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Electrolysing molten lead(II) bromide

Demonstration

This experiment demonstrates that conduction is only possible where lead(II) bromide is molten, and that metallic lead and bromine are the products of electrolysis of the molten electrolyte. This is a demonstration that can be done as an introduction to studying electrolysis.
 

Lesson organisation


This demonstration doesn’t need too much preparation and the apparatus involved is very straightforward. It must be done in a fume cupboard as bromine (VERY TOXIC and CORROSIVE) is produced. A safer alternative may be the Electrolysis of zinc chloride. The demonstration takes about 30-40 mins.

Chemicals

Lead(II) bromide (TOXIC), about 20 g

Universal indicator (FLAMMABLE), a few drops

Refer to Health & Safety and Technical notes section below for additional information.

Apparatus

Eye protection

The teacher will require:

Access to a fume cupboard

Porcelain crucibles, about 2.5 cm in diameter, 2

Graphite rod electrodes, about 15 cm long, 2

Rubber bung with two holes about 1 cm apart to fit the graphite rods

DC power supply adjustable to 12 V (Note 1)

Ammeter, 0-5 A, ideally a large demonstration model

Leads and crocodile clips

Pair of metal tongs

Large wooden board such as dissecting board

Health & Safety and Technical notes


Read our standard health & safety guidance

Wear eye protection and consider wearing gloves when handling the lead bromide. Wash hands after the demonstration.  

Lead(II) bromide, PbBr2(s), (TOXIC, DANGEROUS FOR THE ENVIRONMENT)  - see CLEAPSS Hazcard.

Bromine, Br2(l) and (g), (VERY TOXIC, CORROSIVE, DANGEROUS FOR THE ENVIRONMENT)  - see CLEAPSS Hazcard.

1 You will need a rheostat if the power pack is not adjustable. A 12 V, 5 W bulb mounted on a holder could be used if a large ammeter is unavailable.

Apparatus set-up

 

Procedure


Before the demonstration
a Assemble the apparatus in a fume cupboard. During the experiment highly toxic bromine vapour is produced.

b The students should be given an opportunity to see the melting lead(II) bromide, and to observe that no conduction is possible until it melts.

c The graphite rods should reach almost to the bottom of the crucible, because the lead(II) bromide shrinks considerably when it is melted. This is why the crucible should be filled to overflowing with lead(II) bromide powder. Gloves should be worn when handling the lead(II) bromide. Ensure that the two-holed bung is as far up the graphite rods as possible, to prevent it burning or melting when strong heat is applied from below.
 

The demonstration
a Safety glasses must be worn throughout this demonstration.

b Set the power supply at 12 V, and short-circuit the rods to demonstrate that the circuit is working. Separate the rods and lower them into the powder. Demonstrate that the solid electrolyte does not conduct.

c Light the Bunsen burner and use a roaring flame to heat the crucible. The powder will take a few minutes to melt. This is because lead(II) bromide is not a good conductor of heat, and has a high thermal capacity. Eventually the powder melts and the current flows.

d Continue heating the crucible. The current rises as more and more of the solid melts. If necessary, top up the crucible with fresh powder. When all the powder has melted, use the power supply and/or rheostat to adjust the current to about 1.5 A.

e If a white sheet is placed behind the apparatus, the brown colour of the bromine will be seen. If a piece of moist Universal indicator paper is briefly held in the vapour, it will turn red and may also be partially bleached. Small groups of students can file past the apparatus to confirm that the bromine is being evolved from the anode, and that no gas is evolved from the cathode. They too can do the test with Universal indicator paper.

f Continue to maintain the current at about 1.5 A for 15 minutes. At the end of this time remove the heat from the apparatus, switch off the power supply and remove the graphite rods from the crucible. Use the tongs to pour the molten lead bromide into the second crucible, while keeping the small bead of molten lead in the first. Allow the crucibles to cool.

g When cool, remove the solidified metallic bead of lead and transfer it to a ceramic mat. This bead can be passed amongst the students, who can verify that it can be marked with a fingernail. Students who touch the metal must wash their hands afterwards.

 

Teaching notes


The electrolysis of molten salts is not as complicated as in aqueous solutions, because the electrolysis products of water are not an issue.

Although the graphite rods should be clamped as low as possible in the crucible, they should not be so low as to risk a short-circuit in the pool of liquid lead which collects at the bottom.

The solidified lead(II) bromide can be scraped out of the second crucible afterwards, so that it can be used again for this demonstration.

Lead(II) bromide is used because it melts at an unusually low temperature for an ionic compound (373 °C). Lead can be fairly safely handled afterwards (taking care to ensure that hands are washed after any contact with the metal). Bromine is a coloured acidic gas with a characteristic smell.

Electrolysis is not possible with solid lead(II) bromide. This is because the ions are held in a three-dimensional lattice, unable to move freely to the electrodes. Melting enables the ions to become mobile and to travel to the respective electrodes.

At the cathode (-) molten lead is formed:

Pb2+(l) + 2e- → Pb(l)

At the anode, gaseous bromine is evolved:

2Br-(l) → Br2(g) + 2e-

If there is time, you may like to show that heating alone is insufficient to cause the lead(II) bromide to decompose.

Health and Safety checked October 2007 

 

Weblinks


BBC Bitesize

GCSEScience

Channel4 - this site outlines the contents of a 4Learning programme on Electrochemistry.

Science pages - this site has questions on the electrolysis of lead(II) bromide.

 

Page last updated on 03 January 2012