction. Why reactivity decides the extraction method, the reduction of iron oxide by carbon in the blast furnace, and why very reactive metals need electrolysis.
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What this dot point is asking
SEAB wants you to relate how a metal is extracted from its ore to its position in the reactivity series, describe the extraction of iron in the blast furnace, and explain the two main methods: reduction by carbon for less reactive metals and electrolysis for very reactive metals. The reactivity series, met in the previous dot point, is the key that decides which method works.
The answer
Reactivity decides the method
Most metals occur in nature as compounds (ores), usually oxides, because they have reacted with oxygen. Extracting the metal means reducing the compound (removing the oxygen or adding electrons). How easily this can be done depends on the metal's reactivity:
- Very reactive metals (above carbon: potassium, sodium, calcium, magnesium, aluminium) hold onto their oxygen so strongly that carbon cannot remove it. They are extracted by electrolysis.
- Moderately reactive metals (below carbon: zinc, iron, lead) can be reduced by carbon (or carbon monoxide), which removes the oxygen.
- Unreactive metals (gold, silver) occur native (as the free metal) and need little or no extraction.
The dividing line is carbon: carbon can only reduce the oxide of a metal less reactive than itself.
Extraction of iron in the blast furnace
Iron is below carbon in the reactivity series, so it is reduced by carbon in a blast furnace. The raw materials are iron ore (haematite, iron(III) oxide), coke (carbon) and limestone (calcium carbonate), with hot air blasted in. The key stages:
- Coke burns in the hot air: , releasing heat.
- More coke reacts with the carbon dioxide to form carbon monoxide: .
- Carbon monoxide reduces the iron(III) oxide to molten iron: .
- The limestone decomposes and removes sandy impurities as slag, which floats on the molten iron and is run off.
The molten iron runs to the bottom and is tapped off.
Reduction and oxidation
Reduction is the loss of oxygen (or gain of electrons); oxidation is the gain of oxygen (or loss of electrons). In the blast furnace the iron oxide is reduced (loses oxygen) while the carbon monoxide is oxidised (gains oxygen to become carbon dioxide). In electrolysis, the metal ions are reduced by gaining electrons at the negative electrode.
Why reactive metals need electrolysis
A metal more reactive than carbon (such as aluminium) bonds to oxygen too strongly for carbon to break. Electricity, in electrolysis, supplies the energy to force the decomposition: the metal ions gain electrons at the negative electrode and are deposited as the metal. This is why aluminium is extracted by electrolysis of molten aluminium oxide, even though it is more expensive than reduction by carbon.
Examples in context
Example 1. The cost of aluminium. Because aluminium is more reactive than carbon, it must be extracted by electrolysis, which uses a great deal of electricity and makes the metal relatively expensive. This is why recycling aluminium, which avoids re-extraction, saves so much energy, linking chemistry to sustainability.
Example 2. Gold found as the metal. Gold is so unreactive that it does not combine with oxygen in nature, so it is found native as nuggets and flakes. It needs no chemical reduction, only physical separation, which is why gold was one of the first metals used by people, a direct consequence of its place at the bottom of the reactivity series.
Try this
Q1. State the method used to extract a metal that is more reactive than carbon. [1 mark]
- Cue. Electrolysis.
Q2. Name the three solid raw materials added to a blast furnace. [1 mark]
- Cue. Iron ore (haematite), coke (carbon) and limestone (calcium carbonate).
Q3. Explain why carbon can be used to extract iron but not aluminium. [2 marks]
- Cue. Iron is less reactive than carbon, so carbon can remove the oxygen from its oxide; aluminium is more reactive than carbon, so carbon cannot reduce its oxide, and electrolysis is needed instead.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksIron is extracted in a blast furnace. (a) Name the main ore of iron and the three raw materials added to the furnace. (b) Write a word equation for the reduction of iron(III) oxide by carbon monoxide.Show worked answer →
(a) The main ore is haematite (iron(III) oxide). The three raw materials are iron ore (haematite), coke (carbon) and limestone (calcium carbonate). Hot air is also blasted in.
(b) Iron(III) oxide + carbon monoxide iron + carbon dioxide.
Markers reward haematite as the ore, the three raw materials (ore, coke, limestone), and a correct word equation for the reduction by carbon monoxide.
Original4 marksAluminium is extracted by electrolysis, but iron is extracted by reduction with carbon. (a) Explain why aluminium cannot be extracted by reduction with carbon. (b) State what this tells you about the relative positions of aluminium and carbon in terms of reactivity.Show worked answer →
(a) Aluminium is more reactive than carbon, so carbon cannot remove the oxygen from aluminium oxide (carbon cannot reduce a more reactive metal's oxide). Aluminium therefore has to be extracted by electrolysis, which can decompose the compound using electricity.
(b) It tells you that aluminium is above carbon in reactivity, while iron is below carbon, which is why carbon can reduce iron oxide but not aluminium oxide.
Markers reward the point that carbon cannot reduce the oxide of a metal more reactive than itself, and that aluminium is above carbon while iron is below it.
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