Relate the method of extracting a metal to its position in the reactivity series, describing reduction of iron in the blast furnace and extraction of aluminium by electrolysis

What this dot point is asking

SEAB wants you to link the method of extracting a metal to its place in the reactivity series, and to describe the two main industrial methods: reduction with carbon in the blast furnace (for iron) and electrolysis (for aluminium). The central principle is that the more reactive a metal, the harder it is to extract, so the more energy the method needs.

The answer

Method depends on reactivity

A metal's compound must be broken down (reduced) to get the metal. How easily this can be done depends on the metal's reactivity:

• metals below carbon in the reactivity series (iron, zinc, copper) can be extracted by heating their oxide with carbon, because carbon is more reactive and reduces the oxide,
• metals above carbon (aluminium, magnesium, sodium) cannot be reduced by carbon and are extracted by electrolysis,
• the least reactive metals (gold, silver) occur native (as the free metal) and need little or no extraction.

Reduction with carbon: the blast furnace

Iron is extracted from iron(III) oxide (haematite) in a blast furnace. The key reactions are:

1. coke burns in the hot air blast: $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$,
2. carbon dioxide is reduced by more coke to carbon monoxide: $\text{CO}_2 + \text{C} \rightarrow 2\text{CO}$,
3. carbon monoxide reduces the iron oxide: $\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$.

Limestone (calcium carbonate) is added to remove the main impurity, sandy silicon dioxide. It decomposes to calcium oxide, which reacts with the silicon dioxide to form molten slag (calcium silicate) that floats on the iron and is run off.

Reduction and oxidation

Extraction by carbon is a reduction: the metal oxide loses oxygen. The carbon (or carbon monoxide) is the reducing agent and is itself oxidised. Defining reduction as loss of oxygen makes the blast-furnace chemistry clear.

Electrolysis of aluminium

Aluminium is above carbon, so carbon cannot reduce its oxide; electrolysis is used. Aluminium oxide is dissolved in molten cryolite to lower the melting point, then electrolysed:

• at the cathode (negative): $\text{Al}^{3+} + 3\text{e}^- \rightarrow \text{Al}$ (reduction),
• at the anode (positive): $2\text{O}^{2-} \rightarrow \text{O}_2 + 4\text{e}^-$ (oxidation).

The process uses a lot of electrical energy, which is why aluminium is expensive to produce and valuable to recycle.

Examples in context

Example 1. Why recycling aluminium saves so much energy. Extracting aluminium by electrolysis is hugely energy-intensive, so recycling used aluminium uses only a small fraction of that energy. Understanding that the extraction method is electrolysis explains why recycling this particular metal is so worthwhile.

Example 2. The history of metals. Gold and copper were used in ancient times because they are unreactive and were found native or easily extracted, while aluminium was a precious rarity until electricity made electrolysis possible. The reactivity series explains the order in which civilisations could use each metal.

Try this

Q1. State the method used to extract a metal that is more reactive than carbon, and give one example. [2 marks]

• Cue. Electrolysis (of the molten oxide); for example aluminium (or sodium, magnesium).

Q2. Write the equation for the reduction of iron(III) oxide by carbon monoxide. [2 marks]

• Cue. $\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$.

Q3. Explain why aluminium oxide is dissolved in molten cryolite before electrolysis. [2 marks]

• Cue. Cryolite lowers the melting point of the aluminium oxide, saving energy (so it does not have to be heated to its very high melting point).