Electrolysis of ionic compounds

Inertelectrodes do not react with the electrolyte or the products formed during electrolysis. They just provide a surface for ions to gain or lose electrons so they form products. Graphite (a form of carbon) and platinum are commonly used to make inert electrodes.

During electrolysis of a molten ionic compound:

  • positive metal ions are attracted to the negative electrode, where they gain electrons and form metal atoms
  • negative non-metal ions are attracted to the positive electrode, where they lose electrons and form non-metal atoms

Non-metal atoms formed at the positive electrode, the anode, usually join together by covalent bonding to form simple molecules.

curriculum-key-fact
During the electrolysis of a molten ionic compound, a metal forms at the cathode and a non-metal forms at the anode.

Example

During electrolysis of molten lead(II) bromide, PbBr2(l):

  • Pb2+ ions gain electrons at the cathode and become Pb atoms
  • Br- ions lose electrons at the anode and become Br atoms, which pair up to form Br2 molecules

Overall, lead forms at the negative electrode and bromine forms at the positive electrode.

Molten lead(II) bromide heated in beaker while electrodes inserted. Positive lead ions are attracted to the negative electrode, while negative bromide ions are attracted to the positive electrode.Lead and bromine form during the electrolysis of molten lead(II) bromide

Example

Predict which products form at the anode and the cathode during the electrolysis of molten calcium chloride. Explain your answer.

Calcium will form at the cathode and chlorine will form at the anode.

This is because positive calcium ions are attracted to the negative electrode (cathode), where they gain electrons to form calcium atoms. At the same time, negative chloride ions are attracted to the positive electrode (anode). They lose electrons to form chlorine atoms, which join by covalent bonds to form chlorine molecules.

Question

Predict which products form at each electrode during the electrolysis of molten aluminium oxide.

Aluminium will form at the negative electrode and oxygen will form at the positive electrode.