Electrolysis of Molten Lead(II) Bromide - Virtual Lab

Q: What is formed at each electrode, and why?

Lead at the cathode and bromine at the anode. Pb2+ is positive so it is attracted to the negative electrode, where it gains two electrons: Pb2+ + 2e− → Pb. Br− is negative so it goes to the positive electrode and loses its extra electron: 2Br− → Br2 + 2e−.

1 Heat itBunsen on

2 It melts371 °C

3 Switch on6 V supply

4 ProductsPb and Br₂

Cold solid Light the Bunsen first

Nothing happening

Temperature20°C

Ammeter0.00A

Charge passed0C

Lead at cathode0.000g

Bromine at anode0.0cm³

20 °CMELTING POINT 371 °C700 °C

Controls

What this experiment shows

Lead(II) bromide is an ionic solid, and while it is solid it will not conduct: the ions are locked in a lattice and nothing can carry charge. Heat it past 371 °C and the same ions are still there, but now they are free to move - and the lamp in the circuit lights. That is the whole point of the experiment. It is the standard demonstration that an electrolyte conducts because ions move, not because electrons flow through it. Once the current is running, the lead ions go to the negative electrode and are reduced to lead metal, and the bromide ions go to the positive electrode and are oxidised to bromine.

PbBr₂(l) → Pb(l) + Br₂(g)

What you need

Lead(II) bromide, a white ionic solid
A crucible on a pipe-clay triangle, on a tripod and gauze
A Bunsen burner - the salt must reach 371 °C
Two graphite (carbon) electrodes, which conduct without reacting
A 6 V d.c. supply, with a lamp and an ammeter in series
A fume cupboard, because bromine vapour is toxic

How it is done

Put the lead(II) bromide in the crucible and dip the two graphite electrodes in.
Connect them to the 6 V supply through the lamp and the ammeter.
Switch the supply on while the salt is still solid, and note that nothing happens.
Heat the crucible until the salt melts - the temperature stalls at about 371 °C while it does.
Watch the lamp light, and record what forms at each electrode.
Switch off, and let it cool in the fume cupboard.

What you should see

Solid lead(II) bromide does not conduct: the lamp stays dark and the ammeter reads zero.
The white solid melts to a clear liquid at about 371 °C.
As soon as it is molten the lamp lights and the ammeter reads a current.
A grey bead of molten lead collects at the negative electrode and runs to the bottom.
Red-brown bromine vapour with a sharp smell comes off the positive electrode.
Turn the burner off and it freezes again - and the lamp goes out.

Where marks are lost

Saying the solid has no ions. It has exactly the same ions; they simply cannot move.
Saying electrons flow through the electrolyte. Inside the melt the charge is carried by ions; electrons only travel through the wires and the electrodes.
Getting the electrodes the wrong way round. Positive ions go to the negative electrode - the cathode - because opposite charges attract.
Writing Br⁻ → Br + e⁻. Bromine is diatomic, so it is 2Br⁻ → Br₂ + 2e⁻.
Giving the lead as a solid. The cell is above 327 °C, lead’s melting point, so the lead you get is a liquid: Pb(l).
Leaving the state symbols off. The examiner wants PbBr₂(l), not PbBr₂(s).

Questions students ask

Why must the lead(II) bromide be molten?

Because an electrolyte only conducts if its ions can move. In the solid the ions are fixed in the lattice, so no charge can be carried and no current flows. Melting frees them, and the lamp lights the moment it happens.

What is formed at each electrode, and why?

Lead at the cathode and bromine at the anode. Pb²⁺ is positive so it is attracted to the negative electrode, where it gains two electrons: Pb²⁺ + 2e⁻ → Pb. Br⁻ is negative so it goes to the positive electrode and loses its extra electron: 2Br⁻ → Br₂ + 2e⁻.

Which electrode is oxidation and which is reduction?

Oxidation is loss of electrons, so it happens at the anode - the positive electrode. Reduction is gain of electrons, so it happens at the cathode. That is true of every electrolysis.

Why graphite electrodes?

Graphite conducts electricity but is inert here, so it carries the current without being attacked by the bromine or the molten lead. A reactive metal electrode would join in and spoil the result.

How much lead do you get for a given charge?

One mole of electrons is 96 500 C, and each lead atom needs two of them. So 1 930 C is 0.02 mol of electrons, which gives 0.01 mol of lead - 2.07 g - and 0.01 mol of bromine, which is 240 cm³ at room conditions.

Why does this experiment need a fume cupboard?

Bromine vapour is toxic and corrosive and lead compounds are poisonous. It is never done on an open bench.

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Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.