Transition Metals, Alloys & CorrosionEdexcel GCSE Chemistry: Revision notes
Section 1
What are transition metals?
Most metals in the periodic table are transition metals — the block between Group 2 and Group 3. Iron, copper, gold, silver, nickel and zinc are typical examples.
Typical properties of transition metals:
- High melting point
- High density
- Form coloured compounds (e.g. copper compounds are often blue, iron compounds green or orange)
- The metals and their compounds show catalytic activity (they act as catalysts)
Iron is the classic example used across the specification: it forms coloured compounds (green Fe²⁺, orange-brown Fe³⁺) and iron compounds act as catalysts in some industrial reactions.
Iron(II) compounds are usually pale green; iron(III) compounds are usually orange-brown.
Section 2
What is corrosion and how does iron rust?
Corrosion is the oxidation of a metal, usually reacting with oxygen (and often water) to form a compound on its surface, which weakens and damages the metal over time.
Rusting of iron specifically needs both oxygen AND water present at the same time — if either is missing, iron will not rust.
Rusting can be prevented by:
- Exclusion of oxygen — e.g. coating with paint, oil or grease to form a physical barrier
- Exclusion of water — keeping the metal dry
- Sacrificial protection — attaching a more reactive metal (e.g. zinc or magnesium) to the iron; the more reactive metal corrodes/oxidises instead of the iron, because it loses electrons more readily
If asked to explain sacrificial protection, always state that the more reactive metal loses electrons more easily than iron, so it is oxidised preferentially — this is the marking point examiners look for, not just 'it protects the iron'.
Students often say rusting only needs oxygen or only needs water — the mark scheme requires BOTH oxygen and water to be named.
Section 3
How does electroplating work?
Electroplating uses electrolysis to coat one metal with a thin layer of another metal. It is used to:
- Improve the appearance of an object (e.g. a shiny chromium or gold layer)
- Improve resistance to corrosion (coating a reactive metal with a less reactive, unreactive one)
In electroplating, the object to be plated is made the cathode (negative electrode) in an electrolytic cell containing a solution of ions of the plating metal; the plating metal ions are reduced (gain electrons) and deposit onto the object's surface.
Section 4
Why are metals converted into alloys?
An alloy is a mixture of a metal with one or more other elements (often other metals or carbon).
Pure metals consist of atoms of a single size, arranged in regular layers that can slide over each other easily when a force is applied — this is why pure metals are often soft and easily bent.
In an alloy, the added atoms are usually a different size to the main metal atoms. This distorts the regular layers, making it harder for the layers to slide over one another. This is why alloys are usually stronger and harder than the pure metal.
Think of pure metal atoms as identical marbles stacked in neat rows that slide past each other easily; adding different-sized atoms is like mixing in some larger stones — the layers can no longer slide smoothly, so the structure resists deformation.
Section 5
Why is iron alloyed, and what are examples of alloys and their uses?
Pure iron is too soft and bends too easily for most structural uses, so it is alloyed with carbon and other metals to make alloy steels, which are much stronger and more useful.
Examples of metal uses linked to their alloys:
| Metal / Alloy | Property | Use |
|---|---|---|
| Aluminium | low density, resists corrosion | aircraft bodies |
| Magnalium (Al + Mg alloy) | low density, stronger than pure aluminium | aircraft/car parts |
| Copper | good electrical conductor, malleable | electrical wiring |
| Brass (Cu + Zn alloy) | harder than copper, resists corrosion | door fittings, musical instruments |
| Gold | unreactive, malleable, attractive | jewellery (often alloyed with other metals for strength) |
The uses of a metal or alloy are always related to its properties, and vice versa — if you know the use, you should be able to explain which property makes it suitable.
When asked to 'explain how the uses of a metal are related to its properties', always name the specific property first, then link it directly to the specific use — a bare list of properties without the link loses marks.
Must Know
- Transition metals: high melting point, high density, form coloured compounds, show catalytic activity (e.g. iron)
- Corrosion is oxidation of a metal by its environment; rusting of iron needs BOTH oxygen and water
- Rusting prevention: exclude oxygen, exclude water, sacrificial protection (more reactive metal is oxidised instead)
- Electroplating uses electrolysis to coat a metal for appearance or corrosion resistance
- Alloys are stronger/harder than pure metals because different-sized atoms distort the regular layers, stopping them sliding
- Know specific alloy examples: alloy steels (iron), magnalium (aluminium+magnesium), brass (copper+zinc), and link each to a property and a use
That's the notes covered.
Carry on to the next subtopic.