ELECTROLYSIS of MOLTEN LEAD BROMIDE

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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

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?

Reminders: Electrolysis (of lead bromide) is a way of splitting up (decomposition) of the compound (lead bromide) using electrical energy. The electrical energy comes from a d.c. (direct current) battery or power pack supply. A conducting liquid, containing ions, called the electrolyte (molten lead bromide), must contain the compound (lead bromide) that is being broken down. The electricity must flow through electrodes dipped into the electrolyte to complete the electrical circuit with the battery. Electrolysis can only happen when the circuit is complete, and an electrical current (electricity) is flowing, then the products of electrolysing molten lead(II) bromide are released on the electrode surfaces where they can be collected. Electrolysis always involves a flow of electrons in the external wires and electrodes and a flow of ions in the electrolyte and there is always a reduction at the negative cathode electrode (which attracts positive ions, cations) and an oxidation at the positive anode electrode (which attracts negative ions, anions) and it is the ions which are discharged to give the products. These revision notes on the electrolysis of molten lead bromide and other molten salts should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.

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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.

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– ==> Br2(g)

 or  2Br–(l) ==> Br2(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

SUMMARY OF PRODUCTS FROM THE ELECTROLYSIS OF LEAD(II) BROMIDE

with inert carbon electrodes

Electrolyte negative cathode product negative electrode

cathode half-equation

positive anode product positive electrode

anode half-equation

molten lead(II) bromide

PbBr2(l)

molten lead Pb2+(l) + 2e– ==> Pb(l) bromine vapour

2Br–(l) – 2e– ==> Br2(g)

 or  2Br–(l) ==> Br2(g) + 2e–

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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– ==> Cl2(g)   (an oxidation electrode reaction - electron loss at anode)

 or  2Cl–(aq) ==> Cl2(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. 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)

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(l) – 4e– ==> O2(g)   (an oxidation electrode reaction - electron loss at anode)

or  2O(l)  ==> O2(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


ELECTROCHEMISTRY INDEX:  1. INTRODUCTION to electrolysis - electrolytes, non-electrolytes, electrode equations, apparatus 2. Electrolysis of acidified water (dilute sulfuric acid) and some sulfate salts and alkalis 3. Electrolysis of sodium chloride solution (brine) and bromides and iodides 4. Electrolysis of copper(II) sulfate solution and electroplating with other metals e.g. silver 5. Electrolysis of molten lead(II) bromide (and other molten ionic compounds) 6. Electrolysis of copper(II) chloride solution 7. Electrolysis of hydrochloric acid 8. Summary of electrode equations and products 9. Summary of electrolysis products from various electrolytes 10. Simple cells (batteries) 11. Fuel Cells e.g. the hydrogen - oxygen fuel cell 12. The electrolysis of molten aluminium oxide - extraction of aluminium from bauxite ore & anodising aluminium to thicken and strengthen the protective oxide layer 13. The extraction of sodium from molten sodium chloride using the 'Down's Cell' 14. The purification of copper by electrolysis 15. The purification of zinc by electrolysis 16. Electroplating coating conducting surfaces with a metal layer 17. Electrolysis of brine (NaCl) for the production of chlorine, hydrogen & sodium hydroxide AND 18. Electrolysis calculations


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