Chemguide: Core Chemistry 14 - 16

The electrolysis of molten ionic compounds

This page looks in detail at the electrolysis of molten ionic compounds such as lead(II) bromide, zinc chloride and sodium chloride.

If you haven't recently done so you should first read the page introducing electrolysis. I will assume that you are familiar with the ideas on that page.

Look at two bits of video to start with, and then I will summarise the main points afterwards.

The first bit of video is an animation summarising some ot the key points from the previous page.

The second one shows two of these reactions being done experimentally. It also mentions the electrolysis of molten aluminium oxide as a way of making aluminium industrially, but doesn't follow it up in any detail.

You will meet this later in the course in a section dealing with large-scale chemistry.

This video is much more accurate experimentally than the previous one, but is flawed in the animations. For example, it doesn't show that there are twice as many bromide ions as there are lead ions. It also forgets to pair up the bromine atoms to make bromine molecules.

The electrolysis of molten lead(II) bromide

Key facts

• Nothing happens until the lead(II) bromide is molten.

• Brown bromine gas is formed at the anode (positive electrode).

• A bead of molten lead is formed underneath the cathode (negative electrode).

Equations

The lead(II) ions are reduced to lead atoms by gaining electrons.

Pb²⁺(l) + 2e^-     Pb(l)

The bromide ions are oxidised to bromine by losing electrons.

2Br^-(l)     Br₂(g) + 2e^-

Overall:

PbBr₂(l)     Pb(l) + Br₂(g)

The electrolysis of molten zinc chloride

Key facts

• Nothing happens until the zinc chloride is molten.

• Chlorine gas is formed at the anode (positive electrode). The gas is so pale that you probably won't spot the colour. You can, however, test for it because it bleaches litmus paper.

• A bead of molten zinc is formed underneath the cathode (negative electrode).

Equations

The zinc ions are reduced to zinc atoms by gaining electrons.

Zn²⁺(l) + 2e^-     Zn(l)

The chloride ions are oxidised to chlorine by losing electrons.

2Cl^-(l)     Cl₂(g) + 2e^-

Overall:

ZnCl₂(l)     Zn(l) + Cl₂(g)

The electrolysis of molten sodium chloride

You are probably unlikely to see this in the lab because it is quite difficult to melt any reasonable quantity of sodium chloride in a crucible using a normal Bunsen burner.

Key facts

• Nothing happens until the sodium chloride is molten.

• Chlorine gas is formed at the anode (positive electrode). The gas is so pale that you probably won't spot the colour. You can, however, test for it because it bleaches litmus paper.

• Any sodium formed at the cathode (negative electrode) floats to the top and immediately burns either in the air or in the chlorine being produced at the anode. You will see flashes of orange flame.

Equations

The sodium ions are reduced to sodium atoms by gaining electrons.

Na⁺(l) + e^-     Na(l)

The chloride ions are oxidised to chlorine by losing electrons.

2Cl^-(l)     Cl₂(g) + 2e^-

Overall:

2NaC(l)     2Na(l) + Cl₂(g)