Electrolysis of molten lead(II) bromide

The electrolysis of molten lead(II) bromide is the cleanest introduction to electrolysis in the Form 5 Redox Equilibrium chapter, because there is no water to complicate the products. It shows that an ionic compound conducts only when its ions are free to move, and that the current splits the compound into its elements, lead at one electrode, bromine at the other. Work through the method, then use the Paper 3 section to secure the electrode reactions.

Aim

To investigate the electrolysis of molten lead(II) bromide, to identify the products formed at the cathode and the anode, and to explain the changes as reduction and oxidation.

Apparatus and materials

• Crucible and pipe-clay triangle
• Tripod stand and Bunsen burner
• Two carbon (graphite) electrodes
• Connecting wires, a battery or d.c. power supply, a switch and a light bulb or ammeter
• Lead(II) bromide powder
• Safety goggles; access to a fume cupboard

Procedure

1. Place a quantity of lead(II) bromide powder into the crucible.
2. Stand the crucible on the pipe-clay triangle on the tripod.
3. Dip the two carbon electrodes into the powder, keeping them apart, and connect them through the switch, the bulb (or ammeter) and the battery.
4. Close the switch and observe the bulb while the lead(II) bromide is still solid.
5. Light the Bunsen burner and heat the crucible strongly until the lead(II) bromide melts.
6. Observe the bulb again once the compound is molten, and watch each electrode closely.
7. Record what forms at the cathode and at the anode, then switch off and stop heating.

Expected observations

• While the lead(II) bromide is solid, the bulb does not light, showing that the solid does not conduct electricity.
• Once the lead(II) bromide is molten, the bulb lights, showing that the molten compound conducts electricity.
• At the cathode, a grey, shiny bead of molten lead forms.
• At the anode, a brown, pungent, choking vapour of bromine is released.

Describe the state changes and the products you see; do not quote a made-up current or mass.

Inference and conclusion

An ionic compound such as lead(II) bromide conducts electricity only when it is molten, because only then are its ions free to move and carry the charge; in the solid the ions are locked in a lattice. When the current flows, the compound is decomposed. The lead(II) ions move to the cathode, where they gain electrons and are reduced to lead atoms, which collect as a grey bead. The bromide ions move to the anode, where they lose electrons and are oxidised to bromine molecules, released as brown vapour. Because reduction happens at the cathode and oxidation at the anode, electrolysis is a redox process. The electrode half-equations are Pb²⁺ + 2e⁻ → Pb at the cathode and 2Br⁻ → Br₂ + 2e⁻ at the anode.

Science process skills (Paper 3 style)

• Hypothesis. “Lead(II) bromide conducts electricity and is decomposed by it only when molten, not when solid.” State a testable relationship between state and conduction.
• Variables. The manipulated variable is the state of the lead(II) bromide (solid or molten); the responding variable is whether the bulb lights and whether products form; the controlled variables are the electrodes used, the voltage of the supply and the amount of compound.
• Making an inference. From the bulb lighting only after melting, infer that the ions must be free to move for the compound to conduct.
• Identifying products. A grey bead means lead is discharged at the cathode; a brown pungent gas means bromine is discharged at the anode.
• Operational definition. “An electrolyte is a substance that conducts electricity and is decomposed when molten or in aqueous solution, shown here by molten lead(II) bromide conducting and breaking down into lead and bromine.”

Safety precautions

• Heat the crucible in a fume cupboard, because the bromine vapour released at the anode is toxic and chokes the lungs.
• Wear safety goggles, because molten lead(II) bromide is hot and can spit.
• Do not touch the crucible or tripod during or soon after heating, because they stay very hot.
• Keep the carbon electrodes apart, so the circuit is not short-circuited and heating stays even.

Common errors

• Not heating strongly enough, so the compound does not fully melt and the bulb never lights, making it seem that no electrolysis occurs.
• Letting the electrodes touch, which short-circuits the cell and prevents proper discharge at each electrode.
• Inhaling the bromine vapour by leaning over the crucible instead of working in a fume cupboard.
• Confusing the electrodes, and writing that bromine forms at the cathode or lead at the anode.
• Assuming water is involved and expecting a gas at the cathode; here the compound is molten, so lead itself is discharged.

How we help

Our online one-to-one SPM Chemistry lessons, taught in English, tie each observation to its half-equation, so you can say why the bulb only lights when molten and write both electrode reactions without hesitating. Fees start from RM50 per hour, and a paid one-hour trial lesson lets you test the approach first. Because molten electrolysis sets the pattern, reduction at the cathode, oxidation at the anode, that carries into every aqueous electrolysis in SPM Chemistry, getting it right here makes the harder cases with water much easier to predict.