The Reactivity Series

The displacement principle

• A more reactive metal will displace a less reactive metal from its compound
• This applies to two situations:
  • Heating a metal with a solid metal oxide
  • Adding a metal to an aqueous solution of a metal salt
• The reactive metal takes the oxygen, or the negative ion, leaving the less reactive metal as the free element

Displacement from metal oxides

• Heat zinc powder with copper(II) oxide:

Zn(s) + CuO(s) → ZnO(s) + Cu(s)

- Zinc takes the oxygen, leaving copper as the brown solid product
- Zinc is the **reducing agent** (it removes oxygen); copper(II) oxide is the **oxidising agent**

• The thermite reaction uses aluminium to extract iron from iron(III) oxide:

2 Al(s) + Fe₂O₃(s) → 2 Fe(l) + Al₂O₃(s)

- The reaction releases enough heat to melt the iron, which is why thermite was used to weld railway tracks

• No reaction occurs if the added metal is less reactive than the metal in the oxide (e.g. silver added to lead(II) oxide does nothing)

Displacement from salt solutions

• Add a strip of magnesium to blue copper(II) sulfate solution:

Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)

- The blue colour fades as colourless magnesium sulfate forms
- Pink-brown copper plates onto the magnesium and any loose metal sinks to the base of the beaker

• Add iron filings to copper(II) sulfate solution:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

- Pale-green iron(II) sulfate forms, copper coats the iron

• A metal placed in a salt solution of a more reactive metal gives no reaction (e.g. copper in zinc sulfate does nothing)