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School Chemistry notes: Describing and explaining the electrolysis of molten lead bromide

ELECTROLYSIS of MOLTEN LEAD BROMIDE

PbBr2 and CaCl2, ZnCl2, Al2O3 (re-edit)

(Suitable for AQA, Edexcel and OCR GCSE and advanced level chemistry students - ELECTROCHEMISTRY revision notes on electrolysis, cells, experimental methods, apparatus)


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

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?

Sub-index for this page on electrolysis

(a) The electrolysis of molten lead(II) bromide

(b) The electrolysis of molten anhydrous calcium chloride

(c) The electrolysis of molten anhydrous zinc chloride

(d) The electrolysis of molten aluminium oxide

(e) Learning objectives for this section on electrolysis

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diagram demonstrating the electrolysis of lead bromide in a crucible with carbon graphite electrodes school laboratory demomstartion experiment with PbBr25(a). The electrolysis of molten anhydrous 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!

diagram explaining the electrolysis of lead bromide electrode equations showing formation of products lead at cathode bromine at anode

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

Cathode (-): Pb2+(l) + 2e ==> Pb(l)

half-equation: positive ion reduction by electron gain, lead ions to lead atoms

This is a reduction reaction because the positive lead ions gain electrons to form neutral lead atoms.

(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, bromide ions to bromine molecules.

Anode (+): 2Br(l) – 2e ==> Br2(g)

 or  2Br(l) ==> Br2(g) + 2e

half-equation: negative ion oxidation by electron loss

This is an oxidation reaction because the negative bromide ions lose electrons to form neutral bromine molecules.

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.

3. It takes 2 electrons to reduce on lead(II) ion to a lead atom (reduction) and it takes the removal of 1 electron from each of 2 bromide ions (oxidation) to form a bromine molecule. Therefore for the same quantity of electric current the mole ratio of Pb : Br2 is 1 : 1.

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)

reduction, electron gain

bromine vapour

2Br(l) – 2e ==> Br2(g)

 or  2Br(l) ==> Br2(g) + 2e

oxidation, electron loss

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(b) to (d) Electrolysis of other molten ionic compounds

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

(b) The electrolysis of molten anhydrous calcium chloride CaCl2(l)

Electrode equations:

(i) solid/molten calcium formed at the cathode

Cathode (-): Ca2+(l) + 2e ==> Ca(s)

half-equation: a reduction electrode reaction - electron gain at cathode, positive calcium ions to neutral calcium atoms

(ii) chlorine gas formed at the anode

Anode (+): 2Cl(aq) – 2e ==> Cl2(g)

half-equation: an oxidation electrode reaction - electron loss at anode, negative chloride ions to neutral chlorine molecules

 or  2Cl(aq) ==> Cl2(g) + 2e

This is the basis for the industrial production of calcium metal

 

(c) The electrolysis of molten anhydrous zinc chloride

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

Electrode equations:

cathode (-):  Zn2+ + 2e ==> Zn

half-equation: a reduction electrode reaction - electron gain, zinc ions to neutral zinc atoms

anode (+):  2Cl ==> Cl2 + 2e

or  2Cl(aq) – 2e ==> Cl2(g)

half-equation: an oxidation electrode reaction - electron loss, negative chloride ions to neutral chlorine molecules

 

(d) The electrolysis of molten aluminium oxide Al2O3(l)

You can't do this in a school laboratory, it is an industrial process!

Electrode equations:

(i) molten aluminium is formed at the negative cathode

Cathode (-): Al3+(l) + 3e ==> Al(l)

half-equation: a reduction electrode reaction - electron gain at cathode, positive aluminium ions to neutral aluminium atoms

(ii) oxygen gas is formed at the positive electrode

Anode (+): 2O2–(l) – 4e ==> O2(g)

half-equation: an oxidation electrode reaction - electron loss at anode, negative oxide ions to neutral oxygen molecules

or  2O2–(l)  ==> O2(g) + 4e

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

See The extraction of aluminium from purified molten bauxite ore

The mole ratio of Al : O2 is 4 : 3 since the overall electrolysis equation for the industrial process is:  2Al2O3(l)  ==> 4Al(l)  +  3O2(g)


(e) Learning objectives for the electrolysis of molten lead bromide, calcium chloride, zinc chloride and aluminium oxide

Know that electrolysis requires a conducting solution of ions (electrolyte of molten lead bromide, calcium chloride, zinc chloride) and two inert solid conducting electrodes e.g. graphite (carbon) or platinum (expensive!).

Know that the electrolyte here contains free moving metal ions and non-metal ions from the melted salt.

Know that electrolysis will only happen if a d.c. electrical current is passed through the molten salt and reduction and oxidation reactions occur on passage of the electric current.

Where practicable in a school or college laboratory, be able to describe the apparatus required to electrolyse molten salts and be able to explain and understand the formation of the electrolysis products by:

knowing that the positive ions are reduced by electron gain and discharged at the negative cathode as metal atoms (lead, zinc, calcium),

knowing that the negative chloride or bromide ions are oxidised by electron loss and discharged at the positive anode as chlorine or bromine molecules,

and be able to write out the electrode equations (half equations) for the formation of neutral metal atoms by electron gain reduction and chlorine or bromine gas molecules from the oxidation of bromide or chloride ions by electron loss.

From the electrode equations, be able to explain why the mole ratio e.g. of lead/zinc/calcium atoms to bromine/chlorine molecules is theoretically 1  : 1

Know how to test for chlorine gas formed from the electrolysis of a chloride salt using inert electrodes and recognise brown vapour formed at the anode is an indication that bromine was formed in the electrolysis of molten bromide salts.


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