The ELECTROLYSIS of MOLTEN LEAD BROMIDE PbBr₂

(Suitable for AQA, Edexcel and OCR GCSE chemistry students)

ELECTROCHEMISTRY revision notes on electrolysis, cells, experimental methods, apparatus, batteries, fuel cells and industrial applications of electrolysis

5. The electrolysis of molten lead(II) bromide and the products of electrolysing other halide salts

A simple method of investigating the electrolysis of molten lead(II) bromide is described. The formation of the products of electrolysing molten lead bromide is fully explained with the appropriate electrode equations. What are the products of the electrolysis of molten lead bromide?

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5. The electrolysis of molten lead(II) bromide

The products of electrolysing lead bromide are lead metal and bromine gas

You can electrolyse molten compounds as long as they are ionic compounds, so that on melting, there free ions to move to carry the current to facilitate the electrolysis process of splitting the compound into its constituent elements.

Electrolysing molten salts with carbon electrodes

Inert carbon (graphite) electrodes are dipped into molten salt which has been strongly heated in a crucible. It is difficult to collect the gases at the electrodes! The salts may be very high melting, so sometimes a small amount of another salt impurity is added to lower the melting point.

The electrolyte molten lead(II) bromide PbBr₂(l), provides a high concentration of lead(II) ions Pb²⁺ and bromide ions Br^– to carry the current during the electrolysis process.

Remember that melting an ionic compound breaks down the strong ionic bonding sufficiently to allow the ions to freely move around and carry the electric current.

The electrolysis will only take place when electricity is passed through the molten lead bromide.

This is a good teacher demonstration in the school laboratory – brown vapour and silvery lump provide good evidence of what's happened.

At the end of the experiment its best to pour the molten salt onto cold ceramic surface. Let the residue cool and break it up to find the silvery lump of lead - to prove you do get lead from the electrolysis of lead bromide, the orange-brown vapour of bromine is pretty obvious and pretty obnoxious! PLEASE do in a fume cupboard!

The electrode reactions and products of the electrolysis of the molten ionic compound lead bromide (the electrolyte) are illustrated by the theory diagram above.

This is quite a simpler electrolysis situation where the ionic compound lead bromide on melting provides a highly concentrated mixture of positive lead ions and negative bromide ions.

The half-equations for the electrolysis of lead(II) bromide.

(a) The negative cathode electrode reaction for the electrolysis of molten lead(II) bromide

The positive lead(II) ions are attracted to the negative electrode and are discharged to form molten lead

Pb²⁺_(l) + 2e^– ==> Pb_(l)

positive ion reduction by electron gain

This is a reduction reaction because the lead ions gain electrons.

(b) The positive anode electrode reaction for the electrolysis of molten lead bromide

The negative bromide ions are attracted to the positive anode electrode and discharged to form bromine vapour.

2Br^–_(l) – 2e^– ==> Br_2(g)

 or  2Br^–_(l) ==> Br_2(g) + 2e^–

negative ion oxidation by electron loss

This is an oxidation reaction because the bromide ions lose electrons.

Extra comments on the electrolysis of lead bromide and other molten ionic compounds

1. Overall equation for the electrolysis of molten lead bromide: PbBr₂(l) ==> Pb(l) + Br₂(g)

2. Electrolysis of molten bromide salts_(l) or their concentrated aqueous solution_(aq) or conc. hydrobromic acid_(aq) to make bromine

Electrolysis of other molten ionic compounds

Each provides a positive ion and a negative ion, and this molten mixture of ions constitutes the electrolyte.

(a) molten calcium chloride CaCl₂(l)

Electrode equations:

(i) solid/molten calcium formed at the cathode

Ca²⁺_(l) + 2e^– ==> Ca_(s)   (a reduction electrode reaction - electron gain at cathode)

(ii) chlorine gas formed at the anode

2Cl^–_(aq) – 2e^– ==> Cl_2(g)   (an oxidation electrode reaction - electron loss at anode)

 or  2Cl^–_(aq) ==> Cl_2(g) + 2e^–

This is the basis for the industrial production of calcium metal

(b) Molten anhydrous zinc chloride gives zinc (+) and chlorine (–)

This is a safer salt to use than lead chloride, less toxic, but chlorine is a very harmful gas.

You should end up with a small lump of zinc, but chlorine gas is given off, so this electrolysis needs to be done in a fume cupboard where you can safely see the evolution of the green gas which bleaches litmus paper white.

(If you use blue litmus, chlorine is an acidic gas and turns the litmus red before bleaching it white because it is a powerful oxidising agent).

Electrode equations:

cathode (-):  Zn²⁺ + 2e ==> Zn   (a reduction electrode reaction - electron gain)

anode (+):  2Cl ==> Cl₂ + 2 e   (an oxidation electrode reaction - electron loss)

(c) molten aluminium oxide Al₂O₃(l)

You can't do this in a school laboratory!

Electrode equations:

(i) molten aluminium is formed at the negative cathode

Al³⁺_(l) + 3e^– ==> Al_(l)   (a reduction electrode reaction - electron gain at cathode)

(ii) oxygen gas is formed at the positive electrode

2O^2–_(l) – 4e^– ==> O_2(g)   (an oxidation electrode reaction - electron loss at anode)

or  2O^2–_(l)  ==> O_2(g) + 4e^–

The industrial method for the extraction of aluminium from its ore uses electrolysis.

See 12. The extraction of aluminium from purified molten bauxite ore

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