Seneca
Sign up

Electrolysis 2

Science Practicals: GCSE, topic 2.1.5

Electrolysis - Extracting Metals

Electrolysis is used to extract reactive metals from molten ores (melted materials containing metals). It is used to extract metals such as aluminium, which are more reactive than carbon.

At the anode (positive electrode)

At the anode (positive electrode)

  • Carbon dioxide is formed at the anode.
  • The anode is usually made of carbon because it is a good conductor and is cheap.
At the cathode (negative electrode)

At the cathode (negative electrode)

  • The aluminium is formed at the cathode.
Disadvantage

Disadvantage

  • Lots of energy is needed to:
  • Melt the solid ionic compound to allow the ions to flow.
  • To produce the electrical current.
  • All this energy costs money, and a lot of it!

Electrolysis of Aqueous Solutions

The products of the electrolysis of aqueous solutions are difficult to predict because the water molecules in the solution split up to form hydrogen (H+) and hydroxide (OH-) ions.

At the anode (positive electrode)

At the anode (positive electrode)

  • What is formed at the anode depends on if halide ions are present:
  • If halide ions are present, the respective halogen forms.
  • If halide ions are absent, oxygen forms.
At the cathode (negative electrode)

At the cathode (negative electrode)

  • What is formed at the cathode depends on the reactivity of the metal:
  • If the metal's more reactive than hydrogen, hydrogen is produced.
  • If the metal's less reactive than hydrogen, the metal is produced.

Half Equations

During electrolysis, different reactions take place at the different electrodes. Half equations show the reactions that happen at each electrode. The electrolysis of lead bromide is an example of this:

At the anode (positive electrode)

At the anode (positive electrode)

  • Oxidation reactions happen i.e. negatively charged ions lose electrons.
  • E.g. 2Br- → Br2 + 2e-
At the cathode (negative electrode)

At the cathode (negative electrode)

  • Reduction reactions happen i.e. positively charged ions gain electrons.
  • E.g. Pb2+ + 2e- → Pb

Test yourself on Electrolysis 2

  • Why does electrolysis need such large amounts of energy?

  • Why does electrolysis cost a lot of money to carry out?

  • Why is the anode used in electrolysis usually made of carbon?

  • An aqueous solution of copper(II) sulfate (CuSO4) contains Cu2+, SO42-, H+ and OH- ions. What happens when it is electrolysed?

  • In the electrolysis of aqueous solutions, what products can form at the anode?

Practise this topic for free