What makes the transition metals distinctive, and how are alloys and corrosion controlled?
Transition metals, alloys and corrosion: the properties of transition metals compared with Group 1, the structure and uses of alloys, the conditions needed for rusting, and methods of preventing corrosion.
A focused answer to Edexcel GCSE Chemistry topic 5 (separate chemistry), covering the typical properties of transition metals compared with Group 1 metals, why alloys are harder than pure metals, common alloys and their uses, the conditions required for iron to rust, and methods of preventing corrosion including barriers and sacrificial protection.
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What this dot point is asking
Edexcel wants you (in separate chemistry) to describe the typical properties of transition metals and compare them with Group 1, explain why alloys are harder than pure metals and give common alloys and uses, state the conditions needed for rusting and name the product, and describe methods of preventing corrosion including barriers and sacrificial protection. This is separate-science content for topic 5.
Transition metals compared with Group 1
The transition metals occupy the central block of the periodic table (such as iron, copper, nickel and zinc). They differ markedly from the Group 1 alkali metals:
For example, iron is used as a catalyst in the Haber process, copper compounds are blue or green, and transition metals are dense and strong enough for construction.
Alloys
A pure metal has layers of identically sized atoms (ions) that can slide over one another, so it is relatively soft. In an alloy, the atoms of the added element are a different size, which distorts the layers so they cannot slide as easily, making the alloy harder. Common alloys include:
- Steel (iron with carbon, and sometimes other metals): hard and strong, used in construction and tools.
- Brass (copper and zinc): used for fittings and instruments.
- Bronze (copper and tin): used for statues and bearings.
Corrosion and rusting
You can show that both water and oxygen are needed using test tubes: iron rusts only in the tube with both water and air, not in boiled water (no oxygen) sealed with oil, nor in dry air with a drying agent.
Preventing corrosion
There are two main strategies:
- Barrier methods keep out water and oxygen, for example painting, oiling, greasing or coating with plastic. The barrier must be unbroken to work.
- Sacrificial protection attaches a more reactive metal (such as zinc or magnesium) to the iron. The more reactive metal is oxidised in preference, so it corrodes instead of the iron, and protects the iron even if the surface is scratched. Galvanising coats iron with zinc, which acts as both a barrier and sacrificial protection.
Try this
Q1. Give two properties of transition metals that differ from Group 1 metals. [2 marks]
- Cue. Harder and stronger; higher melting point (also denser, less reactive, coloured compounds, catalysts).
Q2. Name the two substances needed for iron to rust. [2 marks]
- Cue. Water and oxygen.
Q3. Explain why galvanising protects iron even if the zinc layer is scratched. [2 marks]
- Cue. Zinc is more reactive than iron, so it is oxidised in preference (sacrificial protection), protecting the iron even where the coating is broken.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksIron rusts when it is exposed to the air. State the two substances needed for iron to rust, name the product, and explain how coating the iron with zinc prevents rusting even if the coating is scratched.Show worked answer →
A 4-mark rusting and protection question.
Iron needs both water and oxygen to rust (1 mark for both). The product is hydrated iron(III) oxide, which is rust (1 mark). Coating with zinc gives sacrificial protection: zinc is more reactive than iron, so the zinc is oxidised in preference to the iron (1 mark), which means the iron is protected even if the coating is scratched because the zinc corrodes instead (1 mark).
Markers reward naming both water and oxygen, and explaining sacrificial protection by the more reactive metal corroding first.
Edexcel 20213 marksExplain, in terms of structure, why an alloy such as brass is harder than the pure metals it is made from.Show worked answer →
A 3-mark alloy-structure explanation.
A pure metal has layers of atoms (ions) of the same size that can slide over one another easily, so it is relatively soft (1 mark). An alloy contains atoms of different sizes (1 mark), which distort the regular layers so they cannot slide over each other as easily, making the alloy harder (1 mark).
Markers reward the contrast between the regular layers in a pure metal and the distorted layers in an alloy.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)