Transition Metals, Alloys & Corrosion Notes

Edexcel GCSE Chemistry: Revision notes

Key facts

  • Transition metals have high melting points and high densities, form coloured compounds and act as catalysts.
  • Rusting needs both oxygen and water.
  • Rust is prevented by excluding oxygen, excluding water, or by sacrificial protection.
  • Electroplating coats an object for appearance or to resist corrosion.
  • Alloys are harder than pure metals because different-sized atoms stop the layers sliding.

Transition metals

Transition metals sit in the block between Groups 2 and 3, and share a set of typical properties.

Iron, copper, nickel, silver and gold are typical transition metals. They have high melting points and high densities, and they form coloured compounds. The metals and their compounds often act as catalysts.

The colour can tell you which ion is present. Iron forms two ions with different coloured compounds.

1234567012341H2He3Li4Be5B6C7N8O9F10Ne11Na12Mg13Al14Si15P16S17Cl18Ar19K20Ca21Sc22Ti23V24Cr25Mn26Fe27Co28Ni29Cu30Zn31Ga32Ge33As34Se35Br36Kr
  • Metals
  • Non-metals
  • Semi-metals
The transition metals fill the block between Group 2 and Group 3.

Iron(II), FeX2+\ce{Fe^2+}

Charge:
2+
Colour of compounds:
Pale green

Iron(III), FeX3+\ce{Fe^3+}

Charge:
3+
Colour of compounds:
Orange-brown

Which property is NOT typical of a transition metal?

Rusting

Iron rusts only when oxygen and water are both present, so removing either one stops it.

Corrosion is the oxidation of a metal. Rusting is the corrosion of iron and needs oxygen and water together.

To prevent it:

  • exclude oxygen with paint, oil or grease
  • exclude water by keeping the iron dry
  • use sacrificial protection: attach a more reactive metal such as zinc or magnesium

Air and water

Oxygen present:
Yes
Water present:
Yes
Iron rusts:
Yes

Boiled water, oil layer

Oxygen present:
No
Water present:
Yes
Iron rusts:
No

Dry air

Oxygen present:
Yes
Water present:
No
Iron rusts:
No
  1. 1

    Zinc joined to iron

    a more reactive metal

  2. 2

    Zinc loses electrons first

    it is oxidised in preference

  3. 3

    Iron is protected

    the zinc corrodes instead

Sacrificial protection

An iron nail sits in boiled water under a layer of oil. Will it rust?

Electroplating

Electroplating uses electrolysis to coat an object with a thin layer of another metal.

Electroplating is used to improve appearance, such as shiny chromium or gold, and to resist corrosion by coating a reactive metal with an unreactive one.

The object is the cathode (negative electrode). The electrolyte contains ions of the plating metal, which gain electrons there and deposit as a thin layer.

  1. 1

    Object is the cathode

    connected to the negative terminal

  2. 2

    Electrolyte

    a solution of the plating metal ions

  3. 3

    Ions are reduced

    they gain electrons at the cathode

  4. 4

    Thin layer forms

    metal deposits on the surface

Electroplating an object

At which electrode is the object being plated placed?

Alloys

Different-sized atoms in an alloy distort the layers, so they cannot slide easily and the alloy is harder.

An alloy is a mixture of a metal with other elements, often other metals or carbon.

In a pure metal the atoms are the same size, in regular layers that slide over each other easily, so pure metals are soft. In an alloy the added atoms are a different size. They distort the layers and make sliding harder.

Pure metal

Alloy

Regular layers in a pure metal, and the distorted layers in an alloy.

Why is an alloy usually harder than the pure metal?

Uses of metals and alloys

A use always follows from a property, so name the property first and then link it to the use.

Pure iron is too soft for most structures, so it is alloyed with carbon and other metals to make alloy steels. Gold is unreactive, malleable and attractive, so it is used for jewellery, often alloyed for strength.

For each use, state the property that makes it suitable.

05101520AluminiumIronCopperGoldMetalDensity (g/cm³)
Approximate densities: aluminium is far less dense than iron or copper, which is why it is used for aircraft bodies.

Aluminium

Property:
Low density; resists corrosion
Use:
Aircraft bodies

Magnalium

Property:
Low density; stronger than aluminium
Use:
Aircraft and car parts

Copper

Property:
Good electrical conductor; malleable
Use:
Electrical wiring

Brass

Property:
Harder than copper; resists corrosion
Use:
Door fittings, instruments

Why is copper used for electrical wiring?

Try an exam question

Zinc blocks are attached to the iron hull of a ship. Explain how this stops the hull from rusting.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.