Relate the method of extracting a metal to its reactivity, describe the extraction of iron in the blast furnace, and explain the rusting of iron and how to prevent it

What this dot point is asking

The syllabus wants you to relate the method of extracting a metal to its reactivity, to describe the extraction of iron in the blast furnace, and to explain the rusting of iron and how to prevent it. The key idea is that reactive metals hold on to their oxygen tightly and need powerful methods to extract, while rusting is a corrosion reaction that needs both air and water. Both topics flow from the reactivity series.

The answer

Extraction method depends on reactivity

A metal is usually found in nature combined in an ore, and extracting it means removing the other elements (often oxygen). How this is done depends on the metal's place in the reactivity series:

• Very reactive metals (above carbon, such as aluminium) are extracted by electrolysis, because they hold their oxygen too tightly for carbon to remove.
• Moderately reactive metals (below carbon, such as iron and zinc) are extracted by heating their oxide with carbon, which removes the oxygen (reduction).
• Unreactive metals (such as gold) are found uncombined (native) and need little or no extraction.

Reduction with carbon

When a metal oxide is heated with carbon, the carbon takes the oxygen away from the metal. Removing oxygen is called reduction, so the metal oxide is reduced to the metal. Carbon can only do this for metals less reactive than itself.

Extraction of iron in the blast furnace

Iron is extracted from iron oxide in a blast furnace:

1. Coke (carbon) burns in hot air to form carbon dioxide, which reacts with more coke to form carbon monoxide.
2. The carbon monoxide reduces the iron oxide to molten iron: iron(III) oxide + carbon monoxide $\rightarrow$ iron + carbon dioxide.
3. Limestone removes sandy impurities as molten slag, which floats on the iron and is run off.

Rusting and how to prevent it

Rusting is the corrosion of iron to form hydrated iron oxide (rust). Both oxygen and water must be present. Ways to prevent rusting:

• Barrier methods: painting, oiling, greasing, or coating with plastic to keep out air and water.
• Galvanising: coating with zinc. Zinc is more reactive than iron, so it corrodes in preference and protects the iron even if the coating is scratched (sacrificial protection).

Examples in context

Example 1. Why aluminium was once more precious than gold. Aluminium is common in the ground but very reactive, so before electrolysis was available it could not be extracted cheaply and was extremely valuable. Once electrolysis made extraction possible, it became an everyday metal, showing how reactivity controls how easily a metal is obtained.

Example 2. Galvanised roofing. Corrugated steel roofing is coated with zinc to stop it rusting. Even if the surface is scratched, the more reactive zinc corrodes first and protects the steel beneath, which is why galvanised sheets last for many years outdoors.

Try this

Q1. State the method used to extract a metal that is less reactive than carbon, such as zinc. [1 mark]

• Cue. Heating its oxide with carbon, which reduces the oxide by removing the oxygen.

Q2. Name the two conditions needed for iron to rust, and one way to prevent it. [2 marks]

• Cue. Oxygen (air) and water are both needed; prevent it by coating with paint or oil, or by galvanising with zinc.

Q3. Explain why aluminium cannot be extracted by heating its oxide with carbon. [2 marks]

• Cue. Aluminium is more reactive than carbon and holds its oxygen more strongly, so carbon cannot remove the oxygen; electrolysis is used instead.